Endothermic and
Exothermic Reactions
Chemical Reactions
In a chemical reaction, chemical substances (reactants) are changed and
one or more new substances (products) are made.
For example, hydrogen and oxygen molecules react with one another to
produce water molecules.
reactants
products
Chemical Reactions
What observations might tell us that a chemical reaction is taking place?
Fizzing / bubbling / effervescence
If a gas is produced in the reaction,
bubbles or fizzing will be observed.
A change of colour
If the product is a different colour to the
reactant(s), then a colour change will be
observed as the reaction progresses.
A change in temperature
All chemical reactions involve a transfer of
energy, which can result in an increase or
decrease in the temperature of the
surroundings.
Energy Changes in Reactions
When a chemical reaction happens, energy is transferred between the
reaction mixture (known as the system) and its surroundings.
The ‘surroundings’ includes the air around the system, the glassware and
anything else that is not part of the reaction itself.
surroundings
system
Energy Changes in Reactions
Chemical reactions can only occur when reacting particles collide with each
other with enough energy.
The minimum amount of energy that particles must have to react is called
the activation energy.
Energy is also needed to break chemical bonds in reactants.
Energy Changes in Reactions
Energy is transferred from the surroundings to the system to break the
chemical bonds in the reactants.
reactants
energy taken in
from surroundings
to break bonds
Energy Changes in Reactions
When new products are formed, new chemical bonds are made and energy
is transferred from the system to the surroundings.
reactants
products
energy released to
the surroundings
when bonds form
Energy Changes in Reactions
The total amount of energy at the end of a reaction is the same as the total
amount of energy at the beginning of a reaction.
We can say that energy is conserved in chemical reactions.
Energy has been transferred between the system and the surroundings.
This means that the relative amount of energy in the system and in the
surroundings will have changed.
Exothermic Reactions
In an exothermic reaction, the
overall transfer of energy is from
the system to the surroundings.
This means that more energy is
released when new bonds are
formed than is taken in to break
bonds.
Therefore, the energy stored in
the chemical bonds of the
products is less than the energy
stored in the chemical bonds of
the reactants.
The overall transfer of energy to
the surroundings causes the
temperature of the surroundings
to increase.
surroundings
system
Exothermic Reactions
Some examples of exothermic reactions
include:
•
combustion
•
neutralisation
•
many oxidation reactions
•
respiration
Some everyday uses of exothermic reactions
include:
•
self-heating cans
•
hand warmers
Self-Heating Cans
Self-heating cans contain food or drinks that are usually consumed when
hot, such as coffee or soup.
The self-heating can makes use of
an exothermic reaction to warm
up the food or drink without the
need for an external heat source,
such as an oven or stove.
Self-heating cans can be useful
when camping, for example, when
proper cooking and heating
facilities are not readily available.
Self-Heating Cans
A compartment within the self-heating can contains chemicals, one of which
is usually water, separated by a membrane.
food or drink to
be heated
chemicals
water
membrane
button
Self-Heating Cans
When the button is pressed, the membrane between the chemicals is
pierced, and the chemicals mix.
food or drink to
be heated
chemicals
water
membrane
button
Self-Heating Cans
An exothermic reaction takes place, releasing energy to the surrounding
food or drink.
food or drink to
be heated
chemicals
water
membrane
button
Hand Warmers
Disposable hand warmers are designed to release energy over a long
period of time to keep a person’s hand warm.
Hand warmers contain iron filings, water and
other chemicals, sealed in a packet made of a
microporous material. Microporous materials
have many tiny holes in them that allow air to
pass through.
They are kept in a sealed, air-tight package
until they are ready to be used. When the
outer packaging of the hand warmer is
removed, air moves into the hand warmer
through the tiny holes.
Oxygen from the air reacts with the iron
filings inside the hand warmer in an oxidation
reaction. The reaction is exothermic.
Endothermic Reactions
In an endothermic reaction, the
overall transfer of energy is from
the surroundings to the system.
This means that more energy is
taken in to break bonds than is
released when new bonds are
made.
Therefore, the energy stored in
the chemical bonds of the
products is greater than the
energy stored in the chemical
bonds of the reactants.
