APChemistryCollege BoardTopic QuestionsUnit 7: Equilibrium7.3 Reaction Quotient & Equilibrium Constant

Reaction Quotient & Equilibrium Constant (College Board AP Chemistry)

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1a1 mark

CaCO3(s)  rightwards arrow over leftwards arrow CaO(s) + CO2(g)

When heated, calcium carbonate decomposes according to the equation above. In a study of the decomposition of calcium carbonate, a student added a 50.0 g sample of powdered CaCO3(s) to a 1.00 L rigid container. The student sealed the container, pumped out all the gases, then heated the container in an oven at 1100 K . As the container was heated, the total pressure of the CO2(g) in the container was measured over time. The data are plotted in the graph below.

q4-june-2014---ap-chemistry

The student repeated the experiment, but this time the student used a 100.0 g sample of powdered CaCO3(s). In this experiment, the final pressure in the container was 1.04 atm, which was the same final pressure as in the first experiment.

Calculate the number of moles of CO2(g) present in the container after 20 minutes of heating.

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1b1 mark

The student claimed that the final pressure in the container in each experiment became constant because all of the CaCO3(s) had decomposed. Based on the data in the experiments, do you agree with this claim? Explain.

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1c1 mark

After 20 minutes some CO2(g) was injected into the container, initially raising the pressure to 1.5 atm. Would the final pressure inside the container be less than, greater than, or equal to 1.04 atm? Explain your reasoning.

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1d1 mark

Are there sufficient data obtained in the experiments to determine the value of the equilibrium constant, Kp , for the decomposition of CaCO3(s) at 1100 K ? Justify your answer.

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2a1 mark

CH3OH (g)   rightwards arrow over leftwards arrow   CO (g) + 2 H2 (g)                    ∆H ° = 90.0 kJ / molrxn

Methanol vapor decomposes to form carbon monoxide gas and hydrogen gas at high temperatures in the presence of a platinum catalyst, as represented by the balanced chemical equation given.

Are the hydrogen atoms oxidized or are they reduced in the forward reaction? Justify your answer in terms of oxidation numbers.

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2b1 mark

In the following box, draw the complete Lewis electron-dot diagram for the carbon monoxide molecule in which every atom obeys the octet rule. Show all bonding and nonbonding valence electrons.

C   O

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2c3 marks

The values of the standard molar entropies of the compounds involved in the reaction are given in the following table.

Substance S° (J/(K·mol))
CH3OH (g) 240.
CO (g) 198
H2 (g) 131
i)
Use the data in the table to calculate the value of the standard entropy change, ∆S°, in J/(K·molrxn), for the reaction.

ii)
Calculate the value of ∆G°, in kJ/molrxn, for the reaction at 375 K. Assume that ∆H ° and ∆S° are independent of temperature.

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2d1 mark

The following particle-level diagram shows a representative sample of the equilibrium mixture represented by the equation given.

q2d-june-2022---ap-chemistry

Use information from the particle diagram to calculate the partial pressure of CO at equilibrium when the total pressure of the equilibrium mixture is 12.0 atm.

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2e1 mark

Write the expression for the equilibrium constant, Kp, for the reaction.

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2f1 mark

CH3OH(g)    rightwards arrow over leftwards arrow   CO(g) + 2 H2 (g)

The reaction system represented by the equation is allowed to achieve equilibrium at a different temperature. The following table gives the partial pressure of each species in the equilibrium mixture.

 Substance Partial Pressure at Different Temperature
CH3OH(g) 2.7 atm
CO(g) 4.2 atm
H2 (g) 8.4 atm

 

Use the information in the table to calculate the value of the equilibrium constant, Kp, at the new temperature.

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2g2 marks

The volume of the container is rapidly doubled with no change in temperature. As equilibrium is re-established, does the number of moles of CH3OH (g) increase, decrease, or remain the same? Justify your answer by comparing the value of the reaction quotient, Q, with the value of the equilibrium constant, Kp.

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