Edexcel A Level Chemistry:复习笔记5.1.1 Deducing Kp Expressions
Kp Expressions
- We have seen previously that equilibrium reactions can be quantified by reference to an equilibrium expression and equilibrium 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
- Solids?are ignored in equilibrium expressions
- The?Kc?of a reaction is constant and only changes if the?temperature?of the reaction changes
Homogeneous reactions
- In the generic example above, if all the substances are gases, we can show the equation with that state symbol
aA (g) + bB (g) ? cC (g) + dD (g)
- We can write a different equilibrium expression in terms of the?partial pressure?of the gases
- This equilibrium constant is called?Kp?and is defined as follows
Equilibrium expression linking the partial pressures of reactants and products at equilibrium
Heterogeneous reactions
- For heterogenous reactions, solids?and?liquids?are ignored in?Kp?equilibrium expressions
- The?Kp?of a reaction is constant and only changes if the?temperature?of the reaction changes
Worked Example
Write a?Kp?expression for the following equilibria and deduce the units of?Kp?:
- N2?(g) + 3H2?(g)?? 2NH3?(g)
- CaCO3?(s) ?? CaO?(s) + CO2?(g)
Answer 1
Answer 2
Kp?=?pCO2(g) = kPa
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