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Chemical reactions UPM
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The study here focuses on chemical equilibrium sometimes named Chemical Thermodynamics or. Thermochemistry The kinetic problem reaction rates of paramount importance in practice can only be. tackled after its equilibrium Thermodynamics has been settled. From the many different types of chemical reactions we later analyse in detail the combustion process. leaving aside acid base reactions and organic and inorganic synthesis reactions We also leave aside. electrical effects in electrolytes ionic systems electrodes in solution or in gas and plasmas ionised. Stoichiometry, A chemical reaction process is specified by a so called stoichiometric equation for a given reaction. Creactants C,i or 0 i M i, H 2 O or 0 H 2 O H 2 12 O 2 where the first form is preferred for kinetic studies when. e g H 2 12 O 2, a direction in the process is implicit i e initial reactants left converting into products right whereas. the last form is more simple for equilibrium studies where no direction is privileged Notice that. microscopically the reaction is always on both directions to the right and to the left in 9 1 the rates. being balanced when at equilibrium, A stoichiometric equation specifies what chemical species transform the Mi there may be others that are. needed as the catalysts but do not transform globally and in what proportions they transform the i. are called stoichiometric coefficients The same reactants may react differently to give other products. under different conditions e g H2 O2 2OH or H2 O2 H2O2 and even the same reaction may be written. with different stoichiometric coefficients e g 2H2 O2 2H2O or H2 1 2 O2 H2O so it is imperative to. refer to a concrete reaction and not just to the reactants But a given stoichiometric reaction as 9 1 is not. just a label it is the mass conservation equation if Mi is the molar mass of species i and may be the set of. elementary conservation equations when the molecular form of Mi is considered e g for. 0 H2 1 2 O2 H2O 1 1 2 1 mass conservation is 0 iMi 1 2 1 2 32 18 with Mi 2 32 18. g mol conservation of elements are 0 iMi 1 2 1 2 0 2 for hydrogen atoms and. 0 iMi 1 0 1 2 2 1 for oxygen atoms, The state of equilibrium of an isolated system be it reactive or not is the one that maximises the entropy.
of the system and for a system in contact with an infinite atmosphere at T and p constant it is the one that. minimises its Gibbs potential nothing new but we have to introduce new variables and new reference. states to deal with the state of a reactive system. Extent of reaction and affinity, When a mixture with initial composition ni0 reacts within a closed system control mass the degree of. advancement at any later time or extent of the reaction is defined as. Chemical reactions 2,for a given 0 i M i 9 2, i e it is a reduced amount of substance that is independent of the particular species considered the state. variable of chemical progress The variation of extent with time is the reaction rate. but this is not a thermodynamic variable of equilibrium and depends not only on the state of the system. but on the presence of catalysts, To simplify the analysis of equilibrium states G Gmin for a system at T and p constant we introduce the. chemical affinity A defined as,SdT Vdp i dni,A i i such that dG SdT Vdp Ad 9 4. i e at T and p constant dG Ad a positive affinity A 0 makes the system to evolve to the right of. 9 1 and to advance the extent of the reaction 0 such as dG Ad 0 We see that affinity is the. combination of chemical potentials that has the same role in reactions as the individual chemical. potentials in mixtures to mark the natural tendency to evolve. Notice that a single isolated chemical reaction can only proceed if its affinity is positive A 0 at a rate. dictated by its kinetics But if A 0 it may run uphill if external exergy is applied e g forced by solar. radiation ozone formation photosynthesis by electricity electrolysis or by more than one reaction. taking place at the same time in this case Thermodynamics only forces that dG Ard r 0 so that. reactions with negative affinity may proceed coupled to other reaction of positive affinity and at a rate. such that G Ar r 0 what is actually the rule in biological reactions In all these cases of forcing a. reaction uphill against its affinity it is necessary that the activation energy for the reverse downhill. reaction be high to avoid immediate decomposition, Exercise 1 Affinity dependence on extent of reaction.
Enthalpy of formation and absolute entropy, Energy was defined only as increments E W Q 0 Chapter 1 and a reference state had to be assigned for. the system e g specific internal energy u or enthalpy h equal to 0 or other fixed value at a given state. liquid at the triple pint or any other in Chapter 7 Mixtures we adopted an energy reference state for. each of the conservative components in a mixture but this is no longer possible in reactive systems since. chemical species are not conservative However atomic species are conservative in chemical reactive. systems so that only references for chemical elements can be arbitrarily chosen Accordingly we choose. as reference for enthalpy the most stable natural form of the chemical elements at standard temperature. and pressure in Chemistry traditionally taken as T 298 15 K 25 C and p 100 kPa Notice that the. Chemical reactions 3, traditional superscript symbol for standard state is the load line sign proposed by S Plimsoll in mid. 19th century to mark midway between the bow and the stern vessel waterline at nominal load we use the. symbol for typographical reasons, Thus for instance we adopt h h T p 0 for diatomic hydrogen as obtained from water 1H with 115. ppm of 2H h T p 0 for monatomic helium from natural gas 4He with 1 4 ppm of 3He