Chem 4U - Ch.5,6 - Practice Test
Note: These are sample questions for you to practice on before the
unit test. Format / mark value is different on actual test. Answers
appear at the end of this document.
Multiple Choice
Identify the letter of the choice that best completes the statement or answers the question.
____
____
____
____
____
1. An endothermic reaction is one where
a. heat is transferred from the surroundings into a system
b. heat is transferred from a system into the surroundings
c. kinetic energy is transformed into potential energy
d. there is no transfer of heat
e. none of the above
2. In an exothermic reaction, heat flows into the surroundings because
a. the potential energy of the chemicals increases
b. the kinetic energy of the chemicals decreases
c. the chemical potential energy is converted into heat energy
d. the chemical kinetic energy is converted into heat energy
e. none of the above
3. A chemical system in which neither energy nor matter can flow into or out of a system is described as
a. a closed system
d. a chemical system
b. an open system
e. none of the above
c. an isolated system
4. Which of the following is not a form of kinetic energy in a chemical system?
a. moving electrons within an atom
b. the vibration of atoms connected by chemical bonds
c. the rotation of molecules that make up the chemical system
d. the translation of molecules within the chemical system
e. the energy of protons and neutrons in atomic nuclei
5. Given the following thermochemical data:
1. C2H2(g) + 5/2 O2(g)  2CO2(g) + H2O(l) H = –1.30  103 kJ
2. C2H6(g) + 7/2 O2 (g)  2CO2(g) + 3H2O(l) H = –1.56  103 kJ
3. H2(g) + 1/2 O2(g)  H2O (l) H = –2.86  102 kJ
What is H for the following reaction?
C2H2(g) + 2H2(g)  C2H6(g)
____
____
a. –2.60  101 kJ
d. –5.46  103 kJ
2
b. –3.12  10 kJ
e. 2.60  101 kJ
2
c. –5.72  10 kJ
6. When solid ammonium chloride, NH4Cl, is added to water, the temperature of the solution decreases. Which
statement best describes this observation?
a. The reaction is exothermic
b. Heat is released from the system, so it feels cooler.
c. NH4Cl(s) + H2O(l)  NH4Cl(aq) + 33.6 kJ
d. NH4Cl(s) + H2O(l)  NH4Cl(aq) H = +33.6 kJ
e. NH4Cl(s) does not dissolve in water.
7. Using the standard heats of formation, what is the enthalpy change per mole of CO2 for the following
reaction?
2
CO(g) + /2 O2(g)  CO2(g)
1
____
8.
____
9.
____ 10.
____ 11.
____ 12.
____ 13.
a. –506 kJ
d. +506 kJ
b. –284 kJ
e. –617 kJ
c. +284 kJ
Based on the heat of formation of MgO, how much energy is required to decompose 48.6 g of MgO?
a. 603 kJ
d. 60.3 kJ
b. 725 kJ
e. 1504 kJ
c. 301 kJ
Consider the following exothermic reactions:
I. 2H(g)  H2(g) + heat
II. H2(g)  H2(s) + heat
III. H2(l)  H2(s) + heat
IV. 2H + 3H  4He + 1n + heat
What is the order of the reactions in magnitude of their expected H's?
a. II < III < IV < I
d. IV < II < I < III
b. IV < I < II < III
e. I < III < II < IV
c. III < II < I < IV
A small 2.0-g candle is consumed in heating 100 g of water from 25ºC to 75ºC. If 200 g of water were heated
by the same candle, the temperature of the water would change from 25ºC would be about
a. 25ºC
d. 100ºC
b. 50ºC
e. 150ºC
c. 75ºC
A small 2.0-g candle is consumed in heating 100 g of water from 25ºC to 75ºC. If you ignore loss of heat to
the air, the energy produced per gram of the candle would be
a. 8.56 kJ
d. 2.62 kJ
b. 5.23 kJ
e. 20.65 kJ
c. 10.46 kJ
What is the range of the enthalpy change for a nuclear reaction?
a.
d.
b.
e.
c.
What is the range of the enthalpy change for a chemical reaction?
a.
d.
b.
e.
c.
____ 14. The following property can be measured to determine the rate of the reaction
a. change in mass
b. change in colour
c. change in volume
d. change in pressure
e. all of the above depending on the reaction
____ 15. Within a reaction with 2 reactants and 2 products, the reaction rate can be measured with respect to
a. all 4 substances
d. only 1 of the substances
b. 3 of the substances
e. not enough information to determine
c. 2 of the substances
____ 16. A catalyst is only effective if it
a. is present in large quantities
b. is added before the reactants come in contact with each other
c. is specific to the particular reaction
d. is heated before it is added to the reaction
e. all of the above
3
____ 17. If for the reaction
, the rate law is determined to be
a. the order of the reaction is 0
b. increasing the concentration of Y will have no effect on the rate
c. increasing the concentration of X will have no effect on the rate
d. increasing the concentration of Y will increase the rate of the reaction
e. there is no way to determine the value of k
, then
____ 18. If for the reaction
, the rate law is determined to be
a. doubling the concentration of Y will double the rate of the reaction
b. halving the concentration of Y will double the rate of the reaction
c. doubling the concentration of X will double the rate of the reaction
d. halving the concentration of X will double the rate of the reaction
e. only changes to the concentration of Y will affect the rate of the reaction
____ 19. The following graph represents a reaction of order
, then
a. 0
d. 3
b. 1
e. not enough information
c. 2
____ 20. Ineffective collisions are collisions that involve particles
a. without enough energy to react
b. with the wrong orientation
c. that rebound from the collision unchanged
d. that cannot react
e. all of the above
____ 21. The amount of energy required for a reaction to begin is known as
a. enthalpy change
d. kinetic energy
b. reaction energy
e. potential energy
c. activation energy
____ 22. Consider the above reaction mechanism. The rate-law equation from this reaction would be
a.
