MCAT General Chemistry Question 201: Answer and Explanation
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Question: 201
5. Which of the following best describes how a catalyst might speed up a reaction?
- A. CH3COOH(l) → CH3CHO(l) + ½ O2(g)
- B. C2H5OH(l) + ½ O2(g) → CH3CHO(l) + H2O(l)
- C. CH3CHO(l) + ½ O2(g) → CH3COOH(l)
- D. C2H5OH(l) + O2(g) → CH3COOH(l) + H2O(l)
Correct Answer: A
Explanation:
A The change in enthalpy (∆H) for a reaction is equal to the sum of ∆Hf° for products minus the sum of ∆Hf° for reactants: ∆H = Σ ∆Hf (products) – Σ ∆Hf (reactants). For choice A, the lone reduction reaction, ∆H = (–77.8 kJ/mol) – (–484.50 kJ/mol). This is a positive value indicating that enthalpy is absorbed and the reaction is endothermic. Note that the heat of formation of diatomic oxygen gas or any element in its naturally occurring form is defined as 0. The other answer choices, which are all oxidation reactions, will yield a negative value indicating they are exothermic.