h 0 for. metallic lithium h 0 for carbon in the form of graphite not as diamond or fullerenes with 98 9 of. C and 1 1 of 13C etc, Standard state enthalpies for non elementary species are called enthalpies of formation hf h T p 0. and can be evaluated in the simplest case by calorimetric measurement of the reaction of formation of the. compound from its elements hr i h fi or in many cases indirectly evaluated by Hess rule see. below the most accurate results are obtained by electrochemical measurements in redox systems A. tabulation of standard enthalpies of formation is presented aside including the chemical elements for. which the surname formation added to the standard enthalpy is irrelevant hf h h T p 0 Notice. that standard enthalpies of formation are usually negative because the formation reaction is usually. exothermic use as a mnemonic the case of water which formation reaction coincides with the. combustion reaction H2 O2 H2O and hf 286 kJ mol for alkanes for instance they are all negative. with absolute values increasing with molecular size 75 kJ mol for CH4 126 kJ mol for n C4H10 250. kJ mol for n C8H18 etc The value for the enthalpy of formation of a liquid is always algebraically smaller. than that of its vapour state the difference being the vaporization enthalpy hf g hf l hLV e g. hf C8H18 l 250 kJ mol against hf C4H10 g 208 kJ mol you may use as mnemonic the values for. water hf H2O l 286 kJ mol and hf H2O g 242 kJ mol hLV H2O 2442 kJ kg 44 kJ mol. Notice that sometimes it is postulated that enthalpies of formation are zero for the chemical elements at. any temperature a fact that can be easily demonstrated after 9 11 without any assumption since for the. trivial reaction ME ME hr T cp ME T cp ME T 0, We also adopted an entropy reference state for the working substance Chapter 2 or for each of the.
conservative components in a mixture Chapter 7 and we could proceed now similarly as for enthalpies. and adopt an entropy reference for the conservative entities elementary species but there is an. experimental fact explained by the information measuring entropy in 2 1 that makes another choice. preferable entropies at 0 K may be taken equal to zero not only for elementary species but for any perfect. crystalline compound on the basis that all quantum states are at the singly defined ground level This. conclusion is so important that it is known as the Third Principle or Third Law of Thermodynamics S 0. at T 0 or better S 0 when T 0 since T 0 is an asymptotic limit. Entropies referred to this universal reference state are known as absolute entropies and are computed. from the general expression ds cp T dT vdp 4 8 by means of thermal capacities and phase transition. enthalpies beginning with an extrapolation of the Debye model for solids at cryogenic temperatures. thermal capacities tend to zero at T 0 K as cv 12 4 5 R T TD 3 where TD is Debye s temperature in the. Chemical reactions 4,c1T 3 c2T t, A tabulation of absolute entropies at standard conditions is presented aside Notice that standard absolute. entropies are always positive because all terms in 9 5 are positive values increasing with molecular size. in a similar way as molar cp increases with molecular size more energy levels available However when. thermochemical data of ions are included negative values for s may appear because the reference taken. for ions is s H aq 1 mol litre T p 0, It is customary to include in the thermochemical tabulation not only hf and s but also the standard. Gibbs function of formation gf although it is redundant since. g f hf T i si for the reaction of formation of a compound from its elements 9 6. Notice that in the case of the chemical elements the reaction of formation M M implies isi 0 and. hence hf 0 gf 0 but our initial choice of h 0 for elements does not imply g 0 instead from. definition G H TS g h T s T s 0 but it is the standard Gibbs function for the reaction of. formation gf what matters in reactive systems, The Third Principle of Thermodynamics states that entropy changes tend to zero at very low. temperatures ds T 0 0 a law of Nature first found by Walther Hermann Nernst in 1911 from. experiments with vapours at higher temperature solids at low temperature and galvanic cells A corollary. of it is that T 0 0 since 1 V V T p 1 V S p T and ds T 0 0 Notice that the ideal gas model. does not satisfies the Third Principle for them 1 T as T 0. Additionally one may consider states of a system at 0 K different from the perfectly ordered crystalline. structure e g molecular dipoles oriented at random back and forth instead of ordered one way in which. case a residual entropy s k pilnpi Rln2 8 3 0 69 5 8 J mol K should by assigned to the reference. state at T 0 in the case of H2O a residual entropy s Rln 3 2 8 3 0 4 3 4 J mol K at T 0 is added to. account for the proton disorder possible around oxygen atoms each O atom is surrounded by four H. atoms in a near tetrahedral shape with the two of them belonging to the water molecule being closer than. the two others forming the H bonds,Enthalpy of reaction. Chemical reactions give way to vigorous transfers between the potential energy associated to the position. of the atoms and the kinetic energy associated to the microscopic velocities of the molecules translation. rotation and vibration thus producing a macroscopic heating or cooling temperature change the most. common case being for reactions to be exothermic i e to give off heat when maintained at constant. temperature Several new equilibrium variables are defined to analyse reactive systems e g the enthalpy. of reaction hr,Chemical reactions 5, Let a given chemical reaction 0 iMi be at equilibrium with a generic extent with a Gibbs function.
