Question Video: Identifying the Direction of the Reversible Reaction Chemistry

Which of the following statements about reversible reactions is true? [A] A reversible reaction is often a combustion reaction. [B] A reversible reaction is indicated by the symbol ⟶ in a chemical equation. [C] A reversible reaction is a chemical reaction that can proceed in both directions. [D] A reversible reaction always involves hydrated and anhydrous salts. [E] A reversible reaction is endothermic in both directions.

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Video Transcript

Which of the following statements about reversible reactions is true? (A) A reversible reaction is often a combustion reaction. (B) A reversible reaction is indicated by a single double-headed arrow in a chemical equation. (C) A reversible reaction is a chemical reaction that can proceed in both directions. (D) A reversible reaction always involves hydrated and anhydrous salts. (E) A reversible reaction is endothermic in both directions.

A reversible reaction is a reaction that not only goes in the forward direction where the reactants combine to form the products, but also the reverse direction where the products combine to form the reactants. We can indicate this in our chemical equation by using two single-headed arrows. With this information, answer choice (C) is clearly the correct answer. A reversible reaction is a chemical reaction that can proceed in both directions, the forward direction and the reverse direction. But let’s take a quick look through our other answer choices to see why they’re false.

Answer choice (A) says that a reversible reaction is often a combustion reaction. An example of a combustion reaction is the combustion of methane, where methane burns in oxygen to produce carbon dioxide and water. If this reaction were reversible, it would have to go in the reverse direction, where carbon dioxide reacts with water to form methane and oxygen. It’s hard to imagine this process happening. Since combustion reactions don’t typically go in the reverse direction, they’re not often reversible. Answer choice (B) says that reversible reactions are indicated by a single double-headed arrow. But we’ve seen how reversible reactions are indicated by two single-headed arrows in a chemical equation.

Answer choice (D) says that a reversible reaction always involves hydrated and anhydrous salts. There are multiple examples of reversible reactions that involve hydrated and anhydrous salts. For example, anhydrous copper(II) sulfate can react with water to form hydrated copper(II) sulfate. This reaction can be reversed when the hydrated copper(II) sulfate is heated. But there are plenty of examples of reversible reactions that don’t involve these hydrated salts, for instance, the reaction of any weak acid or base with water such as hydrofluoric acid. So reversible reactions sometimes involve hydrated and anhydrous salts, but not always.

Our final answer choice says that a reversible reaction is endothermic in both directions. Recall the endothermic means that the enthalpy change for the reaction is positive. This means that, overall, the reaction takes in energy. Looking at the sketch of an energy diagram, we can see that the forward reaction where A plus B reacts to form C plus D is endothermic since the change in enthalpy is positive. But what about the reverse direction where C plus D reacts to form A plus B? In this case, we would get a negative change in enthalpy, meaning that the reverse reaction is exothermic not endothermic. So a reversible reaction will be endothermic in one direction, but exothermic in the other direction.

So of the statements we looked at, the only one that was true about reversible reactions is that a reversible reaction can proceed in both directions.

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