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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 H2-H1 is a positive value; since H1 is less than H2.
Graphically this can be denoted as follows;-
figchanges22720201125.JPG
Examples:
(i). When NH4NO3 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 H1 is less than H2 giving a positive Δ H1 showing an endothermic reaction.
(ii) N2(g)+O2(g)2NO(g);ΔH=+91KJMol-1


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