Explanations
1. T, T
(Fill in CE.) In an exothermic reaction,
the DH value is negative—energy is given off in
the course of the reaction, and the products are at a lower energy
than the reactants. The first statement is true since you see that
the DH value is negative, and the second statement
is also true: the enthalpy value of the products is less than that
of the reactants in an exothermic reaction. The second statement
is the reason for the first, so fill in the CE oval.
2. C
This question asks you to do a little arithmetic. The
formula for finding enthalpy from bond energies is bonds broken
minus bonds formed (this translates into the energy of the reactants
broken minus the energy of the products formed). You need to draw
the structures so that you see what is going on:
Bonds broken: 4 C—H bonds; 1 C=C bond; 1 Cl—Cl bond
Bonds formed: 4 C—H bonds; 1 C—C bond; 2 C—Cl bonds
The C—H bonds cancel each other out, so you don’t need
to include them in your calculations.
So, DH = (612 + 243) - [347 + 2(341)]
= -174. Choice C is closer to this.
3. B
After a quick survey of the answer choices, it is evident
that choices A, C, and D are
all saying essentially the same thing—the reaction is exothermic.
Reread the question—the temperature is going down, so the reaction
is endothermic. Choice B is the correct answer.
4. C
To solve this problem, you must first convert the grams
of ethanol to moles. The mass of ethanol, C2H5OH,
is about 46 g/mol. Since the initial mass is 45.00 grams, you have
about 1 mole of ethanol. Multiply 1 mole by the DH value
given in the equation, and the answer is C.
5. D
If you remembered the formula for calculating energy
from specific heat, this problem isn’t hard at all. If not, that
formula is q = mCpDT.
The total mass in the solution would be 500 mL + 500 mL = 1000 mL
= 1000 g (since the density is 1.0 g/mL). The specific heat is 4.184
J/g ºC (round to 4). The change in temperature is 5.0ºC. Essentially,
you have 20
1000 = 20,000 J. The only answer that is
close to this value is D.
6. C
Remember the laws of thermodynamics—a decrease in entropy
means that a system becomes more organized. Before looking at the
answer choices, decide whether each statement represents a system
becoming more or less organized, then make your selections. Statement
I gets more organized—solids represent the most organized state.
II is becoming more disorganized—vapors are gases and are more disorganized
than liquids. III is also more disorganized—vapors are more disorganized
than solids. IV is more organized—when purifying a mixed-up solution
you are organizing the solution. Therefore, I and IV represent a
decrease in entropy, and the answer is C.
7. A
This question tests your knowledge of Gibb’s equation,
DG = DH - TDS.
Before approaching all of the choices and getting confused, decide
what should happen if all conditions are favored. In order for the
reaction to be spontaneous, its DG must be negative. You
know that exothermic reactions, which have negative enthalpy values,
are favored. You also know that entropy is favored and that the
more positive the value of DS, the higher the entropy.
You are looking for something that says little enthalpy, large entropy. Choice A states
just this.
8. E
Answering this question correctly just takes some close
reading. The question tells you that the reaction is exothermic
and spontaneous, which means that DH is negative
and DG is negative. A can be eliminated
immediately because a positive free energy value would mean a nonspontaneous
reaction. B and C could both be true—depending
on temperature—but the question asks which must be
true. Choice D gives a specific temperature. This fact
is insignificant without any other numerical values. So E must
be the answer—a negative DG value means a spontaneous
reaction.
9. C
To solve this problem, you must remember the formula
for calculating Gibb’s free energy using standard values provided.
The general equation is DG = S DG products
- S DG reactants. Making the math easy, this gives
you [150 + (-350)] - [2(-20)] = -160. There is only one answer choice
with this value—C: when DG is negative,
the reaction is spontaneous.
10. B
This reaction is an exothermic equilibrium reaction.
You are looking for the statement that represents an event that’s
least likely to occur. Option A should happen, according
to kinetic molecular theory: the more energy, the faster the particles’
movement. Choice C also seems true—when temperature
is increased for an exothermic reaction, the tendency is a shift
to the reactants. Statement D would definitely be true—the
more energy, the more random the particle movement. Statement E basically
restates choice C—there is a shift of equilibrium to
the reactants, so K decreases. Statement B is
the correct answer. With the reaction shifting to the reactants,
less product will be produced, not more.
