Energetics

Edexcel International GCSE Chemistry· 3.1–3.8 (Section 3(a))· 25 min read

1. Exothermic vs Endothermic Reactions★☆☆☆☆⏱ 5 min

All chemical reactions involve energy transfers between the reacting system and its surroundings. The two key categories of reaction are defined by the direction of this heat transfer.

📘 Definition

Exothermic and Endothermic Reactions

Exothermic reactions release heat energy to the surroundings, causing an increase in the measured temperature of the surroundings. Endothermic reactions absorb heat energy from the surroundings, causing a decrease in measured temperature.

Example:

Combustion of fuels is exothermic; photosynthesis and thermal decomposition of calcium carbonate are endothermic.

📐 Worked Example

A student dissolves ammonium nitrate in water, and the temperature of the solution drops from 21°C to 12°C. State if the dissolving process is exothermic or endothermic, and explain your answer.

1
1. Identify the direction of temperature change: the temperature of the surroundings (water) decreases by 9°C
2
1. Link to energy transfer: a temperature decrease means the system absorbed heat from the surroundings
3
1. Classify the process as endothermic, because heat is taken in by the dissolving salt from the water.

Exam tip:

Always link your classification directly to the observed temperature change if given, rather than just restating the definition, to earn full marks.

2. Calorimetry Experiments & Q = mcΔT Calculations★★☆☆☆⏱ 8 min

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Calorimetry is the standard practical method used to measure heat energy changes during reactions. For reactions in solution (dissolving, neutralisation, displacement), you measure the temperature change of the water/solution. For combustion reactions, you measure the temperature change of water heated by the burning fuel.

Insulate the reaction vessel to reduce heat loss to the air
Record initial temperature of the solution/water before the reaction
Carry out the reaction, stir constantly, and record the maximum/minimum final temperature
Calculate temperature change ΔT = final temperature - initial temperature

📘 Definition

Heat Energy Change Formula

Q = mc\Delta T

Q is the heat energy change, m is the mass of the water/solution in grams, c is the specific heat capacity of the solution (usually given as 4.2 J g⁻¹ °C⁻¹ for water), ΔT is the temperature change in °C.

📐 Worked Example

A student adds 25g of magnesium sulfate to 100g of water, and the temperature rises from 19°C to 32°C. Calculate the heat energy change Q in kJ, using c = 4.2 J g⁻¹ °C⁻¹.

1
1. Calculate ΔT: 32°C - 19°C = 13°C
2
1. Substitute values into Q = mcΔT: Q = 100 g * 4.2 J g⁻¹ °C⁻¹ * 13 °C
3
$Q = 100 \times 4.2 \times 13 = 5460 \text{ J}$
4
1. Convert to kJ by dividing by 1000: Q = 5.46 kJ

Exam tip:

Always use the mass of the water or aqueous solution in the Q = mcΔT calculation, not the mass of any solid reactant added to the solution.

3. Molar Enthalpy Change (ΔH) Calculations★★★☆☆⏱ 7 min

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Molar enthalpy change ΔH is the heat energy change per mole of a specified reactant, and is reported in kJ mol⁻¹. The sign convention is critical: ΔH is negative for exothermic reactions (energy released) and positive for endothermic reactions (energy absorbed).

📘 Definition

Molar Enthalpy Change Formula

\Delta H = -Q \div n

ΔH is molar enthalpy change in kJ mol⁻¹, Q is heat energy change in kJ, n is the number of moles of the limiting reactant. The negative sign ensures the correct sign convention is followed.

📐 Worked Example

In the magnesium sulfate dissolving experiment, Q was 5.46 kJ, and 0.1 moles of magnesium sulfate were dissolved. Calculate the molar enthalpy change ΔH for the dissolving process.

1
1. The reaction is exothermic (temperature increased), so ΔH will be negative
2
1. Substitute values into the formula: ΔH = -5.46 kJ ÷ 0.1 mol
3
$\Delta H = -54.6 \text{ kJ mol}^{-1}$

Exam tip:

You will lose 1 mark per question if you omit the correct positive/negative sign for ΔH, so always check the temperature change direction before finalising your answer.

4. Energy Level Diagrams (Higher Only)★★☆☆☆HL only⏱ 4 min

Energy level (reaction profile) diagrams show the relative energy of reactants and products during a reaction. The y-axis is labelled 'Energy' and the x-axis is labelled 'Reaction Progress'.

For exothermic reactions: the horizontal line for products is lower than the line for reactants, and ΔH is labelled as a downward arrow between the two lines
For endothermic reactions: the horizontal line for products is higher than the line for reactants, and ΔH is labelled as an upward arrow between the two lines

📐 Worked Example

Draw a fully labelled energy level diagram for the endothermic reaction of thermal decomposition of calcium carbonate.

