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Question Video: Identifying the Chemical Equation That Describes the Reaction between Potassium and Chlorine Chemistry • 7th Grade

Which of the following equations properly describes the reaction between potassium and chlorine? [A] 2K(s) + Cl₂(g) ⟶ 2KCl(s) [B] 2K(s) + 2Cl₂(l) ⟶ 2KCl(s) [C] 2K(s) + 2Cl₂(g) ⟶ KCl(aq) [D] 2K(s) + Cl₂(g) ⟶ 2KCl(aq) [E] K(s) + Cl₂(g) ⟶ KCl(s)

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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.

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