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4.1 Endothermic & Exothermic
Reactions that absorb heat energy from the surroundings.
When an endothermic reaction occurs:
a) Heat energy is absorbed and is transferred to the reactants
b) The reaction mixture and the container becomes cold
c) The temperatures of the reaction mixture and the container fall
d) The heat energy chemical energy
Examples of Endothermic reaction involving physical changes The dissolving of ammonia salts such
as ammonium chloride (NH4Cl) and ammonium sulphate, (NH4)2SO4 in water.
The dissolving of crystalline salts such as hydrated copper(II) sulphate, (CuSO4.5H2O), hydrated magnesium sulphate (MgSO4.7H2O) and hydrated sodium carbontae (Na2CO3.10H2O)
Melting process (solid liquid)
Evaporation & boiling processes (liquid gas)
Examples of endothermic reactions involving chemical changes The reaction between acids and sodium
or potassium hydrogen carbonate.
Thermal decomposition of metal carbonates, metal nitrates and ammonium chloride
Photosynthesis
The Energy Level Diagrams for Endothermic Reactions The total energy content of the products
is higher than the total energy content of the reactants. This means that the value of ∆H for an endothermic reaction is positive (+ve).
∆H = Hproducts – Hreactants
= positive if Hproducts > Hreactants
Reactions that release heat energy to the surroundings.
When an exothermic reaction occurs:
a) Heat is released and is transferred from the reactants to the surroundings.
b) The reaction mixture and the container become hot.
c) The temperatures of the reaction mixture and the container rise.
d) Chemical energy heat energy.
Examples of exothermic reactions involving physical changes Condensation process (gas liquid)
Sublimation (gas solid)
Freezing @ solidification process (liquid solid)
The dissolving of alkalis and acids in water
The dissolving of anhydrous salts, such as anhydrous copper(II) sulphate (Cu SO4) and anhydrous sodium carbonate (Na2CO3) in water.
Examples of exothermic reactions involving chemical changes Combustion of fuels
Oxidation of food in respiration process
Rusting of iron
Dissolving soluble bases (metal oxides)in water
Neutralisation reactions between acids and bases
Reaction between acids and metals or metal carbonates
Displacement reaction of a metal from its salt solution by a more reactive metal
Haber process (manufacture of ammonia)
Contact process (production of sulphurtrioxide)
The Energy Level Diagrams for Exothermic Reactions The total energy content of the products
is lower than the total energy content of the reactants. Hence, the ∆H value of an exothermic reaction is negative (-ve).
∆H = Hproducts – Hreactants
= negative if Hproducts < Hreactants