kc calculations

Lesson 8-Kc calculations

Lesson Notes

Kc calculations

Consider the following reaction:

N_2(g)  +  3H_2(g)    ↔      2NH_3(g)

5 mol of N₂ and 5 mol of H₂ are injected into a 500 cm³ sealed empty container. Equilibrium is reached at 400 °C. After analysing the mixture, the mass of NH₃ is 20,4 g.

Calculate the value of the equilibrium constant (K_c) at 400 °C.

	N2	3H2	2NH3
Initial	5	5	0
Used/produced	- 0, 6	- 1, 8	+ 1, 2
Equilibrium	= 4, 4	= 3, 2	= 1, 2
Concentration at equilibrium	= 8,8	= 6,4	= 2,4

n = m × M

= 20,4 / (14+3(1))

= 1, 2 mol

H₂ n = 1,2 ÷ 2 × 3

= 1, 8 mol

N₂ n = 1,2 ÷ 2

= 0, 6 mol

N₂ c = n/V

= 4,4 / (500 ÷ 1000)

= 8,8 mol/dm³

H₂ c = n/V

= 3,2 / (500 ÷ 1000)

= 6,4 mol/dm³

NH₃ c = n/V

= 1,2 / (500 ÷ 1000)

= 2,4 mol/dm³

K_c = [products] / [reactants]

K_c  =  [NH₃]² / [N₂][H₂]³

= [2,4]² / [8,8][6,4]³

= 2,5 × 10^-3

EXAM-BASED QUESTIONS

Lesson 1

Study the following graph for the given reaction:

2AB_3(g)        ↔  2AB_2(g)  +  B_2(g)

At the 4^th minute the temperature was increased to 600 °C. Is the forward reaction exothermic or endothermic? Explain your answer.

Lesson 2

Consider the following reaction:

4NH_3(g)  + 5 O_2(g)    ↔     4NO_(g)   +   6H₂O_(g)        ΔH 

Why does a decrease in temperature not yield much NO gas?

Lesson 3

What does each of the following K_c values indicate:

K_c>  1     

K_c  =  1   

K_c

Lesson 4

Write down the K_c expression for the following reaction:

CaCO_3(s)         ↔  CaO_(s)  +  CO_2(g)

If the K_c value is 0,018, calculate the concentration of CO₂ present.

ANSWERS

Lesson 1

When the temperature is increased the system would like to decrease it by favouring the endothermic reaction.

The products decreased and the reactant increased, thus the reverse reaction was favoured.

Therefore the reverse reaction is endothermic and thus the forward reaction is exothermic.

Lesson 2

The forward reaction is exothermic. The forward reaction is favoured by cooling, but at a low temperature the rates of reactions are slow and it takes a long time for the system to reach equilibrium. So the amount of NO produced in a reasonable time is not economical.

Lesson 3

More products (forward reaction favoured)

Same amount of products and reactants

More reactants (reverse reaction favoured)

Lesson 4

K_c = [products] / [reactants]

K_c  =  [CO₂]/ [1][1]

K_c  =  [CO₂]

CO₂ = 0,018 mol/dm³