Reaction Rates Questions

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March 2014

6 The graph below shows the decomposition of gas P according to the following equation:

6.1 Define the term rate of reaction in words by referring to the graph. (2)
6.1 Rate of change of concentration (of P).


The change in concentration (of P) per unit time / per second. (2)

6.2 At which time, 10s or 30s, does the decomposition take place at a higher rate? Refer to the graph to give a reason for the answer. (2)
6.2 Gradient (of the tangent) at 10s is greater than that at 30s.


The graph has a steeper slope at 10s than at 30s. (2)

6.3 Write down the initial concentration of P(g). (1)
6.3   0,27

6.4 The decomposition is carried out in a 2dm3 container.
Calculate the average rate (in mol.s-1) at which P(g) is decomposed in the first 10s. (6)

6.5 Draw a potential energy diagram for the reaction. Clearly indicate the following on the diagram:

6.6 An increase in temperature will increase the rate of decomposition of P(g).
Explain this statement in terms of the collision theory. (2)
More molecules with sufficient / enough kinetic energy.
More effective collisions per unit time.

March 2013

6 The apparatus shown below is used to investigate the rate at which hydrogen gas is produced when a certain amount of zinc reacts with an excess of a dilute hydrochloric acid solution.

The reaction that takes place is represented by the following balanced equation:

6.1 Write down the name of the flask labelled Z. (1)
6.1 Conical flask


Erlenmeyer flask (1)

6.2 Write down ONE function of the item of apparatus labelled Y in THIS investigation. (1)
6.2 Collect gas produced.
Measure volume of gas produced. (1)

Two experiments are conducted using the apparatus above. The conditions for each experiment are given in the table below.

6.3 FROM THE TABLE ABOVE, write down:

6.3.1 The independent variable for this investigation (1)
6.3.1 Concentration (1)

6.3.2 ONE controlled variable (1)
6.3.2 ANY ONE:
Surface area / State of division (1)

The volume of hydrogen gas produced is measured in each experiment. The graphs below show the results obtained.

6.4 Which graph, P or Q, represents Experiment 2?
Refer to the data given in the table, as well as the shape of the graph, to explain how you arrived at the answer. (3)
6.4 P
Higher (acid) concentration in experiment 2.
Steeper slope / Greater gradient


Higher (acid) concentration in experiment 2.
Same volume of gas produced/collected in a shorter time / faster. (3)

6.5 Give a reason why the rate of hydrogen production slows down towards the end in both experiments. (1)
6.5 Concentration of acid decreases as reaction proceeds.


Surface area of Zn decreases. (1)

6.6 Calculate the mass of zinc used to prepare 0,24 dm3 of hydrogen gas at room temperature.
Assume that 1 mole of hydrogen gas has a volume of 24,04 dm3 at room temperature. (6)


Nov 2013

6 A hydrogen peroxide solution dissociates slowly at room temperature according to the following equation:

During an investigation, learners compare the effectiveness of three different catalysts on the rate of decomposition of hydrogen peroxide. They place EQUAL AMOUNTS of sufficient hydrogen peroxide into three separate containers. They then add EQUAL AMOUNTS of the three catalysts, P, Q and R, to the hydrogen peroxide in the three containers respectively and measure the rate at which oxygen gas is produced.

6.1 For this investigation, write down the:

6.1.1 Independent variable (1)
6.1.1 Type of catalyst (1)

6.1.2 Dependent variable (1)
6.1.2 Rate of reaction (1)

The results obtained are shown in the graph below.

6.2 Which catalyst is the most effective? Give a reason for the answer. (2)
6.2 R
Fastest rate. / Steepest (initial) gradient or slope.
Produces oxygen fastest

6.3 Fully explain, by referring to the collision theory, how a catalyst increases the rate of a reaction. (3)
6.3 A catalyst provides an alternative pathway of lower activation energy.
Thus more molecules have sufficient kinetic energy to react.


