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Reversible Reactions
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A reversible reaction is a chemical change in which the products can be converted
back to the original reactants under suitable conditions
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Most reactions are not reversible (irreversible) and have the usual complete arrow
only pointing to the right
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On cooling the reaction is reversed and solid ammonium chloride reforms
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The decomposition is endothermic (heat absorbed or heat taken in) and the
formation of ammonium chloride is exothermic (heat released or heat given
out)
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 Similarly, ammonium sulphate also sublimes when heated above
235oC and thermally decomposes into ammonia gas and sulphuric acid
vapour
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When the white solid is cooled and water added, blue hydrated copper(II) sulphate is
reformed
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5H2O(s)
CuSO4(s) + 5H2O(g)
The dehydration decomposition to give the white solid is the forward reaction
and the 're-hydration' to reform the blue crystals is the backward reaction
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o

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white anhydrous copper(II)

o
o
o

The thermal decomposition is endothermic as heat is absorbed to drive off the
water
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e
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The reverse reaction is used as a simple chemical test for water i
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white
anhydrous copper(II) sulphate turns blue
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Reversible reactions and Equilibrium

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When a reversible reaction occurs in a closed system an equilibrium is formed, in
which the original reactants and products formed coexist
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Le Chatelier's Principle
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Or
It states that when a change is made to the conditions of a system in dynamic
equilibrium,the system moves so as to oppose that change
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Rule 1b: If the forward reaction forming the product is exothermic, decreasing the
temperature favours its formation (increasing temperature decreases the yield)
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Rule 2b: Decreasing the pressure favours the side of the equilibrium with the most number
of gaseous molecules as shown by the balanced symbol equation
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Rule 3a: If the concentration of a reactant (on the left) is increased, then some of it must
change to the products (on the right) to maintain a balanced equilibrium position
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-2-



e
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nitrogen + hydrogen
ammonia
o or N2(g) + 3H2(g)
2NH3(g)
o If the nitrogen or hydrogen concentration was increased, some of this extra
gas would change to ammonia
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 So in terms of enforced change ==> system response:
 Increasing nitrogen ==> decreases hydrogen and increases
ammonia
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 Increasing ammonia ==> increases nitrogen and hydrogen
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 Decreasing nitrogen ==> increases hydrogen and decreases
ammonia
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Rule 4: A catalyst does NOT affect the position of equilibrium, you just get there faster! A
catalyst usually speeds up both the forward and reverse reaction but there is no way it can
influence the final 'balanced' concentrations
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g
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Dynamic equilibrium - A dynamic equilibrium exists once a reversible reaction ceases to
change its ratio of reactants/products, but substances move between the chemicals at an
equal rate, meaning there is no net change
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At the beginning of the reaction, the
concentrations of A and B were at their maximum
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As A and B react, their concentrations fall
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-3-

In the beginning, there isn't any C and D, so there can't be any reaction between them
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With time, the rate of the reaction between C and D increases:

Eventually, the rates of the two reactions will become equal
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At this point there won't be any further change in the amounts of A, B, C and D in the
mixture
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We have reached a position of dynamic equilibrium
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Temprature;
1
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2
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3
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e
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Eg
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In the above reaction if the temperature increase the backward reaction is favoured
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5
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6
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Pressure
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In a gaseous reaction, increase in pressure favours the side of the reaction where there is
less volume
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N2(g) +

3H2(g)

2NH3( g)

1 mole
At rtp;

3 mole

24 dm3

3 x 24dm3

1 vol

2 mole
2 x 24 dm3

3 vol

2 vol

4 mole

2 vol

2
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This means more ammonia is going
to be present in the equilibrium mixture
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A decrease in pressure favours the backward reaction
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2 SO2(g) + O2(g)
3 vol

2SO3(g)
2 vol

Increase in pressure in the above reaction shift the equilibrium to the right
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Concentration;
1
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In industry this is done by cooling the mixture
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3
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