2.3.7 measuring position of equilibrium

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Reactivity 2.3.7 - the equilibrium constant and Gibbs energy change, , can both be used to measure the position of an equilibrium reaction

see 2.3.5 the reaction quotient, Q

the sign of is used to predict the spontaneity of the reaction at a specific composition

calculation of from thermodynamic data

use

rate of reaction and equilibrium

the magnitude of gives no information on the rate of reaction.

considering a reaction that is assumed to occur in a single step: (see 2.2.6, 2.2.7 and 2.2.8 reaction mechanisms)

only in this case, the rate laws can be written from the stoichiometry of the reaction:

where rate constant for the forward reaction and rate constant for the backward reaction

the equilibrium constant expression for this reaction is:

at equilibrium, the rate of forward reaction is the same as the rate of backward reaction, so:

the equilibrium constant is the ratio of the rate constants of forward and backward reactions.

• if is large and reaction progresses towards completion
• if is small and the reaction barely takes place
• increasing the concentration of reactants increases the rate of the forward reaction. following le chatelier’s principle, stays constant
• adding a catalyst increases the values of and by the same factor, so stays constant
• from the Arrhenius equation, , for the forward and backward reactions is different, so they are differently affected by temperature
  • for an endothermic reaction where , increase in temperature has a greater effect on increasing than , so increases as temperature increases