Chapter 15

Kinetics

Kinetics Deals with the rate of chemical

reactions Reaction mechanism – steps

that a reaction takes Haber Process: uses iron oxide

as a catalyst N2 + 3H2 2NH3

Spontaneous – will occur on its own – not necessarily fast

Reaction Rate Change in the concentration of a

reactant or product over time Rate = Δ[A]/Δt A- Molarity (final – initial) Units – moles/L/s or M/s Reactants can have neg. rates –

change to a +

Average rates – M changes over a given time period

Instantaneous rate – found at any given second.

Rate is affected by 2 factors: 1. Coefficients in the balanced

equation. 2. Time

Rate Laws Concerned only with the

concentration of the reactants Rate = k[A]n

k = rate constant; n = order – must be determined by experimental data

Rate is directly related to concentration

2 types of rate laws:

1. differential - (rate law) – shows how reaction depends on concentration.

2. Integrated – shows how concentration depends on time

Determining the order of the differential rate law:

Order is not the same as the coefficient in the balanced equation

Following method only works for differential rates (concentration and rate are given)

First you must find the order

To determine the order: If there are 2 or more reactants: Find where one of the reactants

concentrations remains the same will the other reactant’s concentration changes.

Compare rates by the division method demonstrated.

It is easier if the larger concentration goes on top.

But whichever experiment goes on top for the concentration, the rate for the same experiment must be on top in the other side.

Order: 1st order – concentration

doubles – rate doubles 2nd order – concentration

doubles – rate quadruples 3rd order – concentration

doubles – rate increases by a factor of 8

0 order – concentration doubles – rate stays the same

Do the same thing for each reactant in the table. Then put them together in one rate law.

Overall reaction order – sum of all the orders

General pattern for units: L(n-1)/mol(n-1)*s N is the overall reaction order To calculate the rate constant –

plug #’s of any row back in the equation and solve for k

Integrated rate laws:

Use this method when concentration and time are given

Must graph integrated rate laws to determine the order.

Slope is related to the rate constant.

To determine the concentration at a certain time, plug back into the appropriate equation for the rate law to solve. The initial concentration is the M at time = 0.

To determine the rate, plug back into the rate law to solve.

Pseudo- first order rate law – if more than 1 reactant

Can determine the rate law by graphing one reactant at a time.

Then put them together in one rate law.

Factors affecting reaction rates:

1. Concentration – (from rate laws)

2. Temperature – speed up when temp. is increased

3. Catalyst – speeds up a reaction without being used up itself

How does temperature affect the reaction rate?

Collision model – molecules must collide to react

Higher the temp – higher the KE – more collisions

Two requirements for reactants to collide:

1. The molecules must be oriented correctly to allow bond formation

2. The collisions must have enough energy to produce a reaction

Activation Energy - Ea

Energy required to break a chemical bond and produce a new one in a chemical reaction

To be broken the KE must be high enough to overcome its bond and convert it to PE in the new product

If the Ea is low – reaction happens fast

If the Ea is high – reaction happens slow

Arrhenius Equation Ln(k) = -Ea/R(1/T) +ln(A) Ln(k) – y axis; (1/T) – x axis When graphed a straight line

should form. R= 8.31J/mol*K Units for Ea = J/mol

Slope = -Ea/R

Ln(k2/k1) = Ea/R(1/T1 – 1/T2) **Use this form if temperature

changes** Catalyst – lower the Ea

Does not affect the overall energy difference of the reaction

2 types of catalyst: A. Homogeneous – same phase as

the reacting molecule B. Heterogenous – different phase

(usually 2 gases being absorbed on the surface of a solid)

Reaction Mechanisms: Series of steps for a chemical

reaction Must meet 2 requirements: 1. Sum of the elementary steps

must give overall balanced equation for the reaction

2. The mechanism must agree with the experimentally determined rate law

Elementary steps – reaction whose rate law can be written by looking at the # of species colliding

3 types of molecularity (same as the order)

A. Unimolecular – 1 molecule – 1st order

B. Bimolecular – 2 species – 2nd order

C. Termolecular – 3 species – 3rd order – rare since all have to hit at the same time

Intermediate – a species that is neither in the overall reactant or product but is used up during the reaction

To determine the rate law look at the rate determining step.

Rate determining step – in multi-step reaction it is the slowest step