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Rate Equations

2017 Paper Two

Question Two
Looking at past AQA A-level Chemistry questions, I wanted to go beyond the answers and focus on thought processes, ideas and tips that will help in examinations.

The series will help you to spot weaknesses and help with revision, or it can serve as addenda to your notes.If you haven't gone through the paper, please look up the questions at, or click the direct link below, and have a go...

AQA A-level CHEMISTRY Paper 2 Organic and Physical ChemistryMonday 19 June 2017

This is quite subjective with the mark scheme asking for a straight line through (0.00, 0.50) and cutting the x-axis at between 5 and 12.5 seconds. Make sure your tangent touches the curve – you will lose the mark for this, and the next part, if you don’t!Also, make the calculation of the gradient easier by not drawing a small tangent!

If the tangent is missing/doesn’t touch the curve, the mark can't be awarded. The examiners want to see a correct calculation for a gradient, with an expected value between 0.04 and 0.1.

Notice that the tangent shows a negative gradient.
For my tangent, I have chosen x values of 0 and 5.0.
The gradient I created has a value of…

The initial rate is 0.054 (units are given on the paper)

The ratio is the key to this question
From the graph, the concentration of A, originally, was 0.50 (t = 0)
We are told that the initial rate increases by 1.7 in the repeat experiment…

k and [B] can be cancelled out…

Rearranging, the new concentration of A is given by...

Make sure to take the square root for both sides. Examiners noted this as an issue in the original exam.

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