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

Meaning of endothermic reactions with examples

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Answer Text:
Endothermic and exothermic reactions:
(a). Endothermic reactions
- Is a reaction accompanied by a fall in temperature and energy is absorbed from the surroundings.
- The enthalpy change is normally positive since heat of products (H Products) is higher than heat of reactants (H reactants).
- Thus #H_2-H_1# is a positive value; since H1 is less than #H_2#.
Graphically this can be denoted as follows;-
figchanges22720201125.JPG
Examples:
(i). When #NH_4NO_3# dissolves in water, the temperature of the solution drops.
Procedure
- A spatula end-full of ammonium nitrate is dropped into a test tube of water.
- The bottom of the test tube is felt with the hand.
Observations:
- The hand feels cold.
Reason:
- Energy is absorbed by the products, cooling the test tube.
Thus #H_1# is less than #H_2# giving a positive Δ #H_1# showing an endothermic reaction.
(ii) #N_2(g) + O_2 (g) to 2NO(g); DeltaH=+91KJMol^-1#