IB DP Chemistry: HL复习笔记7.1.3 Equilibrium Constant Relationships

In the previous section we saw that the concentrations of the substances are raised to the power of the coefficients from the balanced equation

This means the K_c expression is dependent on a specific equation

For example, take the reaction between nitrogen and hydrogen to make ammonia

½N_2(g)    +    1½H_2(g)  ⇌     NH_3(g)

The K_c expression for this reaction is:

If you double the stoichiometry the equation becomes

N_2(g)    +    3H_2(g)  ⇌ 2NH_3(g)

The new K_c expression for this reaction is then:

What is the relationship between these two K_c values? You can probably see that when we double the coefficient the new K_c is the square of the original value:

If we reverse the equation:

2NH_3(g) ⇌ N_2(g)    +    3H_2(g)

K_c  becomes the reciprocal of the original K_c value:

Test your understanding in the following example:

Answer:

The correct option is B.

The original equation has been reversed and halved, so the K_c value must be square rooted and inverted to obtain the reciprocal