G T p in a control mass system Notice that for this reaction to be at equilibrium for any generic. some other constrain must act on the system as in electrochemical reactors We define in this. neighbourhood of equilibrium the volume of reaction vr the internal energy of reaction ur the enthalpy. of reaction hr the entropy of reaction sr the Gibbs function of reaction gr and so on as. vr ur hr sr gr 9 7, For instance the volume of reaction vr is the variation of the volume of the control mass when the. reaction proceeds one extent of reaction when one mole of reactive with stoichiometric coefficient equal. to unity is processed Not to be confused with the molar volume v V n or the change in volume by total. amount of substance by the way the total amount of substance is not conservative and an amount of. substance of reaction nr can be defined similarly to 9 7. for instance for H 2 12 O 2, H 2 O nr 1 1 0 5 0 5 i e for every mole of hydrogen processed half a. mole of mixture is lost, For reactions in a gas phase it is valid even with some condensed phase because its volume is usually. negligible the volume of reaction is with the perfect gas model. PGM nRT p RT C, The internal energy of reaction can be deduced from the enthalpy of reaction a more usual variable as. ur hr pvr hr RT i 9 10, The enthalpy of reaction usually has a small variation with temperature since.
hr 2H 2H C, and all cp being of the same order of magnitude and the i of alternating signs the sum nearly cancels. The entropy of reaction is related to the enthalpy of reaction and the Gibbs function of reaction as always. Chemical reactions 6,r hr Tsr 9 12, and from dG SdT Vdp Ad with 9 7 and the Maxwell relation dA dT dS d sr one gets. g r A hr T T hr T 9 13, that is known as Gibbs Helmholtz equation or van t Hoff equation and that will be used below to. compute equilibrium constants based in the above mentioned fact that hr is nearly a constant. Standard enthalpies entropies and Gibbs functions of reactions are computed from the corresponding. tabulated values in the form,i 1 i 1 i 1,Exercise 2 Redundancy in thermochemical data. Measurement of the equilibrium variables 9 7 is difficult may be done in electrochemical cells and the. usual way to measure e g the enthalpy of reaction is by calorimetry in a steady state control volume But. in order to have common reference states for reactions actual values are corrected to correspond to a. standard reactor interfaces Fig 9 1 with as many inputs as reactants that are assumed to enter. separately and as many outputs as products that are assumed to exit separately in spite of the fact that. actual reactors usually have a common exit The corrections for energy functions are small only mixing. and demixing heats but for entropy and Gibbs function the corrections may be important. Fig 9 1 In a standard reactor each reactive and each product is assumed to enter exit pure by a separate. Enthalpy of reaction as the other variables in 9 7 being a state variable its value for a given reaction is. independent of any reaction sequence thought This additivity of hr is known as Hess rule after G H. Hess a Swiss born professor of chemistry at St Petersburg who empirically discovered it in 1840 e g. hr 393 5 kJ mol for C O2 CO2 hr 110 5 kJ mol for C 1 2 O2 CO hr 283 kJ mol for. CO 1 2 O2 CO2 and 110 5 283 393 5, Sometimes the term heat of reaction is employed to define the heat exchange with the environment If.
the reaction is at constant pressure be it in a control mass or a steady control volume the heat of reaction. Chemical reactions 7, is equal to the enthalpy of reaction and if the volume is kept constant in a control mass system the heat. of reaction is equal to the internal energy of reaction see 9 10 In combustion reactions the heat of. reaction is ascribed to the fuel the oxidiser assumed to be freely available as oxygen in the air and the. term heating value is defined as the negative of the heat of reaction to have positive values. Exercise 3 Enthalpy of formation from heat of combustion. Exergy of reaction and exergy of reactants, Exergy Chapter 3 is a state function of the system plus the atmosphere and the balance equations. 7 13 14 apply for any process inclusive of chemical reactions The novelty here is that we want to. know the exergy of substances not existing in the reference atmosphere and thus we need chemical. reactions to synthesise them The analysis is based in the fact that for a process at constant T and p. exergy change equals Gibbs function change T p G Chapter 4 for a steady flow system the. minimum useful work to advance its extent by one mole or the maximum useful work obtained is thus. its Gibbs function of reaction wu min dG d T p gr r. We want to know the minimum work to get a pure chemical component from a reference environment. For a substance in the reference atmosphere Table 3 3 equation 7 15 i RuT lnxi0 directly gives. their molar exergy of separation shown in Table 9 1 e g Ar RuT lnxAr 8 3 298 ln0 0095 11 5. kJ mol except for water vapour where 7 16 must be used. H2O RuT ln xH2Op p T RuT ln 8 3 298 ln0 6 1 26 kJ mol. Table 9 1 Molar exergy of separation of species in the reference atmosphere at 100 kPa. At 288 K and 60 RH At 298 K and 60 RH, Component Molar fraction Molar exergy Molar fraction Molar exergy. N2 0 7720 0 62 kJ molN2 0 7651 0 66 kJ molN2,O2 0 2080 3 8 kJ molO2 0 2062 3 9 kJ molO2. H2O 0 0102 1 2 kJ molH2O 0 0190 1 3 kJ molH2O,Ar 0 0095 11 kJ molAr 0 0094 12 kJ molAr.
CO2 0 0003 20 kJ molCO2 0 0003 20 kJ molCO2, For a substance not in the reference atmosphere e g CO a chemical reaction must be set to produce it. from the existing ones e g CO2 CO 1 2 O2 and the process arranged as in Fig 9 2. Fig 9 2 Example of synthesising a substance out of the reference atmosphere. Chemical reactions 8, Assuming everything is done at the standard temperature and pressure the exergy of demixing CO2. minimum work required from a mechanical reservoir is 20 kJ molCO2 Table 9 1 The exergy to. produce the reaction CO2 CO 1 2 O2 at standard conditions is its Gibbs function. e g for CO2 CO 1 2 O2 r,g f CO 1 2 g f O g f CO 137 0 394 257. kJ molCO2 257, kJ molCO Finally in the mixer one may get an exergy maximum obtainable work contrary to the one for. demixing i e 1 2 3 9 kJ molCO Thus to produce CO from the atmosphere one has to spend a minimum. work of 20 257 2 275 kJ molCO2 that is thence the exergy of CO In general the exergy of synthesising a. component M1 from species Mi in the atmosphere is,g f M i g f M M.
