GRE Subject Test: Chemistry : GRE Subject Test: Chemistry

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #1 : Le Chatelier's Principle And Common Ion Effect

Consider the reaction reaction below.

\(\displaystyle N_2(g)+3H_2(g)\rightarrow2NH_3(g)\)

A student allows the system to reach equilibrium and then removes two moles of hydrogen gas. Which of the following will be a result?

Possible Answers:

The amount of N2 in the reaction vessel will increase

The reaction will shift to the side with fewer total moles of gas

More NH3 will be produced

No change will occur

The reaction will first shift toward the products, then toward the reactants

Correct answer:

The amount of N2 in the reaction vessel will increase

Explanation:

According to Le Chatelier's principle, when a system at equilibrium is disturbed, the system will react to restore equilibrium. In other words, it will seek to undo the stress. Here, if hydrogen gas is removed, the reaction will shift toward the reactants to re-form it. In the process, more nitrogen will be produced.

Example Question #1 : Reaction Equilibrium

A saturated solution of scandium hydroxide, which also contains solid scandium hydroxide, is treated with 0.1N HCl. The addition of acid will __________ the solubility of scandium hydroxide because of __________.

Possible Answers:

decrease . . . Le Chatelier's principle

increase . . . the decreased pH

not affect . . . the common ion effect

increase . . . Le Chatelier's principle

decrease . . . the common ion effect

Correct answer:

increase . . . Le Chatelier's principle

Explanation:

Scandium hydroxide dissociates according to this reaction.

\(\displaystyle Sc(OH)_{3}(s) \rightarrow Sc^{3+}(aq) +3(OH)^{-}(aq)\)

As 0.1N HCl is added, it will quantitatively react with hydroxide anions to produce ScCl3 and water.

\(\displaystyle H^++3Cl^-+Sc^{3+}+OH^-\rightarrow ScCl_3+H_2O\)

Hydroxide will be removed from the above equilibrium, and the system will compensate by shifting the equilibrium to the right, according Le Chatelier's principle. As the equilibrium shifts to the right, more solid scandium hydroxide is hydrolyzed, resulting in increased solubility.

Example Question #2 : Reaction Equilibrium

A system contains iodine atoms that are in equilibrium with respect to the reaction below:

\(\displaystyle 2I_{(g)} \rightleftharpoons I_2_{(g)}\)

The volume of the system is suddenly reduced, leading to an increase in pressure. What effect will this have on the reaction?

Possible Answers:

The reaction shifts to the left

The reaction shifts to the right then to the left

The reaction will not be affected

The reaction shifts to the right

Correct answer:

The reaction shifts to the right

Explanation:

Since the system is originally at equilibrium and a stress is applied, Le Chatelier's principle is to be considered. By increasing the total pressure, the reaction will move in the direction toward which there are less molecules of gas.

\(\displaystyle 2I_{(g)} \rightleftharpoons I_2_{(g)}\)

Looking at the original reaction there are two moles of gas on the left and only one on the right, indicating that the reaction will shift to the right if the pressure is increased.

Example Question #23 : Le Chatelier's Principle And Common Ion Effect

Consider the following saturated solution. Assume it is at equilibrium.  

\(\displaystyle PbCrO_{4}_{(s)}\ \leftrightharpoons\ Pb^{2+}_{(aq)}\ + CrO_{4}^{2-}_{(aq)}\)

\(\displaystyle K_{sp} = 1.8 \cdot 10^{-14}\)

What is the chromate ion concentration after 0.019 moles of lead nitrate is added to 1 L of the above solution?

Possible Answers:

\(\displaystyle 5.2\cdot10^{-12}M\)

\(\displaystyle 1.3 \cdot 10^{-7}M\)

\(\displaystyle 3.4 \cdot 10^{-16}M\)

\(\displaystyle 9.5 \cdot 10^{-13}M\)

Correct answer:

\(\displaystyle 9.5 \cdot 10^{-13}M\)

Explanation:

Us the solubility constant to calculate the answer to this question. We know that:

\(\displaystyle K_{sp} = [Pb^{2+}] \cdot [CrO_{4}^{2-}]\)

and

\(\displaystyle K_{sp} = 1.8 \cdot 10^{-14}\)

Since \(\displaystyle K_{sp}\) is a very small number, we can assume that the concentration of chromate, \(\displaystyle x\), will be very small compared to the concentration of the added lead, 0.019 M. We can use the two known values (\(\displaystyle K_{sp}\) and lead concentration) to solve for the unknown chromate concentration:

\(\displaystyle 1.8 \cdot 10^{-14} = 0.019 M\cdot x\)

\(\displaystyle x = \frac{1.8 \cdot 10^{-14}}{0.019} = 9.5 \cdot 10^{-13}M\)

Example Question #1 : Reaction Equilibrium

Which method can be used to increase the activation energy of a reaction done in the lab?

