Which one among the following is an example of endothermic process?

Correct — C, Photosynthesis. An endothermic process absorbs energy from its surroundings. In photosynthesis a plant takes carbon dioxide and water and builds glucose and oxygen, a set of products that holds far more chemical energy than the reactants — about 2,870 kilojoules for every mole of glucose formed. That energy has to come from somewhere, and it comes from sunlight captured by chlorophyll, which is why photosynthesis stops in the dark. The reaction is the great energy-storing process of the biosphere, and every other item on the list runs the other way, releasing energy.

• (a)Combustion of carbon — Combustion of carbon is the definition of an exothermic reaction. Carbon burning to carbon dioxide gives out about 393 kilojoules per mole, which is why coal is a fuel.
• (b)Mixing acid and alkali — Mixing an acid and an alkali is a neutralisation, and neutralisation is exothermic. Adding sodium hydroxide solution to hydrochloric acid warms the beaker measurably, releasing about 57 kilojoules for every mole of water formed.
• (d)Respiration — Respiration is the reverse of photosynthesis: glucose and oxygen are consumed and carbon dioxide and water released, giving out the energy the plant had stored. That release is what powers every cell, and it is exothermic by definition.

A reaction is exothermic if it releases energy to its surroundings and endothermic if it absorbs energy from them. Combustion, neutralisation, respiration and most oxidation reactions release energy. Photosynthesis, the thermal decomposition of calcium carbonate, the dissolution of ammonium chloride in water and the electrolysis of water all absorb it. A useful check is whether the process needs a continuous energy supply to keep running: photosynthesis stops the moment the light goes out, and decomposing limestone stops the moment the kiln cools.

Photosynthesis and respiration are exact opposites and the pair is worth learning together, since examiners set one against the other. Photosynthesis absorbs light energy and stores it in glucose; respiration releases that stored energy. The other two options are the two most common exothermic reactions in the school syllabus, so if you can classify combustion and neutralisation correctly the item resolves by elimination even without thinking about photosynthesis.

• Endothermic processes absorb energy from their surroundings; exothermic ones release it.
• Photosynthesis stores about 2,870 kilojoules of energy for every mole of glucose formed.
• Combustion, neutralisation and respiration are all exothermic.
• Other common endothermic changes are the thermal decomposition of limestone and the dissolution of ammonium chloride in water.
• Photosynthesis and respiration are chemically the reverse of each other.

• Classifying a reaction by whether it needs heating to start rather than by its net energy flow.
• Treating respiration as endothermic because living things need energy.
• Forgetting that neutralisation warms the mixture.

A one-from-four classification item. The three wrong options are the standard exothermic reactions, so the item can be solved by elimination.

• practice — not a real PYQ

  Which one of the following is an exothermic process?

  • (a)Photosynthesis
  • (b)Electrolysis of water
  • (c)Burning of magnesium ribbon
  • (d)Thermal decomposition of limestone

  Answer(c) Burning of magnesium ribbon — combustion releases energy as heat and light.

• practice — not a real PYQ

  Dissolving ammonium chloride in water cools the beaker. The process is therefore

  • (a)exothermic
  • (b)endothermic
  • (c)neither
  • (d)a combustion

  Answer(b) endothermic — it absorbs heat from the water, which is why the temperature falls.