r i M i 1 i i,for 0 M 1 i M i 9 16,i 2 i 2 i 2, Notice the difference between exergy of a substance and exergy of reaction because it is sometimes. confused in combustion reactions For instance when one carelessly says the available energy of a fuel. one usually refers to the exergy of reaction with pure oxygen and not really to fuel exergy similarly as. when one says carelessly the heating power of a fuel one usually refers to the heating power of the. combustion reaction of that fuel with oxygen pure or in a mixture. Take methane for instance its standard enthalpy for the reaction of combustion after 9 14 is. hr h f CO2 2h f H2O h f CH4 2h f O2 393 52 2 285 93 74 85 2 0 890 3 kJ molCH4 also known. changed of sign as its higher heating value The exergy of methane however after 9 16 is. 2 H 2O r 2 O 2 20 2 1 3 818 2 3 9 831 kJ molCH4 and the exergy of the combustion. reaction is after 9 15 r g r g CO,2 g H 2O g CH,2 g O 2 394 38 2 237 18 50 79 2 0 818 0. kJ molCH4 If we had only the standard atmosphere and not natural gas wells obtaining pure methane. would cost at least 831 kJ molCH4 of work now with this methane we could burn it and get 890. kJ molCH4 of heat and no work afterwards we could run a typical thermal engine with this heat and get. some 300 kJ molCH4 of work 30 of 890 or we could perform the same chemical reaction in an. electrochemical cell and obtain at most 818 kJ molCH4 of work the additional 12 kJ molCH4 of work might. be obtained by reversible mixing of the exhaust gases with the atmosphere although with present day. fuel cell technology one could produce some 450 kJ molCH4 of work 50 of 890 more than with the. thermal engine Notice that the 818 kJ molCH4 of maximum work obtainable refers to the standard. conditions of pure CH4 with pure O2 the maximum work obtainable from methane and air in an ideal fuel. cell would be that minus the work required to obtain the oxygen from the air i e 818 2 3 9 810 kJ mol. A summary on Chemical exergy is available aside,Exercise 4 Fuel cell car. Chemical reactions 9,Equilibrium composition, A chemical reaction will proceed in the presence of an atmosphere until G T p xi min i e until. A T p xi 0 thus establishing a relation between T and p and the composition xi at equilibrium what we. intend to elaborate more here First we separate the affinity dependence on temperature and give it a new. name we only deal with ideal gaseous mixtures,i i T p xi.
i i T p i RT ln,A T p xi 9 17, introducing the standard affinity A T p i e the affinity for pure substances at p and the so called. equilibrium constant K it is not a constant because it depends on temperature but it is non. dimensional in the way,A T p i i T p RT ln K T p,such that at equilibrium A 0 from 9 17 one gets. equilibrium,i ln pi or K T p, But from van t Hoff equation 9 13 and assuming hr constant hr as reasoned in 9 11 lnK can be. approximated as lnK C1 C2 T where C1 is obtained from lnK T p A RT gr RT and C2 from. van t Hoff equation dlnK d 1 T hr R what yields lnK gr RT hr RT 1 T T which can. also be set as lnK sr R hr RT The desired relation between equilibrium composition xi and T and p. for a given reaction 0 iMi is then,exp r r 1 9 20,i 1 p p RT RT T. where the constants are computed from the standard enthalpies and Gibbs functions of formation 9 14. For more precise work experimental values of K T are tabulated for important reactions or most often. the related variable pK pK log10K is tabulated because of the exponential character of K. Notice that the equilibrium equation 9 20 and its equilibrium constant K all refer to a single reaction in. an ideal gas mixture Condensed pure species participating in the reaction like carbon reactions in. solution have not been considered here must not be included in the xi not in the pressure summation. because pressure has a negligible contribution to the chemical potential of condensed matter On the other. hand there can be and usually there are several chemical reactions R to consider in a problem How. many The answer is that as many as chemical components intervening C minus the number of atomic. species E involved i e R C E what is known as Volterra s rule Notice that if electrically charged. components are involved one must add electrical neutrality an additional equilibrium constrain The. Chemical reactions 10, variance V number of degrees of freedom of a reactive system in equilibrium with P phases is then.
V 2 C P R 2 E P what is an extension of the traditional Gibbs phase rule analysed in detail in. Chapter 2 Entropy Mind that sometimes F for freedom is used instead of V variance and instead. of P for phases,Exercise 5 Equilibrium composition. Stability of the equilibrium Principle of Le Ch telier. We saw in Chapter 4 that for an equilibrium state to be stable d2S 0 if isolated d2G 0 if in contact with. the atmosphere it has to be thermally stable implies positive isobaric thermal capacity it has to be. mechanically stable implies positive isothermal compressibility and it has to be chemically stable what. implies as above that if perturbed the system evolves trying to counteract the perturbation meaning. If T increases in an exothermic reaction the extent of reaction decreases to produce less. energy and the contrary for an endothermic reaction The demonstration is based on van t. Hoff equation A T 1 T p hr, If p increases in a reaction with nr 0 that generate amount