3.1Unit 1 · Physical & Organic Chemistry
Energetics — Edexcel IGCSE Chemistry Modular - UNIT 1 Revision Notes & Topical Questions
15 min read 16 parts Lesson 1 of 5 1 quiz
Lesson summary
Distinguish exothermic and endothermic reactions and draw energy level diagrams.Q = m × c × ΔT (Q in J, m in g, c = 4.2 J/g/°C for water).ΔH = −Q ÷ n for an exothermic reaction (kJ/mol).1 cm3 of dilute solution is assumed to have a mass of 1 g.Exothermic ΔH is negative; endotherm…
- Calorimetry
- Energetics calculations
- Bond energy
- Practical – temperature changes
- Energy is conserved
- Reducing heat loss with fuels
Part 1 of 7Key ideas
What the symbols mean:
Q = m × c × ΔT (Q in J, m in g, c = 4.2 J/g/°C for water).
What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
ΔH = −Q ÷ n for an exothermic reaction (kJ/mol).
1 cm3 of dilute solution is assumed to have a mass of 1 g.
Exothermic ΔH is negative; endothermic ΔH is positive.
ΔH = energy to break bonds − energy released making bonds.
Polystyrene cups and lids reduce heat loss; heat loss makes experimental values smaller than true values.
For fuel combustion, heat a known mass of water in a copper can and weigh the burner before and after.
Part 2 of 7Learning objectives
1
Distinguish exothermic and endothermic reactions and draw energy level diagrams.
2
Carry out calorimetry experiments and calculate heat energy changes using Q = mcΔT.
3
Calculate molar enthalpy changes from experimental data.
4
Use average bond energies to calculate ΔH for a reaction.
Part 3 of 7Essential knowledge
Exothermic and endothermic
Exothermic and endothermic in an exothermic reaction energy is transferred to the surroundings and temperature increases (combustion, neutralisation, respiration). In an endothermic reaction energy is taken in and temperature decreases (thermal decomposition of CaCO3, dissolving ammonium nitrate).
The heat energy change at constant pressure is the enthalpy change, ΔH. It is negative for exothermic and positive for endothermic reactions and is usually given in kJ/mol.
Calorimetry
Calorimetry reactions in solution are carried out in an insulated polystyrene cup; for fuels, the flame heats water in a metal can. The temperature change of the water or solution is used to calculate heat transferred.
Energetics calculations
Q = mcΔT, where m is the mass of the water or solution (not the reactants), c = 4.2 J/g/°C and ΔT is the temperature change. Convert to kJ, then divide by the moles of the limiting reactant to find ΔH.
Energy level diagrams
Energy level diagrams for an exothermic reaction the products are drawn lower than the reactants with a downward arrow labelled ΔH; for an endothermic reaction the products are higher. Activation energy is shown as a hump from reactants to the peak.
Bond energy
Bond energy breaking bonds is endothermic and making bonds is exothermic. ΔH = sum of bonds broken − sum of bonds formed. If more energy is released forming bonds than is absorbed breaking them, the reaction is exothermic.
Practical – temperature changes
Practical – temperature changes for displacement, dissolving, neutralisation and combustion, measure initial and extreme temperatures. Keep volumes and concentrations fixed, stir, use a lid, and repeat for reliable averages. Values are usually lower than data-book values because of heat loss and incomplete combustion.
Energy is conserved
Energy cannot be created or destroyed in a reaction. If a reaction gives out heat to the surroundings, the products must store less energy than the reactants by exactly that amount; in an endothermic reaction the products store more.
Assumptions in solution calorimetry
When calculating Q = mcΔT for a solution we assume it has the same density (1 g/cm3) and specific heat capacity as water, that the cup absorbs no heat, that heat losses are negligible and that the reaction is complete. One reactant is added in excess to ensure this.
Reducing heat loss with fuels
Place the metal can close to the flame, put a lid on it and use a draught shield around the burner. Stir the water and record the highest temperature. Values are still low due to heat loss, heat absorbed by the can and incomplete combustion.
