Module K: Kinetics

Nov 24-26Module K: Kinetics Rate of chemical changeKINETICS = RATE OF CHEMICAL CHANGEThe reaction rate is defined either as the increase in the concentration of a product per unit time or as the decrease in the concentration of a reactant per unit time. For the reaction: 2N2O5(g) 4NO2(g) + O2(g)

Reaction rate

Reaction rateThe rate of formation of O2 is the rate of formation of NO2 and the rate of decomposition of N2O5.

2N2O5(g) 4NO2(g) + O2(g)

Reaction rate

The dependence of the reaction rate on the concentration of each reactant is given by an equation called the rate law. For the generic reaction: aA + bB cCRate = k [A]m[B]n

m and n are orders of the reaction with respect to that reactant. (A and B, respectively) k is the rate constant

The exponents m and n in the rate law indicate how sensitive the rate is to changes in [A] and [B], and they are generally unrelated to the coefficients a and b in the balanced equation. These can only be determined experimentally. Rate lawRate = k [A]m[B]n

If m = 1, then the reaction is first order with respect to A.If n = 3, then the reaction is third order with respect to B.

Rate law becomes:Rate = k [A][B]2The overall order of the reaction will be the sum of the orders with respect to each reactant. overall order = 1 + 2 = 3. Order of reactionAn exponent of 1 means that the rate depends linearly on the concentration of the corresponding reactant: if m =1, and [A] is doubled, the rate doubles. If m = 2, and [A] is doubled, [A]2 quadruples and the rate increases by a factor of 4. If m is zero, the rate is independent of the concentration of A because any number raised to the zeroth power equals one ([A]0 = 1 ).Order of reaction

For the reaction: 2 NO(g) + O2(g) 2 NO2(g)

the following rate data were collected using various concentrations of reactants:

Determine the order with respect to each reactant and determine the rate law of the reaction. Determination of rate law