Everything Science Grade 12

3.7 CHAPTER 3. REACTION RATES

The meaning of Kcvalues ESCBB

The formula for Kchas the concentration of the products in the numerator and the
concentration of reactants in the denominator. So a high Kcvalue means that the
concentration of products is high and the reaction has a high yield. We can also say
that the equilibrium lies far to the right. The opposite is true for a lowKcvalue. A
low Kcvalue means that, at equilibrium, there are more reactants than products and
therefore the yield is low. The equilibrium for thereaction lies far to the left.

Calculations made easy
When you are busy withcalculations that involvethe equilibrium constant, the follow-
ing tips may help:

1. Make sure that you always read the questioncarefully to be sure of what you are
   being asked to calculate. If the equilibrium constant is involved, makesure that
   the concentrations youuse are the concentrations at equilibrium, and not the
   concentrations or quantities that are present at some other time in the reaction.


2. When you are doingmore complicated calculations, it sometimes helps to draw
   up a table like the one below and fill in the mole values that you know or those
   you can calculate. Thiswill give you a clear picture of what is happening in the
   reaction and will makesure that you use the right values in your calculations.

Reactant 1 Reactant 2 Product 1

Start of reaction

Used up

Produced

Equilibrium

Example 2: Calculating Kc

QUESTION

For the reaction:

SO 2 (g) + NO 2 (g)→ NO(g) + SO 3 (g)

the concentration of thereagents is as follows:

[SO 3 ] = 0. 2 mol.dm−^3

[NO 2 ] = 0. 1 mol.dm−^3

[NO] = 0. 4 mol.dm−^3

[SO 2 ] = 0. 2 mol.dm−^3