Chemical Kinetics and Equilibrium
This domain tests how reaction rates are measured and explained, how rate laws are deduced from experiment, and how equilibrium constants and Le Chatelier's principle predict the response of a system to changing conditions.
Rates and orders
- Rate is the change in concentration per unit time, with units mol dm−3 s−1; for a reactant it is quoted as a positive value from the decrease.
- Order is experimental and is not taken from the stoichiometric coefficients. Order with respect to a reactant is found by varying one concentration at a time.
- Zero order: rate constant, so a concentration–time plot is linear. First order: constant half-life. Second order: a plot of 1/[A] against time is linear.
Collision theory and Arrhenius
- A reaction occurs when collisions have energy at least equal to Ea and correct orientation. Rate rises with concentration, surface area and temperature, because a greater fraction of molecules exceeds Ea.
- k = Ae−Ea/RT; a plot of ln k against 1/T has gradient −Ea/R. Catalysts lower Ea.
Equilibrium
- Equilibrium is dynamic: forward and reverse rates are equal and concentrations remain constant, but they are not equal to each other.
- Kc = [products]^coefficients/[reactants]^coefficients, omitting pure solids and liquids.
- K changes only with temperature. A catalyst speeds both directions equally and does not shift the position of equilibrium.
- Le Chatelier: raising concentration of a reactant drives the forward reaction; increasing pressure favours the side with fewer moles of gas; increasing temperature favours the endothermic direction.
Worked example
Doubling [A] while [B] is fixed quadruples the rate, so the reaction is second order in A; doubling [B] doubles the rate, so it is first order in B. Rate = k[A]²[B]. When rate = 2.0 × 10−3 mol dm−3 s−1 at [A] = [B] = 0.10 mol dm−3, k = 2.0 × 10−3/(0.0100 × 0.10) = 2.0 dm6 mol−2 s−1.
Traps that cost marks
- Deriving the rate law from the balanced equation rather than from data.
- Giving k units that do not match the overall order.
- Claiming a catalyst increases yield or shifts equilibrium.
- Adding an inert gas at constant volume and expecting a pressure shift; partial pressures of the reactants are unchanged.
- Forgetting that K is unaffected by pressure and concentration changes.