Explaining what's happening
2 marks · EasyExplain, using collision theory, why a reaction between magnesium ribbon and excess hydrochloric acid is faster at the start than at the end.
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At the start, there are more acid particles and more frequent collisions with magnesium so the reaction is faster. As acid is used up, there are fewer acid particles and fewer collisions per second so the reaction slows down.
Also accept: At the beginning, the high concentration of acid causes more frequent collisions, speeding up the reaction. Later, as acid is used up, collisions decrease and the rate slows.
Common mistakes: Only mentioning more reactant without explaining collision frequency.
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Marking points
M1 · 1 mark · States that there are more acid particles or higher concentration at the start (so more collisions).
A1 · 1 mark · Links more frequent collisions at the start and fewer collisions as acid is used up to changes in reaction rate.
Worked solution
At the start, there are many acid particles, so collisions with magnesium happen more frequently (higher collision rate), making the reaction fast.
As the reaction goes on, acid particles are used up, so there are fewer collisions with magnesium each second, and the reaction slows down.