Multiple Choice – RATES OF REACTION TEST
Identify the choice that best completes the statement or answers the question.
A
1. Consider the reaction between zinc granules (lumps) and hydrochloric acid:
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
By altering the conditions under which the reaction is carried out, we can
change the rate at which hydrogen has is produced. Which of the following
changes will NOT increase the rate of the production of hydrogen gas?
A Increasing the concentration of the acid
B Increasing the mass of zinc
C Increasing the temperature
D Using powdered zinc instead of zinc granules
ANSWER: B
0/1
POINTS:
FEEDBACK: Changing the total amount of either reactant will not change the
rate of the reaction, although it will of course affect the amount
of products produced. However, as zinc is a solid reactant,
increasing its total surface area will increase its rate of reaction,
since the reaction can only occur at its surface.
pp284, 290–3
REF:
A
2. Which of the following factors will increase the frequency of collisions between
reactant particles in a chemical reaction between two gases that are sealed in a
container?
A Increasing the pressure on the gases
B Introducing an inert gas into the container
C Cooling the container
D Increasing the volume of the container
ANSWER: A
1/1
POINTS:
FEEDBACK: Under higher pressure the gas particles will be pushed closer
together and will collide more often.
p276
REF:
A
3. The activation energy for a particular reaction:
A depends on the temperature at which the reaction occurs.
B always has a positive value.
C is unique for that reaction and can never be changed.
D is the energy absorbed or released in a chemical reaction.
ANSWER: B
0/1
POINTS:
FEEDBACK: The activation energy of a particular reaction does not depend on
the temperature but can be lowered by the addition of a catalyst.
It is the energy required to break the relevant bonds of the
reactants, so is always positive.
p280
REF:
A
4. The following list shows some of the factors that affect the rate of a chemical
reaction:
I The activation energy for the reaction
II The mean speed of the reactant particles
III The concentrations of the reactant particles
IV The orientation of the reactant particles when they collide
Which factors will affect the probability that a collision between reactant
particles is successful, for a particular reaction?
A I and II
B I and IV
C II and III
D I, II and IV
ANSWER: D
0/1
POINTS:
FEEDBACK: A successful collision is one in which the particles have collided
at the right orientation and with sufficient energy for the
necessary bonds to break (which is measured by the activation
energy for the reaction). The concentrations of the reactant
particles will affect the rate of collisions but not the percentage
of collisions that are successful.
pp275–6
REF:
A
5. Which statement about the heat content of substances in reactions is
CORRECT?
A The heat content of a substance is its chemical potential energy.
B The higher the heat content of a substance, the more stable it is.
C In exothermic reactions, the reactants have a higher heat content than the
products.
D In any reaction, the heat content of the reactants is always zero.
ANSWER: C
0/1
POINTS:
FEEDBACK: The heat content of a substance is the total energy it possesses.
The lower it is the more stable the substance, but substances
cannot have zero heat content. It is just a convention, for
convenience, to assign a value of zero for the heat content of
reactants when drawing an energy profile diagram.
pp279–81
REF:
A
6. Magnesium reacts with hydrochloric acid as shown in the following
thermochemical equation:
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
H = -107 kJ mol-1
Which statement about this reaction is INCORRECT?
A This reaction requires three particles to collide simultaneously.
B As this reaction occurs, the container will get hot.
C The products of this reaction are more stable than the reactants.
D This reaction will only occur on the surface of the magnesium.
ANSWER:
A
1/1
POINTS:
FEEDBACK: The probability of three particles colliding simultaneously at the
right orientation would be negligible. Instead, the reaction would
occur in a series of steps. The equation only shows the net result
of these steps, which are known as the reaction mechanism.
p277
REF:
A
7. Consider the following thermochemical equation:
2H2(g) + O2(g)  2H2O(l)
H = -484 kJ mol-1
This equation tells us that when 2 mol H2 is burnt in excess oxygen:
A 484 kJ of heat energy is absorbed.
B 484 kJ of heat energy is released.
C 968 kJ of heat energy is absorbed.
D 968 kJ of heat energy is released.
ANSWER: B
0/1
POINTS:
FEEDBACK: The negative sign of H tells us that energy is released in the
reaction – that is, it is exothermic. The equation tells us that
when 2 mol H2 reacts with 1 mol O2 (producing 2 mol H2O), 484
kJ energy is released. We assume, unless told otherwise, that the
energy released is in the form of heat only.
p280
REF:
A
8. Consider pure oxygen gas confined in two sealed flasks of equal volume. The
amount of oxygen in each container is 1 mol. The temperature of Flask X is 100
°C and of Flask Y is 200 °C. Which statement about these oxygen molecules is
CORRECT?
A Many of the molecules in Flask X will be moving faster than many of the
molecules in Flask Y.
B All the molecules in Flask X will be moving more slowly than those in
Flask Y.
C The mean kinetic energy of the molecules in Flask X will be half that of
those in Flask Y.