d.
b.
e. impossible to tell from this information
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c.
____ 23. The theoretical effect of an increase in temperature can be explained in terms of collision theory because it
affects
I. the collision geometry involved in the reaction
II. the total number of collisions that occur
III. the fraction of collisions that are effective
IV. the required activation energy for a reaction
a. both I and IV
d. both III and IV
b. I, II and III are true
e. II only
c. both II and III
____ 24. The theoretical effect of an increase in the initial concentration of a reactant can be explained in terms of
collision theory because it affects
I. the collision geometry involved in the reaction
II. the total number of collisions that occur
III. the fraction of collisions that are effective
IV. the required activation energy for a reaction
a. both I and IV
d. both III and IV
b. I, II and III are true
e. II only
c. both II and III
____ 25. The theoretical effect of a catalyst can be explained in terms of collision theory because it affects
I. the collision geometry involved in the reaction
II. the total number of collisions that occur
III. the fraction of collisions that are effective
IV. the required activation energy for a reaction
a. both I and IV
d. both III and IV
b. I, II and III are true
e. II only
c. both II and III
Short Answer
26. Explain the transfer of energy from an exothermic reaction to the surroundings of the system.
27. What are two reasons that energy changes in chemical systems are measured at standard conditions of
temperature and pressure such as SATP?
28. Describe the process by which a medical cold pack works.
29. Dry ice is solid carbon dioxide. It does not melt, but instead it sublimes directly to the gas state.
a. Is this change endothermic or exothermic?
b. What sign would the
have?
c. Explain the transfer of energy involved in this change.
30. Describe what must happen in a reaction in order to use the following instrument to determine the rate of the
reaction.
a. downward displacement of water.
b. conductivity apparatus
c. spectrophotometer
31. Draw a typical rate of reaction graph (Concentration of Reactant vs. Time) and indicate how to determine an
instantaneous rate of reaction.
32. Explain the purpose of a catalytic converter on an automobile.
33. What is the rate law?
34. What is the deference between an order of reaction and the overall order of a reaction?
35. Define the term half-life. Give an example.
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36. Differentiate between an effective collision and an ineffective collision.
37. Within the Rate Law Equation the effect of concentration on the rate is easily noted. However, we know that
temperature, chemical nature of reactants, and catalysts also have an effect on the rate. How is this accounted
for in the rate law equation?
Problem
38. The combustion of methanol is shown by the following equation:
i. Given the data which follows:
a. Find the heat of reaction for the equation above.
b. State the molar heat of combustion of methanol.
c. State whether the reaction is endothermic or exothermic.
ii. What mass of water could be heated from 20.00 C to 35.00 C by the burning of 2.57 mol of methanol?
(
)
39. What is the Ho in kilojoules for the combustion of 1 mol of ethanol, C2H5OH(l), to form gaseous carbon
dioxide and gaseous water?
40. Consider the decomposition of hydrogen peroxide reaction. If the rate of decomposition of hydrogen peroxide
at a particular temperature is determined to be 0.098 mol/(L·s), what are the rates of reaction with respect to
the products of the decomposition reaction?
41. Use the following data to calculate the reaction rate law for the system
Experiment
NO (mol/L)
H2 (mol/L)
Initial Rate of Reaction (mol/(L·s))
1
0.001
0.004
0.002
2
0.002
0.004
0.008
3
0.003
0.004
0.018
4
0.004
0.001
0.008
5
0.004
0.002
0.016
6
0.004
0.003
0.024
42. The radioisotope carbon-14 used in dating archeological artifacts has a half-life of 5730 a. What is the rate
constant for this isotope?
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Chem4U-Ch.5,6-Practice Test
Answer Section
MULTIPLE CHOICE
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
A
C
C
E
B
D
B
B
C
A
C
C
B
E
E
C
B
C
E
E
C
C
C
E
D
SHORT ANSWER
26. ANS:
- some of the chemical potential energy in the system is converted initially to increase the kinetic energy of
the products
- through collisions, this kinetic energy is transferred to the particles in the surroundings, which results in a
raise in temperature (i.e., average kinetic energy of the molecules) of the surroundings
27. ANS:
1. In order to control variables and to allow comparisons between different reactions.
2. Under these conditions, the enthalpy change of a chemical system is the change in the chemical potential
energy of the system because the kinetic energy of the system's molecules stay constant.
