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.
