Eduqas A-Level Chemistry: C2 (Chemical Change) overview

What this topic demands

Topic C2 of Eduqas A-Level Chemistry introduces the three great themes of physical chemistry, equilibrium, energetics and kinetics, plus the board's distinctive treatment of the wider impact of chemistry. The treatment here is qualitative and explanatory; the quantitative tools come later. The examiners reward clear reasoning chains and precise definitions.

This guide walks through C2 in specification order and sets out the exam patterns Eduqas repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Equilibrium and acids

Simple equilibria and acid-base reactions (C2.1) introduces dynamic equilibrium in a closed system, Le Chatelier's principle (the response to changes in concentration, pressure and temperature), and the Bronsted-Lowry theory of acids as proton donors and bases as proton acceptors, with conjugate pairs and the strong/weak distinction. Industrial equilibria such as the Haber process show the compromise between yield and rate.

Energetics

Thermochemistry (C2.2) defines enthalpy change and its sign, the standard enthalpies of reaction, formation and combustion, and the calorimetry method ($q = mc\Delta T$). Hess's law allows indirect calculation through enthalpy cycles, and mean bond enthalpies give estimates of $\Delta H$. Knowing why experimental values fall short of data-book values is a common discussion point.

Kinetics and sustainability

Rates of reaction (C2.3) explains rate through collision theory and the Maxwell-Boltzmann distribution: concentration, pressure and surface area raise collision frequency, temperature mainly raises the fraction exceeding the activation energy, and catalysts lower the activation energy.

The wider impact of chemistry (C2.4) is the green-chemistry topic: atom economy and percentage yield as efficiency measures, the role of catalysts and renewable feedstocks, and the trade-offs that decide whether a process is sustainable.

How this topic is examined

A typical Eduqas profile for C2:

• Equilibrium. Le Chatelier predictions with explanations referring to moles of gas and the sign of $\Delta H$, and the industrial compromise.
• Energetics. Calorimetry calculations, Hess-cycle and formation-data problems, and bond-enthalpy estimates with the correct sign convention.
• Kinetics. Maxwell-Boltzmann sketches and explanations for the temperature and catalyst effects, and the factors affecting rate.
• Sustainability. Atom-economy calculations and evaluation of competing industrial routes.

Check your knowledge

A mix of recall, calculation and explanation questions across C2. Attempt them under timed conditions, then check against the solutions.

1. State Le Chatelier's principle. (2 marks)
2. For $\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightleftharpoons 2\text{NH}_3(\text{g})$, $\Delta H < 0$, predict the effect of increasing temperature on the yield of ammonia. (1 mark)
3. Calculate the enthalpy change for $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$ given bond enthalpies $\text{H-H} = 436$, $\text{Cl-Cl} = 242$, $\text{H-Cl} = 431\ \text{kJ mol}^{-1}$. (2 marks)
4. Explain why a catalyst increases the rate of reaction. (2 marks)
5. Define atom economy. (1 mark)
6. State the two conditions for a successful collision. (2 marks)