1. Kinetics 1.1 The rate of chemical reactions 1.2 The rate expression and order of reaction 1.3 Determining the rate equation 1.4 The rate determining step

1.1 Rate of chemical reactions Learning objectives: 1.Explain what rate of reaction means. 2.Calculate the rate of a reaction using experimental results.

Rate of Reaction = Speed of the reaction

How do we measure speed of reaction? Definition – change in concentration (of either reactant or product) with unit time Unit – mol dm -3 s -1, moles per dm 3 per second Usually follow the change in product at constant temperature.

Measuring Rate Experimentally 1.Measure concentration at regular time intervals. 2.Plot concentration over time on a graph. 3.Measure the gradients of the tangents to the line. The gradient of the tangent line is the rate of the reaction.

1.2 The rate expression Learning Objectives: 1.Write a rate expression. 2.Describe the order of a reaction. 3.Calculate the units for the rate constant.

Rate expression Tells us how the concentrations of each substance in the chemical equation contributes to the rate. What might affect the rate of a reaction? Each reaction will have a different rate equation depending on the particular reaction and the conditions.

The rate expression and rate constant Example: Rate = k[A][B] 2 Rate constant = varies depending on the temperature

The order of reaction Definition – describes how much a species contributes to the overall rate Example:Rate = k[A][B] 2 The rate is first order with respect to [A], doubling A doubles the rate. The rate is second order with respect to [B], doubling B quadruples the rate. The overall order of the reaction is the sum of the orders for all species. The overall order is three.

Units of the rate constant The units for the rate constant are unfortunately NOT constant. They need to be calculated using the rate expression.

1.3 Determining the rate equation Learning Objective: 1.Determine the order of the rate equation by experiment. a.using rate/concentration graphs. b.using the initial rate method.

Using a Rate-Concentration Graph Start with a concentration-time graph. Draw tangents to the curve at various intervals to determine the rate at particular concentrations. Plot a graph of rate-concentration. The shape of the graph will tell you the order of the reaction. This only works with respect to one particular species, not the overall order of the reaction.

Initial Rate Method For this method a series of experiments is done with varying starting concentrations. The initial rate is measured for each experiment. A pair of experiments is compared to see what effect changing the concentration of one species has on the rate. ie: if I double concentration of A, does the rate 1) stay the same (zero order), 2) also doubles (first order), or 3) quadruples (second order)

Practice Time! For the following data determine: 1.Order with respect to [NO] 2.Order with respect to [O 2 ] 3.Rate equation 4.Overall order of the reaction

Experiment Number Initial [NO]Initial [O 2 ]Initial rate 11.0 x x x x x x x x x x x x 10 -4

Experiment Number Initial [NO]Initial [O 2 ]Initial rate 11.0 x x x x x x x x x x x x 10 -4

Experiment Number Initial [NO]Initial [O 2 ]Initial rate 11.0 x x x x x x x x x x x x 10 -4

When we doubled the [NO], the rate increased x4. The rate is second order with respect to [NO]. When we doubled the [O 2 ], the rate also doubled. The rate is first order with respect to [O 2 ]. The rate expression is:rate = k[NO] 2 [O 2 ] The overall rate is third order.

Calculating the rate constant To find out the rate constant, simply plug in values from any of the experiments and solve for k. Try it now. Don’t forget units! k = 7 x 10 5 mol -2 dm 6 s -1 Remember that the k value is only valid for a specific temperature.

The effect of temperature on k Small changes in temperature produce large changes in reaction rates. General rule of thumb: 10 K rise = doubles k Example: 2HI  I 2 + H 2 Temperature (K)k/mol -1 dm 3 s x x x x x 10 -3

Why?

1.4 The rate determining step Learning Objectives: 1.Describe what is the rate determining step of a reaction. 2.Describe the connection between the rate equation and the reaction mechanism.

The rate determining step Most reactions take place in multiple steps. We represent these reactions by an overall chemical equation. One step must follow another because the products of one step are the reactants of the next. The slowest step is the rate determining step as it causes a “bottleneck”.

The rate equation can be used to find the rate determining step Example: Nucleophilic Substitution of C 4 H 9 Br with OH - Overall equation: C 4 H 9 Br + OH -  C 4 H 9 OH + Br - Is this a one step mechanism? Or two steps (forms carbocation as an intermediate)? Step 1: C 4 H 9 Br  C 4 H Br - Step 2: C 4 H OH -  C 4 H 9 OH

Three isomers of C 4 H 9 Br. 1-bromobutane reacts with rate equation rate = k[C 4 H 9 Br][OH - ] Which mechanism does this suggest? 2-bromo-2-methylpropane reacts with rate equation Rate = k[C 4 H 9 Br] Which mechanism does this suggest? In the two step mechanism, which is the rate determining step?