CHAPTER 1

Rate of Reaction

Learning Outcomes

You should be able to : state what rate of reaction is, identify observable changes to reactants or products for determining rate of reaction, determine the average rate of reaction, determine the rate of reaction at any given time from a graph, solve numerical problems involving average rate of reaction. solve numerical problems involving rate of reaction at any given time.

What is rate of reaction?

Rate of reaction:

The speed at which reactants are converted into products in a chemical reaction

As the reaction proceeds:

The amount of reactants decreases

The amount of products increase

Classification: • Fast reaction : the time taken is short

the rate of reaction is high

• Slow reaction : the time taken is long

the rate of reaction is low

Measuring the rate of reaction

Observable changes ( visible changes )

Suitable changes which can be observed in a chemical reaction:

a) volume of gas liberated

b) time taken for the precipitate formed

c) change in mass during the reaction

d) colour changes

e) temperature changes

f) pressure changes

Rate of reaction = change in selected quantity

time taken

Example :

Rate of reaction = decrease in mass of CaCO3

time taken

OR

Rate of reaction = increase in volume of CO2

time taken

Measurement rate of reaction

1. Average rate of reaction :

Average value of the rate of reaction within a specified period of time.

a) Average rate of reaction in the first 3 minutes,

b) Average rate of reaction in the third minute

c) Overall average rate of reaction

2. Rate of reaction at a given time:

The actual rate of reaction at that instant.

Also known as :

The instantaneous rate of reaction

Rate of reaction = Gradient of the

at a given time curve at that instant

Step 1

Draw a tangen (line DE) on the curve at time t.

Step 2

Draw the right-angled triangle DEF.

Step 3

Measure the length of DF and EF.

Step 4

Calculate the gradient of the tangent DE.

Total volume of gas (cm3)

Time (s)

t

D

E F

Rate of reaction at time t, = gradient of curve at time t = length of DF length of EF

cm3 s-1