MCAT General Chemistry Question 78: Answer and Explanation

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Question: 78

3. Carbonated beverages are produced by dissolving carbon dioxide in water to produce carbonic acid:

CO2 (g) + H2O (l) ? H2CO3 (aq)

When a bottle containing carbonated water is opened, the taste of the beverage gradually changes as the carbonation is lost. Which of the following statements best explains this phenomenon?

  • A. The change in pressure and volume causes the reaction to shift to the left, thereby decreasing the amount of aqueous carbonic acid.
  • B. The change in pressure and volume causes the reaction to shift to the right, thereby decreasing the amount of gaseous carbon dioxide.
  • C. Carbonic acid reacts with environmental oxygen and nitrogen.
  • D. Carbon dioxide reacts with environmental oxygen and nitrogen.

Correct Answer: A

Explanation:

Carbon dioxide gas evolves and leaves the bottle, which decreases the total pressure of the reactants. Le Chatelier's principle explains that a decrease in pressure shifts the equilibrium to increase the number of moles of gas present. This particular reaction will shift to the left, which in turn will decrease the amount of carbonic acid and increase the amount of carbon dioxide and water. Oxygen and nitrogen are not highly reactive and are unlikely to combine spontaneously with carbon dioxide or carbonic acid, as in choices (C) and (D).

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