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Delivery Guide (Learner Resource)
Equilibria
H070
H470Topic
TopicTitle
Title
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• Most chemical reactions can only go one way. The reaction of
methane and oxygen for example:
CH4(g) + O2(g)  CO2(g) + 2H2O(l)
• It is almost impossible to return the carbon dioxide and water to the
original methane and oxygen.
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• Some chemical reactions, however, will go backwards and forwards
depending on the conditions.
CoCl 2.6H2O(s)  CoCl 2(s) + 6H2O(l)
pink
blue
This is the symbol for used for reversible reactions.
CoCl2·6H2O(s)
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CoCl2(s) + 6H2O(l)
Equilibrium
• Reversible reactions reach a balance point, where the amount of
reactants and the amount of products formed remains
constant.
• Most common misconception is that they are equal. This is not the
case. They remain constant.
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Position of the equilibrium
• Equilibrium can “lie” to the left or right.
• This is “in favour of products or the forward reaction” or
“in favour of reactants or the reverse reaction”
• Meaning that once equilibrium has been reached there could be
more products or more reactants in the reaction vessel.
LEFT
RIGHT
(REACTANTS)
(PRODUCTS)
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Le Chatelier’s principle involving
AMOUNT or CONCENTRATION
• If you increase A or B you will make more C or D so it will FAVOUR
the forward reaction and the equilibrium will LIE to the right.
A B
C D
• If you increase C or D you will make more A or B so it will FAVOUR
the reverse reaction and the equilibrium will LIE to the left.
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Le Chatelier’s principle involving
TEMPERATURE
• If you increase the temperature of an EXOTHERMIC REACTION
the reaction will try to cool it down so will FAVOUR the reverse
reaction and equilibrium will LIE to the left
A
B
C
D
HEAT
• If you cool down an EXOTHERMIC REACTION it will try to increase
the heat so will FAVOUR the forward reaction and equilibrium
will LIE to the right.
• NB: If it is exothermic forward, it will be endothermic in reverse.
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Le Chatelier’s principle involving
PRESSURE
• If you increase the pressure it will favour the reaction with the
smallest number of moles. In this case it would FAVOUR the
reverse reaction and equilibrium will LIE to the left.
A
B
C
D
E
• If you lower the pressure it will favour the reaction with the most
number of moles. In this case it would FAVOUR the forward
reaction and equilibrium would LIE to the right.
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Strong and weak acids
• A strong acid is one which is FULLY IONISED in water. It will have a
high hydrogen ion concentration
in this case ALL of the H+ is made so strong acid pH1
• A weak acid is one which is NOT fully ionised and is in equilibrium.
It has a low hydrogen ion concentration
In this case there are SOME H+ but not ALL so this makes the
acid weaker (ie pH3) .
EQUILIBRIUM DOES NOT LIE TO THE RIGHT.
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• http://www.youtube.com/watch?v=iWpfHkWr5DY
• http://www.youtube.com/watch?v=zVZXq64HSV4
• http://www.youtube.com/watch?v=0XQVXFL4uoo
• http://www.youtube.com/watch?v=YMqyG9QG6oc
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