Topic 5 of 10

Energy Changes

Understand how energy is transferred in chemical reactions — from exothermic combustion to endothermic thermal decomposition, bond energies, and electrochemical cells.

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Exothermic & Endothermic Reactions

Exothermic Reactions

An exothermic reaction transfers energy to the surroundings, usually by heating. The temperature of the surroundings increases.

  • Combustion: burning fuels (CH₄ + 2O₂ → CO₂ + 2H₂O)
  • Neutralisation: acid + alkali → salt + water
  • Oxidation: metals reacting with oxygen

Everyday uses: self-heating cans, hand warmers.

Endothermic Reactions

An endothermic reaction takes in energy from the surroundings, usually by heating. The temperature of the surroundings decreases.

  • Thermal decomposition: CaCO₃ → CaO + CO₂
  • Dissolving ammonium nitrate: caused by breaking ionic bonds.
  • Citric acid + sodium hydrogencarbonate

Everyday uses: instant cold packs for sports injuries.

All reactions involve both bond breaking (endothermic — requires energy) and bond making (exothermic — releases energy). Whether the overall reaction is exothermic or endothermic depends on which process transfers more energy.

Reaction Profiles

Reaction profiles are energy diagrams showing the energy of reactants and products.

Exothermic Profile

Products are at a lower energy level than reactants. The energy difference is released to the surroundings. The "hump" represents the activation energy (Eₐ) — the minimum energy needed to start the reaction.

Endothermic Profile

Products are at a higher energy level than reactants. The energy difference is taken in from the surroundings.

Effect of Catalysts on Profiles

A catalyst provides an alternative reaction pathway with a lower activation energy. On the profile, the "hump" is smaller, but the overall energy change (ΔH) remains the same.

When drawing reaction profiles, always label: reactants, products, activation energy (Eₐ), and the overall energy change. The x-axis is "Progress of reaction" and the y-axis is "Energy".

Bond Energy Calculations (HT)

Chemical reactions involve breaking old bonds (endothermic) and making new bonds (exothermic).

Overall energy = Energy to break bonds − Energy released making bonds
  • If more energy is released making bonds than is needed to break bonds → exothermic (negative value).
  • If more energy is needed to break bonds than is released making bonds → endothermic (positive value).

Calculate the energy change for: H₂ + Cl₂ → 2HCl

Bond energies: H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol, H–Cl = 431 kJ/mol

Breaking: 1 × H–H + 1 × Cl–Cl = 436 + 242 = 678 kJ

Making: 2 × H–Cl = 2 × 431 = 862 kJ

Overall: 678 − 862 = −184 kJ/mol (exothermic)

A negative answer = exothermic (energy released). A positive answer = endothermic (energy absorbed). Always show your working clearly in the exam.

Chemical & Fuel Cells (HT)

Chemical Cells (Batteries)

A chemical cell produces a voltage when two different metals are dipped into an electrolyte and connected by a wire. Electrons flow from the more reactive metal to the less reactive metal through the external circuit.

The greater the difference in reactivity between the two metals, the greater the voltage.

Hydrogen Fuel Cells

Hydrogen fuel cells react hydrogen with oxygen to produce water and electricity — with no other waste products.

2H₂ + O₂ → 2H₂O + electrical energy

Advantages

  • Only product is water — no CO₂ greenhouse gas emissions.
  • Do not need recharging — continuous supply of fuel.

Disadvantages

  • Hydrogen is difficult and expensive to store and transport (must be compressed or liquefied).
  • Hydrogen is most commonly produced from natural gas (fossil fuel) — so not truly zero-carbon.
  • Platinum catalysts are expensive.