Hess's law and enthalpy cyclesEdexcel A-Level Chemistry: Flashcards
What these 14 flashcards ask
- State Hess's law.
- Why is Hess's law true?
- Define standard enthalpy change of formation.
- Define standard enthalpy change of combustion.
- What is ΔfH of an element in its standard state?
- What is ΔcH of CO₂(g) and H₂O(l)?
- Formula for ΔrH from formation data?
- Formula for ΔrH from combustion data?
- A step has ΔH = −90 kJ mol⁻¹. What is ΔH for the reverse step?
- What do you do to ΔH if the equation is doubled?
- In a calorimetry experiment, what mass is used in q = mcΔT?
- Why is a cooling curve extrapolated in a Hess's law practical?
- Why can Hess's law be needed for the decomposition of CaCO₃?
- How is the sign of ΔH found from a temperature change?
Exam questions on Hess's law and enthalpy cycles
- A student wants the enthalpy change for a reaction that cannot be measured directly in a calorimeter, so she plans to use Hess's law with other reactions whose enthalpy changes can be measured.Define the term standard enthalpy change of formation.2 marks
- Ethanol, C₂H₅OH, is made on a large scale for use as a biofuel. Its standard enthalpy change of formation cannot be measured directly, but standard enthalpy changes of combustion are available: carbon (graphite) −394 kJ mol⁻¹, hydrogen −286 kJ mol⁻¹ and ethanol(l) −1367 kJ mol⁻¹.Use an enthalpy cycle to calculate the standard enthalpy change of formation of ethanol.2 marks
- Iron is extracted from iron(III) oxide in a blast furnace. One of the key reactions is Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g). Standard enthalpy changes of formation are: Fe₂O₃(s) −824 kJ mol⁻¹, CO(g) −111 kJ mol⁻¹, CO₂(g) −394 kJ mol⁻¹.Calculate the standard enthalpy change of reaction.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).