Rate LawsRate Laws

Rate LawsRate Laws

Rate = k [A]Rate = k [A]xx [B] [B]yy

[A] and [B] represent the [A] and [B] represent the concentrations of reactantsconcentrations of reactants

x and y often (but not always) x and y often (but not always) represent the coefficients in front of represent the coefficients in front of [A] and [B][A] and [B]

k is a constant specific to the reaction. k is a constant specific to the reaction.

Rate LawsRate Laws

Rate = k [A]Rate = k [A]xx [B] [B]yy

kk A constant specific to the reaction. A constant specific to the reaction. Rate constants may have a variety of Rate constants may have a variety of

strange units using L, mol, and sec. strange units using L, mol, and sec. This is so the rate will always work out This is so the rate will always work out in mol/L sec.in mol/L sec.

Rate LawsRate Laws

For example:For example:

2NO + O2NO + O22 2 NO 2 NO22

The rate expression is:The rate expression is:

Rate = k [NO]Rate = k [NO]22[O[O22]]

Rate LawsRate Laws

For example:For example:

2 N2 N22OO5 5 4 NO 4 NO22 + O + O22

The rate expression isThe rate expression is

Rate = k [NRate = k [N22OO55]]22

Rate LawsRate Laws

Order in reactions: Order in reactions:

refers to the exponents on the refers to the exponents on the compounds in the rate law compounds in the rate law

Rate = k [NO]Rate = k [NO]22[O[O22]]

This reaction is second order for NO This reaction is second order for NO and first order forand first order for OO22

Rate LawRate Law

This rate expressionThis rate expression

Rate = k [NO]Rate = k [NO]22[O[O22]]

is third order overall.is third order overall.

Rate LawsRate Laws

If a reactant is first order, doubling the If a reactant is first order, doubling the concentration will double the rate.concentration will double the rate.

If a reactant is second order, doubling If a reactant is second order, doubling the concentration will quadruple the the concentration will quadruple the rate.rate.

If changing the concentration has no If changing the concentration has no effect the chemical does not appear in effect the chemical does not appear in the rate law.the rate law.

Rate LawsRate Laws

To determine the rate constant, To determine the rate constant, one must collect data using one must collect data using different concentrations of different concentrations of reactants.reactants.

Rate LawsRate Laws

For example:For example:

HH22OO22 + 2HI + 2HI 2H 2H22O + IO + I22

TrialTrial [H[H22OO22]] [HI][HI] RateRate

11 0.1 M0.1 M 0.1 M0.1 M .0076.0076

22 0.1 M0.1 M 0.2 M0.2 M .0152.0152

33 0.2 M0.2 M 0.1 M0.1 M .0152.0152

Rate LawsRate Laws

If [HI] doubles, the rate doubles, so If [HI] doubles, the rate doubles, so the rate expression is first order the rate expression is first order for HI.for HI.

If [HIf [H22OO22] doubles, the rate doubles, ] doubles, the rate doubles, so the rate expression is first so the rate expression is first order for Horder for H22OO22..

Rate LawsRate Laws

The rate expression must beThe rate expression must be

Rate = k [HRate = k [H22OO22] [HI]] [HI]

Rate LawsRate Laws

Plug in data from any trial to solve for Plug in data from any trial to solve for k:k:

.0076 = k (0.1M) (0.1M).0076 = k (0.1M) (0.1M)

.0076 = .01k.0076 = .01k

K = .76K = .76

Rate LawsRate Laws

If a reaction has two or more steps, If a reaction has two or more steps, the rate-determining step is the the rate-determining step is the slowest one.slowest one.

Rate LawsRate LawsRate-determining StepRate-determining Step

2 NO2 NO(g)(g) + 2H + 2H22(g)(g) N N22(g)(g) + 2 H + 2 H22OO(g)(g)

Elementary StepsElementary Steps

2 NO 2 NO N N22OO22 (fast) (fast)

NN22OO22 + H + H22 N N22O + HO + H22O + HO + H22 (slow) (slow)

NN22O + HO + H22 N N22 + H + H22O (fast)O (fast)

Rate LawsRate LawsRate-determining StepRate-determining Step

Which is the rate-determining step?Which is the rate-determining step?

2 NO 2 NO N N22OO22 (fast) (fast)

NN22OO22 + H + H22 N N22O + HO + H22O + HO + H22 (slow) (slow)

NN22O + HO + H22 N N22 + H + H22O (fast)O (fast)________________________________________________________________________________________________________________________________________________________________________

2 NO2 NO(g)(g) + 2H + 2H22(g)(g) N N22(g)(g) + 2 H + 2 H22OO(g)(g)

Rate LawsRate LawsRate-determining StepRate-determining Step

Which is the rate-determining step?Which is the rate-determining step?

2 NO 2 NO N N22OO22 (fast) (fast)

NN22OO22 + H + H22 N N22O + HO + H22O + HO + H22 (slow) (slow)

NN22O + HO + H22 N N22 + H + H22O (fast)O (fast)________________________________________________________________________________________________________________________________________________________________________

2 NO2 NO(g)(g) + 2H + 2H22(g)(g) N N22(g)(g) + 2 H + 2 H22OO(g)(g)