SAT Subject Test Chemistry

PART 2

REVIEW OF MAJOR TOPICS

CHAPTER 8

Chemical Reactions and Thermochemistry

Practice Exercises

1. A synthesis reaction will occur spontaneously after the activation energy is provided if the heat of formation of the product is

(A) large and negative

(B) small and negative

(C) large and positive

(D) small and positive

2. The reaction of aluminum with dilute H2SOcan be classified as

(A) synthesis

(B) decomposition

(C) single replacement

(D) double replacement

3. For a metal atom to replace another kind of metallic ion in a solution, the metal atom must be

(A) a good oxidizing agent

(B) higher in the activity series than the metal in solution

(C) lower in the electromotive chart than the metal in solution

(D) equal in activity to the metal in solution

4. One reason for a double displacement reaction to go to completion is that

(A) a product is soluble

(B) a product is given off as a gas

(C) the products can react with each other

(D) the products are miscible

5. Hydrolysis will give an acid reaction when which of these is placed in solution with water?

(A) Na2SO4

(B) K2SO4

(C) NaNO3

(D) Cu(NO3)2

6. A salt derived from a strong base and a weak acid will undergo hydrolysis and give a solution that will be

(A) basic

(B) acid

(C) neutral

(D) volatile

7. Enthalpy is an expression for the

(A) heat content

(B) energy state

(C) reaction rate

(D) activation energy

8. The ΔH0f of a reaction is recorded for

(A) 273 K

(B) 298 K

(C) 373 K

(D) 473 K

9. The property of being able to add enthalpies is based on the

(A) Law of Conservation of Heat

(B) First Law of Thermodynamics

(C) Law of Constants

(D) Einstein equation, E = mc 2

10. A ΔHreaction of –100 kJ/mole indicates the reaction is

(A) endothermic

(B) unstable

(C) in need of a catalyst

(D) exothermic

Directions:

Before attempting to answer the following questions (11–15), you may want to review the directions for this type of question.

Every question below contains two statements, I in the lefthand column and II in the right-hand column. For each question, decide if statement I is true or false and whether statement II is true or false, and fill in the corresponding T or F ovals in the answer spaces. *Fill in oval CE only if statement II is a correct explanation of statement I.

I

 

II

11.   If the heat of formation of a compound is a large number preceded by a minus sign, the reaction is exothermic

BECAUSE

the First Law of Thermodynamics states that a negative heat of formation is associated with an exothermic reaction.

12.   The burning of carbon with excess Oto form COwill go to completion

BECAUSE

when a reaction results in the release of a gas that is allowed to escape, the reaction will go to completion.

I

 

II

13.   The heat of formation of a compound can be calculated by algebraically adding two or more thermal reaction equations

BECAUSE

Hess’s Law states that a heat of reaction can be arrived at by the algebraic summation of two or more other thermal reactions.

14.   Entropy can be described as the state of disorder of a system

BECAUSE

when high amounts of energy are released from a reaction, the reaction is said to be exothermic.

15.   The reaction in which HgO is heated to release Ois called a decomposition

BECAUSE

in a decomposition reaction, the original compound is broken apart into equal numbers of atoms.

Answers and Explanations

1. (A) A large negative heat of formation indicates that the reaction will give off a large amount of energy and will self-perpetuate after getting the activation energy necessary for it to start, like burning paper.

2. (C) Because the aluminum replaces the hydrogen to form aluminum sulfate, this is classified as a single-replacement reaction.

3. (B) For a metal to replace another metallic ion in a solution, it must be higher in the activity series of elements than the metal in solution.

4. (B) For a double displacement to go to completion, a product or products must either deposit as a precipitate or leave the reaction as a gas.

5. (D) The only salt that hydrolyzes to a strong acid and a weak base is Cu(NO3)2. This would give an acid solution. All the others are salts of strong acids and strong bases, so they would hydrolyze to a relatively neutral solution.

6. (A) Salts of strong bases and weak acids react with water to give a basic solution. An example would be Na2CO3, because it hydrolyzes into NaOH, a strong base, and H2CO3, a weak acid.

7. (A) Enthalpy is defined as the heat content of a system.

8. (B) The ΔH with the superscript zero indicates that the process was carried out under standard conditions, which are 298 K and 1 atmosphere pressure.

9. (B) The property of being able to add enthalpies comes from Hess’s Law of Heat Summation and is based on the First Law of Thermodynamics, which simply says that the total energy of the universe is constant and cannot be created or destroyed.

10. (D) Because the heat of the reaction is negative, it indicates that energy (heat) is given off when this reaction occurs. It is therefore an exothermic reaction.

11. (T, F) While it is true that a negative heat of formation is associated with an exothermic reaction, the First Law of Thermodynamics states that the total energy of the universe is constant and cannot be easily created nor destroyed. It is a convention of chemistry that energy given off from a combination reaction is given a negative value.

12. (T, T, CE) When a combination reaction results in a gas that is allowed to be released, a precipitate that drops to the bottom of the container, or a nonionizing product, the reaction will go to completion.

13. (T, T, CE) Both statements are true and Hess’s Law is the basis for arriving at a heat of reaction by algebraically adding two or more thermal equations.

14. ( T, T) Both statements are true, but the second does not explain the first, and there is no cause and effect relationship. Entropy is a measure of a system’s randomness or disorder. Enthalpy is related to the change of heat content in a reaction. Negative enthalpies indicate exothermic reactions.

15. (T, F) In the decomposition of HgO, Hg and Oare the products. The equation is 2HgO → 2Hg + O2(g)

It is not always true that the number of atoms of each product is equal. It happens in this reaction because there is an equal number of atoms of Hg and O in the formula. In decomposing water you have: 2H2O → 2H2(g) + O2(g) because there are twice as many atoms of hydrogen than oxygen in the original formula.