What does m represent in q = mcΔT?

The mass of the solution or water that is heated, not the mass of the reactants added.

Define specific heat capacity.

The heat energy needed to raise the temperature of 1 g of a substance by 1 K.

How do you convert q into a molar enthalpy change?

Convert q to kJ, then divide by the moles of the limiting reagent: ΔH = q ÷ n.

What sign does ΔH have if the temperature rises?

Negative: the reaction is exothermic and heat is released.

What assumption is made about the density of a dilute solution?

It is 1.00 g cm⁻³, so 50.0 cm³ has a mass of 50.0 g.

Why is a polystyrene cup with a lid used for reactions in solution?

It is a good insulator with a low heat capacity, so it reduces heat transfer with the surroundings.

Give two reasons why a calorimetry combustion value is less exothermic than the data book value.

Heat is lost to the surroundings or absorbed by the calorimeter, and combustion may be incomplete.

How is the mass of fuel burned found in a spirit burner experiment?

Weigh the burner before and after, and subtract.

Why is a cooling curve extrapolated back to the time of mixing?

It gives the temperature change that would have occurred with no heat exchange with the surroundings.

A thermometer is read to ±0.1 °C. What is the uncertainty in ΔT?

±0.2 K, because two readings are used.

Why does using more reactant reduce the percentage uncertainty in ΔT?

The temperature change is larger, so the fixed thermometer uncertainty is a smaller percentage of it.

CalorimetryAQA A-Level Chemistry: Flashcards

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State the equation used to calculate a heat change in calorimetry.

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State the equation used to calculate a heat change in calorimetry.: q = mcΔT, with q in J, m in g, c in J g⁻¹ K⁻¹ and ΔT in K.
What does m represent in q = mcΔT?: The mass of the solution or water that is heated, not the mass of the reactants added.
Define specific heat capacity.: The heat energy needed to raise the temperature of 1 g of a substance by 1 K.
How do you convert q into a molar enthalpy change?: Convert q to kJ, then divide by the moles of the limiting reagent: ΔH = q ÷ n.
What sign does ΔH have if the temperature rises?: Negative: the reaction is exothermic and heat is released.
What assumption is made about the density of a dilute solution?: It is 1.00 g cm⁻³, so 50.0 cm³ has a mass of 50.0 g.
Why is a polystyrene cup with a lid used for reactions in solution?: It is a good insulator with a low heat capacity, so it reduces heat transfer with the surroundings.
Give two reasons why a calorimetry combustion value is less exothermic than the data book value.: Heat is lost to the surroundings or absorbed by the calorimeter, and combustion may be incomplete.
How is the mass of fuel burned found in a spirit burner experiment?: Weigh the burner before and after, and subtract.
Why is a cooling curve extrapolated back to the time of mixing?: It gives the temperature change that would have occurred with no heat exchange with the surroundings.
A thermometer is read to ±0.1 °C. What is the uncertainty in ΔT?: ±0.2 K, because two readings are used.
Why does using more reactant reduce the percentage uncertainty in ΔT?: The temperature change is larger, so the fixed thermometer uncertainty is a smaller percentage of it.

Exam questions on Calorimetry

A student mixes 25.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid with 25.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution in a polystyrene cup. The initial temperature of both solutions is 20.5 °C and the highest temperature reached is 27.3 °C. Assume that the density of the solution is 1.00 g cm⁻³ and that its specific heat capacity is 4.18 J g⁻¹ K⁻¹.
Calculate the molar enthalpy change of neutralisation, in kJ mol⁻¹, for this reaction. Include the sign.2 marks
A student measures the enthalpy of combustion of ethanol using a spirit burner to heat 100 g of water in a copper calorimeter. The mass of the burner and ethanol falls from 215.30 g to 214.84 g during the experiment, and the temperature of the water rises from 19.0 °C to 38.0 °C. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹ and the data book value for the enthalpy of combustion of ethanol is −1367 kJ mol⁻¹. The relative formula mass, Mr, of ethanol is 46.0.
The experimental value is much less exothermic than the data book value. Suggest two reasons for this.2 marks
A student adds excess zinc powder to 50.0 cm³ of 0.500 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup fitted with a lid. The temperature rises from 19.5 °C to 45.0 °C. The thermometer has an uncertainty of ±0.1 °C on each reading. Assume that the solution has a density of 1.00 g cm⁻³ and a specific heat capacity of 4.18 J g⁻¹ K⁻¹. The equation for the reaction is Zn + CuSO₄ → ZnSO₄ + Cu.
Calculate the molar enthalpy change for this reaction, in kJ mol⁻¹, per mole of copper(II) sulfate.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).