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I'm really confused with the explanation for this one. First of all, the explanation seems to be answering a question that asks "which of the following is NOT true". Nonetheless, I would appreciate anyone that can give some insight about the problem below.
I was thinking that the oxidation number of Cr decreases with the addition of ethanol and increases with the addition of acetic acid (Le Chatlier's Principle). What am I mixing up here? The answer is supposed to be D (because it's supposed to be the only invalid one).
3 H3CCH2OH + 2 K2Cr2O7 + 8 H2SO4 →
3 H3CCO2H + 2 Cr2(SO4) 3 + 2 K2SO4 + 11 H2O
117. As the result of the treatment of potassium dichromate in sulfuric acid with ethanol and acetic acid, the oxidation state of chromium:
A. increases with the addition of either ethanol or acetic acid.
B. decreases with the addition of either ethanol or acetic acid.
C. increases with the addition of ethanol and remains constant with the addition of acetic acid.
D. decreases with the addition of ethanol and remains constant with the addition of acetic acid.
D is the best answer. Ethanol is oxidized into acetic acid over the course of the reaction, which results in the loss of two C-H bonds and the gain of two C-O bonds. This makes choice A a valid statement, so it is not the best answer. Chromium gets reduced when chromate oxidizes a primary or secondary alcohol. When chromium is reduced, its oxidation state is lowered, which makes choice B a valid statement, and thereby eliminates it as the best answer. Sulfate acts as a spectator ion, and it undergoes no change in any facet, including oxidation state. This makes choice C a valid statement, and thereby eliminates it as the best answer. All of the Hs in the product molecules have an oxidation state of +1, just as H+ has. This means that H+ underwent no change in oxidation state, meaning it was not reduced. This makes choice D an invalid statement, so it is the best answer. The best answer is D.
I was thinking that the oxidation number of Cr decreases with the addition of ethanol and increases with the addition of acetic acid (Le Chatlier's Principle). What am I mixing up here? The answer is supposed to be D (because it's supposed to be the only invalid one).
3 H3CCH2OH + 2 K2Cr2O7 + 8 H2SO4 →
3 H3CCO2H + 2 Cr2(SO4) 3 + 2 K2SO4 + 11 H2O
117. As the result of the treatment of potassium dichromate in sulfuric acid with ethanol and acetic acid, the oxidation state of chromium:
A. increases with the addition of either ethanol or acetic acid.
B. decreases with the addition of either ethanol or acetic acid.
C. increases with the addition of ethanol and remains constant with the addition of acetic acid.
D. decreases with the addition of ethanol and remains constant with the addition of acetic acid.
D is the best answer. Ethanol is oxidized into acetic acid over the course of the reaction, which results in the loss of two C-H bonds and the gain of two C-O bonds. This makes choice A a valid statement, so it is not the best answer. Chromium gets reduced when chromate oxidizes a primary or secondary alcohol. When chromium is reduced, its oxidation state is lowered, which makes choice B a valid statement, and thereby eliminates it as the best answer. Sulfate acts as a spectator ion, and it undergoes no change in any facet, including oxidation state. This makes choice C a valid statement, and thereby eliminates it as the best answer. All of the Hs in the product molecules have an oxidation state of +1, just as H+ has. This means that H+ underwent no change in oxidation state, meaning it was not reduced. This makes choice D an invalid statement, so it is the best answer. The best answer is D.
