Which one of the following is not an exothermic reaction?
- (a)Dissolution of calcium oxide in water
- (b)Decomposition of vegetable matter into compost
- (c)Decomposition of lead nitrate
- (d)Dilution of sodium hydroxide with water
Correct — C, Decomposition of lead nitrate. Heating lead nitrate breaks it down into lead(II) oxide, nitrogen dioxide and oxygen, and the heat has to be supplied continuously for the reaction to go on — that makes it endothermic, and it is the standard school example of a thermal decomposition. The other three all release energy: quicklime slaking in water gives out enough heat to boil the water, composting is slow oxidation by microbes that warms a compost heap from within, and diluting sodium hydroxide releases a large heat of solution.
- (a)Dissolution of calcium oxide in water — Strongly exothermic. Quicklime added to water gives slaked lime, calcium hydroxide, with a hissing sound and enough heat to raise the water to boiling — the textbook demonstration of an exothermic combination reaction.
- (b)Decomposition of vegetable matter into compost — Exothermic, even though the word 'decomposition' appears in it. Microbial breakdown of plant matter is a slow oxidation that releases energy, which is why the middle of a compost heap is measurably hot.
- (d)Dilution of sodium hydroxide with water — Exothermic, and dangerously so at high concentration. This is why laboratory practice is to add the alkali to water in small amounts with stirring, never the other way round.
An exothermic reaction releases energy to the surroundings; an endothermic one absorbs it. Most combination reactions, all combustions, respiration and the neutralisation of an acid by an alkali are exothermic. Thermal decomposition is the standard endothermic case, because a compound is being pulled apart into simpler substances and the energy for that has to come from outside — which is why such reactions usually carry the word 'heat' above the arrow and stop the moment heating stops.
The examiner has planted the word 'decomposition' in two options, one endothermic and one not, and that is the whole design of the item. Composting is called decomposition in ordinary speech but is chemically an oxidation that gives out heat; the decomposition of lead nitrate is a thermal decomposition and takes heat in. The reliable test is not the name of the process but the direction of energy flow: ask whether the reaction would continue on its own once started, or whether it stops as soon as the burner is removed.
- Heating lead nitrate gives lead(II) oxide, brown nitrogen dioxide fumes and oxygen; it is endothermic.
- Calcium oxide plus water gives calcium hydroxide with a large release of heat.
- Composting is microbial oxidation and warms the heap; it is exothermic.
- Dissolving or diluting sodium hydroxide releases a large heat of solution.
- Most thermal decompositions are endothermic; most combination and combustion reactions are exothermic.
The trap is the word 'decomposition', which appears in two options; only one of them is a thermal decomposition.
- Marking the composting option because its name contains 'decomposition'.
- Assuming dilution is always energy-neutral; strong alkalis and concentrated acids release a great deal of heat.
- Forgetting that photosynthesis, not respiration, is the endothermic member of that pair.
A not-an-exothermic item that hides the answer behind vocabulary — two options are called decomposition and only one takes energy in.
Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)₂) is an example of
- (a) Displacement reaction.
- (b) Endothermic reaction.
- (c) Decomposition reaction.
- (d) Exothermic reaction.
Answer(d) Exothermic reaction.
Settles option (a) of this question from an official key: slaking quicklime is exothermic, so it cannot be the answer to a not-exothermic item.
Manganese is extracted by heating manganese dioxide with aluminium powder. Which one of the following statements with regard to the reaction is correct?
- (a) The reaction is exothermic.
- (b) The reaction is endothermic.
- (c) Manganese is produced as a solid.
- (d) Manganese is more reactive than aluminium.
Answer(a) The reaction is exothermic.
Asks the same judgement in the opposite direction — deciding whether a named reaction gives out heat or takes it in, using the reaction type rather than any measured value.
- practice — not a real PYQ
The brown gas evolved when lead nitrate is heated strongly is
- (a)nitrogen
- (b)nitrogen dioxide
- (c)nitrous oxide
- (d)oxygen
Answer(b) nitrogen dioxide — lead nitrate decomposes to lead(II) oxide, brown NO2 and oxygen.
- practice — not a real PYQ
Which one of the following processes is endothermic?
- (a)Burning of natural gas
- (b)Respiration in living cells
- (c)Photosynthesis in green plants
- (d)Neutralisation of an acid by an alkali
Answer(c) Photosynthesis in green plants — it absorbs light energy to build glucose, the reverse of respiration.