The short answer
Exothermic vs endothermic, the meaning of ΔH, and using bond energies and Hess's law.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Every reaction absorbs or releases energy, and thermochemistry tracks it with enthalpy change (). The sign of tells the whole story: negative means energy is released, positive means energy is absorbed.
The sign of ΔH
Exothermic: — heat is released to the surroundings (they warm up); products sit lower in energy than reactants. Endothermic: — heat is absorbed (surroundings cool); products sit higher.
Bonds: breaking costs, forming pays
Breaking bonds absorbs energy (endothermic); forming bonds releases energy (exothermic). The net enthalpy from bond energies is . Students often flip this — breaking always costs energy.
Exothermic vs endothermic
Exothermic
- Releases heat
- is negative
- Products lower in energy than reactants
- e.g. combustion, neutralisation
Endothermic
- Absorbs heat
- is positive
- Products higher in energy than reactants
- e.g. photosynthesis, melting ice
Worked example
A reaction breaks bonds totalling 800 kJ of energy and forms bonds releasing 950 kJ. Is it exothermic or endothermic, and what is ?
Check yourself
A reaction has kJ/mol. What does this tell you?
Key takeaways
- ΔH < 0 = exothermic (releases heat); ΔH > 0 = endothermic (absorbs heat).
- Breaking bonds absorbs energy; forming bonds releases it.
- ΔH = (bonds broken) − (bonds formed).
- Energy diagrams: exothermic products sit lower, endothermic sit higher.
- Hess's law: ΔH is path-independent — sum the steps.
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