Exothermic and Endothermic Reactions and Energy Profiles

Energy Changes in Reactions

Every chemical reaction involves a transfer of energy between the reacting chemicals and their surroundings. This energy is usually transferred as heat, but can also be transferred as light, sound or electrical energy.

Exothermic Reactions

An exothermic reaction is one that transfers energy to the surroundings, causing the temperature of the surroundings to increase.

The energy content of the products is less than the energy content of the reactants. The difference is released as heat.

Examples of Exothermic Reactions

• Combustion (burning fuels): CH₄ + 2O₂ → CO₂ + 2H₂O (releases heat and light)
• Neutralisation: HCl + NaOH → NaCl + H₂O (solution gets warmer)
• Oxidation reactions: respiration, rusting
• Hand warmers: crystallisation of a supersaturated solution

Energy Profile for Exothermic Reactions

An energy profile diagram shows energy on the y-axis and progress of reaction on the x-axis.

For an exothermic reaction:

• Reactants start at a higher energy level
• Products end at a lower energy level
• There is a hump between them representing the activation energy (Ea)
• The overall energy change (ΔH) is negative — energy is given out

The height of the hump above the reactants is the activation energy — the minimum energy reactant particles must have to react.

Endothermic Reactions

An endothermic reaction is one that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.

The energy content of the products is more than the energy content of the reactants. Energy is absorbed from the surroundings.

Examples of Endothermic Reactions

• Thermal decomposition: CaCO₃ → CaO + CO₂ (requires continuous heating)
• Citric acid + sodium hydrogencarbonate (the solution gets cold)
• Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Dissolving ammonium nitrate in water (used in cold packs)

Energy Profile for Endothermic Reactions

For an endothermic reaction:

• Reactants start at a lower energy level
• Products end at a higher energy level
• There is still an activation energy hump
• The overall energy change (ΔH) is positive — energy is taken in

Activation Energy

Activation energy (Ea) is the minimum amount of energy that colliding particles must have in order for a reaction to occur. It is shown on the energy profile as the height of the energy barrier above the reactants.

All reactions — both exothermic and endothermic — require activation energy to get started. Even burning a fuel (very exothermic) needs an initial spark or flame to provide the activation energy.

A catalyst lowers the activation energy by providing an alternative reaction pathway. This is shown on the energy profile as a lower hump.

Required Practical: Investigating Energy Changes

Method (Temperature Change on Neutralisation or Dissolving)

1. Measure a fixed volume of acid (or water) using a measuring cylinder and place in a polystyrene cup (acts as an insulator)

2. Record the starting temperature

3. Add the alkali (or solid) and stir

4. Record the temperature every 30 seconds for a set time

5. Calculate the temperature change

Calculating Energy Change

q = mcΔT

Where:

• q = energy transferred (in joules, J)
• m = mass of solution (in grams — assume 1 cm³ = 1 g for dilute solutions)
• c = specific heat capacity of water (4.18 J/g/°C)
• ΔT = temperature change (°C)

Example

50 cm³ of acid + 50 cm³ of alkali. Temperature rises from 20°C to 26.5°C.

m = 100 g (total volume, assuming density = 1 g/cm³)

ΔT = 6.5°C

q = 100 × 4.18 × 6.5 = 2717 J = 2.717 kJ

Exam Tips

• Exothermic = temperature goes UP, energy is released, ΔH is negative
• Endothermic = temperature goes DOWN, energy is absorbed, ΔH is positive
• On energy profiles, always label: reactants, products, activation energy (Ea), and overall energy change
• A catalyst lowers Ea but does not change the overall energy change
• In the required practical, the polystyrene cup acts as an insulator to reduce heat loss to the surroundings