Video: Identifying the Change in Oxidation State of Chlorine in the Reaction 2HCl(aq) → H₂(g) + Cl₂(g), and Classifying It as an Oxidation or a Reduction

In the given reaction, chlorine is _. 2HCl(aq) ⟶ H2(g) + Cl2(g) [A] oxidized from an oxidation state of −1 to 0 [B] reduced from an oxidation state of −1 to 0 [C] oxidized from an oxidation state of 0 to −1 [D] reduced from an oxidation state of 0 to −1 [E] not changing its oxidation state

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Video Transcript

In the given reaction, chlorine is... And we are given the equation 2HCl aqueous going to H2 gas plus Cl2 gas with possible answers (A) oxidized from an oxidation state of minus one to zero. (B) Reduced from an oxidation state of minus one to zero. (C) Oxidized from an oxidation state of zero to minus one. (D) Reduced from an oxidation state of zero to minus one. Or (E) not changing its oxidation state.

Aqueous HCL is also called hydrochloric acid, H2 gas, hydrogen gas, and Cl2 gas, chlorine gas. In a redox reaction, oxidation states change. In a redox reaction, both reduction and oxidation occur. Reduction is the gain of an electron or electrons by a substance in a reaction. And oxidation is the loss of an electron or electrons by a substance in a reaction. We can remember this using the acronym OIL RIG. O-I-L means oxidation is loss of electrons, and R-I-G means reduction is gain of electrons.

To indicate the number of electrons lost or gained by an element in a substance in a reaction, we use oxidation states. Oxidation state is a number used to represent the number of electrons lost or gained by an element in a compound. The oxidation state of a neutral atom is zero. It is positive when an atom loses an electron or electrons. And it is negative when an atom gains an electron or electrons. The oxidation state of an element by itself is also zero. So, for the element molecules H2 and Cl2, we write an oxidation state of zero above the formula.

For HCl, the hydrogen atom is covalently bonded to the chlorine atom. However, we need to treat this compound as an ionic compound in order to determine the oxidation state of each atom. If HCl was ionic, a hydrogen atom in the first group of the periodic table would lose one electron to form a one plus ion. A chlorine atom would gain one electron to form a one minus ion. Typically, we write the number before the charge, H1+, and in Cl1−, when writing an ion symbol. But for an oxidation state, we write the charge before the number. So, the numbers written above the formulas are the oxidation states of each atom.

Let’s look at how the oxidation states change to determine what happens to chlorine in this reaction. Each hydrogen atom gains one electron to gain an oxidation state of zero. Hydrogen undergoes reduction or is reduced. Each chlorine atom loses one electron to gain an oxidation state of zero. So, chlorine is oxidized or undergoes oxidation. We can see that chlorine’s oxidation state changes from minus one to zero. So, in the given reaction, chlorine is (A) oxidized from an oxidation state of minus one to zero.

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