Rates of Reaction Review Problems

CHEM 504

Rates of reaction review problems

1

The winner of a science contest will be the group of students that produces the largest quantity of hydrogen gas in the shortest amount of time.

Each group of students receives 1g of magnesium and 500 mL of hydrochloric acid having a concentration of 0.5 mol/L. and is given the following equation :

Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)

Conditions :

-The quantities of magnesium and acid cannot be increased.

-A catalyst is not available.

In a brainstorming session, your group identifies two experimental conditions which are likely to increase the rate of reaction. What are these two conditions?

2

Zinc, Zn, reacted with hydrochloric acid, HCl. The following table shows the volume of hydrogen gas formed during the reaction as a function of time.

What is the average rate of formation of hydrogen gas between the 25th and 75th second?

3

A student had to determine the average reaction rate when hydrochloric acid, HCl(aq), is combined with magnesium, Mg(s). This reaction is represented by the following chemical equation :

Mg(s) + 2 HCl(aq)  Mg Cl2(aq) + H2(g)

It took 3.0 min for the concentration of HCl(aq) to drop from 1.0 mol/L to 0.60 mol/L.

What is the average reaction rate in moles of HCl(aq) per litre per second?

4

The electrolysis of water produces hydrogen gas, H2(g), according to the following chemical equation :

H2O(l)  H2(g) + O2(g)

A scientist must produce 0.50 mol of O2(g) using an apparatus that decomposes 4.50 g of water per hour at room conditions.

How much time is needed to produce the required amount of oxygen gas?

5

Jasmine conducts the following experiment.

Materials:

-Wax candle (C25H52(s))

-Ethanol burner (CH3CH2OH(l))

-Matches

-Balance

-Timer

Procedure:

-Mass the candle and burner.

-Burn both for 5 minutes.

-Take the final mass of the candle and burner.

Results:

Table 1: Comparison of the rates of combustion for the wax and the ethanol burner

What factor is responsible for the difference in the rates of combustion between the wax and the ethanol burner? Justify your answer.

6

Hydrogen gas (H2(g)) is produced during the following reaction:

Mg(s) + 2 HNO3(aq)  Mg(NO3)2(aq) + H2(g)

A student plots the graph below using the results obtained during the following experiment.

What is the average rate of production of H2(g) in kPa/s between the 2nd and 3rd minute?

7

Hydrochloric acid, HCl, reacts with calcium carbonate, CaCO3, to produce calcium chloride, CaCl2, water, H2O, and carbon dioxide, CO2.

The equation for this reaction is:

HCl(aq) + CaCO3(s)  CaCl2(aq) + H2O(l) + CO2(g)

At Standard Temperature and Pressure (STP), you record the volume of CO2, produced as a function of time.

a)Plot the graph of the volume of CO2 produced as a function of time.

b)Calculate the average reaction rate between 5 and 10 minutes.

8

The graph below shows two potential energy curves for a hypothetical reaction.

The reaction is:X + Y  Z

The two curves show the reaction pathways with and without the addition of a catalyst.

a)How many steps exist in the reaction mechanism for the forward catalyzed reaction?

b)What numbered points on the graph represent the peak energy of the rate determining steps for the forward uncatalyzed and catalyzed reactions respectively?

c)What is the activation energy of the forward uncatalyzed reaction (in kJ/mol)?

d)What is the activation energy of the reverse catalyzed reaction (in kJ/mol)?

e)What is the H of the reverse reaction (in kJ/mol)?

9

A student was studying the electrolysis of 1 L of aqueous aluminum chloride (AlCl3(aq)) according to the following equation:

Using the graph above, calculate the average rate of formation of chlorine gas from 10 s to 20 s.

10

A student conducted an experiment to measure the rate of the following reaction between calcium carbonate and hydrochloric acid:

CaCO3(s) + 2 HCl(aq)  CO2(g) + CaCl2(aq) + H2O(l)

At the end of 30 seconds, the marble slab was not significantly smaller than it was at the start.

