Everything Science Grade 12

CHAPTER 3. REACTION RATES 3.1

Example 1: Reaction rates

QUESTION

The following reaction takes place:

4Li + O 2 → 2Li 2 O

After two minutes , 4 gof Lithium has been used up. Calculate the rateof the

reaction.

SOLUTION

Step 1 : Calculate the numberof moles of Lithium that are used up in

the reaction.

n =

m

M

=

4

6. 94

= 0. 58 mol

Step 2 : Calculate the time (in seconds) for the reaction.

t = 2× 60 = 120 s

Step 3 : Calculate the rate of the reaction.

Rate of reaction =

moles of lithium used

time

=

0. 58

120

= 0. 005

The rate of the reactionis 0.005 mol· s−^1

Exercise 3 - 2

1. A number of different reactions take place. The table below shows the number
   of moles of reactant thatare used up in a particular time for each reaction.

Reaction Moles used up Time Reaction rate

1 2 30 secs

2 5 2 mins

3 1 1.5 mins

4 3.2 1.5 mins

5 5.9 30 secs