D The mean speed of the molecules in Flask X will be half that of those in
Flask Y.
ANSWER: A
1/1
POINTS:
FEEDBACK: The Maxwell-Boltzmann distribution curves for a gas at two
different temperatures show that at each temperature the
particles present have a range of speeds. Hence many of the
particles in the cooler sample will be moving at a faster speed
than many of the particles in the hotter sample. The mean kinetic
energy of a sample of a gas will be half that of a sample that is at
twice the absolute temperature, not twice the Celsius
temperature.
p283
REF:
A
9. A student was following the reaction between marble chips and hydrochloric
acid by performing it in an open conical flask over an electronic balance. The
equation for the reaction is:
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + CO2(g) + H2O(l)
In one test, 50 mL of HCl of concentration 0.5 mol L-1 was added to 10.0 g
marble chips. The mass of the flask was recorded at regular intervals over eight
minutes. The graph obtained is shown below.
What is the best explanation for the shape of this graph?
A Over time the rate of production of CO2 becomes slower and slower,
until it is too hard to detect.
B The mass drops as CO2 escapes the flask, then steadies when bubbling
stops.
C The rate of CO2 production decreases over time, reaching zero when the
HCl runs out.
D The CO2 is produced at a constant rate until the HCl runs out.
ANSWER: C
0/1
POINTS:
FEEDBACK: The rate of production of CO2 is shown by the gradient of the
graph. The graph becomes less steep over time because the
reaction slows down. If the CO2 were produced at a constant
rate, this part of the graph would be a straight line, not a curve.
We begin with 0.1 mol CaCO3, which requires 0.2 mol HCl to
react. But we only have 0.025 mol HCl, so the HCl is the
limiting reagent. The graph flattens when the HCl has run out.
pp290–1
REF:
A
10. Which proposal of the kinetic molecular theory does not hold for any real gas?
A The collisions between gas particles are elastic.
B Gas particles move in straight lines until they undergo a collision.
C The mean kinetic energy of gas particles is proportional to their absolute
temperature.
D There are no forces between gas particles.
ANSWER: A
1/1
POINTS:
FEEDBACK: If no forces operated between gas particles, they could not
liquefy when cooled and/or subjected to high pressure. However,
the forces are negligible when the gases are at high and low
temperatures since the gas particles spend less time near each
other in this state. (The strength of the electrostatic forces of
attraction is only significant when particles are very close to one
another.)
p283
REF:
A
11. Which of the following describes the key reason why a catalyst increases the
speed of a reaction?
A It causes the rate of collisions between the reactant particles to increase.
B It provides an alternative reaction mechanism with lower activation
energy.
C It pins down the particles of one of the reactants onto its surface.
D It makes the reactant particles collide at the right orientation.
ANSWER: B
0/1
POINTS:
FEEDBACK: Some catalysts such as wire gauzes and enzymes may work by
binding reactant particles to their surface. In some of these cases,
such as in reactions between gases, this increases the probability
of a collision between the reactant particles. In some cases of
solid catalysts, such as in the synthesis of certain polymers
(polymer production), the catalyst may ensure that collisions
occur at a certain orientation so the desired product is formed.
But B is true for all catalysts.
p285
REF:
A
12. Consider the energy profile diagram for the reaction:
CO(g) + NO2(g)  CO2(g) + NO(g)
From this energy profile we can deduce that for this reaction:
A H = +225 kJ mol-1
B H = -225 kJ mol-1
C H = +375 kJ mol-1
D H = -375 kJ mol-1
ANSWER: B
0/1
POINTS:
FEEDBACK: The enthalpy change for the reaction is the difference in energy
content between the reactants and the products. In this case 225
kJ of energy will be evolved as each mole of CO reacts. It will
require 150 kJ to break the bonds in the reactants and 375 kJ of
energy will be released as new bonds are created to form the
products.
pp278–80
REF:
A
13. Consider the reaction in which hydrogen peroxide decomposes in the presence
of some chopped liver, which can catalyse this reaction:
2H2O2(aq)  2H2O(l) + O2(g)
In one experiment, this reaction was monitored by two sets of students: Group 1
and Group 2. They measured the mass of oxygen gas that evolved at regular
intervals over time. There was just one difference between the reaction
conditions they used. The graphs they obtained are shown next.
The different results obtained by the two groups could be explained if the
difference in the reaction conditions they used was that:
A Group 1 added a greater amount of hydrogen peroxide to their flask.
B Group 2 chopped their piece of liver into smaller pieces.
C Group 1 used a higher concentration of hydrogen peroxide.
D Group 2 sat their reaction flask in hot water.
ANSWER: C
0/1
POINTS:
FEEDBACK: Changing the amount of reactant will change the amount of
product but not the rate at which it reacts. The use of smaller
pieces of catalyst or higher temperature by Group 2 would have
increased the reaction rate and produced a steeper curve for
Group 2.
pp289–93
REF:
A
14. Compared with heterogeneous catalysts, homogeneous catalysts:
A are generally more expensive.
