Measuring enthalpy changesEdexcel A-Level Chemistry: Flashcards
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What is the equation for the energy transferred to water?
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- What is the equation for the energy transferred to water?
- Q = m × c × ΔT.
- What is the specific heat capacity of water?
- 4.18 J g⁻¹ K⁻¹.
- In Q = mcΔT, what does m represent?
- The mass of water or solution that is heated.
- What are the units of Q in Q = mcΔT?
- Joules, J.
- How do you calculate ΔH from Q?
- ΔH = −Q / n, with Q converted to kJ and n in moles of the limiting reagent or fuel burned.
- What sign does ΔH have if the temperature rises?
- Negative: the reaction is exothermic.
- Why is a polystyrene cup used for reactions in solution?
- It is a good insulator, so it reduces heat loss to the surroundings.
- How is heat loss corrected in a solution experiment?
- Plot temperature against time and extrapolate the cooling line back to the time of mixing.
- Why are experimental enthalpies of combustion less exothermic than data book values?
- Heat loss, energy absorbed by the can, incomplete combustion and evaporation of fuel.
- What assumption is made about dilute solutions in Q = mcΔT?
- They have the density (1.00 g cm⁻³) and specific heat capacity of water.
- How can heat loss be reduced in a spirit burner experiment?
- Use a draught shield, lid or insulation and keep the flame close to the can.
- What measurements are needed to find ΔcH of an alcohol with a spirit burner?
- Mass of water, temperature change of the water and mass of fuel burned.
- How can an enthalpy change that cannot be measured directly be found?
- Indirectly, from other reactions that can be measured.
Exam questions on Measuring enthalpy changes
- A student measures the enthalpy change of combustion of propan-1-ol (Mr = 60.0) using a spirit burner to heat 150 g of water in a copper can. The temperature of the water rises by 28.0 K when 0.60 g of propan-1-ol is burned. The specific heat capacity of water is 4.18 J g⁻¹ K⁻¹.The data book value is −2021 kJ mol⁻¹. Give two reasons why the experimental value is less exothermic than this.2 marks
- A student measures the enthalpy change of neutralisation by mixing 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 an insulated polystyrene cup. The temperature rises from 20.5 °C to 27.2 °C. Assume the solutions have density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.Calculate the enthalpy change of neutralisation, in kJ mol⁻¹, including the sign.2 marks
- A student measures the enthalpy change for Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). She adds 2.00 g of zinc powder (Ar = 65.4) to 50.0 cm³ of 0.500 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup and stirs. The temperature rises from 19.8 °C to 45.6 °C. Assume the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.Calculate the amount, in mol, of copper(II) ions in the solution and the energy transferred to the solution, in kJ.3 marks
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).