Video Transcript
In the following chemical reaction, the forward reaction is endothermic. Hydrated CuSO4 reversibly produces solid CuSO4 and liquid water. What must be true about the reverse reaction? (A) The reverse reaction is exothermic. (B) The reverse reaction is also endothermic. (C) The reverse reaction absorbs heat from the surroundings. (D) The reverse reaction has a greater overall energy change. Or (E) the reverse reaction has a lesser overall energy change.
This question is asking about a reversible reaction as indicated by the double-headed arrow in the chemical equation. We are told that the forward reaction is endothermic. In other words, it absorbs heat from the surroundings. In a reversible reaction, the forward reaction is the one that proceeds from left to right. In this case, it takes the reactant, a hydrated crystal, and separates it into an ionic compound and liquid water. The reverse reaction is the one that goes in the other direction from right to left, where the products recombine to form the reactants. In this case, the reverse reaction is where the water recombines with the solid copper sulfate to form the initial reactant, the hydrated crystal. We can call this reaction a reversible dehydration reaction.
The forward reaction removes water from or dehydrates the crystal, but we can also go in the reverse direction when we add the water back in. We know that the forward reaction is endothermic, or it absorbs heat. And this question is asking us to draw a conclusion about the reverse reaction. To answer this question, we need to know how the energy change shifts when we go from the forward reaction to the reverse reaction.
Well, if we know that the forward reaction is endothermic, or in other words it absorbs heat, the reverse reaction has to do the opposite thing. It must release heat. In other words, it’s exothermic. So the correct answer is choice (A), the reverse reaction is exothermic.
Choice (B) and (C) describe the forward reaction, which is endothermic and absorbs heat from the surroundings, but not the reverse reaction, which is exothermic and releases heat into the surroundings. To be thorough and eliminate choices (D) and (E) as well, we can say that there is the same intensity of energy change in each direction of the reaction. In other words, if the reaction absorbs a certain amount of energy to produce copper sulfate and water, it will release that same amount of energy when the copper sulfate and water are recombined to form the initial crystal. The amount of energy is the same, but the sign is different. So it’s incorrect to say that the reverse reaction would have a greater or lesser overall energy change than the forward reaction. It has the same energy change.
So when the forward reaction is endothermic, what must be true about the reverse reaction? The correct answer is choice (A), the reverse reaction is exothermic.
