Topic 7: Equilibrium

rxn in which products can react with one another under suitable conditions to produce rxns

• conc. of reactants and products don’t change over time • backward & forward rxns are occurring simultaneously • with the rate of both reactions equal

equilibrium set up in physical processes e.g. melting of solids, evaporation of bromine

equilibrium set up in chemical processes e.g. decomposition of CaCO3

• bromine is a volatile liquid with a boiling point close to room temp • significant number of particles will have enough energy to evaporate • concentration of bromine vapour increases in the closed system • as the vapour can’t escape, many of its particles will condense back

at a given temp, the ratio of conc of products (raised to the power of molar coefficients) to conc of reactants (raised to the power of molar coefficients) is a constant this constant is called Kc NOTE: in aqueous rxns, the conc of the solvent won’t appear in the equilibrium constant expression as its conc won’t change

• if Kc > 0 at a given temp, products are favoured over reactions • if Kc

non-reversible rxns

• denoted by Q • measures relative amount of reactants & products during a rxn at a particular point in time • helps in figuring out which direction a rxn is likely to proceed using pressure/conc of reactants

Kc: describes rxn at equilibrium Q: describes rxn not at equilibrium

• if Kc > Q: rxn is in favour of products (forward) • if Kc = Q: equilibrium • if Kc

inverts the value of Kc 1/Kc

square the expression Kc Kc^2

square root of Kc Kc^(1/2)

multiply the 2 Kc values Kc1 x Kc2

a system of equilibrium, when subjected to change, will respond to minimise the effect of the change

• concentration • pressure • change in temp • catalyst

• increase in product conc: backwards reaction favoured • decrease in product conc: forwards reaction favoured • increase in reactant conc: forwards reaction favoured • decrease in reactant conc: backwards reaction favoured

• increase in pressure = shift towards side with less no of moles • decrease in pressure = shift towards side with higher no of moles

• increase in temp = shift to endothermic side (ΔH = +tive) • decrease in temp = shift to exothermic side (ΔH = -tive) • due to temp having diff effects on forward/backward rxns due to diff activation energies

• catalysts have no overall effect on equilibrium position - it only speeds up the attainment of equilibrium

N2 (g) + 3H2 (g) 2NH3 (g) [ΔH = -93 kJ/mol]

Favourable: • low temp • high pressure Actual: - moderate conditions 450 degrees and 200 atm

low temp = slow rate of rxn = more time taken to produce yield = inefficient high pressure = risk of explosion = costly to rebuild infrastructure