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All 11 Flashcards — Bond enthalpies
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Question
What is bond enthalpy?
Answer
The energy needed to **break one mole** of a particular bond in the **gaseous** state (always a positive value).
Question
Is breaking a bond endothermic or exothermic?
Answer
**Endothermic** — breaking a bond always **costs** (absorbs) energy.
Question
Is making a bond endothermic or exothermic?
Answer
**Exothermic** — forming a bond always **releases** energy.
Question
Formula for ΔH from bond enthalpies?
Answer
$\Delta H = \Sigma(\text{bonds broken}) - \Sigma(\text{bonds made})$.
Question
What does a negative ΔH mean?
Answer
The reaction is **exothermic** — more energy was released making bonds than was used breaking them.
Question
What does a positive ΔH mean?
Answer
The reaction is **endothermic** — breaking bonds cost more energy than was released making them.
Question
Why are bond enthalpies 'average' values?
Answer
A bond (e.g. C–H) exists in many molecules with slightly different strengths, so the booklet gives an **average**; ΔH is therefore an **estimate**.
Question
When can bond enthalpies be used for ΔH?
Answer
Only when **all species are gaseous**, because bond enthalpy is defined for the gaseous state.
Question
Which bonds do you need to count?
Answer
Only the bonds that **break or form** — unchanged bonds (spectator bonds) cancel out.
Question
Stronger bond means higher or lower bond enthalpy?
Answer
**Higher** — a larger bond enthalpy means a stronger bond that needs more energy to break.
Question
Why does bond-enthalpy ΔH differ from the experimental value?
Answer
Because the bond enthalpies are **averages**, so the calculated ΔH is only an **estimate**.
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Topic 4.2 hub
Energy cycles in reactions
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