Exothermic reactions
2 marks · NormalState whether the following reactions are exothermic or endothermic. Give a reason for each answer.
Dissolving potassium nitrate in water causes the temperature of the solution to fall.
Combustion of methane gas produces heat and a flame.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Endothermic, because the temperature decreases as heat is absorbed from the surroundings.
(b) Exothermic, because heat is released, causing a temperature increase and flame.
Common mistakes: Incorrectly describing the direction of energy transfer
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly identifies 'endothermic' and gives a valid reason (temperature falls/heat absorbed).
B2 (b) · 1 mark · Correctly identifies 'exothermic' and gives a valid reason (heat/flame produced, temperature rises/heat released).
Worked solution
a. The reaction is endothermic. The temperature falls because heat is absorbed from the surroundings.
b. The reaction is exothermic. Combustion releases heat, producing a flame and increasing the temperature.