AP Chemistry/Kinetics

TERMINOLOGY

Rate of reaction - change of concentration of reactants/products per unit time

Activation energy - energy required to break the bonds of the reactants at the start of the reaction Rate equation - experimentally determined equation showing the relationship between the rate constant and concentrations of reactants raised to their respective powers

Order of reaction(with respect to a reactant) - is the power to which the reactant concentration is raised in a rate equation

Overall order of reaction - sum of powers to which concentrations of reactants are raised in a rate equation

Rate constant - proportionality constant whose magnitude shows how fast the reaction proceeds

Reaction mechanism - series of steps undergone by a reaction

Rate determining step - slowest step in the reaction mechanism of a reaction

Catalysis - process of speeding up the rate of a reaction using a catalyst. It involves the lowering of activation energy

Homogeneous catalysis - catalysis using a catalyst which is in the same phase with reactants e.g the reaction of S₂O₈^2- with I^- catalysed by Fe²⁺ or Fe³⁺ in an aqueous state as shown below:

          S₂O₈^2- + 2I^- → I₂ + 2SO₄^2-

Using Fe³⁺ : 1. 2Fe³⁺ + 2I^- → I₂ + 2Fe²⁺

        2. 2Fe²⁺ + S₂O₈^2- → 2Fe³⁺ + 2SO₄^2-

WORKED EXAMPLE

1 The mechanism of a reaction is as shown below:

H₂O₂ + I^- → H₂O + IO^- (slow)

H⁺ + IO^- → HIO (fast)

HIO + H⁺ + I^- → H₂O + I₂ (fast)

a Deduce the equation of the reaction

b Write down the rate equation

ANSWER

1(a) step1: cancel out what appears on both sides of the reaction mechanism

 step2: equate what remains on the LHS to what remains on the RHS:

      H₂O₂ + 2I^- + 2H⁺ → 2H₂O + I₂

b rate equation: k[H₂O₂].[I^-]. *{taken from the slowest step/rate determining step}