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3.1Physical Chemistry

Energetics — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 14 parts Lesson 4 of 9 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Energetics: Exothermic, Endothermic and Energy Profiles — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe energy transfers in chemical reactions, draw energy profiles and carry out simple calorimetry practicals.Chemical reactions involve energy changes because bonds break and form.

  • activation energy
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Calorimeter

Part 1 of 6Energy in chemical reactions

Chemical reactions involve energy changes because bonds break and form. The total energy of a closed system is conserved, so energy lost by the reacting chemicals is gained by the surroundings. Reactions are classified as exothermic (energy to the surroundings, temperature rises) or endothermic (energy from the surroundings, temperature falls).

Exothermic examplesEndothermic examples
Burning fuels, respirationPhotosynthesis (light energy absorbed)
Neutralisation of acid with alkaliThermal decomposition of limestone
Reaction of metals with acidDissolving ammonium nitrate or potassium chloride
Rusting (slow), hand warmersReaction of citric acid with sodium hydrogencarbonate

Part 2 of 6Energy profile diagrams

An energy profile compares the energy of reactants and products.

The vertical axis is energy; the horizontal axis is the course of reaction.
For exothermic reactions products sit below reactants.
For endothermic reactions they sit above.
Activation energy
A hump between represents the activation energy.
A catalyst lowers the hump. ΔH is the vertical difference between reactants and products, negative for exothermic and positive for endothermic.

Part 3 of 6Practical: measuring temperature changes

1
Place 25 cm3 of one reactant (for example 1.0 mol/dm3 hydrochloric acid) in a polystyrene cup inside a beaker.
2
Record the initial temperature of both solutions; they should be equal.
3
Add the other reactant (alkali), put on a lid and stir with the thermometer.
4
Record the highest (or lowest) temperature reached and calculate the temperature change.

Polystyrene is a good insulator and the lid reduces heat loss, giving more accurate results. For a fuel, burn it in a spirit burner under a copper can containing a known mass of water and measure the temperature rise. Results lower than expected arise from heat loss, incomplete combustion and evaporation.

Part 4 of 6Comparing fuels

To compare fuels fairly

  • use the same mass of water and the same distance between the flame and can, burn the same mass of fuel, and calculate energy per gram.
  • Alcohol fuels such as ethanol give less energy per gram than hydrocarbons like octane but are renewable if made from sugar cane;
    • hydrogen releases the most energy per gram and produces only water.

Part 5 of 6Energy changes in everyday life and industry

Energy changes matter in daily life.

1
Self-heating food cans have an exothermic reaction separating the layers of calcium oxide and water.
2
Hand warmers rely on the oxidation of iron powder.
3
Fuel choice depends on the energy released per gram, cost, availability and pollutants: coal and petrol release carbon dioxide and sulfur dioxide,
  • while hydrogen releases only water but is difficult to store.
4
The decomposition of calcium carbonate (CaCO3 → CaO + CO2) needs continuous heat, which is why lime kilns burn large amounts of fuel.

The idea of conservation of energy underlies all of this: energy is not created or destroyed, only transferred from the chemical store of the reactants to thermal energy of the surroundings (exothermic), or from the surroundings to the chemical store of the products (endothermic).

Energy changes in everyday life and industry
When you describe a temperature change in an exam you should say "the energy is released to the surroundings" rather than "the reaction makes heat".
Always link a measured temperature change to the direction of energy transfer and then to the sign of ΔH.

Part 6 of 6Worked examples

  1. Sodium hydroxide solution is added to hydrochloric acid and the temperature rises from 20 °C to 27 °C. Classify the reaction and explain. [2]2 marks

  2. Draw the energy profile for an endothermic reaction. [3]3 marks

  3. A student wants to improve accuracy in the polystyrene cup experiment. Suggest two changes. [2]2 marks

  4. Give one use of an endothermic reaction. [1]1 mark

Key terms

TermMeaning
ExothermicReleases energy to the surroundings
EndothermicAbsorbs energy from the surroundings
Activation energyEnergy needed to start the reaction
Energy profileDiagram of energy changes during a reaction
CalorimeterInsulated container for measuring heat changes
SurroundingsEverything outside the reacting chemicals

Avoid these mistakes

1
Confusing temperature rise with an endothermic reaction.
2
Stating that the energy is destroyed.
3
Drawing the activation energy from products to hump.
4
Not stirring in calorimetry.

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