The enthalpy of neutralisation of a strong acid by a strong alkali is almost constant at about minus 57 kJ per mole because
- A. all acids contain the same number of hydrogen atoms
- B. the salts formed have similar masses
- C. the reaction is always carried out at the same concentration
- D. in every case the same reaction occurs, hydrogen ions combining with hydroxide ions to form water
Explanation
Strong acids and strong alkalis are fully ionised, so the spectator ions take no part and the only chemical change is the formation of water from its ions, which releases the same energy whatever salt is produced. With a weak acid the value is smaller, because some energy is absorbed in ionising the acid first. This constancy is strong evidence for the ionic theory of neutralisation.
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