28. ANS:
- a medical cold pack operates on the principle that certain salts dissolve endothermically in water
- as the salt dissolves, it absorbs energy from the water when they dissolve in water
- as a result, the cold pack becomes cold
29. ANS:
a. endothermic
b. positive
c. the energy required for the reaction would be taken from the air surrounding the dry ice
as particles from the air collide with the dry ice, energy would be passed to the ice causing it to sublime
7
30. ANS:
a. a gas must be produced in the reaction
b. ions must be produced or used up during the reaction
c. colour would have to change - either production of a colour or removal of a colour
31. ANS:
32.
33.
34.
35.
36.
37.
On this graph, the instantaneous rate of reaction is the slope of the curve (or the tangent to the curve) at that
point.
ANS:
- the catalytic converter on an automobile uses finely divided platinum and palladium to speed the
combustion of the exhaust gases so that a higher proportion of the exhaust products will be relatively
harmless, completely oxidized substances
- for example, toxic carbon monoxide is oxidized to carbon dioxide before being emitted to the atmosphere
ANS:
The rate, r, will always be proportional to the product of the initial concentrations of the reactants, where
these concentrations are raised to some exponential values. This can be expressed as
.
ANS:
- these may be the same. (i.e., if there is only one reactant or one reactant that affects the rate of the
reaction)
- in a reaction where two or more reactants affects the rate of the reaction, the order of the reaction refers to
each specific reactant and the overall order of the reaction is the sum of the individual orders of reaction
for each reactant
ANS:
The half-life is the time required for one half of the sample to react (e.g., if you start with 100 g of a reactant
with a half-life of 5 minutes; then after 5 minutes, there would be 50g of a reactant left).
ANS:
- in an effective collision, the particles collide with sufficient energy and correct orientation for a reaction
to occur, the formation of products occurs from this collision
- in an ineffective collision, the particles collide without sufficient energy and/or correct orientation for the
reaction to occur, the reactants are unchanged by this collision
ANS:
- this is accounted for in the rate constant, k
-
k can be calculated by the Arrhenius Equation,
where
is the activation energy in J
A is a constant related to the geometry of the molecules
R is the gas constant
8
T is the temperature (in K)
PROBLEM
38. ANS:
i. a.
=[(2  –393 kJ) + (4  –242 kJ)] – [2  –638 kJ]
= –478 kJ
The heat of reaction for the equation is –478 kJ
b. Since this is for 2 moles of methanol,
= –478 kJ /2 mol
= –239 kJ/mol
The molar heat of combustion of methanol is –239 kJ/mol
c. Since the value is negative, the reaction is exothermic
ii.
since q = –
q = 614 kJ
= 150 C
c= 4.184 kJ/kg0 C
9.78 kg of water could be heated with 2.57 mol of methanol.
39. ANS:
First write and balance the combustion equation.
C2H5OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(g)
Hess's law for this equation is:
Ho = [(2)CO2(g) + (3)H2O(g)] – [C2H5OH(l) + (3)O2(g)]
= [ (2)–393.5 + (3)–241.8 kJ/mol] – [ –235.2 + (3)0 kJ/mol]
= [–787 –725.4 kJ/mol ] – [ –235.2 kJ/mol ]
= –1512.4 + 235.2 kJ/mol
= –1277.2 kJ/mol
9
The reaction for the combustion of ethanol is exothermic by 1277.2 kJ/mol.
40. ANS:
Write a balanced equation for the reaction
The rate of production of water is 0.098 mol/(L·s) and the production of oxygen gas is 0.049 mol/(L·s).
41. ANS:
From the equation we can write the partial rate law as
Using experiments 1 and 2, the concentration jumps from 0.001 to 0.002 moles/L. IT DOUBLES!! The rates
for these same experiments. The rate jumps from 0.002 to 0.008 moles/L seconds. IT QUADRUPLED!!.
The exponential constant 'm' for the [NO] is the mathematical relationship between these two values. i.e.
2m = 4 therefore m = 2 because 22 = 4
Look at experiments 4 and 5. The H2 concentration DOUBLES and the rate DOUBLES.
2n = 2 therefore n = 1 since 21 = 2
So the rate law expression can be rewritten as
rate = k [NO]2 [H2]1
Now to determine the value of 'k'. Choose any one of the experiments.
Using experiment 1. Using the rate law, above fill in the values from the data table.
0.002 mol/L sec = k (0.001 mol/L)2 * (0.004 mol/L)
0.002 mol/L sec = k * (0.000001 mol2 /L2) * (0.004 mol/L)
0.002 mol/L sec = k * 0.000 000 009 mol3/L3
k = 0.002 mol/L sec
0.000 000 004 mol3/L3
= 500,000 sec/mol2 L2 or sec mol-2 L-2
Therefore the rate law equation for this reaction is
rate = 500,000 sec mol-2 L-2 [NO mole/L]2 [H2 mol/L]
42. ANS:
First write the rate equation:
10
Then apply the equation that relates half-life and rate constant
The rate constant for Carbon–12 is 1.21 x 10-4 a-1