1TENTH EDITIONGENERAL CHEMISTRYPrinciples and Modern ApplicationsPETRUCCIHERRINGMADURABISSONNETTEPETRUCCI HERRING MADURA BISSONNETTE Principles of Chemical Equilibrium15Reaction Mixtures in Dynamic Chemical EquilibriumReversible ReactionsO(l)H(g)CO(s)CaCO(aq)HCO2(aq)Ca223-32Reaction Mixtures in Dynamic Chemical EquilibriumReversible ReactionsO(l)H(g)CO(s)CaCO(aq)HCO2(aq)Ca223-32Reaction Mixtures in Dynamic Chemical EquilibriumReversible ReactionsReversible chemical reactions can go in either direction, depending on the conditionsdepending on the conditions.Dynamic chemical equilibrium: a state in which the opposite reactions occur at equal rates so that there is no change of concentrations of species.O(l)H(g)CO(s)CaCO(aq)HCO2(aq)Ca223-32⇌Net ReactionThe change of concentrations of species due to unequal rates of the reactions in opposite directions, when the system is not at equilibrium.
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2The Reaction Quotient and the Equilibrium ConstantReaction Quotient (Q)For a reaction represented by a A + b B ⇌c C + d DEXAMPLE : Reaction between hydrogen and iodine gases at 425 °Cto produce hydrogen iodide gas badcBADCQ222IHHIQH2(g)+I2(g)⇌2HI (g)Initial(mol L-1 )0.01750.017500Concentration at time 1(mol L-1 )0.01500.01500.00500.111Concentration at time 2 (mol L-1 )0.01000.01000.01502.25Concentration at time 3 (mol L-1 )0.00370.00370.027656The Reaction Quotient and the Equilibrium ConstantLaw of equilibrium: for a given reaction at a specified temperature, all equilibrium mixtures have the same value of the reaction quotient (Q) This numerical value). This numerical value of Qis called the equilibrium constant (K).The Reaction Quotient and the Equilibrium ConstantThe form of Qand KFor reactions involving solids, concentration terms for the solids are not incl ded in e pressions forQKsolids are not included in expressions for Q or K.CaCO3(s) ⇌CaO(s) + CO2(g)For reactions taking place in aqueous solution, the molar concentration of water is not included in expressions for Q 2COQor K.NH3(aq) + H2O (l) ⇌NH4+(aq) + OH-(aq)34NHOHNHQThe Reaction Quotient and the Equilibrium ConstantActivity-based Equilibrium ConstantsReaction quotients are most correctly expressed asReaction quotients are most correctly expressed as functions of activities of species rather than as concentrations. Activities are measures of effective concentration, and as a ratio compared with 1 mol L-1, are unitless.In sufficiently dilute solutions (which is concerned inIn sufficiently dilute solutions (which is concerned in this text), there is negligible difference between the concentrations and activities of species.