of substance the extent decreases. to produce less moles and the contrary for a nr 0 reaction The demonstration is based on. A p T vr that for gas reactions reduces to A T lnp T R i. If one reactive or product ni increases adding a little the extent changes so that this substance. is partially consumed For ideal mixtures A T lnni T p R i jxj. The first statement of this general stability criterion is due to H Le Ch telier who in 1884 proposed it for. precisely these changes in chemical reactions The most general idea is that stable systems are stable. because they resist an imposed stress i e they respond with an opposing strain unstable systems. Exercise 6 Town gas manufacture, The importance of these qualitative prediction arguments should not be underestimated they yield most. valuable information with minimum effort as may be appreciated in the following example. Consider for instance the oxidation by oxygen of an elementary material M e g hydrogen aluminium. carbon We can put in general that reaction as M O2 MO2 and it might correspond to a quick. combustion process a slow oxidation or a controlled electrochemical process The following questions. can be answered based exclusively on the standard thermochemical data. Question 1 Is M O2 MO2 a natural or an artificial process Answer it only depends on gr or A gr. A 0 means it can take place spontaneously i e without exergy input whereas if A 0 it can only happen. when aided For instance for carbon C O2 CO2 is a natural process at usual T p conditions that takes. place slowly if left alone or quickly if properly triggered but does not demands exergy as can be. deduced from A gr igf I 393 52 0 0 kJ mol The same happens for 2H2 O2 2H2O but not for. Chemical reactions 11, 1 2 N2 O2 NO2 since in the latter case A gr igf I 33 18 0 0 kJ mol It might be argued that. there are several other nitrogen oxides that could be formed but all of them require an exergy source to. be formed The problem of the well known NOx pollution is another question once they are formed at. high temperatures inside combustion devices their dissociation rate is too slow to our convenience. towards the innocuous equilibrium state, Question 2 What effect may a catalyst have Answer adding a catalyst makes absolutely no difference to.
the equilibrium state thus a reaction with A 0 can naturally progress and if A 0 it requires assistance. independently of the catalyst However catalysts can widely change the reaction rate For instance a. suitable catalyst may accelerate the slow dissociation rate of NO2 urea is being tried on diesel engines. and platinum catalyses the slow oxidation of hydrogen to yield a rapid combustion but catalysts cannot. change the direction only the speed both the forward and backward speeds of the dynamic process. since their ratio is the equilibrium constant that is not changed. Question 3 What is the effect of pressure on the decomposition N2O4 2NO2 Answer pressure shifts the. equilibrium to the left towards the smaller amount of substance since A T lnp T R i R 2 1 0. more pressure less affinity The synthesis of ammonia N2 3H2 2NH3 is performed at high pressure. 10 MPa for the same reason whereas it is better e g to carry out natural gas. reforming CH4 H2O 3H2 CO at low pressure, Question 4 What is the effect of an inert gas on the chemical equilibrium Answer it depends on whether. the inert gas is added at constant volume increasing the pressure or at constant pressure increasing the. volume at constant volume there is no effect because concentrations of the reactants ci ni V remain the. same both ni and V whereas at constant pressure the equilibrium will shift towards increasing the. amount of substance since the effect is equivalent to a decrease in pressure for the reactant species. considered alone without the inert one thence for instance adding argon to an isobaric reactor where. N2 3H2 2NH3 is taking place would yield less ammonia. Question 5 What is the effect of temperature on the voltage of an electric battery Answer batteries and. fuel cells yield more electricity higher voltage the lower the temperature because they are exothermic. processes and thus A T 1 T p hr 0 and A T p 0 However other effects like mass transport and. ionic conduction are faster at higher temperatures and this may more than offset the drop in open circuit. voltage Recall that we only analyse equilibrium states and not reaction rates For the same reason the. synthesis of ammonia N2 3H2 2NH3 that is also exothermic and thus would yield more at low. temperatures d dT 0 is actually performed at some 650 K to increase the rate d dT 0. Question 6 Why iron ore is reduced with coal and aluminium ore is not Answer because a plot of. affinities versus temperature shows that aluminium affinity for oxygen is greater than that of carbon. which is greater than that of iron and thus carbon can get hold of the oxygen in iron oxide but not in. aluminium oxide Fig 9 3 It may be interesting to have a closer look at such a plot which is known as. Ellingham diagram First of all what is plotted is A T p defined by 9 18 and computed with the. model developed in 9 20 i e,Chemical reactions 12,A T p RT ln K T p RT r r 1 hr Tsr. showing that plots are straight lines with a slope equal to sr and the ordinate at the origin equal to hr. both values easily found for each reaction from the standard thermochemical values. Let us begin with a trivial case the reaction of liquid water to produce vapour water line 6 in Fig 9 3. for which hr hf H2O g hf H2O l 241 82 285 83 44 kJ mol and. sr s H2O g s H2O l 188 72 69 95 119 J mol Liquid and vapour are at equilibrium when A 0. corresponding to the well known boiling point Tb 373 K Tb hr sr 44000 119 370 K with this. approximate model, As for the oxides it is evident from Fig 9 3 line 7 that NO2 is unstable at any temperature it requires. an exergy supply to be formed whereas the others are stable i e materials have positive affinities for. their oxides But the affinity decreases with temperature because entropy decreases by fixing oxygen gas. into a solid compound and copper oxide becomes unstable above 1750 K. To see why iron ore can be reduced with coal i e to analyse the reaction. 