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IB DP Chemistry: HL復習筆記7.1.3 Equilibrium Constant Relationships

Category: IB課程, 教材筆記, 福利干貨 Date: 2022年7月27日下午5:30

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

?N_2(g)??? +??? 1?H_2(g)? ?? ? ?NH_3(g)

The?K_c?expression for this reaction is:

7.1.3-Kc-expression-for-NH3

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:

7.1.3-Kc-expression-for-NH3-doubled

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:

7.1.3-Kc-relationships-squared

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:

7.1.3-Kc-expression-for-NH3-reversed

Test your understanding in the following example:

Worked Example

K_c?for 2NO₂?(g) + F₂?(g) ? 2NO₂F (g) is 7.1 × 10³²What is?K_c?for the following reaction, at the same temperature?

NO₂F (g) ?? NO₂?(g) + ?F₂ (g)

7.1.3-Worked-Example-Question6

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

Exam Tip

You must use square brackets in equilibrium constant expressions as they have a specific meaning, representing concentrations. In an exam answer you would lose the mark if you used round brackets

轉載自savemyexams

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