What are reversible
Chemistry KS4 science
GCSE/IGCSE/O level Revision Notes
PART A REVERSIBLE REACTIONS
What is a reversible reaction? This page introduces
you to the idea of a reversible reaction. Examples of the reversible reactions
described include the thermal decomposition of ammonium chloride, hydrated
copper sulfate, reaction of bismuth chloride with water, formation of ammonia
and the thermal decomposition of limestone. These revision notes on reversible
reactions, should prove useful for the new AQA GCSE chemistry, Edexcel GCSE
chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) &
(5-1) science courses.
A Reversible Reactions (this page) * B Reversible reactions
C Haber Synthesis of ammonia
* D(a) The Uses of ammonia-nitric acid-fertilisers
Fertilisers-environmental problems * E
The nitrogen cycle
Advanced A Level Notes - Chemical Equilibrium
Advanced A Level Notes - Nitrogen & Ammonia
- Introduction examples
- A reversible reaction is a chemical
change in which the products can be converted back to the original reactants under suitable conditions.
- In a reversible reaction, changing the
reaction conditions e.g. concentration, pressure or temperature will change
the net direction the reaction goes i.e. more to the right (forward) or more
to left (backward).
- It also means a reversible reaction does not go to
completion in either direction and all components, original reactants or
- and ALL co-exist in the reaction mixture (see notes on
- This means the reaction can go in either
- A + B ==> C + D or written
as C + D ==> A + B
- A reversible reaction is shown by the sign
- a half-arrow to the right (direction
of forward reaction),
- and a half-arrow to the left (direction
of backward reaction).
- It is really important you understand
that the terms right & left AND forward & backward are
used in the context of how the equation is presented.
- Most reactions are not reversible (irreversible)
and have the usual complete arrow
only pointing to the right.
Five examples 1a to 1e of reversible reactions are
Part A contd. 1a The thermal decomposition of ammonium
On heating strongly above 340oC, the white solid ammonium chloride,
thermally decomposes into a mixture of two colourless gases
ammonia and hydrogen chloride.
- On cooling the reaction is reversed and solid ammonium chloride reforms.
- This is an example of sublimation but
here it involves both physical and chemical changes.
- When a substance
sublimes it changes directly from a solid into a gas without melting
and on cooling reforms the solid without condensing to form a liquid.
But, more importantly in this section, it is
an example of a
ammonia + hydrogen chloride
NH3(g) + HCl(g)
Heat moves the reaction to the right
(referred to as the forward reaction) and cooling moves the reaction to the
left (referred to as the backward reaction).
So, the thermal
decomposition of ammonium chloride into ammonia and hydrogen chloride is the forward reaction, and the
formation of ammonium chloride from ammonia and hydrogen chloride is the backward reaction.
The terms forward (L to R)
and backward (R to L) must be used in the context of the direction
reversible reaction equation is written.
This is a simple Pyrex test tube
experiment for the school laboratory, the solid ammonium chloride
changes to a colourless gas (endothermic) but the crystals reform higher
up on the cooler surface of the test tube (exothermic).
By putting damp neutral litmus paper
at the top of the tube and half-way down you might see if it change blue from the alkaline
ammonia and red from the acidic hydrogen chloride.
The molecules diffuse
at different rates so you might some separation to allow the observation
of both litmus changes - ammonia (NH3 Mr = 17)
diffuses faster than hydrogen chloride (HCl Mr = 36.5) and
should get to the top of the tube first.
It is a rule that the smaller the
molecular mass of a gas molecules the greater the average speed of the
molecules, so the faster the molecules spread out - diffuse.
For the experiment to show this see
gases - diffusion
- Reversing the reaction conditions reverses the direction of chemical change, typical of a reversible reaction.
- Thermal decomposition means using 'heat' to 'break down' a molecule into smaller ones.
- The decomposition of ammonium chloride into
ammonia and hydrogen chloride is endothermic (heat absorbed or heat taken in
from the surroundings - the hot test tube).
formation of ammonium chloride from ammonia and hydrogen chloride
is exothermic (heat released or heat given out to the surroundings
- the cooler parts of the test tube).
- This means if the direction of chemical change is reversed, the energy change is also reversed.
- Ammonium fluoride (decomposes >180oC),
ammonium bromide (decomposes >450oC) and
ammonium iodide (decomposes >550oC), with a similar formula, all
sublime in a similar physical-chemical way
when heated, so the equations will be similar i.e. just swap F, Br or
I for the Cl, in other words ...
+ HX(g) (where X represent F, Br or I)
- Similarly, ammonium sulphate
also sublimes when heated above 235oC and thermally
decomposes into ammonia gas and sulphuric acid vapour.
- For more on
sublimation, see the States of Matter
Part A contd. 1b The thermal decomposition of hydrated copper(II)
- On heating the blue solid, hydrated copper(II) sulphate, steam is given off and the white solid of anhydrous copper(II) sulphate is formed.
- When the white solid is cooled and water added, blue hydrated copper(II) sulphate is reformed
i.e. a reversible reaction.+ heat
white anhydrous copper(II) sulphate +
CuSO4(s) + 5H2O(g)
decomposition to give the white solid is the forward endothermic reaction and the
're-hydration' to reform the blue crystals is the backward exothermic reaction.
