Everything Science Grade 12

3.7 CHAPTER 3. REACTION RATES

[O 2 ] =

3 mol· dm−^3

6

= 0. 5 mol· dm−^3

Example 4: Equilibrium calculations

QUESTION

Initially 1.4 moles of NH 3 (g) is introduced into asealed 2.0 dm−^3 reaction

vessel. The ammonia decomposes when the temperature is increased to600K

and reaches equilibriumas follows:

2 NH 3 (g)� N 2 (g) + 3H 2 (g)

When the equilibrium mixture is analysed, the concentration of NH 3 (g) is 0.3

mol·dm−^3

1. Calculate the concentration of N 2 (g) and H 2 (g) in the equilibrium mix-
   ture.


2. Calculate the equilibrium constant for the reaction at 900 K.

SOLUTION

Step 1 : Calculate the number of moles of NH 3 at equilibrium.

c =

n

V

Therefore,

n = c× V = 0. 3 × 2 = 0. 6 mol

Step 2 : Calculate the number of moles of ammonia that react (are ’used

up’) in the reaction.

Moles used up = 1. 4 − 0 .6 = 0. 8 mol

Step 3 : Calculate the number of moles of product thatare formed.

Remember to use the mole ratio of reactants toproducts to do

this. In this case, the ratio of NH 3 :N 2 :H 2 = 2 : 1 : 3. Therefore,

if 0.8 moles of ammonia are used up in the reaction, then 0.4

moles of nitrogen are produced and 1.2 molesof hydrogen are

produced.

Step 4 : Complete the followingtable