# A2 Equilibria

Kc

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• Created by: Michelle
• Created on: 27-09-09 16:14

## Review from As

Equilibrium= steady rate reached by a chemical reaction where foward and backward reactions both occur at equal rates.

HETERGENEOUS- different phases

HOMOGENEOUS- all the same phase

Le chatelier's principle- IF A REACTION IS AT EQUILIBRIUM AND REACTION CONDITIONS ARE CHANGES THE EQUILIBRIUM POSITION WILL MOVE IN THE DIRECTION WHICH OPPOSES THE APPLIED CHANGE.

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## calculating Kc

• Write a balanced equilibrium equation
• use the equation the write the Kc expression
• if necessary calculate any unknown equilibrium concentrations using C-n/v
• substitute all known values
• calculate the unknown

REMEMBER- its always right over left

aA + bB---->. cC + d [D]

so Kc = [C]^c [D]^d

[A]^a [B]^b

ALWAYS REMEMBER UNITS (Mol/dm^3) and if there are no units write 'NO UNITS'

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## Value of Kc

The Value of Kc is only Affected by A CHANGE IN TEMPERATURE

it is not affected by a change in CONCENTRATION

PRESSURE

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## Equilibrium constants in gaseous systems

Equilibrium constants are normally expressed in terms of concentrations but for reactions involving gases we express the amount of gas in terms of partial pressure

PARTIAL PRESSURE =MOLE FRACTION X TOTAL PRESSURE

MOLE FRACTION IS FOUND USING THIS EQUATION

E.g the mole fraction of A in a mixture of A and B is;

Mole fraction of A=num of moles of A

Moles of A + moles of B

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