A comprehensive in-depth, specification led, revision booklet of the 2015 spec chemistry AS level course. Written during AS studies by an AS student, meaning it is written in an easily comprehensible stance for any student wishing to find extra resources to revise from.
1. know that many reactions are readily reversible and that they can reach a state of dynamic
equilibrium in which:
i the rate of the forward reaction is equal to the rate of the backward reaction
ii the concentrations of reactants and products remain constant
Irreversible and Reversible reactions
When a mixture of hydrogen and oxygen in a 2:1 molar ratio is ignited, water is produced. There is
very little, if any, uncombined hydrogen or oxygen remaining at the end of the reaction. We often
describe such reactions as ‘irreversible’.
Most combustion reactions fall into this category since they are highly exothermic. That is, ∆H is
large and negative.
However many reactions, particularly organic chemistry, for which ∆H is small. These reactions
may not go to completion. At the end of the reaction, detectable amounts of the reactants remain,
mixed with the product. Such reactions are called ‘reversible’ reactions.
The reaction between hydrogen and iodine
If a mixture of hydrogen and iodine in a 1:1 ratio is heated to 573 K in a closed container, about
90% of the hydrogen and iodine reacts to form hydrogen iodide. Provided that the reaction mixture
remains closed in the container at 573 K, 10% of the hydrogen and iodine will remain unreacted no
matter how long we leave the reaction mixture.
If a sample of hydrogen iodide is heated to 573 K in a closed container, it partially decomposes,
and produces an identical equimolar solution of hydrogen and iodine seen in the from the
reactants.
This reaction is clearly seen to be reversible, and when there is no more change in concentrations
of products or reactants then it is said to be in ‘equilibrium’.
The symbol ⇌ is used to show a reversible reaction.
H2(g) + I2(g) ⇌ 2HI(g)
When the reaction is written in this way, the reaction between hydrogen and iodine is referred to a
the forward reaction. And the decomposition of hydrogen iodide into hydrogen and iodine is called
the backwards reaction.
How is equilibrium established?
When a mixture of hydrogen and iodine is heated, the two gases start to react and form hydrogen
iodide. With increasing time, the concentrations of hydrogen and iodine decreases so the rate of
the forward reactions decreases.
As soon as some hydrogen iodide is formed, it slowly starts to decompose. With increasing time,
however, the concentration of hydrogen iodide increases, so the rate of the backward reaction
increases.
Eventually the rates of the forward and backward reaction become equal and after this point there
is NO FURTHER CHANGE in the concentrations of the reactants or products. The system is now
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