Fill in reasonable values for the times required for the evolution of CO2 gas in this second experiment in the table below.

What was the average rate of reaction in the second experiment? Include units.

11

Consider the following chemical reaction:

2 Fe(s) + 3 CO2(g)  Fe2O3(s) + 3 CO(g)

The graph below illustrates the moles of iron present as a function of time in seconds for the reaction.

Plot a graph of the moles of CO(g) present, as a function of time.

12

An energy distribution curve is shown below.

A catalyst is added and the gas sample is cooled.

In your Answer Booklet, modify the graph to indicate the changes that would occur.

Justify the changes you made to the graph.

13

The wine (CH3CH2OH = alcohol) inside a bottle oxidizes in contact with air (O2) to be transformed into vinegar (CH3COOH). The equation can be represented by :



Which of the following changes would increase the rate of the oxidation of wine?

1.Increase the surface area in contact with air (O2).

2.Decrease the air pressure.

3.Place the bottle in the refrigerator.

4.Add the catalyst MnO2 to the wine.

14

To compare the rates of reaction, you put 100g of calcium carbonate, CaCO3(s), into eight solutions of hydrochloric acid HCl(aq). You group them in pairs as shown in the four situations below :

In which situation (1, 2, 3 or 4) will the beaker on the right react more rapidly than the beaker on the left?

15

Below are three kinetic energy curves representing the same chemical reaction.

The area under each curve is the same.

Which factor accounts for the variations in reaction rate between the following pairs of curves?

16

Give one justification for your choice based upon the ''collision theory'' of reactions.

In each of the following pairs of reactions, choose the one that will be the slower.

(Assume that all other variables are constant in all reactions.)

1.A)The burning of a strip of magnesium, Mg(s), in air.

B)The burning of finely powdered magnesium, Mg(s), in air.

2.A)The oxidation of hydrogen peroxide, H2O2(l), in air.

B)The oxidation of hydrogen peroxide, H2O2(l), in air, with a catalyst.

3.A)The reaction of 5.0 g of powdered zinc metal, Zn(s), with 1.0 HCL(aq) at 50°C.

B)The reaction of 5.0 g of powdered zinc metal, Zn(s), with 1.0 HCL(aq) at 20°C.

4.A)The reaction of hydrogen gas, H2(g), with oxygen, O2(g), in air to produce water vapour, H2O(g).

B)The reaction of hydrogen gas, H2(g), with pure oxygen, O2(g), to produce water vapour, H2O(g).

5.A)Pb+2(aq) + 2 I(aq)  PbI2(s)

B)C11H22O11(s) + 11 O2(g)  11 CO2(g) + 11 H2O(g)

17

You are interested in comparing the rate of some chemical reactions that occur in nature.

In each of the following pairs, which reaction will have the slowest rate?

1.a) The formation of coal in the earth.

b) The formation of water vapour during a forest fire.

2.a) The burning of a cigarette in air.

b) The burning of a cigarette in pure oxygen gas.

3.a) The combustion of a match.

b) The combustion of sugars in the human body.

18

It is difficult to compare the rates of different chemical reactions. However it is possible to predict and to make a comparison of the rates of various reactions at room temperature.

Which of the following four chemical reactions would be the slowest at room temperature?

19

Which energy distribution graph below shows the effect of an increase in temperature on the rate of this reaction?

20

The automobile engine (internal combustion engine) is powered by gasoline and air.



Among the factors proposed below, which does NOT modify the rate of combustion of gasoline?

21

An ecologist, experimenting with atmospheric equilibrium, wishes to promote the reproduction of highly photosynthetic algae.

After having selected 5 appropriate species of algae, he decides to measure their rate of photosynthesis.

Which of the following procedures can be used to make the appropriate calculations for equal quantities of each species of algae?

1.Measure the mass of CO2 used up per unit of time.

2.Measure the variation in pH value in the water per unit of time.

