Chapter 7.1 Exothermic and endothermic reactions

Question 1

What is the definition of an exothermic reaction?

Answer 1

A reaction in which energy is transferred from the reacting substances to their surroundings.

Question 2

What is the definition of an endothermic reaction?

Answer 2

A reaction in which energy is transferred to the reacting substances from their surroundings is called an endothermic reaction.

Question 3

How much energy do the products of an endothermic reaction have?

Answer 3

In endothermic reactions the products have a higher energy content than the reactants, so energy is transferred form the surroundings.
CaCO3 (s) ==> CaO (s) +CO2 (g)
The energy transferred from the surroundings = 178 KJ/mol.

Question 4

How much energy do the products of an exothermic reaction have?

Answer 4

The products of exothermic reactions have a lower energy content than the reactant. The actual difference in energy are usually expressed in kilojoules per mole (KJ/mol).
CH4(g) +2O2(g) ==> CO2(g) +2H2O (l)
The energy transferred to the surroundings= 890 KJ/mol.

Question 5

Give four examples of an exothermic reaction.

Answer 5

• Any combustion reaction
• A neutralisation reaction
• Rusting of iron
• A thermite reaction (An exothermic redox reaction between a metal and metal oxide as reactants, such as aluminium and iron oxide)

Question 6

Give four examples of an endothermic reaction.

Answer 6

• Heat absorbing processes - melting ice cubes and melting solid salts for example.
• Converting frost to water vapour
• Melting
• Boiling and evaporation

Question 7

How is an exothermic reaction expressed in chemical equation?

Answer 7

Chemical equation: reactants → products + energy.

Question 8

How is an endothermic reaction expressed in chemical equation?

Answer 8

Chemical equation: reactants + Energy → Products