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 Form 4 Chemistry: Energy changes in chemical and physical processes lessons

Meaning of exothermic reactions with examples

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Answer Text:
Exothermic reactions:
- Are reactions accompanied by a rise in temperature and energy is liberated to the surroundings.
- The enthalpy change is normally negative, since heat of reactants (#H_1#) is normally higher than heat of products (#H_2#).
- Thus,# H_2-H_1# gives a negative value since #H_1# is higher than# H_2#.
Exothermic reactions can be represented graphically as follows;-
- In exothermic reactions #H_2# is lower than #H_1#.
(i). Manufacture of ammonia in the Haber process;
i.e. #N_2(g) + 3H_2 (g) 2NH_3(g); ΔH=-46Kjmol^(-)#
(ii) Dissolving sodium hydroxide pellets in water,
-The temperature of the resulting NaOH(aq) is higher than the temperature of water at room temperature.
-This implies the internal temperature of products is lower than the reactants‟ original temperature.
-Thus (H products – H reactants) =-ve value, an exothermic reaction.