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