2Fe2O3 s 3C s 4Fe s 3CO2 g it is enough to plot the basic reaction 4 3 Fe O2 2 3 Fe2O3 and. C O2 CO2 taking care to arrange the stoichiometric coefficients to allow for the same amount of. oxygen and subtract i e look at the relative affinities for T 950 K carbon has more affinity than iron. for oxygen and this is the principle of operation for the blast furnace. Notice by the way that hydrogen would work similarly as reducer of iron ore and that aluminium ore. cannot be reduced by either within a practical temperature range availability of charcoal since the Stone. Age replaced by coal since the 17th c is what has dictated the preference of carbon as the metallurgist s. reducer of choice It is apparent from Fig 9 3 that aluminium would be a better reducer of iron oxide than. carbon what is demonstrated in the thermite reaction Fe2O3 2Al Al2O3 2Fe but aluminium is expensive. to get in spite of Al being 8 by mass of Earth crust against 5 Fe or 0 02 C Commercial. production of aluminium was started by H Deville in 1854 by reduction of aluminium chloride produced. by carbo chlorination of alumina with sodium metal produced by reduction of sodium carbonate with. carbon an ingenious example of achieving a desired result by multiple steps where the direct process. carbon reduction of alumina is not feasible, Notice also that we have only considered one of the iron oxide hematite Fe2O3 and only one of the. carbon oxides carbon dioxide CO2 amongst other simplifications e g we have extrapolated beyond the. phase changes as above the melting point of CuO at 1500 K or below the condensation point of water at. Chemical reactions 13, Fig 9 3 Some examples of affinity variation with temperature.
1 43 Al s O 2 g,3 Al 2 O 3 s,2 3 Fe s O 2 g,3 Fe 2 O 3 s. 3 2H 2 g O 2 g 2H 2 O g,4 C s O2 g CO 2 g,5 2Cu s O 2 g 2CuO s. 6 H 2 O l H 2 O g,7 2 N 2 g O2 g,Electrochemical systems Fuel cells. We have only considered chemical reactions in absence of external force fields Gravitational forces do. not introduce new chemical behaviour except at the huge intensities inside stars but electrical forces may. have a profound influence on chemical reactions particularly when ions are involved. A fuel cell is an open system electrochemical reactor fed with a fuel and an oxidiser and given off. electricity heat and reaction products Figure 9 4 shows the concept of a fuel cell in comparison with a. normal chemical reactor, Fig 9 4 A chemical reactor and a fuel cell an electrochemical reactor with an electrolyte sandwiched. between two porous electrodes,Chemical reactions 14.
Taking as example the reaction of hydrogen the fuel most used to study fuel cells because of its fast. kinetics with the oxygen in the air the most ready oxidiser in a normal reactor a combustor in this. case the reaction started for instance by a spark globally corresponds to H2 1 2 O2 H2O and develops. violently may explode greatly rising the temperature of the system However in the electrochemical. reactor a fuel cell the gradient of electrochemical potential at the electrodes forces the diffusion and. ionisation of the substances in the anode a porous conducting catalyst the fuel decomposes as. H2 2H 2e until an equilibrium concentration would be reached for a given temperature and pressure. but the electrons are left to flow through a conductor to an electrical load and finally to a cathode and the. ions are left to diffuse through an electrolyte to the cathode At the cathode a porous conducting. catalyst the oxidiser is reduced by the electrons as O2 4e 2O2 and the ions combine to yield 2H O2. H2O although the combined reaction O2 4e 4H 2H2O is preferred to represent this process because. the O2 ion is not found alone contrary to the H ion that travels alone through the electrolyte. Globally the reaction is H2 1 2 O2 H2O in both cases for the combustor and for the fuel cell and thus. they have the same enthalpy 286 kJ mol and exergy 237 kJ mol of reaction but in the typical. combustor all the chemical energy goes to thermal energy whereas in a fuel cell part of the chemical. energy already goes to electrical work and only the rest goes to the products or as output heat for non. adiabatic reactors in any case i e for reactions at 25 C and 100 kPa the combustor output is q 286. kJ mol and w 0 whereas the fuel cell output is w wmax 237 kJ mol and q 49 kJ mol at 25 C and 100. kPa Notice that the enthalpy of reaction varies only a little with temperature but the exergy of reaction. the Gibbs function of reaction diminish roughly linearly with increasing temperatures. gr hr Tsr hr gr hr T T as shown in Fig 9 5 where the electrical efficiency e defined as. the work output at a given temperature divided by the heat output at standard conditions higher heating. value is presented for a hydrogen fuel cell, Fig 9 5 Thermodynamic limit to the electrical efficiency of a hydrogen oxygen fuel cell at 100 kPa. solid line dash lines correspond to other possible definition of this efficiency. Each electrochemical reaction has its own electromotive voltage that at equilibrium must satisfy. gr zF where z is the number of electrons involved in the reaction equation and F the Faraday constant. F 96485 C mol For instance for the hydrogen oxygen reaction 1 23 V at 298 K and 100 kPa or. 1 03 V at 1000 K that is why fuel cells are usually packed in stacks to reach the desired voltage. The variation of voltage with temperature d dT can be deduced as follows From gr hr Tsr and Maxwell. relation sr A T gr T we get gr hr T dgr dT and dgr dT hr gr T and this variation can be.

