Chemistry › Energetics
Energetics
Unit C04 · 4 lessons.
Some enthalpy changes can be measured in a polystyrene cup and most cannot. The standard enthalpy changes are defined here, a temperature change is turned into an enthalpy change with q = mcΔT, Hess's law cycles reach the values no experiment gives directly, and mean bond enthalpies give an estimate where nothing else is available.
Two slips account for most lost marks. The mass in q = mcΔT is the mass of the solution being heated, not the mass of the solid added, and the answer needs its sign, negative for a reaction that warms its surroundings. A value from mean bond enthalpies also differs from a data-book value, because each bond enthalpy is an average over many compounds and assumes every species is a gas.
On the written papers it is set on AQA Papers 1, 2 and 3, OCR A Papers 1 and 3, Edexcel Papers 1, 2 and 3 and Cambridge Papers 1 and 2.
Specification sections: AQA 3.1.4.1, 3.1.4.2, 3.1.4.3, 3.1.4.4 · OCR A 3.2.1 · Edexcel 8.1 to 8.11, 6.38, 12.23 · CAIE 5.1, 5.2
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Enthalpy change, standard conditions and the definitions
Enthalpy and enthalpy change at constant pressure, the sign of ΔH for exothermic and endothermic reactions, enthalpy profile diagrams and activation energy, standard conditions and standard states, the standard enthalpies of formation, combustion and reaction, the enthalpy of neutralisation and its definition per mole of water formed, with their definitions and equations, fractions of O₂ in combustion equations, ΔrH° per equation against ΔfH° per mole for ammonia, and why alcohols release less heat than alkanes on burning.
- Measuring an enthalpy change: calorimetry
The equation q = mcΔT with the mass of water or solution and c = 4.18 J g⁻¹ K⁻¹, converting q to ΔH per mole with the correct sign, ΔH = −mcΔT ÷ n, unequal amounts of acid and alkali and the limiting reagent, why strong acids all give about −57 kJ mol⁻¹ and weak acids less, the polystyrene cup method for neutralisation and displacement, AQA required practical 2 with the cooling-curve extrapolation, the spirit burner method for combustion, and the heat losses and other errors that make measured values too small.
- Hess's law and enthalpy cycles
Hess's law and why it holds, enthalpy cycles built from enthalpies of formation and from enthalpies of combustion, the two formulae and the direction of the arrows in each, finding the enthalpy of formation of methane and of carbon monoxide, and finding the enthalpy change for the hydration of anhydrous copper(II) sulfate from two measured enthalpies of solution, and the thermal decomposition of potassium hydrogencarbonate from its reaction and that of potassium carbonate with hydrochloric acid.
- Mean bond enthalpies
Bond dissociation enthalpy and mean bond enthalpy and why most bond enthalpies are averages, bond breaking as endothermic and bond making as exothermic, calculating ΔH as bonds broken minus bonds made, working back to an unknown bond enthalpy, the mean C–H bond enthalpy in methane from enthalpies of atomisation, bond length and the unreactivity of nitrogen, the bromination of methane, and why the answers differ from data-book values.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
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