Name : _________________________________________ (C3) Calculating rates
Rate of reaction is a number !
You might calculate it so that you can compare the rate at different points in the reaction.
►Measuring rate of reaction (I) : calculation
Rate = amount of change in the “dependent variable”
time measured for
This leads to the question : why is the rate so high
(34 cm3/min) at the start, but so much lower by the 4th minute (2 cm3/min)……….…?
►Measuring rate of reaction (II) : Tangent-method on a graph
This method is useful if you’re given some practical results in the form of a graph and asked to
calculate the rate of reaction …………
If you’ve got a curve (which many rates graphs are), it’s hard to measure the “gradient” of the curve.
One way is to draw a tangent - a straight line (drawn with a ruler) that touches the curve at a particular
point. Using the steps below, this can help you to calculate the gradient of that curve and the gradient
is the rate of reaction at that point….
Tangent
Straight line
just touching
the curve
QUESTIONS
Q1] Zinc metal reacts with hydrochloric acid to release the gas hydrogen.
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g)
This release of gas can be measured to monitor how quickly a reaction is proceeding.
In a reaction between 1.5g of zinc powder and 50mls of 2.0M hydrochloric acid at room
temperature, the following results were obtained:
a) What was the dependent variable in this experiment ? [1]
b) On the graph, label the following points on the curve with the appropriate letter:
A – point where reaction is fastest &
B – point where no more gas is formed (reaction has finished) [2]
c i) Sketch a 2nd curve (starting at zero-zero) on the same axes to show the reaction involving
1.5g of zinc granules (lumps) and 50mls of 2.0M hydrochloric acid at room temperature. [2]
c ii) Suggest why both reactions (represented by the 2 curves on the graph) would eventually
produce the same total volume of gas ? [1]
d i) Draw a “tangent” on the original curve (using a ruler) to help you to calculate the initial rate of
reaction [1]
d ii) Complete the “triangle” (by adding 2 sides to the tangent on the graph)and use the data to
calculate the initial rate of reaction ? [2]
Rate of reaction = Change in Y-axis reading
Change in X-axis reading
d iii) What are the units for the rate of reaction ? [1]
e) Using ideas about particles and successful collisions per second, suggest in your own words why
both of the graph lines above curve, rather than carrying on in a straight lines ? [2]
f) In this reaction, it was planned so that the acid would be in “excess” – it wouldn’t all be used up in
the reaction (some would be left over, unreacted, at the end) – suggest why ? [1]
hint : good idea to use the phrase “limiting reactant” here !
g) Suggest an independent variable that you could change to make the first reaction (1.5g of zinc
powder and 50mls of 2.0M hydrochloric acid ) go slower , other than using lumps of zinc rather
than powder, and explain why it would slow down……..? [3]
Q2] A reaction was carried out between a different metal (magnesium ribbon) and Hydrochloric acid.
The following results were collected and recorded, by reading off the volume of gas in the syringe
every 10 seconds for 1 minute 30 seconds:
Time (secs) 0 10 20 30 40 50 60 70 80 90
Volume of hydrogen 0 18 30 40 48 53 57 58 58 58
gas collected in syringe (cm3)
a) Why is it important to record the volume of gas result at time 0 (zero) seconds ? [1]
b) Which measuring device (gas syringe or stopwatch) had the highest resolution ? Explain [2]
c) Using the data in the table, calculate the rate of reaction for:
i) the first 10 second period of the reaction (including units) ? [2]
ii) the 2nd 10 second period of the reaction ? [1]
iii) the 5th 10 second period of the reaction ? [1]
d) Calculate the “Average” rate for this reaction (including units) ? [1]
e) Why is the average rate of reaction less “useful” than the rate of reaction at each 10 second
interval ? [1]
3] In the reaction in Q1, Zinc metal reacts with hydrochloric acid.
Zn (s) + 2HCl (aq) ZnCl2 (aq) + H2 (g)
A different dependent variable was measured to monitor the rate of the reaction, resulting in the
following graph of the results.
a) Which dependent variable might have been measured during this reaction ? [1]
b) Suggest how it was measured in the experiment. [2]
c) Suggest how this new method of measuring the rate of reaction might lead to results with a higher
resolution than in the original experiment in Q1. [2]
C3 Rates calculations
How did I do ?
How can I improve ?
Grade How can I achieve this ? evidence Achieved ? needs
(Q ?) (teacher) more work
/ X (student)
CALCULATE rate by drawing a tangent on a curve and 1d i & ii
using it to calculate rate of reaction.
EXPLAIN (using particles) why the rate changes with 1g
changing temperature or concentration
CALCULATE the rate of reaction using data (at a specific 2c i, ii, iii
point in a reaction) and record result to appropriate number
A* of d.p.
EXPLAIN (in own words) that rate depends on frequency of 1e
successful of collisions (per sec) between particles 1g
EXPLAIN (in own words) that rate depends on energy of 1g
collision between particles (if temp)
EVALUATE & EXPLAIN resolution of data 2b + 3c
USE information to work out when reaction has finished 1b & 1ci
DRAW sketch graph to show how rate changes with 1c i
changing surface area of reactant
RECOGNISE that when the “limiting reactant” is used up, the 1f
reaction stops
UNDERSTAND that rate of reaction depends on number of 1e & 1g
C successful collisions between particles per second
Recognise & use the idea that amount of product formed is 1 c ii
related to amount of reactant used
UNDERSTAND common units used for rate of reaction (and 1 d iii
work out what they will be given primary data)
EVALUATE value of average rate of reaction versus interval 2e
rate of reaction
Grade achieved = _____________ Target grade = ________________
To improve, I need to :
