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Questions 1 - 10
1

Which of the following will increase the pH of an buffer solution?

I. Removing carbonic acid

II. Adding sodium bicarbonate

Both I and II

I only

II only

Neither of these options

Explanation

To answer this question we need to look at the reaction below:

An increase in the pH will result in a decrease in the concentration of hydrogen ions (). Using Le Chatelier’s principle we can find out which answer choices will decrease .

Removing carbonic acid will decrease the concentration of . To maintain equilibrium, the reaction will shift to the left and make more reactants from products; therefore, there will be a decrease in the and an increase in pH.

Recall that salts like sodium bicarbonate, or , will dissociate in water and form ions. Sodium bicarbonate will form sodium () and bicarbonate () ions. This side reaction will result in an increase in the bicarbonate ion concentration. Le Chatelier’s principle will shift the equilibrium of the given reaction to the left and, therefore, decrease the . Adding sodium bicarbonate will increase the pH.

2

Which of the following statements best describes ionic compounds?

3-D arrays of charged particles

Formed when molecules share electrons

Neutral particles that donate electrons

Malleable compounds that lack structural stability

Explanation

The definition of ionic compounds are three-dimensional arrays of atoms held together by strong ionic bonds. Ions are charged particles that have either gained or lost a certain number of electrons. They have great crystalline strength because of the strong electrostatic forces between the ions.

3

Which of the following is a classic example of a first-order reaction?

Radioactive decay

A collision between 2 reactant molecules

A change in temperature

None of the other answers

Explanation

First order reactions have rates that are directly proportional to only 1 reactant. In radioactive decay, the rate of decrease of a radioactive material is proportional only to the amount of the material.

4

Which of the following is a law of thermodynamics?

ΔH (system) + ΔH (surroundings) = ΔH (universe)

ΔS (system) + ΔS (surroundings) = ΔS (universe)

ΔE (surroundings) = ΔE (system)

The entropy of the universe is always decreasing

Explanation

The second law of thermodynamics states that the entropy of the system and the entropy of the surroundings is equal to the entropy of the universe. The rest of the answer choices are not one of the fundamental laws of themodynamics.

5

Consider the typical phase diagram of a compound given below.

Phase diagram

Which of the following lines or points on the diagram represents a situation in which the rate of vaporization of the compound is equal to its rate of condensation?

Line C

Line A

Line B

Point D

None of these

Explanation

In this question, we're presented with a phase diagram and are asked to determine where on the graph the rate of vaporization equals the rate of condensation.

First, it's important to realize that when the rate of vaporization and condensation are equal, we have an equilibrium of liquid and gas phases. In other words, for a given temperature and pressure, the rate at which the liquid evaporates into a gas is exactly equal to the rate at which the gas condenses into a liquid.

On a phase diagram, the area of the upper left portion of the diagram represents the solid state. The middle portion of the diagram represents the liquid state. The bottom and right most part of the diagram represents the gas phase.

Furthermore, each line on the diagram represents the specific combination of temperature and pressure in which a given compound will exist in equilibrium between two phases. The point where all three lines intersect, however, represents the triple point. This tells us the temperature and pressure in which the compound will exist in an equilibrium between all three states.

Because we are looking for the equilibrium line that represents equilibrium of vaporization and condensation, we want the line that separates the liquid portion of the diagram from the gas portion. Based on the identification of regions on the diagram discussed above, that would be line C as shown in the diagram. Line A represents equilibrium between solid and gas (sublimation rate = deposition rate). Line B represents equilibrium between solid and liquid (melting rate = freezing rate).

6

Given the following equation (2A+B --> 3C). Which of the following correctly displays the rate of the reaction?

I. -Δ\[A\]/2Δt

II. Δ\[B\]/Δt

III. Δ\[C\]/3Δt

I only

II only

I and III only

II and III only

I, II, and III

Explanation

The rate based on concentration is related to the coefficients in front of the compounds. Based on the reactants the rate should be negative (because change in concentration for the forward reaction will be negative) and based on the products should be positive. This means that II is incorrect. The rate for each compound in the reaction should be divided by it's coefficient to make it all related to 1M, showing that I and III are correct.

7

The following ReDox reaction takes place in acidic solution:

Fe2+ + Cr2O72– → Fe3+ + Cr3+

What is the sum of coefficients in this redox reaction?

36

35

34

33

37

Explanation

When you balance the redox reaction in acidic conditons, there are 6Fe2+, 1 Cr2O72–, 14 H+, 6 Fe3+, 2 Cr3+, and 7 H2O. Don't forget to add the 1 in front of the Cr2O72–

8

Calculate the standard cell potential of the following reaction:

Cd(s) + MnO2 (s) + 4 H+ (aq) + -> Cd2+ (aq) + Mn2+ (aq) + 2 H2O (l)

Given:

MnO2 (s) + 4 H+ (aq) + 2e- -> Mn2+ (aq) + 2 H2O (l) Eo = 1.23 V

Cd2+ (aq) + 2 e- -> Cd (s) Eo = -0.40 V

1.63 V

0.83 V

-1.63 V

-0.83 V

0.0 V

Explanation

Eocell = Eo cathode - Eoanode

Eocell = 1.23 – (-0.40) = 1.63 V

9

Consider the following reaction:

Cu_{(s)}+2Ag^{+}{(aq)}\rightarrow Cu^{2+}{(aq)}+2Ag_{(s)}

What is the oxidizing agent, and what is the reducing agent?

Silver is the oxidizing agent and copper is the reducing agent

Silver is the reducing agent and copper is the oxidizing agent

Silver is the oxidizing agent and there is no reducing agent

No redox chemistry occurs

Silver is the reducing agent and there is no oxidizing agent

Explanation

Let's break down the reaction into two separate reactions:

Cu_{(s)}\rightarrow Cu^{2+}_{(aq)}+2e^- and

2Ag^+{(aq)}+2e^- \rightarrow 2Ag{(s)}

We can see that copper loses electrons, while silver gains electrons. Recall that oxidation is loss and reduction is gain, with regard to electrons. Copper is oxidized and silver is reduced.

However, this question asks for the oxidizing agent and reducing agent. Recall that the oxidizing agent is reduced, while the reducing agent is oxidized. Since copper is oxidized, it is the reducing agent. Similarly, since silver is reduced, it is the oxidizing agent.

10

Which of the following is a law of thermodynamics?

ΔH (system) + ΔH (surroundings) = ΔH (universe)

ΔS (system) + ΔS (surroundings) = ΔS (universe)

ΔE (surroundings) = ΔE (system)

The entropy of the universe is always decreasing

Explanation

The second law of thermodynamics states that the entropy of the system and the entropy of the surroundings is equal to the entropy of the universe. The rest of the answer choices are not one of the fundamental laws of themodynamics.

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