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Week 9Chemical EquilibriaElley [email protected]: 2275 037Learning objectives9.1 explain that many chemical reactions do not lead to complete conversion of reactantsto products but to equilibrium mixtures containing both.9.2 write expressions for Kc, Kp and Qc.9.3 use the relationship between K and ΔrG ο to calculate one quantity given the other.9.4 predict how an equilibrium mixture will change in response to the addition or removalof a product or reactant or a change in the pressure or temperature.9.5 calculate equilibrium constants and equilibrium concentrations.29.1 Chemical equilibrium• Many chemical reactions result in the virtually complete conversion of reactants intoproducts.• When the reactants and products are of approximately equal stability, the reaction isreversible.• A reversible reaction is one that can go in either direction, from products to reactantsor reactants to products.39.1 Chemical equilibrium4When the number of people going upis the same as the number goingdown, the number of people on eachfloor is constant.9.1 Chemical equilibrium• Rates of the forward and reverse reactions are equal.• There is no net change in the overall composition of the reaction mixture.• Dynamic equilibrium. ‘Reactants’ substances on the left. ‘Products’ substances on the right.59.1 Chemical equilibrium The reaction read from left to right is referred to as the forward reaction. The reaction from right to left is referred to as the reverse reaction.• After some time, the concentrations of reactants and products undergo no furtherchange although both reactions continue.• At this point, the reaction vessel contains a mixture of all reactants and products, andthe reaction is said to be in a state of chemical equilibrium.69.1 Chemical equilibrium7• The forward reaction takes place rapidly at the beginning of the reaction thenslows down as reactant concentrations decrease.• The reverse reaction takes place slowly at the beginning but then speeds upas product concentrations increase.• Ultimately, the forward and reverse rates become equal.9.1 Chemical equilibrium89.1 Chemical equilibrium9• Chemical equilibrium is an active, dynamic condition.• All substances present are appearing and disappearing at the same rate, sotheir concentrations are constant at equilibrium. The reaction is continuing in both directions.• It is not necessary for the concentrations of reactants and products atequilibrium to be equal.9.1 Chemical equilibrium10• The extent to which the forward or reverse reaction is favoured is acharacteristic property of a reaction under given conditions.• It is possible to predict what the equilibrium conditions will be for any givenreaction. The concentrations of reactants and products are fixed at equilibrium.9.2 The equilibrium constant, K, and the reactionquotient, Q11• For a given overall system composition equilibrium concentrations areindependent of direction of approach.9.2 The equilibrium constant, K, and the reactionquotient, Q12Reactions at equilibrium:• The following holds when equilibrium is established:• Equilibrium constant expression. Kc – equilibrium constant. Kc dependent on temperature, always specify temperature when Kcreported.9.2 The equilibrium constant, K, and the reactionquotient, Q13• For a given overall system composition equilibrium concentrations areindependent of direction of approach.9.2 The equilibrium constant, K, and the reactionquotient, Q14The magnitude of the equilibrium constant:• Product concentrations are in the numerator of Kc• The size of Kc gives a measure of how far the reaction proceeds towardscompletion when equilibrium is reached.