3.Measure the solubility of CO2 and the solubility of oxygen in water.

4.Measure the volume of oxygen produced per unit of time.

5.Measure the quantity of energy contained in one mole of C6H12O6.

22

During an experiment dealing with the factors that influence the rate of rust formation (slow oxidation), a team of students obtain the following results :

Which experimental result above best justifies the technological applications below?

a.A manufacturer of electronic equipment inserts a porous bag into his packaging. This bag contains a dessicator. (A drying agent.)

b.A chemist suggests the addition of anti-oxidant wrapping to certain food products before they are put on shelves for sale.

c.Every spring, your father paints the swing.

23

The four graphs below represent the number of molecules of reactants as a function of their kinetic energy. Which graph represents the fastest reaction?

24

The decomposition of sucrose into glucose is a very slow reaction that can be speeded up by a digestive enzyme like sucrase.

Which potential energy diagram correctly illustrates both the reaction with the enzyme and the reaction without the enzyme?

LegendReaction without the enzyme

Reaction with the enzyme

25

The graph below represents a chemical reaction which occurred at first without a catalyst and then occurred a second time with a catalyst.

What are respectively, in kilojoules, the heat of reaction (H) and the activation energy (Ea) of the reaction with a catalyst?

26

During an experiment to determine the rate of a chemical reaction, you increase the temperature of the system and notice an increase in the reaction rate.

Which of the following best explains this increase in the reaction rate?

27

In an experiment that involved measuring the rate of a chemical reaction, 6.35 grams of solid copper reacted with a 1.0 mol/L solution of nitric acid. The reaction lasted 1 min 40 s and occurred at room temperature.

What is the reaction rate in moles of copper per second (mol/s)?

28

To measure the rate of the reaction below,

2 HCl(aq)+Mg(s)H2(g)+MgCl2(aq)

you follow this procedure:

-Record room temperature and atmospheric pressure.

-Set up the apparatus as shown in Figure 1 using 0.05 g Mg(s) and 200 mL of a 1 mol/L solution of HCL.

-Record time taken for the reaction to reach completion H2(g).

-Allow the apparatus to cool for a few minutes to ensure the H2(g) returns to room temperature.

-Record the volume of H2(g) collected.

Figure 1: Apparatus used to collect the gas released during the chemical reaction.

Table 1: Volume of H2(g) collected:

What is the average rate of production of H2(g) for this reaction?

29

The combustion of wood, a hydrocarbon, can be represented by the general equation below:

Wood(s) + O2(g)  CO2(g) + H2O(g) + SOx(g) + NOx(g) + ...

Why does the rate of combustion decrease when less hydrocarbon fuel is supplied?

30

In the laboratory, Jerry was asked to determine the rate of the following reaction:

Mg(s) + 2 HCl(aq)MgCl2(aq) + H2(g)

The reaction involved 0.120 g of magnesium, Mg(s), and an excess of hydrochloric acid, HCl(aq). The reaction was complete after 90.0 s.

What was the rate of formation of the hydrogen gas, H2(g)?

31

The graphs below represent the same chemical reaction.

According to these graphs, which of the following statements is FALSE ?

32

The thermal decomposition of calcium carbonate (CaCO3) produces "quicklime" or calcium oxide (CaO) and carbon dioxide (CO2).

The equation is:

CaCO3(s)  CaO(s) + CO2(g)

A sample of calcium carbonate weighing 0.45 g was heated and reacted as shown in the equation.

After 3.00 minutes, the unreacted calcium carbonate was removed and was found to weigh 0.31 g.

What was the average rate of reaction measured in moles of calcium carbonate reacted per minute?

33

The following kinetic energy distribution curve shows the threshold energy of two reactions labeled A and B.

Which of the following statements are necessarily TRUE about these two reactions?

1.Reaction A has a lower activation energy than reaction B.

2.A greater number of molecules have the threshold energy of reaction A than the threshold energy of reaction B.

3.Reaction A has a larger H than reaction B.

