1. Factors that affect reaction rate  Concentration of REACTANTS  Temperature (kinetic energy of molecules)  Presence of a catalyst  Physical state of reactants (surface area)

2. Higher concentration = more collisions

3. Ways over (or around) the barrier  Temperature increases reaction rate by increasing the fraction of molecules which have sufficient energy to jump the barrier  A catalyst is a way to remove, or at least lower the barrier. A catalyst acts to increase the chemical reaction, but is not consumed itself during the reaction

4. Catalysts modify the pathway  Addition of a chlorine catalyst increases the rate of decomposition of ozone into O 2 – the reason for the destructive influence of these compounds in the atmosphere  Although two barriers are present, both are smaller than the one without the catalyst, and the reaction proceeds more rapidly

5. Without catalysts, there would be no life at all, from microbes to humans  ENZYMES are biological catalysts  Most enzymes are proteins – large molecules  Have correct shape to bring reactant molecules together in correct orientation

6. Natural selection produces prodigious catalysts  Green lines are the amino acid backbone of the enzyme  The white molecule is the fatty acid  Red are iron atoms

7. Catalysts can be solids, liquids or gases

8. The catalyst shape can direct certain products: zeolite molecular sieve Reactant molecules Product molecules

9. Mathematical treatment of reaction rate  In the reaction 2N 2 O 5 = 4NO 2 + O 2 We describe the rate according to the form Rate = k[N 2 O 5 ] n  k is the rate constant  n is the order of reaction with respect to N 2 O 5  NB: n is not the same as the coefficient in the equation!  Rate is normally measured by monitoring growth in concentration of a product, or decline in concentration of a reactant

10. More than one reactant  A + B = products Rate = k[A] m [B] n m and n are individual orders for the reactants A and B. The overall order of reaction = m + n

11. Strategies for determining orders of reaction  Method of initial rates  At t = 0, product concentration is zero  Vary concentration of one reactant, keeping the other(s) constant  Reaction is second order in NO and first order in O 2

12. Effect of temperature: activation energy  As temperature increases rate increases  Arrhenius expression: E A is activation energy  Plot of ln k vs 1/T is constant (T in Kelvin)

13. Activation energy using two temperatures  Measure ratio of rates at two temperatures (Rate is proportional to k)