More molecules have kinetic energy equal to or greater than the activation energy.
Thus more effective collisions occur per unit time.
Rate of effective collisions increases. (3)

In another experiment, the learners obtain the following results for the decomposition of hydrogen peroxide:

6.4 Calculate the AVERAGE rate of decomposition (in of H2O2(aq) in the first 400s. (3)

6.5 Will the rate of decomposition at 600s be GREATER THAN, LESS THAN or EQUAL TO the rate calculated in QUESTION 6.4?
Give a reason for the answer. (2)
6.5 Less than
The concentration of hydrogen peroxide decreases as the reaction proceeds.

6.6 Calculate the mass of oxygen produced in the first 600s if 50 cm3 of hydrogen peroxide decomposes in this time interval. (5)

March 2012

A group of learners use the reaction between zinc and sulphuric acid to investigate one of the factors that affects reaction rate. The equation below represents the reaction that takes place.

They add 6,5 g of zinc granules to excess DILUTE sulphuric acid and measure the mass of zinc used per unit time.

The learners then repeat the experiment using excess CONCENTRATED sulphuric acid.

6.1 Define the term reaction rate. (2)
6.1 Amount of reactants used per unit time.

Amount of products formed per unit time.


Change in concentration of reactants or products per unit time. (2)

6.2 Give a reason why the acid must be in excess. (1)
6.2 To ensure that (nearly) all zinc is used up.
Zinc is a limiting reagent. (1)

6.3 Write down a hypothesis for this investigation. (2)
6.3 Examples:

6.4 Give a reason why the learners must use the same amount of ZINC GRANULES in both experiments. (1)
6.4 To make it a fair test.

Ensure validity/reliability of results. OR

So that the contact (surface) area may not influence the reaction rate. The surface area must not change. OR

It is the controlled variable.


To ensure there is only one independent variable. (1)

The results obtained for the reaction using DILUTE sulphuric acid are represented in the graph below.

6.5 Using the graph, calculate the mass of zinc used from t = 0s to t = 60s. (4)

6.6 Calculate the average rate of the reaction (in gram per second) during the first 60s. (2)

6.7Copy the above graph into your ANSWER BOOK. ON THE SAME SET OF AXES, use a dotted line to show the curve that will be obtained when concentrated sulphuric acid is used. Label that curve P (no numerical values are required). (2)

• Graph P has a steeper slope than the original graph.
• Graph P intercepts with the x axis earlier than original graph.

Nov 2012

6 Calcium carbonate chips are added to an excess dilute hydrochloric acid solution in a flask placed on a balance as illustrated below. The cotton wool plug in the mouth of the flask prevents spillage of reactants and products, but simultaneously allows the formed gas to escape. The balanced equation for the reaction that takes place is:

6.1 Write down the NAME of the gas that escapes through the cotton wool plug while the reaction takes place. (1)
6.1 Carbon dioxide (1)

The loss in mass of the flask and its contents is recorded in intervals of 2 minutes. The results obtained are shown in the graph below.

6.2 From the graph, write down the following:

6.2.1 The coordinates of the point that represents results that were measured incorrectly (1)
(6 ; 3,1)        (1)

6.2.2 How long (in minutes) the reaction lasts (1)
12 minutes     (1)

6.2.3 How long (in minutes) it takes 75% (three quarters) of the reaction to occur (1)
4 minutes       (1)

6.3 The experiment is now repeated using hydrochloric acid of a higher concentration. It is found that the rate of the reaction INCREASES. Use the collision theory to explain this observation. (2)
More particles per unit volume
More effective collisions per unit time (second). (2)

6.4 How would a higher concentration of hydrochloric acid affect the following:

6.4.1 Loss in mass per unit time (1)
6.4.1 Increases (1)

6.4.2 Total loss in mass (1)
6.4.2 Remains the same

6.4.3 Time for the reaction to reach completion (1)
6.4.3 Decreases (1)

6.5 Apart from concentration and temperature changes, write down TWO other changes that can be made to increase the rate of this reaction. (2)
6.5 (2)

6.6 Calculate the mass of calcium carbonate used when the reaction is completed. Assume that all the gas that was formed, escaped from the flask. (5)

March 2011

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V. Gokal