MCAT question on Chemistry

Question

HG₂Cl₂ hv_→ HgCl₂ + Hg(l)

The reaction is

I. a single displacement reaction.

II. a double displacement reaction.

III. a disproportionation reaction.

IV. endothermic.

A. I only

B. II only

C. I and IV only

D. III and IV only

Answer 1

This reaction is not a displacement, either single or double, because neither element is replaced. However, the oxidation state of mercury is changed. On the left side, two mercury atoms have an oxidation state of +2. On the right, one mercury has a oxidation state of +1 and the other has an oxidation state of 0. Therefore, a redox reaction, or disproportionation reaction has occurred. Also, light, indicated as hv, is an energy source that is needed for this reaction. Therefore the reaction is endothermic.

The correct answer is D.