Problem 19 - Chemistry: The Central Science

Question

Indicate whether the following balanced equations involve oxidation-reduction. If they do, identify the elements that undergo changes in oxidation number. (a) \(\mathrm{PBr}_{3}(l)+3 \mathrm{H}_{2} \mathrm{O}(I) \longrightarrow \mathrm{H}_{3} \mathrm{PO}_{3}(a q)+3 \mathrm{HBr}(a q)\) (b) \(\mathrm{Nal}(a q)+3 \mathrm{HOCl}(a q) \longrightarrow \mathrm{NalO}_{3}(a q)+3 \mathrm{HCl}(a q)\) (c) \(3 \mathrm{SO}_{2}(g)+2 \mathrm{HNO}_{3}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\) $$ 3 \mathrm{H}_{2} \mathrm{SO}_{4}(a q)+2 \mathrm{NO}(g) $$

Step-by-Step Solution

Answer

(a) The reaction does not involve oxidation-reduction. (b) The reaction involves oxidation-reduction; Iodine (I) is oxidized from -1 to +5, and Chlorine (Cl) is reduced from +1 to -1. (c) The reaction involves oxidation-reduction; Sulfur (S) is oxidized from +4 to +6, and Nitrogen (N) is reduced from +5 to +2.

Step 1: Assign oxidation numbers

We will assign oxidation numbers to all atoms in the reactants and products of each equation. (a) PBr3: P has oxidation number +3, Br has oxidation number -1. H2O: O has oxidation number -2, H has oxidation number +1. H3PO3: P has oxidation number +3, O has oxidation number -2, H has oxidation number +1. HBr: H has oxidation number +1, Br has oxidation number -1. (b) NaI: Na has oxidation number +1, I has oxidation number -1. HOCl: O has oxidation number -2, H has oxidation number +1, Cl has oxidation number +1. NaIO3: Na has oxidation number +1, I has oxidation number +5, O has oxidation number -2. HCl: H has oxidation number +1, Cl has oxidation number -1. (c) SO2: S has oxidation number +4, O has oxidation number -2. HNO3: H has oxidation number +1, N has oxidation number +5, O has oxidation number -2. H2O: O has oxidation number -2, H has oxidation number +1. H2SO4: H has oxidation number +1, S has oxidation number +6, O has oxidation number -2. NO: N has oxidation number +2, O has oxidation number -2.

Step 2: Determine changes in oxidation numbers

Now, we will check if any elements have undergone changes in their oxidation numbers in each equation. (a) No elements have changed oxidation numbers: P: +3 to +3 Br: -1 to -1 O: -2 to -2 H: +1 to +1 (b) Two elements have changed oxidation numbers: I: -1 to +5 Cl: +1 to -1 (c) Two elements have changed oxidation numbers: S: +4 to +6 N: +5 to +2

Step 3: Identify the elements with changes in oxidation numbers and confirm the reaction type

Based on the changes in oxidation numbers, we can confirm the reaction types and identify the elements involved in oxidation-reduction. (a) No elements have changed oxidation numbers, so the reaction does not involve oxidation-reduction. (b) The elements I and Cl have changed oxidation numbers, so the reaction involves oxidation-reduction. Iodine (I) has been oxidized from -1 to +5, while Chlorine (Cl) has been reduced from +1 to -1. (c) The elements S and N have changed oxidation numbers, so the reaction involves oxidation-reduction. Sulfur (S) has been oxidized from +4 to +6, while Nitrogen (N) has been reduced from +5 to +2.

Key Concepts

Assigning Oxidation Numbers

Understanding how to assign oxidation numbers to the elements in a chemical compound is crucial for analyzing redox reactions. Oxidation numbers, also known as oxidation states, represent the hypothetical charge an atom would have if all bonds to atoms of different elements were completely ionic.

• For an atom in its elemental form, the oxidation number is always zero.
• For monatomic ions, the oxidation number is the same as the charge of the ion.
• Oxygen usually has an oxidation number of -2, except in peroxides where it's -1.
• Hydrogen typically has an oxidation number of +1 when bonded to non-metals and -1 when bonded to metals.
• The algebraic sum of oxidation numbers for all atoms in a neutral compound must be zero; for a polyatomic ion, it must equal the ion's charge.

To clarify the concept, let's revisit our exercise examples.

• In H₂O, hydrogen is +1 (since it's bonded to a non-metal), oxygen is -2.
• For NaI, Na is a Group 1 element and thus has an oxidation number of +1, while I as a halogen here is -1.
• In H₂SO₄, hydrogen is +1, sulfur's oxidation state is deduced to be +6 to satisfy the overall charge balance with four oxygen atoms.

Accurate assignment is the first step towards identifying a redox reaction and thus serves as a fundamental skill in chemistry.

Changes in Oxidation States

Identifying changes in oxidation states is central to determining whether a redox reaction has occurred and understanding the electron transfer processes involved. During a redox reaction, one species undergoes oxidation (loses electrons and increases its oxidation state), while another undergoes reduction (gains electrons and decreases its oxidation state).

• Any change in oxidation number indicates that an element has either lost or gained electrons.
• Oxidation involves an increase in oxidation number, while reduction involves a decrease.
• The substance that gets oxidized is reducing agent, and the substance that gets reduced is the oxidizing agent.

Referring back to our exercise example (b), iodine (I) shows a change from an oxidation number of -1 to +5, signaling it has lost electrons (oxidation), while chlorine (Cl) changes from +1 to -1, indicating it has gained electrons (reduction). Such information is key when balancing redox equations and in understanding the underlying chemistry of the reaction.

Redox Reaction Identification

Identifying whether a chemical reaction is a redox or non-redox process is a critical skill in chemistry. A redox reaction is characterized by the transfer of electrons between chemical species, resulting in changes in oxidation states.

• If there are no changes in the oxidation states of the elements in a chemical equation, it's not a redox reaction.
• Changes in the oxidation numbers indicate a redox reaction, where at least one element gets oxidized and another gets reduced.
• Identifying the oxidizing and reducing agents helps in determining the direction of electron flow.

From our examples, in reaction (a) there are no changes in oxidation numbers, so it's a non-redox reaction. Reactions (b) and (c), however, show changes in the oxidation states (I and Cl in (b), S and N in (c)), which is characteristic of redox reactions. Reaction (b) is a redox reaction where iodine (I) and chlorine (Cl) are the reactants that undergo oxidation and reduction, respectively. Similarly, in reaction (c), sulfur (S) is oxidized, and nitrogen (N) is reduced, making it a redox process.