Energetics: energy changes in chemical reactions
Every chemical reaction involves a transfer of energy between the reacting system and its surroundings. Energetics is the study of these energy changes, and it is one of the most commonly examined areas across the Pearson Edexcel IGCSE Chemistry papers. The 4CH1 specification requires you to classify reactions as exothermic or endothermic, draw energy level diagrams, perform calorimetry calculations and use bond energies to calculate enthalpy changes. These edexcel igcse chemistry notes work through each of those skills with worked examples and a systematic approach.
Key facts
| Term | Definition | Temperature change of surroundings | Examples |
|---|---|---|---|
| Exothermic reaction | A reaction that transfers heat energy to the surroundings | Temperature increases | Combustion, neutralisation, many oxidation reactions |
| Endothermic reaction | A reaction that takes in heat energy from the surroundings | Temperature decreases | Thermal decomposition, citric acid + sodium hydrogencarbonate, photosynthesis |
Calorimetry: measuring energy changes
Simple calorimetry experiments measure the temperature change when a reaction occurs in a known mass of water (or aqueous solution). The heat energy change is then calculated using the expression:
Q = mcΔT
Where:
- Q = heat energy change (in joules, J)
- m = mass of water or solution (in grams, g)
- c = specific heat capacity of water (4.2 J/g/°C)
- ΔT = change in temperature (°C)
Worked example: combustion calorimetry
Step 1: Identify values. m = 100 g, c = 4.2 J/g/°C, ΔT = 34.5 - 21.0 = 13.5 °C
Step 2: Substitute into Q = mcΔT. Q = 100 x 4.2 x 13.5
Step 3: Calculate. Q = 5670 J (or 5.67 kJ)
The reaction is exothermic because the water temperature rose, confirming that heat energy was transferred from the burning ethanol to the water.
Calculating molar enthalpy change
Once you know Q from the calorimetry calculation, you can find the molar enthalpy change (ΔH) by dividing Q by the number of moles of the substance that reacted:
ΔH = Q / n
Where n = number of moles. Continuing the example above:
- Mr of ethanol (C2H5OH) = (2 x 12) + (6 x 1) + 16 = 46
- Moles = mass / Mr = 0.50 / 46 = 0.0109 mol
- ΔH = 5670 / 0.0109 = 520,183 J/mol = 520.2 kJ/mol
For an exothermic reaction, ΔH is given a negative sign: ΔH = -520.2 kJ/mol. For an endothermic reaction, ΔH is positive. The sign tells you the direction of energy transfer.
Energy level diagrams
Energy level diagrams are a visual way to represent exothermic and endothermic reactions. The vertical axis shows energy and the horizontal axis shows the progress of the reaction.
Exothermic reaction diagram
The reactants start at a higher energy level than the products. The difference between the two levels is the enthalpy change (ΔH), which is negative. There is also a hump above the reactant energy level, representing the activation energy: the minimum energy the reactants need to start the reaction. The overall energy released equals the drop from reactants to products.
Endothermic reaction diagram
The reactants start at a lower energy level than the products. ΔH is positive (energy is absorbed). The activation energy hump sits above the reactant level, and the products sit higher still. The overall energy absorbed equals the rise from reactants to products.
| Feature | Exothermic diagram | Endothermic diagram |
|---|---|---|
| Reactant energy vs product energy | Reactants higher than products | Reactants lower than products |
| Sign of ΔH | Negative | Positive |
| Temperature of surroundings | Increases | Decreases |
| Activation energy | Present (shown as a hump) | Present (shown as a hump) |
When drawing these diagrams in the edexcel IGCSE exam, always label: reactants, products, ΔH (with an arrow showing the direction and a sign), and the activation energy (Ea). A diagram without labels will not earn full marks.
Bond energies: breaking and making bonds
This is where energetics connects to the molecular level. Two principles govern every reaction:
- Breaking bonds requires energy (endothermic process). Energy must be supplied to overcome the attraction between bonded atoms.
- Making bonds releases energy (exothermic process). When new bonds form, energy is given out because the atoms move to a more stable, lower-energy arrangement.
The overall enthalpy change of a reaction depends on the balance between these two processes:
- If the energy released by making new bonds is greater than the energy absorbed by breaking old bonds, the reaction is exothermic (ΔH is negative).
- If the energy absorbed by breaking old bonds is greater than the energy released by making new bonds, the reaction is endothermic (ΔH is positive).