This is also a simple Pyrex test tube
experiment for the school laboratory, the blue solid changes to a white
solid (endothermic) and water vapour condenses out higher up on the cooler
surface of the test tube (exothermic). After allowing to cool (to avoid
cracking the test tube!) adding a few drops of water restores the blue
colour of the original crystals.
A nice simple class experiment.
- The 5H2O in the formula of hydrated copper(II) sulphate is called the
water of crystallisation and forms part of the crystal structure when copper(II) sulphate solution is
evaporated and crystals form.
- This crystal structure is broken down on
heating and the water is given off.
- The thermal decomposition is endothermic as
heat is absorbed to drive off the water.
- The reverse reaction is exothermic i.e. on adding water to white anhydrous copper(II) sulphate the mixture heats
up as the blue crystals reform.
- The reverse reaction is used as a simple chemical test for
water i.e. white anhydrous copper(II) sulphate turns blue.
TOP OF PAGE
Part A contd. 1c
The reaction of bismuth chloride with
chloride (BiCl3) is added to water, it dissolves and then
reacts with water to form a white precipitate of bismuth oxychloride
(BiOCl) and a colourless solution of hydrochloric acid (HCl).
+ water ==> bismuth oxychloride + hydrochloric acid
+ H2O(l) ==> BiOCl(s) + 2HCl(aq)
A reaction in which a
molecule reacts with water to give at least two products is called a
acid is added to the mixture, the bismuth oxychloride dissolves to
reform the bismuth chloride solution.
reaction is reversible and what is formed depends on the relative
amounts of hydrochloric acid and water, and so the reaction equation
should be written as:
Bismuth chloride is a
poisonous chemical - teacher demonstration only at GCSE level.
Part A contd. 1d
The formation and decomposition of
The synthesis of
ammonia from nitrogen and hydrogen is a reversible reaction and by
changing the temperature and pressure of the reacting gases you can
make the reaction go one way more than another,
e.g. high pressure
makes more ammonia (forward reaction) and higher temperature causes ammonia to decompose
into hydrogen and nitrogen (backward reaction).
N2(g) + 3H2(g)
The forward reaction
forms the basis of ammonia manufacture by the Haber Process (full
details in section C)
If the pressure is
reduced, or the temperature raised, the reaction tends to reverse to
give a greater proportion of nitrogen and hydrogen.
If the pressure is
increased, or the temperature lowered, the reaction tends to
give a greater proportion of ammonia.
governing these reversible reaction situations are discussed and
explained with examples in Part B.
This is the very important Haber
synthesis reaction for the industrial production of ammonia for
Part A contd. 1e
The formation and hydrolysis of
- Ethyl ethanoate, an ester, is formed by the reaction
of ethanoic acid with ethanol and is another example of a reversible
- ethanoic acid + ethanol
ethyl ethanoate + water
- If the ester is warmed with water or
any dilute acid (faster), it changes back into the original acid and
- This reverse reaction is called hydrolysis (backward
reaction, as written in the symbol equation) ...
- ethyl ethanoate + water ==> ethanoic acid + ethanol
- whereas esterification (forward
reaction, as written in the symbol equation) is
- ethanoic acid + ethanol ==>
ethyl ethanoate + water
- The term hydrolysis is a reaction in
which a molecule reacts with water to give at least two products.
- Esterification simply means an ester
- An ester is particular kind of organic
molecules formed from an organic acid and alcohol.
- For more details of making the ester see
Esters, chemistry and uses including perfumes
Part A contd. 1f
thermal decomposition of limestone (calcium carbonate)
If limestone is strongly
heated above 900oC it decomposes into calcium oxide (lime) and
It the lime (calcium
oxide) is cooled, it recombines with any carbon dioxide present.
reversible reaction can be expressed as ...
CaO(s) + CO2(g)
So, again, by changing
the reaction conditions, in this case changing the temperature, the
reaction can be made to either way, i.e. a class reversible
This is an important industrial
process for making lime for the building industry and agriculture.
Hopefully you now understand what
we mean by a reversible reaction and how changing conditions can change
the direction of reversible reaction AND changing directions changes
whether energy is absorbed or given out. So, now we can move to take
another look at reversible reactions in the context of what we call
chemical equilibrium and look again at some of the reactions already
mentioned on this page.
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A Reversible Reactions * B
Reversible reactions and Equilibrium
Haber Synthesis of ammonia * D(a)
The Uses of ammonia-nitric acid-fertilisers
The nitrogen cycle
Foundation tier (easier) multiple choice QUIZ on ammonia,
nitric acid and fertilisers etc.
Higher tier (harder) multiple choice QUIZ on
ammonia, nitric acid and fertilisers etc.
Advanced A Level Notes - Equilibrium
Advanced A Level Chemistry Notes p-block nitrogen & ammonia
Keywords & formulae:
Explain why heating hydrated copper sulfate is a reversible
reaction? Reversible Reactions Reversible
reactions NH4C <=> NH3 + HC * NH4NO3 <=> N2O + 2H2O * CuSO4.5H2O <=>
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