Everything Science Grade 12

3.7 CHAPTER 3. REACTION RATES

H 2 (g) I 2 2HI

Start of reaction 0.496 0.181 0

Used up 0.1735 0.1735 0

Produced 0 0 0.347

Equilibrium 0.3225 0.0075 0.347

Step 4 : Calculate the concentration of each of the reactants and prod-

ucts at equilibrium.

c =

n

V

Therefore the equilibrium concentrations are asfollows:

[H 2 ] = 0. 3225 mol.dm−^3

[I 2 ] = 0. 0075 mol.dm−^3

[HI] = 0. 347 mol.dm−^3

Step 5 : Calculate Kc

Kc=

[HI]

[H 2 ][I 2 ]

=

0. 347

0. 3225 × 0. 0075

= 143. 47

Exercise 3 - 5

1. Write the equilibriumconstant expression, Kcfor the following reactions:
   (a) 2 NO (g) + Cl 2 (g)� 2 NOCl
   (b) H 2 (g) + I 2 (g)� 2 HI (g)


2. The following reaction takes place:

Fe3+(aq) + 4Cl−� FeCl− 4 (aq)

Kcfor the reaction is 7.5× 10 −^2 mol.dm−^3. At equilibrium, the concentration

of FeCl− 4 is 0.95× 10 −^4 mol.dm−^3 and the concentration of free iron (Fe3+) is

0.2 mol.dm−^3. Calculate the concentration of chloride ions atequilibrium.

3. Ethanoic acid (CH 3 COOH) reacts with ethanol (CH 3 CH 2 OH) to produce ethyl
   ethanoate and water. The reaction is:

CH 3 COOH + CH 3 CH 2 OH→ CH 3 COOCH 2 CH 3 + H 2 O

At the beginning of thereaction, there are 0.5 mol of ethanoic acid and0.5 mol

of ethanol. At equilibrium, 0.3 mol of ethanoic acid was left unreacted. The

volume of the reaction container is 2 dm^3. Calculate the value ofKc.

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