Possible Answers:

Increasing the temperature

Decreasing the temperature

Increasing the pressure

Shaking the reaction flask

Increasing the concentration of reactants

Correct answer:

Decreasing the temperature

Explanation:

The activation energy is the minimum amount of energy required to start a chemical reaction. Based on the collision model, increasing the temperature, pressure, and concentration for a chemical reaction will decrease the activation energy. This is because increasing these factors increases the number of collisions of molecules, which is required for a reaction to occur. Therefore, decreasing the temperature would have the opposite effect causing the activation energy to increase, decreasing the likelihood of a reaction taking place.

Example Question #2 : Reaction Equilibrium


Screen shot 2015 10 06 at 10.37.14 pm

Based on the equilibrium reaction shown, how would adding \(\displaystyle H_{2}\) to the system affect the equilibrium?

Possible Answers:

The effect of adding \(\displaystyle H_{2}\) to the system cannot be predicted

Adding \(\displaystyle H_{2}\) to the system would cause the equilibrium to shift to the right, producing more \(\displaystyle NH_{3}\) in order to reduce the concentration of \(\displaystyle H_{2}\)

Adding \(\displaystyle H_{2}\) would not have any effect on the equilibrium

Adding \(\displaystyle H_{2}\) to the system would cause the equilibrium to shift to the left, producing more \(\displaystyle N_{2}\) in order to reduce the concentration of \(\displaystyle H_{2}\)

Adding \(\displaystyle H_{2}\) to the system would cause the equilibrium to shift to the right, producing more \(\displaystyle N_{2}\) in order to reduce the concentration of \(\displaystyle NH_{3}\)

Correct answer:

Adding \(\displaystyle H_{2}\) to the system would cause the equilibrium to shift to the right, producing more \(\displaystyle NH_{3}\) in order to reduce the concentration of \(\displaystyle H_{2}\)

Explanation:

According to Le Chatelier's principle, if an equilibrium reaction is disturbed by addition of a reactant or product, the equilibrium will shift to counteract the disturbance. Therefore, adding \(\displaystyle H_{2}\) to the system would cause the equilibrium to shift to the right, producing more \(\displaystyle NH_{3}\) in order to reduce the concentration of \(\displaystyle H_{2}\).

Example Question #2 : Reaction Equilibrium

Which method can be used to decrease the activation energy of a reaction?

Possible Answers:

Increasing the pressure

All of these

Adding a catalyst

Increasing the temperature

Increasing the concentration of reactants

Correct answer:

All of these

Explanation:

The activation energy is the minimum amount of energy required to start a chemical reaction. Based on the collision model, increasing the temperature, pressure, and concentration for a chemical reaction will decrease the activation energy. This is because increasing these factors increases the number of collisions of molecules. Molecules must collide to react.

Example Question #11 : Solubility And Solution Equilibrium

Which of the following is true regarding the solubility product constant, \(\displaystyle \small K_{sp}\), for a reaction in the form:

\(\displaystyle aA_{(s)}\rightleftharpoons cC_{(aq)}+dD_{(aq)}\)

Possible Answers:

\(\displaystyle \small K_{sp}\) has units of moles per liter

\(\displaystyle K_{sp}=[C]^c[D]^d\)

\(\displaystyle \small K_{sp}\) represents the ratio of reactants to products as the reaction progresses

The concentration of substance \(\displaystyle \small A\) strongly influences the value of \(\displaystyle \small K_{sp}\)

As more of substance \(\displaystyle \small A\) dissolves in the given solution, \(\displaystyle \small K_{sp}\) increases

Correct answer:

\(\displaystyle K_{sp}=[C]^c[D]^d\)

Explanation:

To determine the solubility product constant, we only need the concentrations and coefficients of the ions. The effective concentration of any pure substance (solid, liquid, or gas) is equal to one by definition, so does not influence the value of \(\displaystyle \small K_{sp}\). The equation for the solubility product constant of this reaction is:

\(\displaystyle K_{sp}=[C]^c[D]^d\)

The units of the solubility product constant will depend on the coefficients of the products. \(\displaystyle \small K_{sp}\) will be a constant for the reaction, and will not change as more solid dissolves or as the reaction progresses.

Example Question #1 : Reactions By Product

Untitled

What is the IUPAC name of the given molecule?

Possible Answers:

None of these

3,3,5-trimethyloctane

4,6-dimethyl-6-ethylpentane

2,2,4-trimethyloctane

3,3,5-trimethylnonane

Correct answer:

3,3,5-trimethyloctane

Explanation:

The longest carbon chain that can be formed is eight carbons. The base molecule is octane.

Using IUPAC rules, substituents should have the lowest possible numbers; thus, we start counting carbons from the right side rather than the left. If you count from the correct side, there are two methyl groups on carbon 3 and one on carbon 5. Thus, the name of the moleculue is 3,3,5-trimethyloctane.

Example Question #1 : Hydrocarbon Products

Untitled

How could you brominate the compound?

Possible Answers:

Bromine gas

Hydrobromic acid

Bromine and UV light

Bromine and peroxides

None of these

Correct answer:

Bromine and UV light

Explanation:

The given molecule is an alkane. The only way to brominate an alkane is with bromine gas and UV light. The energy from the light serves to creat two radical bromines. These radicals are capable of bonding with alkanes. If the given compound were an alkene, either hydrobromic acid or bromine and peroxides would work.

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