Worked example: using bond energies
2H2(g) + O2(g) → 2H2O(g)
Bond energies: H-H = 436 kJ/mol, O=O = 498 kJ/mol, O-H = 464 kJ/mol
Step 1: Bonds broken (reactants)
2 x H-H = 2 x 436 = 872 kJ
1 x O=O = 1 x 498 = 498 kJ
Total energy in (breaking) = 872 + 498 = 1370 kJ
Step 2: Bonds made (products)
Each H2O has 2 O-H bonds. There are 2 H2O molecules, so 4 O-H bonds total.
4 x O-H = 4 x 464 = 1856 kJ
Total energy out (making) = 1856 kJ
Step 3: Calculate ΔH
ΔH = energy in (breaking) - energy out (making)
ΔH = 1370 - 1856 = -486 kJ
The negative sign confirms the reaction is exothermic. More energy is released making the four O-H bonds than is required to break the two H-H bonds and one O=O bond.
Common mistakes in energetics
- Reversing the bond energy formula. The correct approach is: ΔH = bonds broken - bonds made. Students who subtract the wrong way round get the right magnitude but the wrong sign, which misclassifies the reaction.
- Forgetting to count all bonds. In the example above, 2H2O contains four O-H bonds, not two. Always expand the structural formulae and count every bond individually.
- Confusing activation energy with ΔH. Activation energy is the energy needed to start the reaction (the height of the hump). ΔH is the overall energy difference between reactants and products. A reaction can have a high activation energy and still be exothermic if the products are much lower in energy than the reactants.
- Omitting the sign on ΔH. A ΔH value must carry a positive or negative sign. Writing "486 kJ" without specifying "- 486 kJ" loses the mark because the direction of energy transfer is not stated.
Practical calorimetry experiments
The 4CH1 specification lists several types of reaction that can be investigated by calorimetry:
| Reaction type | Typical observation | Exo or endothermic |
|---|---|---|
| Neutralisation (acid + alkali) | Temperature rises | Exothermic |
| Displacement (e.g., zinc + copper sulfate) | Temperature rises | Exothermic |
| Combustion (e.g., burning ethanol) | Water above flame heats up | Exothermic |
| Dissolving ammonium nitrate in water | Temperature drops | Endothermic |
| Citric acid + sodium hydrogencarbonate | Temperature drops | Endothermic |
When describing a calorimetry experiment in the exam, state what was measured (temperature change), what was controlled (mass of water, starting temperature, insulation), and how Q was calculated using Q = mcΔT.
Catalysts and energy level diagrams
A catalyst provides an alternative reaction pathway with a lower activation energy. On an energy level diagram, the catalyst lowers the hump but does not change the energy levels of the reactants or products. This means a catalyst does not change ΔH. It simply allows the reaction to proceed faster because more particles have enough energy to overcome the lower activation barrier.
Self-check questions
- Define the term exothermic reaction.
- A student mixes an acid and an alkali in a polystyrene cup and records a temperature increase of 8.5 °C. The total mass of the solution is 50 g. Calculate Q. (Use c = 4.2 J/g/°C)
- Draw an energy level diagram for an exothermic reaction. Label the reactants, products, ΔH and activation energy.
- Use bond energies to calculate ΔH for: CH4 + 2O2 → CO2 + 2H2O. Given: C-H = 413 kJ/mol, O=O = 498 kJ/mol, C=O = 805 kJ/mol, O-H = 464 kJ/mol.
- Explain why a catalyst lowers the activation energy but does not change the enthalpy change of a reaction.
- State whether each of these processes is exothermic or endothermic: (a) bond breaking, (b) bond making.
The concept of energetics explained at this level underpins much of the quantitative chemistry in the edexcel IGCSE specification. If you are researching what is energetics igcse, the answer spans four interconnected skills: classifying reactions by energy transfer, measuring energy changes by calorimetry, drawing energy level diagrams with correct labels, and using bond energies to predict enthalpy changes from molecular structure.
Edexcel igcse energetics covers everything from simple temperature measurements to multi-step bond energy calculations, so fluency with each method is essential. The edexcel igcse chemistry definition of exothermic and endothermic reactions must be stated precisely, including the direction of energy transfer. For further practice on these edexcel igcse chemistry explained patterns, the Green Bridge CBT platform offers topic-filtered questions on energetics with step-by-step worked solutions. The more calculations you practise under timed conditions, the more automatic the method becomes on exam day.
Edexcel IGCSE energetics explained: exothermic and endothermic reactions, bond energies, enthalpy calculations and energy level diagrams for 4CH1.
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