Energy Changes in Reactions

Energy changes questions appear across Papers 2 and 4 and typically carry 3-5 marks. The topic combines conceptual understanding (exo vs endo) with numerical calculations (bond energies). Both require precision with signs and terminology.

Exothermic reactions

An exothermic reaction transfers energy to the surroundings. The temperature of the surroundings increases.

Delta H is negative (energy is lost by the system).

Common exothermic reactions:

• Combustion (burning fuels)
• Neutralisation (acid + alkali)
• Oxidation reactions (e.g. rusting, respiration)
• Many displacement reactions

Everyday applications: hand warmers, self-heating cans.

Endothermic reactions

An endothermic reaction absorbs energy from the surroundings. The temperature of the surroundings decreases.

Delta H is positive (energy is gained by the system).

Common endothermic reactions:

• Thermal decomposition (e.g. CaCO3 -> CaO + CO2)
• Photosynthesis
• Dissolving some salts (e.g. ammonium nitrate in water)
• Electrolysis (requires continuous energy input)

Everyday applications: cold packs (ammonium nitrate dissolving).

Energy level diagrams

Energy level diagrams are covered in detail at energy level diagrams. The key features:

Exothermic energy level diagram

• Reactants drawn at a higher energy level than products
• Arrow pointing downward shows energy released (Delta H is negative)
• The activation energy (Ea) hump shows the minimum energy needed to start the reaction

Endothermic energy level diagram

• Reactants drawn at a lower energy level than products
• Arrow pointing upward shows energy absorbed (Delta H is positive)
• The activation energy hump is larger because it includes both the activation energy and the energy absorbed

Drawing tips for exams

• Label the axes: y-axis = Energy, x-axis = Reaction progress (or Progress of reaction)
• Label reactants and products
• Show Delta H with a clear arrow and sign (negative for exo, positive for endo)
• Show activation energy (Ea) as the height from reactants to the top of the hump

Bond energy calculations (Supplement)

Bond energy is the energy needed to break one mole of a particular bond in the gaseous state. Breaking bonds requires energy (endothermic). Making bonds releases energy (exothermic).

The calculation method

Energy change = Energy to break bonds - Energy to make bonds

Or equivalently: Delta H = Sum of bond energies broken - Sum of bond energies formed

If the answer is positive, the reaction is endothermic. If the answer is negative, the reaction is exothermic.

Worked example: combustion of methane

CH4 + 2O2 -> CO2 + 2H2O

Bonds broken (in reactants):

• 4 x C-H = 4 x 412 = 1648 kJ
• 2 x O=O = 2 x 496 = 992 kJ
• Total energy to break bonds = 2640 kJ

Bonds made (in products):

• 2 x C=O = 2 x 743 = 1486 kJ
• 4 x O-H = 4 x 463 = 1852 kJ
• Total energy released making bonds = 3338 kJ

Energy change = 2640 - 3338 = -698 kJ/mol

The answer is negative, so the reaction is exothermic. This makes sense: combustion releases heat.

Worked example: decomposition of water

2H2O -> 2H2 + O2

Bonds broken (in reactants):

• 4 x O-H = 4 x 463 = 1852 kJ

Bonds made (in products):

• 2 x H-H = 2 x 436 = 872 kJ
• 1 x O=O = 1 x 496 = 496 kJ
• Total = 1368 kJ

Energy change = 1852 - 1368 = +484 kJ/mol

The answer is positive, so the reaction is endothermic. Decomposition of water requires energy input.

Step-by-step method for the exam

1. Write the balanced equation with structural formulae (showing all bonds)
2. List every bond in the reactants and count them
3. List every bond in the products and count them
4. Look up bond energies from the data provided
5. Calculate: total broken - total formed
6. State whether exothermic (negative) or endothermic (positive)

Common exam mistakes

1. Subtracting the wrong way round: It is bonds broken minus bonds formed. Reversing this gives the wrong sign.
2. Forgetting to count all bonds: CH4 has 4 C-H bonds, not 1. H2O has 2 O-H bonds. Use the structural formula to count.
3. Confusing bond breaking and bond making: Breaking is always endothermic (needs energy). Making is always exothermic (releases energy). Many students reverse this.
4. Forgetting the coefficient in the equation: 2O2 means 2 molecules, so 2 x O=O bonds, not 1.
5. Wrong sign on Delta H: Exothermic = negative, endothermic = positive. Some students write the number without the sign.

Practical measurement

In a simple calorimetry experiment:

• The temperature change of the water is measured
• For exothermic reactions, the water temperature rises
• For endothermic reactions, the water temperature falls
• Energy transferred = mass x specific heat capacity x temperature change (q = mc Delta T)

Sources of error: heat loss to surroundings, incomplete combustion, heat absorbed by the container. These always make the measured value less than the theoretical value.

Worked exam questions

The following bond energies are given: C-H = 412 kJ/mol, Cl-Cl = 242 kJ/mol, C-Cl = 338 kJ/mol, H-Cl = 431 kJ/mol.

Calculate the energy change for: CH4 + Cl2 -> CH3Cl + HCl. State whether the reaction is exothermic or endothermic. [4 marks]

Bonds broken: 1 x C-H = 412, 1 x Cl-Cl = 242. Total = 654 kJ [1]

Bonds formed: 1 x C-Cl = 338, 1 x H-Cl = 431. Total = 769 kJ [1]

Energy change = 654 - 769 = -115 kJ/mol [1]

The reaction is exothermic (negative value / more energy released making bonds than absorbed breaking them) [1]

Draw an energy level diagram for an exothermic reaction. Label the reactants, products, activation energy, and Delta H. [4 marks]

• Reactants labelled at a higher energy level [1]
• Products labelled at a lower energy level [1]
• Activation energy (Ea) shown as arrow from reactants to the top of the energy hump [1]
• Delta H shown as a downward arrow from reactants level to products level, labelled as negative [1]