Energy per gram of fuel
Find the mass of fuel burned by weighing the spirit burner before and after. Energy per gram = Q ÷ mass burned. For example, if 0.50 g of fuel releases 10.5 kJ, it releases 21 kJ/g. Divide by moles instead to get kJ/mol.
Temperature change units
A temperature change in °C has the same size as a change in kelvin, so ΔT can be used in either unit in Q = mcΔT. Q comes out in joules; divide by 1000 for kJ.
Finding an unknown bond energy
Rearrange ΔH = bonds broken − bonds made. For H2 + Cl2 → 2HCl with ΔH = −184 kJ/mol, H–H = 436 and H–Cl = 431: 436 + x − 862 = −184, so the Cl–Cl bond energy x = 242 kJ/mol.
Counting bonds
Draw displayed formulae first so every bond is visible, then multiply by the balancing numbers in the equation. For example, 2HCl means two H–Cl bonds are formed.
Neutralisation temperature practical
Add an acid in 5 cm3 portions to a fixed volume of alkali in a polystyrene cup, recording the highest temperature after each addition. Plot temperature against volume: the peak shows the volume that exactly neutralises the alkali.
Part 4 of 7Process & sequence
1
Measure a known volume of acid (e.g. 25 cm3) into a polystyrene cup inside a beaker.
2
Record the starting temperature with a thermometer.
3
Add the second reactant (alkali, metal powder or fuel heat) and stir.
4
Record the highest (or lowest) temperature reached.
5
Calculate ΔT and use Q = mcΔT.
6
Find moles reacted and divide Q (in kJ) by moles to get ΔH, adding the correct sign.
Finding an enthalpy change by calorimetrywrite these steps in this order; each step is a marking point.
1
Measure volume into polystyrene cup
2
Record starting temperature
3
Add reactant and stir
4
Record extreme temperature
5
Calculate Q = mcΔT
6
Divide by moles for ΔH
Part 5 of 7Worked examples
−57.1 kJ/mol
50 cm3 of 1.0 mol/dm3 HCl is mixed with 50 cm3 of 1.0 mol/dm3 NaOH. The temperature rises by 6.8 °C. Calculate ΔH for the neutralisation.
- Mass of solution = 100 g.
- Q = 100 × 4.2 × 6.8 = 2856 J = 2.856 kJ.
- Moles of water formed = 0.050 × 1.0 = 0.050 mol. ΔH = −2.856 ÷ 0.050 = −57.1 kJ/mol .
- (negative because temperature rose)
−184 kJ/mol
Use bond energies (H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol) to find ΔH for H2 + Cl2 → 2HCl.
- Bonds broken
- 436 + 242 = 678 kJ.
- Bonds made: 2 × 431 = 862 kJ.
- ΔH = 678 − 862 = −184 kJ/mol, so the reaction is exothermic.
Part 6 of 7Summary
1
Exothermic reactions release heat, ΔH negative.
2
Endothermic reactions absorb heat, ΔH positive.
3
Use Q = mcΔT then divide by moles.
4
Bond breaking absorbs, bond making releases energy.
5
Heat loss is the main source of error in calorimetry.
Part 7 of 7Compare: Compare
| Feature | Exothermic | Endothermic |
|---|---|---|
| Heat transfer | Given out to surroundings | Taken in from surroundings |
| Temperature of surroundings | Rises | Falls |
| Sign of ΔH | Negative | Positive |
| Products vs reactants energy | Products lower | Products higher |
| Examples | Combustion, neutralisation, displacement | Thermal decomposition, dissolving NH4NO3, photosynthesis |
| Process | Energy | Effect on ΔH |
|---|---|---|
| Breaking bonds | Absorbed (endothermic) | Adds positive energy |
| Making bonds | Released (exothermic) | Adds negative energy |
| Overall | ΔH = bonds broken − bonds made | Negative if more energy released than absorbed |
Key terms
Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Exothermic reaction · Endothermic reaction · Enthalpy change (ΔH) · Calorimetry · Specific heat capacity (c) · Bond energy · Activation energy · Energy level diagram · Molar enthalpy change
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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