Knowledge Organiser – Chemical Changes and Energy
Science Department
What the specification says…
Exothermic reactions transfer energy to the surroundings. A temperature
increase would occur.
Endothermic reactions take in energy from the surroundings. A temperature
decrease would occur.
 Make sure you know the common examples of each reaction – look in the
specification on the left.
During a chemical reaction: bonds in the reactants are broken and
new bonds are made in the products
•
Energy is absorbed to break bonds.
•
Energy is released when new bonds form.
You need to be able to recognise that the graph on the left shows an exothermic
reaction and the graph on the right shows an endothermic reaction
Remember, all reactions require
activation energy. The value of
this is shown by the pink arrow.
Knowledge Organiser – Chemical Changes and Energy
What the specification says…
A good cross topic link here… this comes up
in ‘Thermal Energy’
Use this equation to work out an energy
change:
Energy transferred = mass of water heated
× specific heat capacity of water ×
temperature rise
Science Department
Higher tier only:
You can calculate the energy change in a reaction using
average bond energies. Bond energy is the amount of energy
needed to break one mole of a particular bond.
To calculate bond energy
1. Add together the bond energies for all the bonds in the
reactants – this is the ‘energy in’.
2. Add together the bond energies for all the bonds in the
products – this is the ‘energy out’.
3. Calculate the energy change = energy in – energy out.
Worked example – an exothermic reaction
Hydrogen and chlorine react to form hydrogen chloride gas:
H−H + Cl−Cl → 2 × (H−Cl)
Bond
Bond Energy
(kJ/mol)
H−H
436
Cl−Cl
243
H−Cl
432
1.Energy in = 436 + 243 = 679 kJ/mol
2.Energy out = 2 × 432 = 864 kJ/mol
3.Energy change = in – out = 679 – 864
= –185 kJ/mol
The energy change is negative,
showing that energy is released to the
surroundings in an exothermic
reaction.
Knowledge Organiser – Chemical Changes and Energy
What the specification says…
TOP TIP: Note that aluminium can be difficult to place in
the correct position in the reactivity series during the
experiments described on the right. This is because its
protective aluminium oxide layer makes it appear to be
less reactive than it really is. When this layer is
removed, the observations are more reliable.
Science Department
In a reactivity series, the most reactive element is placed at the top and the least
reactive element at the bottom. More reactive metals have a greater
tendency to lose electrons and form positive ions.
A reactivity series of metals could include any elements. For example:
You will need to remember which
metals undergo reactions and then
use this knowledge to construct a
reactivity series.
The tables show how the
elements react with water and
dilute acids:
Element
Reaction with dilute acids
Calcium
Very quickly
Magnesium
Quickly
Element
Reaction with water
Zinc
More slowly
Potassium
Violently
Iron
More slowly than zinc
Sodium
Very quickly
Copper
Very slowly
Lithium
Quickly
Silver
Barely reacts
Calcium
More slowly
Gold
Does not react
A more reactive metal will displace a less reactive metal from a
compound.
2Al + Fe2O3 → 2Fe + Al2O3
You might be given an equation like this (or several) and be expected
to work out which metal is most reactive.
Knowledge Organiser – Chemical Changes and Energy
What the specification says…
Extracting metals with carbon
Science Department
Metals which are less reactive
than carbon can be extracted
by reduction with carbon.
The general equation for this
reaction is:
metal oxide + carbon  metal +
carbon dioxide
Note that:
• the oxidising agent is the chemical that causes oxidation
• the reducing agent causes the other chemical to be reduced (in
the example above, carbon is a reducing agent)
HT ONLY:
Oxidation is the loss of electrons from a substance. It is
also the gain of oxygen by a substance. For example,
magnesium is oxidised when it reacts with oxygen to
form magnesium oxide:
2Mg + O2 → 2MgO
Reduction is the opposite of this – gain of electrons and
loss of oxygen. Usually, these take place at the same
time in a reaction = a redox reaction.
Ionic equations show only the atoms/ions involved in a
displacement reaction e.g. Mg + CuSO4  Cu + MgSO4
becomes…
Mg + Cu2+  Cu + Mg2+
Half equations can then be written to show the
oxidation/reduction of a reactant:
e.g. Cu2+ + 2e-  Cu (the copper ion has been reduced)
Knowledge Organiser – Chemical Changes and Energy
What the specification says…
Electrolysis is the process by which ionic substances are
decomposed (broken down) into simpler substances when an
electric current is passed through them
This only works on ionic substances because they contain charged
particles called ions. For example, lead bromide contains
positively charged lead ions and negatively charged bromide ions.
Here you can see what
happens to the ions
during electrolysis. The
ions have to be able to
move so the compound
must be MOLTEN or
DISSOLVED.
Science Department
This is what happens during electrolysis in words (the part in bold is what students
commonly forget to include in their exam answers):
1. Positively charged ions move to the negative electrode during electrolysis. They
receive electrons and are reduced forming elements
2. Negatively charged ions move to the positive electrode during electrolysis. They
lose electrons and are oxidised. The substance that is broken down is called the
electrolyte.