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a skeletal chemical equation for the burning of magnesium in air. 1.1.2 Balanced Chemical Equations Recall the law of conservation of mass that you studied in Class IX; mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of the elements present in the products of a chemical reaction

Chemical ReactionsChemical Reactions

Chemical ReactionsChemical Reactions

Section 9.1 Assessment page 288 7. Explain why it is important that a chemical equation be balanced. Because mass is neither created nor destroyed in chemical reactions, the numbers of atoms of all elements must be equal on both sides of the reaction arrow. 8. List three types of physical evidence that indi-cate a chemical reaction has occurred. Answers may include release or absorption of ...

REACTIONS OF IONS AND MOLECULES IN AQUEOUS SOLUTIONS

REACTIONS OF IONS AND MOLECULES IN AQUEOUS SOLUTIONS

the nature of the chemical reactions they undergo, and the products that form. We will also introduce you to another important class of compounds called acids and bases. These are also com-mon substances that include many household products as well as compounds found in all living creatures. In this

Chemistry Notes for class 12 Chapter 4 Chemical Kinetics

Chemistry Notes for class 12 Chapter 4 Chemical Kinetics

Chemistry Notes for class 12 Chapter 4 Chemical Kinetics The branch of chemistry, which deals with the rate of chemical reactions. the factors affecting the rate of reactions and the mechanism of the reaction. is called chemical kinetics. Chemical Reactions on the Basis of Rate of Reaction 1. Fast/instantaneous reactions Chemical reaction which completes in less than Ips (10-12 s) time, IS ...

LAB : FACTORS INFLUENCING ENZYME ACTIVITY

LAB FACTORS INFLUENCING ENZYME ACTIVITY

LAB : FACTORS INFLUENCING ENZYME ACTIVITY Background Enzymes are biological catalysts capable of speeding up chemical reactions by lowering activation energy. One benefit of enzyme catalysts is that the cell can carry out complex chemical activities at a relatively low temperature.

PHOTOSYNTHESIS - Films

PHOTOSYNTHESIS Films

photosynthesis; many of the latter can be dupli-cated in a highschool lab. The first program, "Seeing the Light," develops the chemical equa-tion that represents photosynthesis, and intro-duces the light and dark reactions and the morphological features of photosynthetic organs. "The Light Reaction" traces the pathways of

7.2 Types of Reactions - Physical Science

7 2 Types of Reactions Physical Science

7.2 Types of Reactions Reading Strategy Previewing Skim the section and begin a concept map like the one below that identi?es types of reactions with a general form. As you read, add the general form of each type of reaction. Key Concepts What are the general types of chemical reactions? How did the discovery of subatomic particles affect

CHAPTER FOUR TYPES OF CHEMICAL REACTIONS AND SOLUTION ...

CHAPTER FOUR TYPES OF CHEMICAL REACTIONS AND SOLUTION

CHAPTER FOUR TYPES OF CHEMICAL REACTIONS AND SOLUTION STOICHIOMETRY Questions 9. "Slightly soluble" refers to substances that dissolve only to a small extent. A slightly soluble salt may still dissociate completely to ions and, hence, be a strong electrolyte. An example of such a substance is Mg(OH) 2 . It is a strong electrolyte, but not very ...

PARTIAL PURIFICATION AND CHARACTERIZATION OF LIPASE ENZYME ...

PARTIAL PURIFICATION AND CHARACTERIZATION OF LIPASE ENZYME

PARTIAL PURIFICATION AND CHARACTERIZATION OF LIPASE ENZYME FROM A Pseudomonas STRAIN Lipase is a triacylglycerol-hydrolyzing enzyme which is catalyzed the hydrolysis of water insoluble free fatty acid and glycerols and also a wide range of chemical reactions. Beside, microbial lipases show regiospecificity and enantioselectivity properties ...

Biomolecules: The Chemistry of Life

Biomolecules The Chemistry of Life

Biomolecules On the following slides, the yellow sections are the main points; summarize the slides in your notebooks. Underlined words = vocabulary! All life depends on chemistry to function. During breathing and eating, the body uses oxygen and other nutrients in chemical reactions to keep you alive.

Classifying Chemical Reactions

Classifying Chemical Reactions

Classifying Chemical Reactions Analyzing and Predicting Products Introduction The power of chemical reactions to transform our lives is visible all around us-in our homes, in our cars, even in our bodies. Chemists try to make sense of the great variety of chemical reactions the same way that biologists organize their knowledge of life, by sorting

Enzymes: The Biological Catalysts of Life

Enzymes The Biological Catalysts of Life

1. Introduction Enzymes are proteins functioning as catalysts that speed up reactions by lowering the activation energy. A simple and succinct definition of an enzyme is that it is a biological catalyst that accelerates a chemical reaction without altering its equilibrium. During the reactions the enzymes themselves undergo transient changes ...

Chapter 8 The Reaction-Diffusion Equations

Chapter 8 The Reaction Diffusion Equations

Chapter 8 The Reaction-Diffusion Equations Reaction-diffusion (RD) equations arise naturally in systems consisting of many interacting components, (e.g., chemical reactions) and are widely used to describe pattern-formation phenomena in variety of biological, chemical and physical sys-tems. The principal ingredients of all these models are ...

Chemical reaction hazards and the risk of thermal runaway ...

Chemical reaction hazards and the risk of thermal runaway

identifies the main hazards of carrying out chemical reactions; provides guidance on how you can ensure a safe operation; and identifies some sources of further information and guidance. The advice is aimed at small and medium-sized companies in the chemical manufacturing industry, although the principles apply equally to all firms.

Chapter 7 TEST CHEMICAL REACTIONS

Chapter 7 TEST CHEMICAL REACTIONS

Chapter 7 TEST CHEMICAL REACTIONS PART 1: MULTIPLE CHOICE. _____ 1. Which of these is not a sign of a chemical reaction? a. A gas is given off. c. Heat is released. b. The material dissolves. d. A color change occurs. _____ 2. The substance that is formed in a chemical reaction is called the a. polymer. c. radical. b. reactant. d. product ...

