Chemistry C2 Topic 5
Revision PowerPoint
5
What I’m Looking For
1) To be able to explain what
happens in exothermic reactions.
2) Give examples of exothermic
reactions.
Energy changes in reactions
This PowerPoint covers Topic 3 Lessons 23 to 24
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Oxidation reactions
Exothermic reactions
When a chemical reacts with oxygen it becomes
oxidised. e.g. carbon burns on a barbecue and is
oxidised to carbon dioxide. C + O2 → CO2
In electrolysis, when a non-metal ion loses
electrons, this is also an oxidation reaction.
2Cl– → Cl2 + 2e–
These are chemical reactions where energy passes
from the reacting chemical into the surroundings,
which warm up. e.g. all combustion reactions;
respiration is living cells; acids being neutralised by
alkalis
Combustion
This is another word for ‘burning’ and we use the
word when we describe fuels burning. Combustion is
an oxidation reaction.
Energy
There are many forms of energy, including thermal energy
(heat) and chemical energy. Fuels and sugary or fatty foods
contain stored chemical energy which can be released by
reacting with oxygen. This can happen in fires or in living cells.
WILF1
Most chemical reactions release energy into the
surroundings. The energy released raises the
temperature of the surroundings, so the
surroundings (the air, the test tube …) becomes
warmer.
In an exothermic reaction some of the chemical
energy in the molecules of reactants is changed
into thermal energy, leaving new chemicals
which have less chemical energy than the
original reactants.
What I’m Looking For
1) To be able to recall that in chemical
reactions energy can be transferred
from or to the surroundings.
2) To be able to recall that if a reaction
is exothermic in one direction it is
endothermic in the opposite
direction.
WILF2
These chemical reactions release energy into
the surroundings:
o the burning of fuels in air or oxygen
o neutralisation reactions between acids
and alkalis
o reactions between very reactive metals
and water, less reactive metals and acid
o using oxygen to change sugar into carbon
dioxide and water inside cells (respiration)
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Reversible reactions
Chemical reactions normally see a change where
reactants are converted into products. In reversible
reactions there is a backward reaction where some of
the product reacts to re-form reactants. Reversible
reactions are unusual and rare.
Endothermic reactions
These are reactions where the reactants take energy
from the surroundings in order to make products. This
causes the surroundings to lose energy and cool
down.
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WILF1
Anhydrous copper sulphate
Endothermic reactions are unusual but important.
When the reactants are changed into the products,
energy is taken from surroundings which then become
cold.
Anhydrous copper sulphate is a white
coloured powder, formula CuSO4
The powder can be made by heating
hydrated copper sulphate (copper sulphate
crystals) strongly.
Hydrated copper sulphate
Hydrated copper sulphate (copper sulphate
crystals) is blue in colour and has water of
crystallisation associated with it. The
formula of the crystals is CuSO4. 5H2O
WILF1(frompreviouslesson)
Exothermic reactions release energy into the
surroundings, which become warm:
o the burning of fuels in air or oxygen
o neutralisation reactions between acids and
alkalis
o reactions between very reactive metals and
water, less reactive metals and acid
o using oxygen to change sugar into carbon
dioxide and water inside cells (respiration)
o
thermal decomposition reactions (e.g. heating
limestone or copper carbonate)
o
electrolysis reactions, where the electrical energy
decomposes the compounds into elements
o
photosynthesis, where light energy is absorbed by
plant leaves in order to make sugar from carbon
dioxide and water
WILF2
Reversible reactions are unusual and quite
rare. The reactants change into products, but
if you change the conditions of the reaction
then some of the products will react together
to make the reactants.
The symbol for a reversible reaction is a
double half arrow and this tells us that a
forward and a backward reaction are both
possible.
WILF2
If copper sulphate crystals are heated strongly, they
lose water of crystallisation as steam. This leaves a
white powder called anhydrous copper sulphate. This is
the forward reaction and it is endothermic!
If you add cold water to the white powder (anhydrous
copper sulphate) then blue hydrated copper sulphate
forms, together with clouds of steam. This is the
backward reaction and it is exothermic!