Science question bank 2011/2012/ Term 3
Formative assessment questions
Grade
Chapter
12
15 Energy and
chemical change
Subject
Sections
Chemistry
15.2,15.3, 15.4
objectives
15.2.1
Define thermochemistry
15.2.2
Describe how the system in thermodynamics is related
to the suuroundings and the universe Universe= system
+ surroundings
15.2.3
Define the enthalpy(H)
15.2.4
Define the enthalpy( heat ) of reaction ∆Hrxn
15.2.5
Calculate the ∆Hrxn using the following formula
∆Hrxn = Hfinal  Hinitial or
15.2.6
∆Hrxn = Hproducts  Hreactants
Explain why the sign of ∆Hrxn for an exothermic
reaction is always negative
15.2.7
Explain why the sign of ∆Hrxn for an endothermic
reaction is always positive
15.3.1
Describe and write a thermochemical equation
15.3.2
Compare between the molar heat of fusion with the
molar heat of vaporization
15.3.3
Classify condensation, solidification, vaporization, and
fusion as exothermic or endothermic processes
15.3.4
Calculate the heat absorbed or released in a chemical
reaction
15.4.1
State Hess’s law
15.4.2
Calculate the enthalpy( heat change) ∆H for a reaction
using Hess’s law
Page 1 of 9
1. How much heat is evolved from 54.0 g glucose (C6H12O6), according to the
equation for calculating heat? (Obj. 15.3.4)
A.
0.842 kJ
B.
8.42 kJ
C.
84.2 kJ
D.
842 kJ
2. The heat content of a system at constant pressure is defined as the.
(Obj. 15.2.3)
A.
Enthalpy
B.
Entropy
C.
Work
D.
Heat
3. How much heat is absorbed in the complete reaction of 3.00 grams of SiO2
with excess carbon in the reaction below? ΔH° for the reaction is +624.7 kJ.
SiO2(g) + 3C(s) → SiC(s) + 2CO(g) (Obj. 15.3.4)
A.
366 kJ
B.
1.13 x 105 kJ
C.
5.06 kJ
D.
31.2 kJ
4. In the cold pack process, 27 kJ are absorbed from the environment per mole
of ammonium nitrate consumed. If 50 g of ammonium nitrate are consumed,
what is the total heat absorbed? (Obj. 15.3.4)
27 kJ + NH4NO3(s) → NH4+(aq) + NO3-(aq)
A.
27 kJ
B.
17 kJ
Page 2 of 9
C.
0.34 kJ
D.
43 kJ
5. In thermochemistry, the specific part of the universe you are studying is
called (Obj. 15.2.2)
A.
System
B.
Area
C.
Enthalpy
D.
Surroundings
6. What is the universe when using a bomb-calorimeter to measure heat
absorbed by a substance in a process? (Obj. 15.2.2)
A.
the substance in the calorimeter
B.
the calorimeter itself
C.
the water in the calorimeter
D.
the calorimeter and all its contents,
7. In which example is the ΔH° positive? (Obj. 15.2.7)
A.
an ice cube melting
B.
condensation forming on cold glass of water
C.
molten lava cooling to form solid rock
D.
water vapor changing directly into frost on windows
8. In which example is the ΔH° negative? (Obj. 15.2.6)
A.
an ice cube melting
B.
condensation forming on cold glass of water
C.
molten lava cooling to form solid rock
D.
water vapor changing directly into frost on windows
Page 3 of 9
9. The amount of energy required to melt one mole of a solid is called ____.
(Obj. 15.3.2)
A.
molar enthalpy of vaporization
B.
molar enthalpy of melting
C.
molar enthalpy of fusion
D.
molar enthalpy of condensation
10. A thermochemical equation specifies about changes in ____. (Obj. 15.3.1)
A.
Temperature
B.
pressure
C.
Enthalpy
D.
molar mass
11. The standard enthalpy of an element in its standard state is ____.
(Obj. 15.3.4)
A.
