3aPhysical Chemistry
Energetics — Edexcel Modular - OL (Chemistry) Revision Notes & Topical Questions
10 min read 16 parts Lesson 1 of 3 2 quizzes
Energetics Edexcel O Level Modular Chemistry notes: know that chemical reactions in which heat energy is given out are described as exothermic, and those in which heat energy is taken in are described as endothermicExperimental values are less exothermic due to heat loss and inco…
- Energetics Edexcel O Level Modular Chemistry: what to do next
- Enthalpy change (ΔH)
- Standard conditions
- Hess’s law
- Bond energy
- Activation energy
Section 1 of 16Process & sequence
- Reactants
- Break bonds (+)
- Atoms
- Make bonds (−)
- Products
Section 2 of 16Keep going
Energetics Edexcel O Level Modular Chemistry: what to do next
These Energetics Edexcel O Level Modular Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Energetics Edexcel O Level Modular Chemistry understanding with the practice questions below, then reread any Energetics Edexcel O Level Modular Chemistry point you were unsure about before moving on.
Section 3 of 16Calorimetry: calculating heat energy
The heat energy transferred to a liquid is found from the temperature change using Q = m × c × ΔT, where m is the mass of water (g), c is the specific heat capacity (4.18 J/g/°C for water) and ΔT is the temperature change (°C).
Example
- burning 1.023 g of propan-1-ol (M = 60.11 g/mol) heated 200 g of water by 30 °C.
- Q = 200 × 4.18 × 30 = 25 080 J, about 25 kJ.
- Combustion is exothermic, so the enthalpy change is written as −25 kJ for this mass.
To find the enthalpy change per mole, divide by the moles burned: ΔH = Q ÷ n.
Section 4 of 16Energy level diagrams
An energy level diagram shows the relative energies of reactants and products. Energy is on the y-axis and progress of the reaction on the x-axis. An arrow shows the direction of change: downwards for exothermic and upwards for endothermic. The vertical gap between the levels is the overall enthalpy change.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy of products | Lower than reactants | Higher than reactants |
| Temperature of surroundings | Rises | Falls |
| Sign of ΔH | Negative | Positive |
You should be able to draw fully labelled diagrams and identify a reaction type from a diagram. Reaction profiles are similar but also show activation energy.
Section 5 of 16Bond breaking and bond making
Energy must be taken in to break bonds, so bond breaking is endothermic. Energy is released when new bonds form, so bond making is exothermic.
In an endothermic reaction the energy needed to break existing bonds is greater than the energy released on making new ones, so products have more energy than reactants and ΔH is positive. In an exothermic reaction the opposite is true and ΔH is negative.
You can also work backwards. In the formation of hydrogen bromide, the energy out for two H–Br bonds (366 kJ/mol each) is 2 × 366 = 732 kJ, and an unknown bond energy such as Br–Br can then be found from the known overall ΔH.
Section 6 of 16Practical: temperature change in neutralisation
To investigate the reaction between HCl and NaOH use a polystyrene calorimeter with a lid, because the lid prevents heat loss.
The maximum temperature occurs at the neutralisation point, where acid and alkali have exactly reacted.
Section 7 of 16Compare
| Enthalpy change | Symbol | Definition | Sign | Example | Note |
|---|---|---|---|---|---|
| Reaction | ΔH⦵r | Molar quantities in equation react | ± | — | Depends on equation |
| Formation | ΔH⦵f | 1 mol compound from elements in standard states | Usually − | C + O₂ → CO₂ | Elements = 0 |
| Combustion | ΔH⦵c | 1 mol burns completely in O₂ | Always − | CH₄ + 2O₂ → CO₂ + 2H₂O | — |
| Neutralisation | ΔH⦵neut | 1 mol water formed from H⁺ + OH⁻ | − | ≈ −57 kJ mol⁻¹ | Strong acid/alkali |
| Atomisation | ΔH⦵at | 1 mol gaseous atoms from element | + | ½Cl₂ → Cl | — |
| Hydration / solution | ΔH⦵hyd / sol | Ions into water / solid dissolves | − / ± | — | A2 extension |
Section 8 of 16Worked examples
1.023 g of propan-1-ol was burned to heat 200 g of water, raising the temperature by 30 °C. Calculate the heat energy change. [2]2 marks
Q = m × c × ΔT = 200 × 4.18 × 30 = 25 080 J ≈ 25 kJ; combustion is exothermic so the enthalpy change is −25 kJ.
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)250 g of water is warmed by 12 °C by burning 0.01 mol of fuel. Calculate ΔH in kJ/mol. [3]3 marks
Q = 250 × 4.18 × 12 = 12 540 J. ΔH = 12 540 ÷ 0.01 = 1 254 000 J/mol = −1254 kJ/mol (negative because the temperature increased).
Explain why bond breaking is endothermic but bond making is exothermic. [2]2 marks
Energy must be supplied to overcome the attraction between atoms to break bonds; energy is released when atoms attract each other and new bonds form.
Two H–Br bonds of 366 kJ/mol each are formed. State the energy out and explain how ΔH is found. [2]2 marks
Energy out = 2 × 366 = 732 kJ. ΔH = energy taken in (bonds broken) − energy given out (bonds formed).
Section 9 of 16Aim and apparatus
Section 10 of 16Method and variables
- The acid volume is the independent variable;
- the highest temperature or temperature rise is the dependent variable.
- Keep alkali volume, concentrations, initial temperatures and apparatus the same.
Section 11 of 16Record and plot results
Section 12 of 16Interpret the pattern
- Around the amount needed to react with all the alkali, the temperature reaches its maximum.
- Beyond that point, extra acid does not cause more neutralisation and may lower the measured temperature by adding cooler liquid.
Section 13 of 16Key terms
| Term | Meaning |
|---|---|
| Enthalpy change (ΔH) | The heat energy change of a reaction at constant pressure, in kJ/mol. |
| Exothermic | A reaction that releases energy to the surroundings (ΔH negative). |
| Endothermic | A reaction that takes in energy from the surroundings (ΔH positive). |
| Bond energy | The energy needed to break one mole of a particular bond. |
| Calorimeter | An insulated container used to measure temperature changes in reactions. |
| Specific heat capacity | The energy needed to raise 1 g of a substance by 1 °C; 4.18 J/g/°C for water. |
Section 14 of 16Syllabus checklist
Section 15 of 16Command terms
Section 16 of 16Exam technique
Marked practiceQuizzes for this lesson
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