4.An increase in temperature will increase the rate of reaction A but not reaction B.

34

Study the kinetic energy distribution curve below:

Which of the following energy distribution curves represents the effect of adding a catalyst and increasing temperature?

Answers and solutions:

1

1.Increase the reactants’ contact surface area by changing the magnesium into little pieces.

2.Increase the temperature of the reactants.

Also :Increase (H+ ) by boiling away some of the acid solution.

2

Example of an appropriate procedure

1.Graph showing the volume of hydrogen gas formed during the reaction as a function of time

Note :The curve must not be a straight line.

2.Volume of hydrogen at the 25th second and the 75th second

At the 25th second, roughly 19 mL of hydrogen gas had been formed.

At the 75th second, roughly 39 mL of hydrogen gas had been formed.

3.Average rate of formation of hydrogen gas between the 25th and the 75th second

ra = = = = 0.40 mL/s

Note :The rate may vary by  0.04 mL/s.

Answer

The average rate of formation of hydrogen gas is 0.40 mL/s.

3

Example of an appropriate process

1.Change in the concentration of HCl(aq)

1.0 mol/L - 0.60 mol/L = 0.40 mol/L

2.Duration of the reaction in seconds

3.0 min  = 180 s

3.Average reaction rate in moles of HCl(aq) per litre per second

Average Rate =

Average Rate = 2.2  10-3

Answer

The average reaction rate is 2.2  10-3 .

4

Example of an appropriate process

1.Number of moles of H2O(l) decomposed in 1 hour

18.02 g 1 mol

4.50 g ?

0.25 mol

2.Number of moles of O2 produced in 1 hour

According to the given equation, 0.25 mol of water will produce 0.125 mol of oxygen gas.

3.Average rate of production of oxygen gas in mol/h

Average Rate =

Average Rate = 0.125 mol/h

4.Time needed to produce the required amount of oxygen gas

t =

t =

t =4.0 h

Answer

The time needed is 4.0 h (240 min or 14 400 s).

5

Explanation of the factor responsible for the difference noted in the rates of reaction

Examples of answers:

Nature of reactants: Wax is a larger molecule than ethanol, therefore there are more chemical bonds to break in the reaction.

Nature of reactants: Wax is a solid, ethanol is a liquid. In general, liquids react faster than solids.

6

Example of solution:

1- =

Answer:The average rate of production of H2(g) between the 2nd and 3rd minute is 0.04 kPa/s.

7

Example of an appropriate and complete procedure

a)

b)The average reaction rate is = 2.1 mL/min

Note: Accept  0.2 mL/min. No marks if the student inverts the axes.

8

Example of an appropriate procedure

a)There are three steps in the forward catalyzed reaction.

b)The peak energy of RDS for the forward uncatalyzed reaction is at point #3.

The peak energy of RDS for the forward catalyzed reaction is at point #8.

c)The activation energy of the forward uncatalyzed reaction is: +500 kJ/mol.

d)The activation energy of the reversed catalyzed reaction is: approximately +425 kJ/mol.

e)The H for the reverse reaction is: + 200 kJ/mol.

9

Example of an appropriate and complete answer

0.37 mol  0.28 mol = 0.09 mol AlCl3

2.0.09 mol AlCl3  = 0.14 mol Cl2

3. = 0.014 Cl2(g)

Answer:The average rate of formation of Cl2(g) between 10 s and 20 s was 0.014 .

10

Example of an appropriate and complete solution

In the second experiment, the marble slab is sliced into two parts. As a consequence, the surface area of the marble will be doubled and the rate will double.

Since this is experimental data, some variation ( 1 sec) in the data and in the answers must be allowed. The final time must be close to 23 sec ( ).

Answer:

The surface area of the four edges of the slab is not doubled when the slab is cut but this will be a very minor factor given that the slab of marble is said to be “very thin”.

Calculate the rate:

Average rate=

=

Answer:The average rate of reaction in the second experiment is .