Energetics I
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8.1 Enthalpy change and standard conditions
Enthalpy change, ΔH: the heat energy change in a reaction at constant pressure, in kJ mol−1.
Standard conditions: 100 kPa pressure and a stated temperature, usually 298 K, with all substances in their standard states. Shown by the symbol ΔH⦵.
Exothermic: energy given out to the surroundings; ΔH is negative; the surroundings get warmer. Combustion, neutralisation and respiration are exothermic.
Endothermic: energy taken in from the surroundings; ΔH is positive; the surroundings cool. Thermal decomposition and photosynthesis are endothermic.
Check yourself: What are standard conditions?
100 kPa and a stated temperature, usually 298 K
8.2 Enthalpy level diagrams and reaction profiles
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8.3 Standard enthalpy changes
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8.4 Measuring enthalpy changes in solution
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8.5 Enthalpy change of combustion
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8.6 Temperature–time graphs: finding the true ΔT
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8.7 Errors and assumptions in calorimetry
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8.8 Hess's law
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8.9 Core Practical 8: an enthalpy change by Hess's law
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8.10 Bond enthalpies
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Extra worked questions
Cover the solution, answer on paper, then mark yourself against each step. Each step shows the mark it earns.
All 8 worked solutions are free for members. The first one is open to everyone.
Heat taken in by a solution
A reaction in 50.0 cm3 of solution raises its temperature by 6.5 K. Calculate the heat energy released, in kJ. Assume c = 4.18 J g−1 K−1 and density 1.00 g cm−3.
Try it first, then show the worked solution
| Substitute | q = 50.0 × 4.18 × 6.5 | 1 |
|---|---|---|
| Evaluate | = 1359 J = 1.36 kJ | 1 |
Answer: 1.36 kJ
Examiner tip: The mass is the solution's mass (50.0 g here), not the mass of a reactant.
Two definitions
Define (a) standard enthalpy of formation and (b) standard enthalpy of combustion.
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Enthalpy of neutralisation
25.0 cm3 of 1.00 mol dm−3 HCl is mixed with 25.0 cm3 of 1.00 mol dm−3 NaOH. The temperature rises by 6.8 K. Calculate the enthalpy change of neutralisation.
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Burning propan-1-ol
Burning 0.600 g of propan-1-ol (M = 60.0 g mol−1) heats 200 g of water by 21.5 K. Calculate the enthalpy of combustion of propan-1-ol, and suggest why it is less exothermic than the data-book value.
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Combustion from formation data
Use these enthalpies of formation (kJ mol−1) to calculate the enthalpy of combustion of ethanol: C2H5OH(l) −277; CO2(g) −394; H2O(l) −286. C2H5OH + 3O2 → 2CO2 + 3H2O
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Formation from combustion data
Calculate the enthalpy of formation of ethane, C2H6(g), using enthalpies of combustion (kJ mol−1): C(s) −394; H2(g) −286; C2H6(g) −1560.
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Burning methane by bond enthalpies
Use mean bond enthalpies (kJ mol−1) C–H 413, O=O 498, C=O 805, O–H 464 to calculate ΔH for CH4(g) + 2O2(g) → CO2(g) + 2H2O(g). Give one reason it differs from the data-book value of −890 kJ mol−1.
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Core Practical 8: potassium hydrogencarbonate
2.50 g of KHCO3 (M = 100.1) added to 30.0 cm3 of excess HCl lowers the temperature by 4.6 K. 2.07 g of K2CO3 (M = 138.2) added to 30.0 cm3 of the same acid raises it by 5.4 K. Calculate ΔH for 2KHCO3 → K2CO3 + H2O + CO2. Assume c = 4.18 J g−1 K−1 and 1.00 g cm−3.
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Video walkthroughs
Narrated walkthroughs of exam-style questions from this topic of the book, showing where each mark comes from. Try the question first, then watch. Captions are available in the player.
Enthalpy of neutralisation
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Hess cycle from enthalpies of formation
Bond enthalpy: burning ethene
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Temperature correction by extrapolation
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Hydrogenation: combustion data vs bond enthalpies
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Recall cards
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