C6 Energy changes in chemical reactions
C6 Energy changes in chemical reactions
1 Describe the meaning of exothermic and endothermic reactions
2 Describe bond breaking as an endothermic process and bond forming as an
exothermic process
3 Draw and label energy level diagrams for exothermic and endothermic
reactions using data provided
4 Interpret energy level diagrams showing exothermic and endothermic reactions
and the activation energy of a reaction
Energy changes in chemical reactions
1 Describe the meaning of exothermic and endothermic reactions
Exothermic reactions
When energy is transferred to the surroundings, this is called
an exothermic reaction, and the temperature of the surroundings
increases.
Exo- meaning out , therm- meaning relating to heat
Examples
combustion reactions
Condensation of water vapour
Respiration
Reaction between acid and metal
many oxidation reactions
most neutralisation reactions
When energy is taken in from the surroundings, this is called
an endothermic reaction and the temperature of the surroundings
decreases. Endo – meaning in
Examples:
Thermal decomposition reactions
Photosynthesis
Cooking an egg
Evaporation of water
the reaction of citric acid and sodium hydrogen carbonate
Everyday uses of endothermic reactions include instant ice packs
which can be used to treat sports injuries.
Exothermic reaction
Mg ribbon is dropped into the conical flask
containing HCl.
Cotton prevents loss of heat produced.
Energy is released during reaction. Chemical
energy of the reactants are converted into
heat energy.
Temperature of the acid increases.
Endothermic reaction
Sodium hydrogen carbonate is
added to the plastic cup containing
dilute HCl.
Energy is absorbed during reaction.
Heat energy of the reactants are
converted into chemical energy.
The products have more energy than
reactants.
Temperature of the acid drops.
Breaking and making bonds
❑ During a chemical reaction:
▪ bonds in the reactants are broken
▪ new bonds are made in the products
❑ Energy is absorbed to break bonds. Bond-breaking is
an endothermic process.
❑ Energy is released when new bonds form. Bond-making is
an exothermic process.
❑ If more heat energy is released when making the bonds than was taken
in, the reaction is exothermic
❑ If more heat energy was taken in when making the bonds than was
released, the reaction is endothermic
Energy diagrams
❑ Energy diagrams show the level of energy of the reactants and of
the products.
❑ The bigger the difference between the energy of the reactants
and the energy of the products, the more energy is given out or
taken in.
❑ It is easy to see from an energy level diagram whether the
reaction is exothermic or endothermic:
▪ in exothermic reactions, the reactants are higher than the
products
▪ in endothermic reactions, the reactants are lower than the
products
An energy level diagram for an exothermic reaction
In an exothermic reaction,
the products are at a lower
energy than the reactants.
The difference between the
energy of the reactants and
the energy of the products
is called the enthalpy
change (∆H) of the reaction.
For an exothermic reaction,
the enthalpy change is
always negative.
An energy level diagram for an endothermic reaction
In an endothermic
reaction, the products
are at a higher energy
than the reactants.
This means that
the enthalpy change of
the reaction (∆H) is
positive.
3 Draw and label energy level diagrams for
exothermic and endothermic reactions using
data provided
Activation energy
When a fuel burns, a spark is needed to start the reaction. A minimum
amount of energy is needed in order for a fuel to burn.
❑ The minimum amount of energy needed for a reaction to take place is
called the activation energy.
High and low activation energy
In order for two substances to react, their particles must collide with
enough energy. The particles in a substance have a range of different
energies.
❑ A low activation energy means that a lot of the particles will collide
with enough energy to react. The reaction will be fast.
❑ A high activation energy means that far fewer particles will collide
with enough energy. The reaction will be slow.
4 Interpret energy level diagrams showing
exothermic and endothermic reactions and the
activation energy of a reaction
A reaction profile for an exothermic reaction
A reaction profile for an endothermic reaction