Here’s what happens when lead bromide is electrolysed
Observation
Explanation
Anode (positive)
Bubbles of brown gas
Bromine vapour (Br2) is
produced
Cathode (negative)
Silvery liquid metal present at
this electrode underneath the
remaining molten electrolyte
Lead (Pb) is produced
It is easy to predict the products of electrolysis of molten electrolytes because they
simply split into their elements:
• the metal is formed at the negative electrode because that is where the positive
metal ions are attracted
• the non-metal element is formed at the positive electrode where the negative
non-metal ions are attracted
 Have a go at the examples below, what would form at each electrode?
Compound
Zinc chloride
Aluminium oxide
Anode (positive)
Cathode (negative)
Knowledge Organiser – Chemical Changes and Energy
What the specification says…
Metals that are more reactive that carbon are extracted
using electrolysis e.g. aluminium.
Copper can be extracted using carbon but is often
purified using electrolysis. The anode (positive
electrode) is made from impure copper and the cathode
(negative electrode) is made from pure copper.
Science Department
Aluminium is the most abundant metal on Earth. But it is expensive,
largely because of the amount of electricity used in the extraction
process.
Aluminium ore is called bauxite. The bauxite is purified to yield a
white powder - aluminium oxide - from which aluminium can be
extracted.
The extraction is done by electrolysis. But first the aluminium oxide
must be melted so that electricity can pass through it. However,
aluminium oxide has a very high melting point (over 2000°C) so it
would be expensive to melt it.
Instead, it is dissolved in molten cryolite - an aluminium compound
with a lower melting point than aluminium oxide. The use of molten
cryolite as a solvent reduces some of the energy costs involved in
extracting aluminium.
The oxygen produced at the positive electrode reacts with the carbon of the electrodes, forming carbon
dioxide, so they gradually burn away. As a result, the positive electrodes have to be replaced frequently.
This adds to the cost of the process.
Knowledge Organiser – Chemical Changes and Energy
What the specification says…
Electrolysing aqueous solutions of ionic compounds can
be more complicated than electrolysing molten
compounds, because the water molecules can provide
hydrogen ions (H+) and hydroxide ions (OH-), in addition
to the ions from the ionic compounds.
At the negative electrode:
Whether you get the metal or hydrogen during
electrolysis depends on the position of the metal in the
reactivity series:
• the metal will be produced if it is less reactive than
hydrogen
• hydrogen will be produced if the metal is more
reactive than hydrogen
Science Department
At the positive electrode
If the negative ion from the ionic compound is simple (eg Cl- or Br-), then that
element is produced. If the negative ion is a complex ion (eg NO3-, SO42-, CO32-),
then oxygen is produced from the hydroxide ion present instead.
The table summarises some of the elements you should expect to get during
electrolysis.
Negative ion in solution
Element given off at positive electrode
Chloride, Cl–
Chlorine, Cl2
Bromide, Br–
Bromine, Br2
Iodide, I–
Iodine, I2
Sulfate, SO42-
Oxygen, O2
Nitrate, NO3-
Oxygen, O2
At the negative and
positive electrodes
This table shows
some common ionic
compounds (in
solution), and the
elements released
when their
solutions are
electrolysed using
inert electrodes, eg
carbon electrodes:
Ionic substance
Element at -
Element at +
Copper chloride, CuCl2
Copper, Cu
Chlorine, Cl2
Copper sulfate, CuSO4
Copper, Cu
Oxygen, O2
Sodium chloride, NaCl
Hydrogen, H2
Chlorine, Cl2
Hydrochloric acid, HCl
Hydrogen, H2
Chlorine, Cl2
Sulfuric acid, H2SO4
Hydrogen, H2
Oxygen, O2
Very dilute solutions of halide compounds
If a halide solution is very dilute (eg NaCl), then oxygen
will be given off instead of the halogen. This is because
the halide ions are outnumbered by the hydroxide ions
from the water.
Knowledge Organiser – Chemical Changes and Energy
What the specification says…
A half-equation shows you what happens at one of the electrodes
during electrolysis. Electrons are shown as e-. A half-equation is
balanced by adding, or taking away, a number of electrons equal
to the total number of charges on the ions in the equation.
When positive metal ions (cations) arrive at the negative
electrode (the cathode), they gain electrons to form neutral metal
atoms. This is called reduction. For example:
Pb2+ + 2e- → Pb
When negative non-metal ions (anions) arrive at the positive
electrode (the anode), they lose electrons to form neutral atoms
or molecules. This is called oxidation. For example:
2Br- → Br2 + 2e-
Science Department
Writing half equations
Cations go to the cathode. They need to gain enough electrons to make
them neutral. So an Al3+ ion needs to gain three electrons:
Al3+ + 3e- → Al
Half-equations for non-metal anions are more difficult to balance. For
example, chloride ions make chlorine gas. Most non-metal elements
formed in electrolysis are diatomic molecules (eg Cl2). For example:
Cl- → Cl2
Add in two electrons to balance the charge so that both sides have the
same charge. The two electrons need to go on the right-hand side, so
that both sides have an overall charge of -2. For example:
2Cl- → Cl2 + 2e-
In the electrolysis of zinc chloride, which is the correct
half equation for the negative electrode (cathode)?
• 2Br- → Br2 + 2e• Zn2+ + 2e- → Zn
• Zn → Zn2+ + 2eWhich half equation shows what happens at the
negative electrode when purifying copper by
electrolysis?
• Cu2+ + 2e- →Cu
• Cu2+ → Cu + 2e• Cu → Cu2+ + 2e-