Bond enthalpiesEdexcel A-Level Chemistry: Flashcards
What these 13 flashcards ask
- Define bond enthalpy.
- Define mean bond enthalpy.
- Is bond breaking exothermic or endothermic?
- Is bond making exothermic or endothermic?
- Formula for ΔH using mean bond enthalpies?
- What does a large bond enthalpy tell you?
- Why are tabulated bond enthalpies called 'mean'?
- Why are bond enthalpy values only estimates?
- What is missing from a bond enthalpy calculation if a product is liquid water?
- How do you find an unknown bond enthalpy x?
- How many N–H bonds are in 2NH₃?
- Which is stronger: C–C (347) or C=C (612)?
- In H₂ + Cl₂ → 2HCl, how many moles of H–Cl bonds are made?
Exam questions on Bond enthalpies
- A student is estimating the enthalpy change for the reaction H₂(g) + Cl₂(g) → 2HCl(g) using mean bond enthalpies (kJ mol⁻¹): H–H 436, Cl–Cl 243, H–Cl 432.Use the bond enthalpy values to explain why this reaction is exothermic.2 marks
- Ethene reacts with hydrogen in the presence of a nickel catalyst: C₂H₄(g) + H₂(g) → C₂H₆(g). Mean bond enthalpies (kJ mol⁻¹): C=C 612, C–C 347, C–H 413, H–H 436. The experimental enthalpy change for the reaction is −137 kJ mol⁻¹.Calculate the enthalpy change for the reaction using the mean bond enthalpies.2 marks
- Ammonia has a standard enthalpy change of formation of −46 kJ mol⁻¹. Mean bond enthalpies (kJ mol⁻¹): N≡N 945, H–H 436. Hydrazine, H₂N–NH₂, which contains one N–N bond and four N–H bonds, decomposes: N₂H₄(g) → N₂(g) + 2H₂(g), ΔH = −95 kJ mol⁻¹.Calculate the mean bond enthalpy of the N–H bond from the standard enthalpy change of formation of ammonia, for ½N₂(g) + 3/2 H₂(g) → NH₃(g).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).