When a chemical reaction occurs there is a net energy transfer between the reaction system and the surroundings. Either energy may be transferred from the reacting system to the surroundings or vice versa.

A. Exothermic reactions - energy is released to the surroundings
When fuel burns
2C₈H₁₈(g) + 25O₂(g)  -->  16CO₂(g) + 18H₂O(g)  DH = - 5470.5 kJ mol^-1

Burning coke or charcoal in air or oxygen
C(s)  +  O₂(g)  -->  CO₂(g)  DH = - 393.5 kJ mol^-1

B. Endothermic reactions - energy is taken in from the surroundings
When coke burns in carbon dioxide in a blast furnace,
C(s)  +  CO₂(g)  -->  2CO(g) DH = + 283  kJ mol^-1

Coke burning in steam to make water gas (a mixture of carbon monoxide and hydrogen
C(s)  +  H₂O(g)  -->  CO(g)  +  H₂(g) DH = +132 kJ mol^-1

Place the labels in their correct position on the diagram (right).
Other questions may be answered on paper and handed to your teacher for marking.