Lesson: Chapter - 9

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 = mC_pDT. 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.

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