C6 Finding Reaction Rates from Graphs (page 146)

Question 1

How do you calculate the ‘Mean Reaction Rate’ from a Graph?

Answer 1

1) Remember, a rate of reaction graph shows the amount of product formed and amount of reactant used up on the y-axis and time on the x-axis.

2) So to find the mean rate for the whole reaction, you, you just work out the overall change in the y-value and then divide this by the total time taken for the reaction.

3) You can also use the graph to find the mean rate of reaction between any two points in time:

example:

(the graph shows the volume of gas released by a reaction, measured at regular intervals. Find the mean rate of reaction between 20 s and 40 s)

Mean rate of reaction = change in y ÷ change in x
= (19 cm³ - 15 cm³) + 20 s.
= 0.2 cm³/s

see graph on page 146)

Question 2

If you’re asked to find the mean rate of reaction for the whole reaction, what do you need to remember?

Answer 2

remember that the reaction finishes as soon as the line on the graph goes flat.

Question 3

How do you find the rate of the reaction at a particular point in time?

Answer 3

you need to rind the gradient (slope) of the curve at that point. The easiest way to do this is to draw a tangent to the curve - a straight line that touches the curve at one point and doesn’t cross it. You then work out the gradient of the tangent.

Please look at graph example on page 146, half way down the page.

Question 4

Magnesium powder was added to the conical flask containing dilute H2SO4. H2 was produced and collected in a gas syringe. The volume of gas released was recorded at 10 second intervals in the following table:

Time (s) 10 20 30 40 50 60
Volume of H2 (cm³) 18 28 34 38 40 41

a) plot these results on a graph and draw a line of best fit (3 marks).

b) Find the rate of the reaction at time = 25 s. (4 marks)

Answer 4

a) please look at answer as its in a diagram on page 245, p146 Q) a

b) Please look at answer on page 245, p.146 Q)b

change in y = 36 - 24 = 12
change in x = 32 - 12 = 20
rate = change in y + change in x = 12 + 20
= 0.6 cm³/s

(1 mark for drawing a tangent at 25 s, 1 mark fr correctly calculating a change in y from the tangent, 1 mark for correctly calculating a change in x from the tangent and 1 mark for a rate between 0.5 cm³/s and 0.7 cm³/s