Triple Chemistry GCSE AQA Grade 9 Unit 5 Energy Changes Notes

Unit 5: Energy Changes
Energy Transfers
 During chemical reactions energy is transferred from the chemicals to/from the
surroundings.
 Inducing a temperature change in many reactions.
 Reversible reactions are exothermic in one direction and endothermic in the opposite.
- Transferring the same amount of energy each way.

EXOTHERMIC ENDOTHERMIC
Reactions accompanied by a temperature rise. Reactions accompanied by a temperature fall.
 Emits energy  Absorbs energy
 Temperature increase in surroundings.  Temperature decrease in surroundings.
 Energy exits chemicals.  Energy enters chemicals.
 Chemical energy in reactants is  HEAT is converted from surroundings to
converted to HEAT energy chemical energy in products, indicated
 Transferred to surroundings and by temperature decrease.
indicated by temperature increase.  Used in some sports injury packs.
 Used in products e.g. self-heating cans  Examples:
and hand warmers. - Photosynthesis
 Examples: - Electrolysis
- Respiration
- Combustion
- Oxidation
- Neutralisation
Heat
Heat




He
at t a
He
He
at at
He

Heat Heat
Heat Heat

a t He
He at
at
He




He
Heat
ta
Heat

, Required Practical: Investigating Energy Changes in Reactions

Method: Independent variable: metal used.
Set up the equipment shown. Dependant variable: temperature change
Take the temperature of the acid. Control variable: type, concentration and volume
Add the metal powder and stir. of acid
Record the highest temperature the reaction Hazards/Risks: there is a low risk of a corrosive acid
mixture reaches. damaging the experimenter’s eye, wear goggles.
Calculate the temperature change for the reaction.
Repeat the experiment using a different metal.




Considerations, Mistakes and Errors: Thermometer
The more reactive the metal, the greater the Lid
temperature change (an evident correlation Polystyrene
between metal reactivity and temp change) cup
Always uncertainty about results obtained when a Cotton
measurement is made. Repeat experiment three Reaction
wool mixture
times to collect an average, work out an
uncertainty too. Large
beaker



Calorimetry: study of energy changes which occur in chemical reactions.Calorimeter: insulated reaction
vessel used to measure temperature changes during chemical reactions.

Energy Level Diagrams (Reaction Profiles)
 Chemical reactions only occur when reacting particles collide with sufficient energy.
 These diagrams show the energy change as the reaction occurs.

Activation energy (Ea): minimum amount of energy needed for particles to react.
Overall energy change (ΔH): total energy change in energy between reactants and products.

EXOTHERMIC REACTION ENDOTHERMIC REACTION


Products
Ea
Reactants
Ea +Δ
H
-ΔH Reactants



Products