B are less easily poisoned.
C require higher pressures.
D are less selective in the reactions they can catalyse.
ANSWER: B
0/1
POINTS:
FEEDBACK: Although homogeneous catalysts such as enzymes can be
poisoned (inhibited), in general this is less likely to occur than
the poisoning of the surface of a heterogeneous catalyst by
impurities in the reaction mixture.
p286
REF:
A
15. One sealed flask contains ammonia molecules, NH3, and another contains
hydrogen chloride molecules, HCl, as shown.
Both gases are at the same temperature. When the stopcocks are opened, a white
smoke is formed where the gases come in contact with one another, due to the
reaction: NH3(g) + HCl(g)  NH4Cl(s)
Where along the connecting tube will the two gases meet, and why?
A X, since the NH3 molecules will have greater kinetic energy than the HCl
molecules
B X, since the NH3 molecules have higher mean speed due to their lower
mass
C Y, since both the NH3 molecules and the HCl molecules have the same
kinetic energy
D Z, since the NH3 molecules will have lower kinetic energy than the HCl
molecules
ANSWER: B
0/1
POINTS:
FEEDBACK: The gases are at the same temperature, so their molecules will
have the same kinetic energy. However, since the mean kinetic
energy is proportional to the mass of the particles and the square
of their mean speed, the greater their mass, the lower their mean
speed must be. Hence, in the time taken to meet, the NH3
molecules will have travelled further.
p283
REF:
A
16. Which of the following statements about why reaction rates are faster at high
temperatures is INCORRECT? At high temperatures:
A the mean speed of the particles increases, hence the collision rate
increases.
B the colliding particles have more kinetic energy on average.
C the activation energy is low, so a high percentage of collisions are
successful.
D a high percentage of colliding particles possess the requisite activation
energy.
ANSWER: C
0/1
POINTS:
FEEDBACK: The activation energy is not changed by a change of
temperature, but can be lowered by the addition of a catalyst.
Reaction rates are generally greater at high temperatures, not
only because the collision rate increases but also because the
proportion of collisions that are successful increases.
p284
REF:
A
17. Fine metal gauzes are often used to catalyse reactions between gases because in
general:
A they slow down the gas particles so they can spend more time near each
other.
B they increase the surface area of contact between the gas particles.
C then the reacting molecules collide with them, they bounce off with
greater energy.
D they form weak bonds with the gas molecules, weakening their covalent
bonds.
ANSWER: D
0/1
POINTS:
FEEDBACK: The gauzes generally work by attracting one of the reactants to
their surface, weakening the covalent bonds within the molecule
that need to be broken in the reaction, and increasing the
probability of the other reactant colliding with them.
pp285–7
REF:
A
18. In many industrial processes that involve reactions between gases, the gases are
passed over beds of solid catalyst at high speed. In these cases the catalyst
should be in the form of:
A large pellets, to maximise the surface area of the catalyst.
B large pellets, so the catalyst is not blown out with the gases.
C a very fine powder, to maximise the surface area of the catalyst.
D small pellets, so the catalyst has a large surface area but is not blown
away.
ANSWER: D
0/1
POINTS:
FEEDBACK: The smaller the particles, the greater the total surface area for a
given mass of solid. But too fine a powder is impractical because
of the increased risk that it will be blown out with the gases.
pp285–6
REF:
A
19. Which statement about catalysts is INCORRECT?
A They take part in the reaction in some way.
B They are consumed during the reaction.
C Their use can lower the temperature required for the process.
D They can be solids or dissolved in solution.
ANSWER: B
0/1
POINTS:
FEEDBACK: Catalysts alter the reaction mechanism and hence do take part in
some way. However, they are regenerated in the end. They are
often used to reduce the temperature and hence the energy
required for a reaction to occur at a suitable, economic rate.
Solid catalysts are known as heterogeneous catalysts and those
in solution, which are used to catalyse a reaction taking place in
solution, are termed homogeneous catalysts.
REF:
A
pp285–7
20. The thermochemical equation for the combustion of hydrogen in oxygen is:
2H2(g) + O2(g)  2H2O(g)
H = -483.6 kJ mol-1
Energy is released in this reaction because:
A the reactants are elements and the product is a compound.
B it requires very little energy to break the covalent bonds in H2 and O2
molecules.
C the covalent bonds in the H2O molecule are stronger than those in H2 and
O2.
D the reaction does not require the presence of a catalyst.
ANSWER: C
0/1
POINTS:
FEEDBACK: Whether energy is absorbed or released in a chemical reaction
depends on the relative strengths of the bonds of the reactants
and the products, not on just the strength of the bonds of one or
the other. Whether the reactants or products are elements or
compounds is not a deciding factor – only the relative strengths
of the bonds within them. Many endothermic reactions will
occur without a catalyst.
pp279–81
REF: