Edexcel A Level Chemistry:復習筆記1.10.4 Deducing Kc Expressions
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:
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 =
- 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
Worked Example
Deducing equilibrium expressions
Deduce the equilibrium 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 1:
-
- Kc =
- [Ag (s)] is not included in the equilibrium expression as it is a solid
- Kc =
Answer 2:
-
- Kc =
- Kc =
Answer 3:
-
- Kc =
- Kc =
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