Home » 國際課程 » A-level課程 » Details

Edexcel A Level Chemistry:復習筆記1.10.4 Deducing Kc Expressions

Category: A-level課程, 教材筆記, 熱門資訊, 福利干貨 Date: 2022年9月9日下午4:20

Deducing Kc Expressions

Equilibrium expression & constant

The?equilibrium expression?links the?equilibrium constant,?Kc, 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

Kc?is defined as follows:

1.7-Equilibria-Equilibrium-Expression

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

The?Kc?of a reaction is specific and only changes if the?temperature?of the reaction changes

Homogeneous systems and?Kc?

A?homogeneous?system is where all of the reactants and products are in the?same physical state, e.g.

CH3COOH (l) + C2H5OH (l) ??CH3COOC2H5?(l) + H2O (l)

For this reaction, all of the reactants and products are in the same, liquid state / phase and will, therefore, all feature in the?Kc?expression
- Kc =

Heterogeneous systems and?Kc?

A?heterogeneous?system is where not all of the reactants and products are in the same physical state, e.g.

CaCO3?(s) ? CaO (s) + CO2?(g)

Solids?are ignored in equilibrium expressions
- This leads to a?Kc?expression of?Kc?= [CO2]

Exam Tip

For?Kc?expressions, it is important that you use square brackets as sometimes examiners are instructed to be strict about the appearance of brackets in expressions

Square brackets implies concentration