Form 4 · Chemistry · KCSE

Energy Changes in Chemical and Physical Processes

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Slide 1 · Learning Outcomes

What you will be able to do

Every chemical reaction and every change of state involves an energy change, usually seen as heat. In this topic you will measure, calculate and explain those changes.

  • Tell exothermic and endothermic processes apart and draw energy level diagrams for them.
  • Use bond energies to calculate the enthalpy change (ΔH) of a reaction.
  • Explain energy changes during melting and boiling using a heating curve.
  • Relate heat of solution to lattice energy and hydration energy.
  • Determine molar heats of neutralisation, combustion and displacement from experimental data.
  • Apply Hess's law to energy cycles.
  • Explain the factors that affect the choice of a fuel and the effects of fuels on the environment.
The one formula you will use all topic:
Heat change (J) = m × c × ΔT
m = mass of solution/water (g, where 1 cm³ ≈ 1 g), c = 4.2 J g⁻¹ °C⁻¹ for water, ΔT = temperature change (°C).
Slide 2 · Exothermic and endothermic

Does it give out or take in heat?

Exothermic

Heat is given out to the surroundings, so the temperature rises. The products have less energy than the reactants: ΔH is negative.

Energy Reactants Products ΔH = −ve

Endothermic

Heat is taken in from the surroundings, so the temperature falls. The products have more energy than the reactants: ΔH is positive.

Energy Reactants Products ΔH = +ve

Activity: classify the process

Tap a card, then tap the box it belongs to.

Exothermic (ΔH −ve)

Endothermic (ΔH +ve)

Slide 3 · Bond breaking and bond forming

Where does the energy come from?

Breaking bonds absorbs energy (endothermic). Forming bonds releases energy (exothermic). The overall ΔH depends on which is bigger.

ΔH = (bonds broken) − (bonds formed)

Activity: bond energy calculator

Bond energies (kJ mol⁻¹): H–H 436, Cl–Cl 242, H–Cl 431, N≡N 945, N–H 391, C–H 413, O=O 498, C=O (in CO₂) 805, O–H 464.

Slide 4 · Energy changes in physical processes

Heating curve of water

When ice is heated steadily, the temperature rises, then stays constant while it melts and again while it boils. The heat absorbed at these flat parts is used to overcome the forces between molecules, not to raise the temperature. It is called latent heat.

Molar heat of fusion: heat absorbed when 1 mole of a solid changes to liquid at its melting point. Water: +6.0 kJ mol⁻¹.
Molar heat of vaporisation: heat absorbed when 1 mole of a liquid changes to gas at its boiling point. Water: +40.7 kJ mol⁻¹.
Why is vaporisation so much bigger than fusion? Melting only loosens the molecules; boiling separates them completely, so far more intermolecular forces (hydrogen bonds in water) must be broken. Freezing and condensation give out the same amounts of heat (exothermic).
Slide 5 · Heat of solution

Why do some salts make water hot and others cold?

Dissolving an ionic solid happens in two imaginary steps:

ΔHsolution = Lattice energy + Hydration energy

Activity: dissolve a salt

Values are approximate textbook data in kJ mol⁻¹.

Rule: if hydration energy is bigger (in size) than lattice energy, dissolving is exothermic (water warms). If lattice energy is bigger, dissolving is endothermic (water cools), as with ammonium nitrate in cold packs.
Slide 6 · Molar heat of neutralisation

Mixing acids and alkalis

Molar heat of neutralisation is the heat evolved when an acid and a base react to form 1 mole of water.

H+(aq) + OH−(aq) → H2O(l) ΔH ≈ −57.3 kJ mol⁻¹

Virtual experiment: 50 cm³ of 2 M acid + 50 cm³ of 2 M base

Pick an acid and a base, then mix. Starting temperature: 22.0 °C.
Why are weak acids/bases lower? They are only partly ionised. Some of the heat released is used up to ionise the rest of the molecules, so less heat is given out overall.
Slide 7 · Heat of combustion and Hess's law

Burning fuels and energy cycles

Molar heat of combustion is the heat evolved when 1 mole of a substance is burnt completely in oxygen.

Activity: spirit-lamp calculator

A lamp of ethanol (C₂H₅OH, M = 46) heats water in a metal can. Change the readings.

Hess's law

The total enthalpy change of a reaction is the same whatever route is taken, as long as the initial and final states are the same.

C(s) + O₂(g) CO₂(g) CO(g) + ½O₂(g) ΔH₁ = −393 kJ ΔH₂ = ? ΔH₃ = −283 kJ
Slide 8 · Fuels

Choosing a fuel

A fuel is a substance that burns in air to produce useful heat. The heating (calorific) value is the heat given out when 1 g of the fuel burns completely.

Activity: compare fuels

Tap a fuel to see its strengths and weaknesses. Bars show heating value in kJ g⁻¹ (approximate).

Tap a fuel.
Factors in choosing a fuel: heating value · cost · availability · ease of transport and storage · rate of burning / ignition point · amount of pollution (smoke, ash, harmful gases).
Environmental effects: CO₂ → global warming; CO is poisonous; SO₂ and NO₂ → acid rain; soot and smoke → respiratory diseases; cutting trees for wood/charcoal → deforestation and soil erosion.
Safety: burn fuels in well-ventilated places; never run a charcoal jiko in a closed room (CO poisoning); store petrol and gas away from flames.
Slide 9 · Practice

Worked example and quick checks

Worked example: heat of displacement

Excess zinc powder was added to 50 cm³ of 0.2 M copper(II) sulphate. The temperature rose by 10 °C. Calculate the molar heat of displacement of copper by zinc. (c = 4.2 J g⁻¹ °C⁻¹, density = 1 g cm⁻³)

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Step 1: Heat evolved = m c ΔT = 50 × 4.2 × 10 = 2100 J = 2.1 kJ
Step 2: Moles of Cu²⁺ = (0.2 × 50) ÷ 1000 = 0.01 mol
Step 3: 0.01 mol gives 2.1 kJ, so 1 mol gives 2.1 ÷ 0.01 = 210 kJ
Answer: ΔH = −210 kJ mol⁻¹ (negative because heat was given out)

Your turn

1. 100 g of water rises by 5 °C. Heat absorbed = J.
2. A thermometer reading falls when a salt dissolves, so the process is .
3. Using the cycle on slide 7, if ΔH₁ = −393 and ΔH₂ = −110, then ΔH₃ = kJ.
4. The temperature stays constant while water boils because heat is used to break intermolecular .
Slide 10 · Quiz

Check your understanding

Part A: Fill in the blanks (auto-marked, 1 mark each)

1. A reaction that gives out heat to the surroundings is .
2. For an endothermic reaction, the sign of ΔH is .
3. Breaking chemical bonds is an process.
4. The heat evolved when 1 mole of water is formed from an acid and a base is the molar heat of .
5. For a strong acid and a strong alkali, this value is about kJ mol⁻¹.
6. Hess's law: the enthalpy change depends only on the initial and states.
7. Heat absorbed when 1 mole of a solid melts at its melting point is the molar heat of .
8. Heat change = mass × specific heat capacity × .
9. Energy released when gaseous ions are surrounded by water molecules is called energy.
10. Burning charcoal in a closed room is dangerous because it produces poisonous carbon .

Part B: Short answers (smart-marked against key points)

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