Overview of Types of Organic Reactions and Basic Concepts ...

Overview of Types of Organic Reactions and Basic Concepts

Overview of Types of Organic Reactions and Basic Concepts of Organic Reaction Mechanisms. SCH 102 Dr. Solomon Derese 2 Reactions of organic compounds can be organized broadly in two ways by: I. What types of reactions and II. How these reactions occur. A chemical reaction is the transformation of one chemical or collection of chemicals into another chemical or collection of chemicals. A ...

Chapter 18 Reaction Rates and Equilibrium

Chapter 18 Reaction Rates and Equilibrium

Reaction Rates and Equilibrium RATES OF REACTION Speeds of chemical reactions can be extremely fast or extremely slow. The rate is a measure of the speed of any change that occurs within an interval of time (which could range from fractions of a second to centuries). In chemistry, the rate of a chemical change or the reaction rate is usually ...

Chemical Reactions

Chemical Reactions

Guided Reading Audio Program, Section 10.1 Cosmic Chemistry Videodisc, Disc 2, Side 3 Using the Internet in the Science Classroom, TCR Chemistry Web site: al.science.glencoe.com Optional Resources Challenge Problems, p. 10 TCR Solving Problems: A Chemistry Handbook, Section 10.1, TCR Spanish Resources 10.1 TCR National Science Content Standards: UCP.3, UCP.5; A.1; B.2, B.3, B.6 AL COS: 18, 31 ...

CHAPTER 5 SOLID STATE DIFFUSION

CHAPTER 5 SOLID STATE DIFFUSION

1 18.03.05 10:36 CHAPTER 5 SOLID STATE DIFFUSION INTRODUCTION Numerous chemical reactions or micro-structural changes in solids take place through solid

Development of a Redox-Free Mitsunobu Reaction Exploiting ...

Development of a Redox Free Mitsunobu Reaction Exploiting

13 For a review on catalytic variants of phosphorus(V)-mediated reactions, see S.P. Marsden in Sustainable Catalysis: Challenges and Practices for the Pharmaceutical and Fine Chemical Industries (Ed.: P.J. Dunn and K.K. Hii, Wiley, New York, 2013, p339.

Skills Worksheet Active Reading - Houghton Mifflin Harcourt

Skills Worksheet Active Reading Houghton Mifflin Harcourt

Skills Worksheet Active Reading Section 1: Energy Flow in Ecosystems ... series of chemical reactions that require carbon dioxide and water. The

Enthalpy of Neutralization - ccri.edu

Enthalpy of Neutralization ccri edu

Enthalpy of Neutralization Introduction Energy changes always accompany chemical reactions. If energy, in the form of heat, is liberated the reaction is exothermic and if energy is absorbed the reaction is endothermic.

Chemical Reactions Chapter 12 Study Guide (Unit 9)

Chemical Reactions Chapter 12 Study Guide Unit 9

Chemical Reactions Chapter 12 Study Guide (Unit 9) 2 | P a g e 12.2 Chemical Calculations In chemical calculations, mole ratios are used to convert between moles of reactant and moles of product, or between moles of products. In a typical stoichiometric problem, the given quantity (starting quantity) is first converted to moles. Then the mole ...

Section 7.1 7.1 Describing Reactions

Section 7 1 7 1 Describing Reactions

7.1 Describing Reactions Reading Strategy Monitoring Your Understanding Preview the Key Concepts, topic headings, vocabulary, and ?gures in this section. List two things you expect to learn. After reading, state what you learned about each item you listed. Key Concepts What is the law of conservation of mass? Why must chemical equations be ...

ACD/ChemSketch Tutorial (ver 12.0) - Free

ACD ChemSketch Tutorial ver 12 0 Free

ACD/ChemSketch Tutorial v Before You Begin Thank you for purchasing ACD/ChemSketch. We have endeavored to produce the easiest to use, most powerful program for drawing chemical structures, reactions, schematic diagrams, and

Molecular Mechanisms Involving Free Radical Reactions of ...

Molecular Mechanisms Involving Free Radical Reactions of

Radical Reactions of Antioxidants and Radioprotectors K. Indira Priyadarsini Radiation Chemistry & Chemical Dynamics Division Bhabha Atomic Research Centre erobic organisms produce a number of reactive free radicals (molecules or atoms having unpaired electrons) continuously in cells during respiration, metabolism and phagocytosis. Out of these, the most important source of free radicals being

EXPERIMENT 5 - Double Replacement Reactions

EXPERIMENT 5 Double Replacement Reactions

Double replacement reactions typically form a product that is either molecular or ionic. Molecular products such as H 2 O remain in solution and may not appear visually, but gaseous molecular substances such as CO 2 are usually identified easily by the appearance of bubbles and/or a new odor. A chemical reaction in which an insoluble product (or precipitate) forms is called a precipitation ...

Thermal Methods of Analysis - Springer

Thermal Methods of Analysis Springer

The wide range of applications of thermal methods of analysis in measuring physical properties, studying chemical reactions and determining the thermal behaviour of samples is of interest to academics and to industry. These applications prompted the writing of this book, in the hope that the

ElectrElectrochemistrochemistryy ...

ElectrElectrochemistrochemistryy

Electrochemistry is the study of production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy