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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.
Examples:
(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.