0.00 calories
B.
0.00°C
C.
0.00 kilocalories
D.
0.00 kJ
12. What is the enthalpy of oxygen in its standard state?(Obj. 15.3.4)
A.
0.00 kJ
B.
15.99 kJ
C.
100.0 kJ
D.
8.00 kJ
13. Two or more thermochemical reactions can be summed to determine the
overall enthalpy changes based on what law? (Obj. 15.4.1)
Page 4 of 9
A.
Boyle’s law
B.
Hess’s law
C.
Gay-Lussac’s law
D.
law of conservation of energy
14. The ____________________ is defined as the system along with the
surroundings. (Obj. 15.2.2)
A.
System
B.
Surrounding
C.
universe
D.
Unisystem
15. The molar enthalpies of condensation and ______________have the same
numerical value. (Obj. 15.3.2)
A.
Vaporization
B.
Freezing
C.
Melting
D.
Sublimation
16. When the change in enthalpy is positive the reaction is _______________.
When the change in enthalpy is negative the reaction is __________.
(Obj. 15.2.6 & 15.2.7)
A.
Small, large
B.
Large, small
C.
exothermic, endothermic,
D.
endothermic, exothermic
Page 5 of 9
17. _________is the study of heat changes that accompany chemical reactions
and phase changes. (Obj. 15.2.1)
A. Physical chemistry
B. Thermochemistry
C. analytical chemistry
D. quantum chemistry
18. The heat required to___________ one mole of a liquid is called its molar
enthalpy of _______(Obj. 15.3.2)
A. vaporize, condensation
B. vaporize, vaporization.
C. condense, freezing
D. freeze, melting
19. The heat required to melt one mole of solid substance is called
its______________.(Obj. 15.3.2)
A. molar enthalpy of fusion
B. molar enthalpy of melting
C. molar enthalpy of freezing
D. molar enthalpy of liquefaction
20. _________is the reaction of a fuel with oxygen. (Obj. 15.3.4)
A. coloration
B. condensation
C. composition
D. Combustion
21. Which equation is a thermochemical equation? (Obj. 15.3.1)
A. 4Fe(s) + 3O2 (g)
B. Fe(s) + 3O2 (g)
2Fe2O3(s)
Fe2O3(s)
Page 6 of 9
ΔH = – 1625 kJ
ΔH = – 1625 kJ
C. 4Fe+ 3O2
2Fe2O3
D. 4Fe(s) + 3O2 (g)
ΔH = – 1625 kJ
2Fe2O3(s)
II Answer the following questions:
1. Explain why ∆H for an exothermic reaction always has a negative value?
(Obj. 15.2.6)
∆Hrxn = Hproducts - Hreactants and H products < H reactants
2. Describe what the system means in thermodynamics, and explain how the
system is related to the surroundings and the universe. (Obj. 15.2.2)
The system contains the process being studied. The surroundings are everything except the system, and the universe is the system and its surroundings.
3.Write a complete thermochemical equation for the combustion of ethanol
(C2H5OH). ∆H comb = -1367 kJ/mol (Obj. 15.3.1)
C2H5OH(l) + 3O2(g) → 2CO2(g)+3H2O(l) ∆Hcomb= -1367
4. Determine Which of the following processes are exothermic? endothermic?
(Obj. 15.3.3)
a.C2H5OH(l) → C2H5OH(g) endothermic
b.Br2(l) → Br2(s) exothermic
c. C5H12(g) + 8O2(g) → 5CO2(g)+6H2O(l) exothermic
d. NH3(g) → NH3(l) exothermic
e.NaCl(s) → NaCl(l) endothermic
5. The reaction A → C is shown in the enthalpy diagram below. Is the reaction
exothermic or endothermic? Explain your answer. (Obj. 15.2.6)
The reaction is exothermic because the product (C) has a lower energy than
the reactant (A).