1
1. Draw y-axis labelled 'Energy' and x-axis labelled 'Reaction Progress'
2
1. Draw a lower horizontal line labelled 'Reactants (CaCO₃)' and a higher horizontal line labelled 'Products (CaO + CO₂)'
3
1. Draw an upward arrow between the two lines, labelled 'ΔH = +ve (endothermic)'

5. Bond Energy Enthalpy Calculations (Higher Only)★★★★☆HL only⏱ 6 min

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Enthalpy changes in reactions come from breaking and making covalent bonds: breaking bonds is endothermic (requires energy input), making bonds is exothermic (releases energy). You can calculate ΔH for a reaction using given bond energy values.

📘 Definition

Bond Energy Enthalpy Formula

\Delta H = \Sigma (\text{bonds broken}) - \Sigma (\text{bonds made})

Σ bonds broken is the total energy required to break all bonds in reactants, Σ bonds made is the total energy released when all bonds in products are formed. Bond energy values are always given in exam questions.

📐 Worked Example

Calculate ΔH for the reaction CH₄ + 2O₂ → CO₂ + 2H₂O using the given bond energies: C-H = 413 kJ mol⁻¹, O=O = 498 kJ mol⁻¹, C=O = 743 kJ mol⁻¹, O-H = 463 kJ mol⁻¹.

1
1. Calculate total bond breaking energy: 4 C-H bonds + 2 O=O bonds = (4413) + (2498) = 1652 + 996 = 2648 kJ mol⁻¹
2
1. Calculate total bond making energy: 2 C=O bonds + 4 O-H bonds = (2743) + (4463) = 1486 + 1852 = 3338 kJ mol⁻¹
3
$\Delta H = 2648 - 3338 = -690 \text{ kJ mol}^{-1}$

Exam tip:

Always count the number of each bond type carefully, especially when there are multiple moles of reactants or products, to avoid arithmetic errors.

6. Common Pitfalls

Wrong move:

Using the mass of solid reactant instead of mass of water/solution in Q = mcΔT

Why:

The heat energy is transferred to/from the water/solution, so its mass is the correct value to use

Correct move:

Always use the mass of the aqueous solution or water in the Q calculation unless explicitly told otherwise

Wrong move:

Forgetting to convert Q from J to kJ before calculating ΔH

Why:

ΔH is almost always required in kJ mol⁻¹, so Q must be in kJ for units to match

Correct move:

Divide your Q value in J by 1000 to convert to kJ before substituting into the ΔH formula

Wrong move:

Calculating ΔH as Σ bonds made minus Σ bonds broken

Why:

This reverses the sign of ΔH, as bond breaking is endothermic (positive) and making is exothermic (negative)

Correct move:

Always calculate ΔH as total bond breaking energy minus total bond making energy

Wrong move:

Omitting the positive/negative sign for ΔH

Why:

Edexcel awards marks for correct sign convention as well as numerical value, so missing signs cost marks

Correct move:

Check if the reaction is exothermic (temp increase, products lower energy) to assign negative ΔH, or endothermic for positive ΔH

Wrong move:

Calculating ΔT as initial minus final temperature

Why:

This gives the wrong sign for Q, leading to an incorrect ΔH sign

Correct move:

Always calculate ΔT as final temperature minus initial temperature, even if the value is negative for endothermic reactions

7. Quick Reference Cheatsheet

Concept	Tier	Key Rule/Formula	Units
Exo/Endo Identification	Core	Temp increase = exothermic, temp decrease = endothermic	°C
Heat Energy Change	Core	Q = mcΔT	Q: J/kJ, m: g, c: J g⁻¹ °C⁻¹, ΔT: °C
Molar Enthalpy Change	Core	ΔH = -Q ÷ moles, exo = -ve, endo = +ve	ΔH: kJ mol⁻¹
Energy Level Diagrams	Higher	Exo: products < reactants, Endo: products > reactants	N/A
Bond Energy ΔH	Higher	ΔH = Σ bonds broken - Σ bonds made	kJ mol⁻¹

8. Frequently Asked

Do I need to memorise the specific heat capacity of water for exams?

The value of c for water (usually 4.2 J g⁻¹ °C⁻¹) is almost always given in exam questions. You only need to recall the Q = mcΔT formula and correct unit conversions.

Why is ΔH negative for exothermic reactions?

The enthalpy change measures the difference in energy between products and reactants. For exothermic reactions, products have lower energy than reactants, so the difference is negative, matching the energy released to the surroundings.

Is activation energy covered in this topic?

No, activation energy and the effect of catalysts on reaction profiles are part of the Reaction Rates topic (S3_T02), not this energetics sub-topic.

Going deeper

official_specEdexcel IGCSE Chemistry 4CH1 SpecificationRefer to section 3(a) for full energetics requirements

What's Next

Now you have mastered all energetics content for Edexcel IGCSE Chemistry, you can move on to related physical chemistry topics that build on this foundation. Next, you will study reaction rates, which links to activation energy (often shown on energy level diagrams) and the effect of temperature on reaction speed. You will also learn about reversible reactions and equilibrium, where enthalpy changes occur in both forward and reverse directions. Make sure to practice past paper calculation questions for energetics, as these are high-mark, frequently tested questions that are easy to score well on with consistent practice.