Question

Determine whether or not each reaction is a redox reaction. For each redox reaction, identify the oxidizing agent and the reducing agent. a. \(4 \mathrm{Li}(s)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{Li}_{2} \mathrm{O}(s)\) b. \(\mathrm{Mg}(s)+\mathrm{Fe}^{2+}(a q) \longrightarrow \mathrm{Mg}^{2+}(a q)+\mathrm{Fe}(s)\) c. \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\mathrm{Na}_{2} \mathrm{SO}_{4}(a q) \longrightarrow \mathrm{PbSO}_{4}(s)+2 \mathrm{NaNO}_{3}(a q)\) d. \(\mathrm{HBr}(a q)+\mathrm{KOH}(a q) \stackrel{(a q)}{\longrightarrow} \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{KBr}(a q)\)

Short answer

a. Redox reaction; Oxidizing agent: O2, Reducing agent: Li.b. Redox reaction; Oxidizing agent: Fe^2+, Reducing agent: Mg.c. Not a redox reaction.d. Not a redox reaction.

Step-by-step solution

Determine Oxidation States for Reaction a

For reaction a: Lithium (Li) starts with an oxidation state of 0 and goes to +1 in Li2O. Oxygen (O2) starts with an oxidation state of 0 and goes to -2 in Li2O. There is a change in oxidation states, indicating a redox reaction.

Identify the Oxidizing and Reducing Agents for Reaction a

Li is oxidized (reducing agent), and O2 is reduced (oxidizing agent) in reaction a.

Determine Oxidation States for Reaction b

For reaction b: Magnesium (Mg) starts with an oxidation state of 0 and goes to +2 in Mg^2+. Iron (Fe^2+) starts with +2 and goes to 0 in Fe. There is a change in oxidation states, indicating a redox reaction.

Identify the Oxidizing and Reducing Agents for Reaction b

Mg is oxidized (reducing agent), and Fe^2+ is reduced (oxidizing agent) in reaction b.

Examine Ionic Exchange for Reaction c

For reaction c: There is no change in oxidation states for any elements as Pb goes from +2 to +2, NO3 remains -1, Na remains +1, and SO4 remains -2. Reaction c involves an exchange of ions without a change in oxidation states, thus it is not a redox reaction.

Determine Oxidation States for Reaction d

For reaction d: HBr and KOH react in a double displacement reaction, forming H2O and KBr. The oxidation states for all elements remain unchanged (H, Br, K, and O maintain their oxidation states). Therefore, reaction d is not a redox reaction.

Key concepts

Oxidation States

Understanding oxidation states is essential for analyzing redox reactions, which involve transfer of electrons between species. The oxidation state, often called oxidation number, is an indicator of the degree of oxidation of an atom in a chemical compound. Simply put, it's like a bookkeeping tool to track electrons during a reaction.

Consider a simple element like Lithium (Li) in its metallic form; it has an oxidation state of 0. When it reacts to form Lithium oxide (Li₂O), Li is assigned an oxidation state of +1, indicating it has lost an electron, which is evident in reaction (a). Oxygen (O₂), which starts with 0 in its molecular form, gains electrons to achieve an oxidation state of -2 when it transforms into oxide ions in Li₂O.

The oxidation state changes are telltale signs of a redox process. Reaction (b), involving Mg and Fe²⁺, exhibits similar changes with Mg going from 0 to +2 and Fe from +2 to 0. To accurately assign oxidation states and analyze redox reactions, follow these guidelines: Elements in their elemental form have an oxidation state of 0; For ions, the oxidation state is the charge of the ion; Oxygen almost always has an oxidation state of -2, except in peroxides where it's -1; Hydrogen is typically +1 when bonded with non-metals and -1 with metals. These rules help us to deduce the correct oxidation states and consequently understand the nature of the reaction.

Oxidizing and Reducing Agents

In addition to identifying whether a chemical reaction is a redox reaction, it’s also crucial to pinpoint the substances that gain or lose electrons. These substances are known as the oxidizing and reducing agents. An oxidizing agent, or oxidant, gains electrons and is reduced, while a reducing agent, or reductant, loses electrons and is oxidized.

In reaction (a), oxygen (O₂) is the oxidizing agent because it accepts electrons from lithium (Li), which is the reducing agent. Similarly in reaction (b), iron (Fe²⁺) serves as the oxidizing agent as it gains electrons to become iron (Fe), and magnesium (Mg) is the reducing agent because it loses electrons to form Mg²⁺. Reducing agents are often metals in reactions because metals tend to lose electrons more readily. Conversely, non-metals typically act as oxidizing agents because they tend to gain electrons.

To further aid in identifying these agents, remember that the substance that is oxidized will have its oxidation state increase, while the substance that is reduced will have its oxidation state decrease. Keeping track of the changes in the oxidation state can guide you to correctly determine which is the oxidizing agent and which is the reducing agent in a redox reaction.

Chemical Reactions Analysis

Breaking down chemical reactions to understand their nature and predict their outcomes is at the heart of reaction analysis. For instance, reactions (c) and (d) of our exercise were different; neither involved a change in the oxidation states of their respective elements and were, therefore, not redox reactions. This shows us that not all reactions are redox reactions and other reaction types exist, like double displacement reactions as seen in reaction (d).

In a redox reaction, by comparing the initial and final oxidation states of the elements involved, we can explain the movement of electrons and identify which element is undergoing oxidation or reduction. It's a bit like detective work, identifying clues in the form of oxidation state changes to solve the mystery of electron flow. For reactions not involving redox processes, such as a simple ionic exchange as in reaction (c), you'll notice that oxidation states remain constant; no electrons are transferred from one element to another.

While the step-by-step solution provided is quite comprehensive, a deeper exploration of the context behind each step can be valuable. Understanding how to deduce oxidation states and recognizing oxidizing and reducing agents are instrumental in analyzing the reaction fully. Also, recognizing the type of reaction, whether redox or not, can help us predict and understand the behavior of different substances when they interact.