6. Explain what is meant by Hess’s law and how it is used to determine
∆H°.(Obj. 15.4.1)
Hess’s law says that if two or more equations add up to an overall equation,
the ∆H°rxn of the overall equation is the sum of the ∆H° values of the
equations that were combined
7. After studying the solution in the flask below answer the following questions:
(Obj. 15.2.2)
Page 7 of 9
a. What is the system?
The system is the solution of Ba(OH)2 and NH4NO3.
b. What are the surroundings?
The surroundings include everything except the solution.
c. What is the universe?
The universe is the solution plus the surroundings.
8. Use Hess’s Law to calculate the energy change for the reaction that produces
SO3. (Obj. 15.4.2)
2S(s) + 3O2(g) → 2SO3(g) ΔH = -792
9. Use Hess’s Law to calculate the energy change for the decomposition of
hydrogen peroxide (H2O2). (Obj. 15.4.2)
2H2O2 (l) → 2H2O (l) + O2 (g)
ΔH = -196 kJ
10. Calculate the enthalpy of a reaction, where the enthalpy of the reactant is
209kJ and the enthalpy of the product is 121.4 kJ. Also, identify the type of the
reaction with respect to its enthalpy. (Obj. 15.2.5)
11. Calculate the enthalpy of the reaction N + 3H
2NH , where the
enthalpy of N is 126.2 kJ, the enthalpy of H is 87.1 kJ, and the enthalpy of
NH3 is 125.8 kJ. (Obj. 15.2.5)
Page 8 of 9
12. Calculate the heat required to melt 64.0 g of solid methanol, given that the
molar enthalpy of fusion is 3.22 kJ/mol and the molar mass of methanol is 32.0
g. (Obj. 15.3.4)
6.44 kJ
13. Calculate the amount of heat evolved when 0.33 mol of sucrose is burnt.
The enthalpy of combustion is –5644 kJ/mol.(Obj. 15.3.4)
1.9 x103 kJ
14. If molar enthalpy of vaporization of ethanol is 38.6 kJ/mol, how many
moles of ethanol are vaporized, when the required heat is 200.72 kJ? (Obj. 15.3.4)
5.20 mol
15. Calculate the enthalpy change with the help of Hess’s law for the
decomposition of hydrogen peroxide, (2H O (l) = 2H O(l) + O (g)), if the
enthalpy of formation of water (2H (g) +O (g)) is –512KJ and vaporization of
hydrogen peroxide (H O (l) = H (g) + O (g)) is 376 kJ. (Obj. 15.4.2)
–136 kJ
16. Calculate the enthalpy change for the production of sulfur
trioxide(2S(s)+2O (g) = 2SO (g)), the main constituent of acid rain. Given the
enthalpy of the formation of sulfur dioxide(2S(g) + 2O (g) = 2SO (g)) is –581
kJ, and the vaporization of sulfur trioxide(2SO3(g) = 2SO (g)+O (g)) is 192 kJ.
(Obj. 15.4.2)
–389 kJ
17. Calculate the enthalpy change of the reaction (Obj. 15.4.2)
2CO(g) + 2NO(g)
2CO (g) +N (g) using thermochemical equations:
a) 2CO(g) + O (g)
b) 2NO(g)
2CO (g)
N (g) + O (g)
The enthalpy of the equation a) is –566.0 kJ and the enthalpy of the equation
b) is –180.6 kJ.
–746.6 kJ
18. Calculate the standard enthalpy change of a reaction using the total
enthalpies of the reactants and the products. The total enthalpy of the
reactants is –912 kJ and the total enthalpy of the products is –82 kJ. (Obj. 15.2.5)
830 kJ
19. Calculate the enthalpy change of the reaction (Obj. 15.4.2)
CH4 (g) + 2Cl2(g)
CCl4 (l) + 2H2 (g), using thermochemical equations:
a) C(s) + 2Cl2 (g)
CCl4 (l)
= –128 kJ
b) CH4 (g)
C(s) + 2H2 (g)
= + 75 kJ
–53 kJ
Page 9 of 9