The overall transfer of energy
from the surroundings causes the
temperature of the surroundings
to decrease.
surroundings
system
Endothermic Reactions
Some examples of endothermic reactions
include:
•
thermal decomposition
•
the reaction between citric acid and
sodium hydrogen carbonate
•
photosynthesis
Some everyday uses of exothermic reactions
include:
•
sports injury packs
•
cooking and baking
Sports Injury Packs
Single-use sports injury packs are designed to limit the body’s response to
injuries such as sprains, strains or bruises. They can also be used for cold
therapy for conditions such as arthritis, which causes inflamed joints.
Sports injury packs contain water
and a sealed tube containing a
powdered chemical.
When the pack is bent, the tube is
broken and the chemicals mix,
resulting in an endothermic
reaction.
The sports injury pack starts to feel
cold and can be held against an
injury to help to reduce swelling and
relieve pain.
Reaction Profiles
A reaction profile (sometimes called an energy level diagram) shows the
relative energies of the reactants and products in a chemical reaction.
Endothermic Reaction Profile
Energy
Energy
Exothermic Reaction Profile
reactants
products
Progress of Reaction
products
reactants
Progress of Reaction
Exothermic Reaction Profile
Energy
In an exothermic reaction, the overall transfer of energy is from the system
to the surroundings, so the energy stored in the chemical bonds of the
products is less than the energy stored in the chemical bonds of the
reactants.
reactants
products
Progress of Reaction
Exothermic Reaction Profile
Some energy is transferred from the surroundings to break the chemical
bonds in the reactants and to provide the energy needed for the reaction to
take place. This is called the activation energy.
Energy
activation
energy
reactants
products
Progress of Reaction
Exothermic Reaction Profile
A larger amount of energy is transferred to the surroundings as new
chemical bonds are made.
Energy
activation
energy
energy
released
reactants
products
Progress of Reaction
Exothermic Reaction Profile
More energy has been released than was taken in. The overall transfer of
energy is to the surroundings. The overall energy change of the system is
negative.
Energy
activation
energy
energy
released
reactants
overall energy
change
Progress of Reaction
products
Endothermic Reaction Profile
Energy
In an endothermic reaction, the overall energy transfer is from the
surroundings to the system, so the energy stored in the chemical bonds of
the products is greater than the energy stored in the chemical bonds of the
reactants.
products
reactants
Progress of Reaction
Endothermic Reaction Profile
A large amount of energy is transferred from the surroundings to break the
chemical bonds in the reactants and to provide the energy needed for the
reaction to take place (the activation energy).
Energy
activation
energy
products
reactants
Progress of Reaction
Endothermic Reaction Profile
Some energy is transferred to the surroundings as new chemical bonds are
made.
Energy
activation
energy
energy
released
products
reactants
Progress of Reaction
Endothermic Reaction Profile
More energy has been taken in than was released. The overall transfer of
energy is from the surroundings. The overall energy change of the system is
positive.
Energy
activation
energy
energy
released
products
reactants
overall energy
change
Progress of Reaction
Reaction Profiles
Exothermic Reactions
activation
energy
reactants
overall energy
change
More energy is taken in than is
released, so the overall energy
change of the system is positive.
Energy
Energy
More energy is released than is
taken in, so the overall energy
change of the system is negative.
Endothermic Reactions
activation
energy
products
reactants
overall energy
change
products
Progress of Reaction
Progress of Reaction
Summary
•
Chemical reactions may be exothermic or endothermic.
•
In an exothermic reaction, the overall transfer of energy is from the
system to the surroundings. This causes the temperature of the
surroundings to increase.
•
Uses of exothermic reactions include self-heating cans and hand
warmers.
•
In an endothermic reaction, the overall transfer of energy is from the
surroundings to the system. This causes the temperature of the
surroundings to decrease.
•
Uses of endothermic reactions include sports injury packs.
•
Reaction profiles can be used to show the relative energies of the
reactants and products, the activation energy and the overall energy
change in a chemical reaction.
•
The overall energy change in an exothermic reaction is negative and the
overall energy change in an endothermic reaction is positive.