Bond enthalpiesAQA A-Level Chemistry: Flashcards
What these 12 flashcards ask
- Is bond breaking endothermic or exothermic?
- Is bond making endothermic or exothermic?
- Define mean bond enthalpy.
- Write the equation for ΔH using mean bond enthalpies.
- Why is the sign of ΔH negative in an exothermic reaction?
- State the phase in which mean bond enthalpy calculations are valid.
- Why is a bond enthalpy answer only approximate?
- Why can the Hess's law value differ from the bond enthalpy value?
- How many bonds are broken in one mole of CH₄?
- Which bond is stronger, C=C or C–C?
- How many N–H bonds are made when 2NH₃ forms?
- Why must liquids be treated differently in bond enthalpy calculations?
Exam questions on Bond enthalpies
- Hydrogen reacts with chlorine in the gas phase: H₂(g) + Cl₂(g) → 2HCl(g). The mean bond enthalpies, in kJ mol⁻¹, are: H–H 436, Cl–Cl 243 and H–Cl 432.Explain, in terms of bonds, why this reaction is exothermic.2 marks
- Ethene reacts with hydrogen in the gas phase in the presence of a nickel catalyst: C₂H₄(g) + H₂(g) → C₂H₆(g). The mean bond enthalpies, in kJ mol⁻¹, are: C=C 612, C–C 347, C–H 413 and H–H 436.Define the term mean bond enthalpy.2 marks
- Methane burns completely in oxygen with all species in the gas phase: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). The mean bond enthalpies, in kJ mol⁻¹, are: C–H 413, O=O 496, C=O 805 and O–H 463. The standard enthalpies of formation, in kJ mol⁻¹, are: CH₄(g) −74.8, CO₂(g) −393.5 and H₂O(g) −241.8.Use the mean bond enthalpies to calculate the enthalpy change for the combustion of methane.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).