• Applies to reactions involving only gasses or species in solution.Worked example 9.115• Write the expression for Kc for the following reaction:• Does this reaction favour reactants or products at equilibrium?9.2 The equilibrium constant, K, and the reactionquotient, Q16Reactions at equilibrium:• The value of the equilibrium constant indicates the position of a reaction atequilibrium. Kc >> 1 indicates the equilibrium reaction mixture contains more productthan reactant (reaction is product favoured). Kc Kc system reacts to use up products and generate more reactantsQc < Kc system reacts to use up reactants and generate more products9.2 The equilibrium constant, K, and the reactionquotient, Q27Quick check28The concentrations of the components of the following reaction mixtureN2 (g) + 3H2 (g) ⇋ 2NH3 (g)were measured to be:[N2] = 4.0 x 10‐2 mol L‐1[H2] = 8.5 x 10‐1 mol L‐1[NH3] = 3.1 x 10‐3 mol L‐1At equilibrium, Kc = 5.1 x 10‐4 for this systemCalculate Qc for the reaction and predict whether it will proceed towardsreactants or products.9.2 The equilibrium constant, K, and the reactionquotient, Q29Manipulating equilibrium constant expressions:• When the direction of an equation is reversed, the new equilibriumconstant is the reciprocal of the original.   3   25 CPCl lPClc K  cK ‘ 3 25PCl Cl PClcc KK‘ 19.2 The equilibrium constant, K, and the reactionquotient, Q30Manipulating equilibrium constant expressions:• When multiplying the stoichiometric coefficients of a reaction, theequilibrium constant is raised to a power equal to that factor.   3   25 CPCl lPClc K   cK 2” 52 23 2PClPCl Cl” 2c cK K9.2 The equilibrium constant, K, and the reactionquotient, Q31Manipulating equilibrium constant expressions:• When chemical equilibria are added, their equilibrium constants aremultiplied.     222221N ON Oc K   cK 422 2 32 2NON O O            42224232224222222N ONON O ONON ON O     cK423 2 42 2NON Oc1 c2 c3K K K 9.2 The equilibrium constant, K, and the reactionquotient, Q32Equilibrium constant expressions for heterogeneous systems:• Homogeneous reaction, all reactants and products are in the same phase.• Heterogeneous reaction, more than one phase exists in reaction mixture:Kc= [H2O][CO2] Do not include the concentrations of pure solids or pure liquids.9.2 The equilibrium constant, K, and the reactionquotient, Q33Equilibrium constant expressions for heterogeneous systems:• For any pure liquid or solid at constant temperature, the ratio of amount ofsubstance to volume of substance is constant.9.3 Equilibrium and Gibbs energy34The relationship between ΔrGθ and K:• If the system is at equilibrium: ΔG = 0 Q = KrG  rG  RT lnQrG  RT ln K9.4 How systems at equilibrium respond tochange35Le Châtelier’s principle:• If an outside influence upsets an equilibrium, the system undergoes a changein a direction that counteracts the disturbing influence and, if possible,returns the system to equilibrium.• Better to compare equilibrium constant, K, and reaction quotient, Q, whenexamining the effect of perturbation to a chemical process at equilibrium.9.4 How systems at equilibrium respond tochange36Le Châtelier’s principle:• When a stress is applied to a system at equilibrium, the equilibrium shifts torelieve the stress. The word “stress” means anything that disturbs the original equilibrium.9.4 How systems at equilibrium respond tochange37Le Châtelier’s principle:9.4 How systems at equilibrium respond tochange38Adding or removing a product or reactant:• When not a pure solid or liquid, removal or addition of a reactant or productinstantaneously alters the concentration of that species in the reactionmixture.• The value of Q changes so that Q ≠ K, the system is no longer at equilibrium.9.4 How systems at equilibrium respond tochange39Adding or removing a product or reactant:  Q = K Q < K Q > Kequilibrium.shift towards products.shift towards reactants.[Cu(H2O)4]2+(aq) + 4Cl‐(aq)⇋ [CuCl4]2‐(aq) + 4H2O(l) Q         242 42 4CuClCu OH Cl9.4 How systems at equilibrium respond tochange40Changing the pressure in gaseous reactions:• Two ways of changing the total pressure: changing the volume of the system adding an inert gas.• Consider the equilibriumN2 (g) + 3H2 (g) ⇋ 2NH3 (g)9.4 How systems at equilibrium respond tochange41Changing the pressure in gaseous reactions:• Option 1: Changing the volume of the system:   Qc2332 2NHN H Vnc  cnQ Vn nV V32 22NH23N H3   cQn nV n 32 22NH 23N HQc ∝

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