R1.2 Energy cycles in reactions (SL)
13 revision questions with answers for R1.2 Energy cycles in reactions (SL). Read them here, or play the set as a class game on the projector. Free to host, and students join with a code.
Bond breaking is always…?
- Exothermic
- EndothermicRight answer
- Spontaneous
- Reversible
What this set covers
Bond enthalpies and Hess's law at SL: bond breaking and forming, average bond enthalpy, calculating ΔH for H₂ + Cl₂, why bond-enthalpy answers are only approximate, and adding or reversing steps in energy cycles.
- Common mix-up
- Students often reverse the bond enthalpy formula: ΔH is the energy of bonds broken minus the energy of bonds formed, not the other way round.
- Revision tip
- Draw the H₂ + Cl₂ reaction showing every bond broken and formed, then check that your ΔH of −184 kJ mol⁻¹ has the correct sign.
All 13 questions and answers
Bond breaking is always…?
Answer: Endothermic
Energy to break one mole of a given bond in gaseous molecules, from many compounds?
Answer: Average bond enthalpy
Bond enthalpy data apply only to substances in which state?
Answer: Gaseous
Expression for ΔH from bond enthalpies?
Answer: Σ(broken) − Σ(formed)
Bond enthalpies H–H 436, Cl–Cl 242, H–Cl 431 kJ mol⁻¹: ΔH for H₂ + Cl₂ → 2HCl?
Answer: −184 kJ mol⁻¹
CH₄(g) → C(g) + 4H(g) needs 1656 kJ mol⁻¹. Average C–H bond enthalpy?
Answer: 414 kJ mol⁻¹
ΔH from bond enthalpies only approximates experiment because bond enthalpies are…?
Answer: Average values
Which of these bonds has the highest bond enthalpy?
Answer: C≡C
A reaction is exothermic when the bonds formed are, overall…?
Answer: Stronger
Total enthalpy change is independent of the route taken: this is…?
Answer: Hess's law
ΔH(A → B) = −100 and ΔH(B → C) = +30 kJ mol⁻¹. ΔH(A → C)?
Answer: −70 kJ mol⁻¹
ΔH for X → Y is +50 kJ. ΔH for Y → X?
Answer: −50 kJ
ΔH for A → B is −30 kJ. ΔH for 2A → 2B?
Answer: −60 kJ
Play it as a class game
Play this set opens it in splycr, where you can host it for your class straight away. Hosting needs no account: put the room code on the projector, and students join from a phone with the code and their first name. Each question plays as multiple choice, and the three wrong options were written for that question.