CCEA A Level Chemistry
CCEA A Level Chemistry 2.8 Energetics Exam Questions
Free CCEA A Level Chemistry revision for 2.8 Energetics: what you need to know, common mistakes and exam-style questions with answers. Covers enthalpy changes and standard conditions, combustion, formation and neutralisation, calorimetry, Hess's law and average bond enthalpies.
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About CCEA A Level Chemistry 2.8 Energetics
Energetics is section 2.8 of Unit AS 2. You need to define exothermic and endothermic changes, standard conditions (100 kPa and 298 K) and standard enthalpy changes of combustion, formation and neutralisation, and draw enthalpy level diagrams. CCEA requires simple experimental methods for measuring enthalpies of combustion and neutralisation and calculations using q = mc delta T. You must apply Hess's law using enthalpy cycles, calculate enthalpy changes from average bond enthalpies, and explain why bond enthalpy values differ from Hess's law values. This section is built on in A2 1 with lattice enthalpy and free energy.
What you need to know
- Definitions Standard enthalpy of combustion: enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions. Standard enthalpy of formation: one mole of a compound formed from its elements in their standard states. Enthalpy of neutralisation: one mole of water formed when an acid reacts with a base.
- Calorimetry q = mc delta T, where m is the mass of water or solution heated (in g), c is 4.18 J g-1 K-1 and delta T is the temperature change. Then delta H = -q / moles, converted to kJ mol-1.
- Experimental methods Combustion: burn a known mass of fuel under a copper calorimeter of water. Neutralisation: mix acid and alkali in a polystyrene cup and record the maximum temperature. Heat loss and incomplete combustion give values less exothermic than data book values.
- Hess's law The enthalpy change of a reaction is independent of the route taken. Use formation data (products minus reactants) or combustion data (reactants minus products) in an enthalpy cycle.
- Bond enthalpies Delta H = sum of bonds broken minus sum of bonds made. Average bond enthalpies are means over many compounds and apply to gases, so results differ from Hess's law values.
Common mistakes in 2.8 Energetics
- Using the mass of the fuel in q = mc delta T. Use the mass of water or solution being heated.
- Forgetting the negative sign for an exothermic reaction. If the temperature rises, delta H is negative.
- Getting Hess's law cycles the wrong way round with combustion data. For combustion data, delta H = sum of reactants minus sum of products.
- Forgetting state symbols or standard states in definitions. Elements must be in their standard states and conditions stated as standard.
- Saying bond enthalpy calculations are inaccurate because of heat loss. They differ because average values are used and substances may not be gaseous.
Try three 2.8 Energetics exam-style questions
1. 50.0 cm3 of 1.00 mol dm-3 HCl was mixed with 50.0 cm3 of 1.00 mol dm-3 NaOH. The temperature rose by 6.8 C. Calculate the enthalpy of neutralisation. (3 marks)
Show the mark scheme answer
q = 100 x 4.18 x 6.8 = 2842 J; moles water = 0.0500; delta H = -2842 / 0.0500 = -56.8 kJ mol-1.
2. Use the enthalpies of combustion C = -394, H2 = -286 and CH4 = -890 kJ mol-1 to calculate the enthalpy of formation of methane. (2 marks)
Show the mark scheme answer
Delta fH = -394 + 2(-286) - (-890); = -76 kJ mol-1.
3. Using average bond enthalpies (kJ mol-1) C-H 413, O=O 498, C=O 805, O-H 464, calculate delta H for CH4 + 2O2 gives CO2 + 2H2O. (3 marks)
Show the mark scheme answer
Bonds broken = 4(413) + 2(498) = 2648; bonds made = 2(805) + 4(464) = 3466; delta H = 2648 - 3466 = -818 kJ mol-1.
2.8 Energetics FAQs
What does CCEA A Level Chemistry 2.8 Energetics cover?
It covers enthalpy definitions, standard conditions, calorimetry, q = mc delta T, Hess's law and average bond enthalpies.
Which unit is 2.8 Energetics examined in?
It is examined in Unit AS 2: Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
What equations do I need for 2.8 Energetics?
q = mc delta T, delta H = -q / moles, and delta H = bonds broken - bonds made. Learn to set out Hess cycles for formation and combustion data.
