# Equilibria

Important points for unit 4 Equilibria, Aqa chemistry.

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## Kc = equilibrium constant

aA + bB + cC (reverse reaction sign) ** +yY +zZ

the above is valid for any system in eqm.

(X)eqm x (y)eqm y (Z) eqm z

(A) eqm a (B) eqm b (C) eqm c

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## expression for an homogeneous system in eqm.

• Homogenous = all in same state
• units of Kc must be worked out by cancelling out.
• 2A+B (reversible reaction sign) 2C
• Kc =(C)2

(A)2(B)

• = mol-1 dm3
• value can only be found by experiment.
• Method:
• write out equation with mol values under reactants (products will be 0 mol initially)
• question will tell you the mol value for one of the reactants at eqm.
• The equation willl show you the ratio. (if 1:1 then the val at eqm will be the same if they have the same inital values)
• Inital - eqm value = the volume of reactant used up.
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## expression for an homogeneous system in eqm. 2

• Look at ratio of products to reactants and use this to calculate vol of products
• The question will tell the volume of the system. Divide the values of each volumes of products (a) and reactants (b) by the total volume of the system.
• Kc = (a +a) / (b+b)
• In some cases the units will cancel out and so Kc will have no units.
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## Predicting effects of changes in temp and eqm cons

• Type of reaction
• endothermic with decrease in temp
• Kc = decrease
• Volume of products =decreases
• Volume of reactants = increases
• Eqm moves to the left.
• endothermic with increase in temp
• Kc = increase
• volume of products = increases
• volume of reactants = decreases
• eqm moves to the right
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## Predicting effects of changes in temp and eqm cons

• exothermic with increase in temp
• Kc = decrease
• volume of products = decrease
• volume of reactants = increase
• eqm moves to the left
• exothermic with decrease in temp
• Kc = increase
• volume of products = increase
• volume of products = increase
• volume of reactants = decrease
• eqm moves to the right.
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## General rule

• For an exothermic reaction (enthalpy = -ve) increasing temp decrease eqm constant
• For an endothermic reaction (enthalpy = +ve) increasing temp increases eqm constant.
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## Changes in concentration or the addition of a cata

Concentration

• addition to conc will cause eqm to remove changes to reduce disturbance
• top and bottom values will change to produce the same values for Kc

Catalyst

• effects forward and backward reaction equally
• eqm position doesn't move
• effects rate at which eqm is obtained.
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