### Video Transcript

Which of the following equations
properly describes the reaction between potassium and chlorine? (A) Two K solid plus Cl2 gas react
to produce two KCl solid. (B) Two K solid plus two Cl2 liquid
react to produce two KCl solid. (C) Two K solid plus two Cl2 gas
react to produce KCl aqueous. (D) Two K solid plus Cl2 gas react
to produce two KCl aqueous. Or (E) K solid plus Cl2 gas react
to produce KCl solid.

Potassium and chlorine react
together to produce potassium chloride, KCl. Potassium is given the symbol
K. And it is a solid at room
temperature, as all metals are, except for mercury. It is reacted with chlorine, which
at room temperature and pressure is a diatomic gas. Option (B) states that chlorine is
a liquid. This is not the case under these
reaction conditions. Thus, option (B) cannot be the
answer to this question. When the solid metal is heated in
the presence of the gas, it produces potassium chloride in solid form.

Options (C) and (D) state that the
potassium chloride is in aqueous form. This is rather unlikely considering
we’re reacting a solid with a gas, and no water is involved in the reaction. Therefore, we can rule out options
(C) and (D) as answers to this question. Both options (A) and (E) use the
correct chemical symbols and the correct state symbols, but they have different
stoichiometric coefficients. So, we need to figure out how to
correctly balance this equation.

Let’s start by counting up how many
we have of each element on each side of the equation. We only have one equivalent of
potassium on the reactant side, so we can do a tally of one. We also only have one equivalent of
potassium on the product side. So, we can do a tally of one for
potassium in the products column. Chlorine is only present in one of
the reactant species, but it has a subscript value of two. So, we can do a tally of two for
chlorine in the reactants column. However, there is only one
equivalent of chlorine on the product side. So, we do a tally of one for
chlorine in the products column.

For the equation to be correct, the
tallies for each element in each column need to be balanced. They’re currently balanced for
potassium but not for chlorine. We need another equivalent of
chlorine on the product side. If we add another equivalent of
potassium chloride, the number of equivalents of chlorine on the product side
increases by one. So, the chlorine atoms are now
balanced. But the number of equivalents of
potassium on the product side also increases by one. So, potassium is now not
balanced. To make potassium balanced again,
we need to increase the number of equivalents on the reactant side. If we add another equivalent of
potassium, then there will be two equivalents of potassium on each side of the
equation. Thus, it is now balanced.

We can now see that the
stoichiometric coefficient for potassium is two, chlorine gas is one, so can be
omitted, and potassium chloride is also two. So, our final equation is two K
solid plus Cl2 gas react to produce two KCl solid. We can see that this matches the
equation in (A), not the equation in (E). So, it seems as though option (A)
is the answer to this question. We can verify this with a general
equation.

The reactions between the halogen
gases, such as chlorine, and the alkali metals, such as potassium, can be summed up
with the equation two M solid plus X two gas react to produce two MX solid. If we replaced M, which represents
the metal, with K and replaced X, which represents the halogen, with Cl, we would
find that this also matched the equation in (A). So, the answer to the question
“Which of the following equations properly describes the reaction between potassium
and chlorine?” is (A). Two K solid plus Cl2 gas react to
produce two KCl solid.