Explaining what's happening
4 marks · NormalA student adds excess powdered magnesium to dilute hydrochloric acid in a beaker. The mixture fizzes and gives off a gas.
State the name of the gas produced in this reaction.
Explain, using collision theory, why the reaction is faster at the start than near the end.
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) Hydrogen · must show hydrogen
(b) — · must show At the start, more acid particles so more frequent collisions, As acid is used up, fewer acid particles so collision rate decreases, Reaction slows because fewer successful collisions per second
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies hydrogen as the gas produced
M1 (b) · 1 mark · States more acid particles at the start / higher concentration at the start
M2 (b) · 1 mark · Mentions more frequent collisions at the start / higher collision rate
A1 (b) · 1 mark · As acid is used, fewer collisions occur so rate decreases / fewer successful collisions per second
Worked solution
a) The gas is hydrogen.
b) At the start, there are more acid particles present. This means there are more collisions with the magnesium per second (higher collision frequency). As the reaction continues, the acid is used up and its concentration falls, so there are fewer acid particles and the collision rate drops. Therefore, the reaction is faster at the start and slows down near the end.