Question detail
Show the comparison working for the energy transferred in H2 + Br2 -> 2HBr using average bond energies in The energy change of reactions (HT only).
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
Question
Type
exam_style
Style
Topic
Exothermic and endothermic reactions
Question
Show the comparison working for the energy transferred in H2 + Br2 -> 2HBr using average bond energies in The energy change of reactions (HT only).
Answer
-103 kJ. The reaction is exothermic.
Explanation
The correct option is -103 kJ, exothermic. Bonds broken: 1 × 436 + 1 × 193 = 629 kJ. Bonds formed: 2 × 366 = 732 kJ. Energy transferred = 629 - 732 = -103 kJ. The sign shows the reaction is exothermic. This comparison question supports the approved objective to (HT only) Calculate the overall energy change as energy needed to break bonds minus energy released when bonds form and keeps bond breaking separate from bond making in The energy change of reactions (HT only).
Common mistake
Mis‑applying the sign convention
Students often add the energy released when bonds form to the energy required to break bonds, giving a positive value for an exothermic reaction.
Remember the formula: ΔE = ΣE(bonds broken) – ΣE(bonds formed). For an exothermic reaction the second term is larger, so ΔE is negative, indicating energy is released.
Related flashcards
Flashcard 1 of 5
Related practice questions
Question 1 of 5
Choose an answer, get feedback, then move sideways through the set.