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IB DP Chemistry: SL復(fù)習(xí)筆記7.1.2 The Equilibrium Law

Category: IB課程, 教材筆記, 福利干貨 Date: 2022年8月4日下午3:27

The Equilibrium Constant

Equilibrium expression & constant

The?equilibrium expression?is an expression that links the?equilibrium constant,?K_c, to the?concentrations of reactants and products?at equilibrium taking the?stoichiometry?of the equation into account
So, for a given reaction:

aA + bB ? cC + dD

the?K_c?is expressed as follows:

1.7-Equilibria-Equilibrium-Expression

Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium

Solids?are ignored in equilibrium expressions
The?K_c?of a reaction is specific to a given equation

Worked Example

Deduce the equilibrium constant expression for the following reactions

Ag⁺?(aq) + Fe²⁺?(aq) ? Ag (s) + Fe³⁺?(aq)
N₂?(g) + 3H₂?(g) ? 2NH₃?(g)
2SO₂?(g) + O₂?(g) ? 2SO₃?(g)

Answer:

Equilibrium-Constant-Concentrations-Worked-Example-equations

The Size of Kc

The size of K_c?tells us how the equilibrium mixture is made up with respect to reactants and products

7.1.2-Equilibrium-Constant-Expression

If?K_c?> 1, the concentration of products is?greater?than the concentration of reactants and we say that the equilibrium lies to the?right hand?side
If?K_c?< 1, then the concentration of reactants is?greater?than the concentration of products and we say that the equilibrium lies to?the left hand?side
K_cis a constant at a specified temperature
Since temperature can affect the position of equilibrium it follows that?K_c?is dependent on temperature

Worked Example

When the following reactions reach equilibrium, state whether the equilibrium mixture contains mostly reactants or products.Assume the value of K_c?corresponds to the temperature of the reaction mixture

Ag⁺?(aq) + Fe²⁺?(aq) ? Ag (s) + Fe³⁺?(aq)? ? K_c?= 7.3 x 10^-26
N₂?(g) + 3H₂?(g) ? 2NH₃?(g)? ? ? ? ? ? ? ? ? ? ? ? ? ?K_c?= 2.6 x 10^-18
2SO₂?(g) + O₂?(g) ? 2SO₃?(g)? ? ? ? ? ? ? ? ? ? ? ? ?K_c?= 5.0 x 10¹³

Answer:

- 1 and 2: As K_c?is very much smaller than 1 the denominator in the equilibrium expression must be much larger than the numerator so the concentration of the reactants is much larger than the concentration of products. The equilibrium mixture contains?mostly reactants
- 3: As K_c?is very much larger than 1 the numerator in the equilibrium expression must be much larger than the denominator so the concentration of the products is much larger than the concentration of reactants. The equilibrium mixture contains?mostly products