Chapter 6: Kinetics6.1: Rates of ReactionsThe rate of a chemical reaction is a measure of the

“speed” of the reactionrate = quantity of a product formed

unit time

rate = quantity of a reactant consumedunit time

in general: rate = Δamount (a reactant or product) Δ time

In symbols: rate = Δ[A] Δt

Or using calculus notation: : rate = d[A] dt

Example: 0.040 mol of a substance is produced in a 2.5 dm3 vessel in 20 seconds. What is the rate of the reaction?

Concentration = 0.040 mol = 0.016 mol dm-3 2.5 dm3

Rate = 0.016 mol dm-3 = 8 x 10-4 mol dm-3 s-1 20 seconds

Example: 22 g of CO2 is produced in 15 seconds in vessel with a capacity of 4 dm3 . What is the reaction rate?

Moles of carbon dioxide = 22g = 0.50 mol 44 g mol-1

[CO2] = 0.50 mol = 0.125 mol dm-3 4 dm3

Rate = 0.125 mol dm-3 = 8.33 x 10-3mol dm-3 s-1 15 seconds

Example: acidified hydrogen peroxide and aqueous potassium iodide react according to the following equation:

2H+ (aq) + H2O2(aq) + 2I- (aq) I2(aq) + 2H2O(l) It was found that the concentration of iodine was 0.60

mol dm-3 after 30 seconds. Calculate the average rate of formation of iodine.

Average rate = [I2]t2 - [I2]t1 t2 - t1

= 0.60 mol dm-3 – 0 mol dm-3 30 sec – 0 sec

= 2 x 10-3 mol dm-3 s-1

2H+ (aq) + H2O2(aq) + 2I- (aq) I2(aq) + 2H2O(l)

For the above reaction notice that for every one iodine molecule formed two water molecules will be formed.

In symbols: Δ[I2] = ½Δ[H2O] Δt Δt

It can also be noted that the rate of formation of iodine is equal to the rate of consumption of hydrogen peroxide.

In symbols: Δ[I2] = - Δ[H2O2] Δt Δt

Given the following general equation:aA + bB → pP + qQ

The rate of a reaction can be defined as:

The graph of concentration vs. Time

The graphs show that the amount of product increases with time and that the amount of reactant decreases with time.

The graphs also show that the rate of a reaction is NOT constant.

The instantaneous rate of a reaction can be determined by taking the slope of the tangent at a particular time.

By drawing tangents at various times, you can determine the reactionrate at those times.Notice that the reaction rate will be at its highest at the

beginning when the concentration of the reactants is at its maximum.

As the reaction proceeds, the concentration of the reactant decreases, and this causes the rate of the reaction to decrease.

Measuring Rates of ReactionsThere are various different ways to monitor the

rate of a reaction. All of them will measure either directly or

indirectly a change in the concentration of either a reactant or product.

The following are some suitable changes that can be monitored:

1. Colour change – one of the reactants or products must have a distinct colour.

rate = Δ colour intensity Δ time

-if a reactant is coloured, the colour of the mixture will fade as the reaction proceeds

-if a product is coloured, the mixture will become more intensely coloured as the reaction proceeds

-a colorimeter can be used to measure colour intensity-example: Cu(NO3)2(aq) + Zn(s) Cu(s) + Zn(NO3)2(aq)

blue grey reddish colourless

2. Change in mass - if a gas is produced, the reaction can be carried out in an open container that is placed on a digital balance.

-as the gas escapes, the mass will decrease-needs to ensure that the gas produced will not

dissolve in the solution-the gas produced should be heavy enough to make a

noticeable changerate = Δ mass

Δ time3. Measure the volume of gas collected - If a gas is

produced, a gas collecting tube or gas synringe can be used to measure the volume of gas collected

4. Change in pH – if one of the reactants or products is either OH- or H+, a pH probe or pH meter can be used to monitor the change in pH

5. Change in conductivity – If there is an overall change in the number of ions, a conductivity probe can be used to monitor the change in conductivity

6. Changes in pressure and volume – if the reactants and products are gases, the reaction can either be carried out at constant pressure and the change in volume can be recorded, or the reaction can be carried out at constant volume and the change in pressure can be recorded . Example:2NO(g) + O2(g) 2NO2(g) As this above reaction proceeds,

if the pressure is kept constant, the volume will decreaseIf the volume is kept constant, the pressure will decrease

7. Withdrawal of samples and titrations - Samples of the reaction mixture can be removed

and titrated with a standard solution to determine the concentration of a particular reactant or product.

- When the sample is removed the reaction needs to be stopped by cooling the mixture or adding a substance to stop the reaction

8. Clock Reactions – one of the products is further reacted with another compound. When that compound is used up, a sharp colour change is observed.