IB DP Chemistry: SL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