Redox Reactions: Videos & Practice Problems

Redox reactions, or oxidation-reduction reactions, are fundamental processes where electrons are transferred between substances, altering their oxidation states. Remembering "LEO the lion says GER" can help you recall that "Lose Electrons Oxidation" (LEO) leads to an increase in oxidation number, making the substance more positive, while "Gain Electrons Reduction" (GER) results in a decrease in oxidation number, making the substance more negative. It's important to note that the substance being oxidized acts as the reducing agent, and the substance being reduced serves as the oxidizing agent. These concepts are crucial for understanding redox reactions and predicting the changes that occur to the oxidation states of elements or compounds involved.

Oxidation and Reduction Reactions

Redox reactions, or oxidation-reduction reactions, are fundamental chemical processes involving the transfer of electrons between reactants. Understanding the concepts of oxidation and reduction is crucial, and a helpful mnemonic to remember these processes is "LEO the lion goes GER." In this phrase, LEO stands for "Lose Electrons = Oxidation," while GER represents "Gain Electrons = Reduction."

When a substance undergoes oxidation, it loses electrons, which are negatively charged. Consequently, losing negatively charged particles results in a more positive charge for the substance. This change can be reflected in the oxidation number, which increases as electrons are lost. For example, if a substance's oxidation number changes from +2 to +4, it indicates oxidation has occurred.

Conversely, reduction involves the gain of electrons. When a substance gains electrons, it becomes more negatively charged, leading to a decrease in its oxidation number. For instance, if a substance's oxidation number shifts from +3 to -1, this signifies a reduction process.

In summary, oxidation is characterized by the loss of electrons and an increase in oxidation number, while reduction is defined by the gain of electrons and a decrease in oxidation number. This understanding is essential for analyzing and predicting the behavior of substances in redox reactions.

Oxidizing and Reducing Agents

In the context of redox (reduction-oxidation) reactions, understanding the roles of reducing and oxidizing agents is crucial. The oxidizing agent is defined as the element or compound that undergoes reduction, meaning it gains electrons. Conversely, the reducing agent is the element or compound that is oxidized, which involves the loss of electrons. This relationship can be remembered using the mnemonic "LEO the lion goes GER," where LEO stands for "Lose Electrons = Oxidation" and GER stands for "Gain Electrons = Reduction." Thus, if a substance is oxidized, it acts as the reducing agent, while a substance that is reduced functions as the oxidizing agent.

To effectively analyze redox reactions, one must identify which species are oxidized and reduced, as well as determine the corresponding oxidizing and reducing agents. This understanding is essential for solving problems related to electron transfer in chemical reactions.

Oxidation and Reduction Reaction Example

In the reaction where solid lithium reacts with zinc ions, we can analyze the changes in oxidation states to determine which reactants undergo oxidation and reduction. The balanced chemical equation is:

2 Li (s) + Zn²⁺ (aq) → 2 Li⁺ (aq) + Zn (s)

Initially, solid lithium has an oxidation number of 0, while the zinc ion (Zn²⁺) has an oxidation number of +2. Upon reaction, lithium is converted to lithium ions (Li⁺), which have an oxidation number of +1. This indicates that lithium's oxidation number increases from 0 to +1, signifying that lithium is oxidized.

Conversely, zinc starts with an oxidation number of +2 and is reduced to its elemental form, zinc (Zn), which has an oxidation number of 0. This decrease in oxidation number from +2 to 0 indicates that zinc is reduced.

In summary, during this reaction, lithium solid is oxidized as its oxidation number increases, while the zinc ion is reduced as its oxidation number decreases. Thus, lithium is the reducing agent, and zinc is the oxidizing agent in this redox process.

Which element is being reduced in the following reaction? 
Cr₂O₇^2- + 3 HNO₂ + 5 H⁺ → 2 Cr³⁺ +  3NO₃^- + 4 H₂O

• A. Cr
• B. H
• C. N
• D. O

Answer: (A)

Identify the oxidizing agent and reducing agent from the following redox reaction. 
Ba (s) + Cl₂ (g) → BaCl₂ (aq)

• A. Ba:oxidizing agent; Cl2 reducing agent
• B. Ba:reducing agent; Cl2 oxidizing agent

Answer: (B)

Which element is oxidized and which is reduced in the following reaction?
Hg (aq) + HgCl₂ (aq) → Hg₂Cl₂

• A. Hg in Hg(aq):oxidized; Cl in HgCl₂(aq):reduced
• B. Hg in Hg(aq):reduced; Cl in HgCl₂(aq):oxidized
• C. Hg in Hg(aq):oxidized; Hg in HgCl₂(aq):reduced
• D. Hg in Hg(aq):reduced; Hg in HgCl₂(aq):oxidized

Answer: (C)

Which of the following represents an oxidation-reduction reaction? 
I. PCl₃ (aq) + Cl₂ (g) → PCl₅ (aq) 
II. 2 AgNO₃ (aq) + Cu (s) → Cu(NO₃)₂ (aq) + 2 Ag (s) 
III. CO₂ (g) + 2 LiOH (aq) → Li₂CO₃ (aq) + H₂O (l)
IV. FeCl₂ (aq) + 2 NaOH (aq) → Fe(OH)₂ (aq) + 2 NaCl (aq) 

• A. I, II, III, IV
• B. I, II and III
• C. III
• D. I and II
• E. IV

Answer: (D)

Here’s what students ask on this topic:

What are redox reactions?

Redox reactions, short for reduction-oxidation reactions, are a family of chemical processes involving the transfer of electrons between two substances. In a redox reaction, one molecule, atom, or ion loses electrons (is oxidized), while another gains electrons (is reduced). These two processes occur simultaneously and are linked; the substance that gives away electrons is called the reducing agent, and the substance that accepts electrons is called the oxidizing agent.

To remember which is which, you can use the mnemonic "OIL RIG": Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). Redox reactions are fundamental to numerous biological processes, such as cellular respiration and photosynthesis, and also to industrial processes like corrosion, combustion, and the functioning of batteries. The change in the oxidation state of the substances involved is a key indicator that a redox reaction has occurred.

What is an oxidizing agent?

An oxidizing agent, also known as an oxidant, is a substance that has the ability to oxidize other substances — in other words, it causes other substances to lose electrons. In a chemical reaction, the oxidizing agent gains electrons and is reduced, while the other substance loses electrons and is oxidized. This process is a fundamental part of redox reactions, which are reactions involving the transfer of electrons between two species.

Oxidizing agents are characterized by their strong tendency to pull electrons away from other substances, and they often contain elements with high electronegativity or have atoms that can attain a more stable electronic configuration by gaining electrons. Common examples of oxidizing agents include halogens like chlorine and fluorine, oxygen, and substances like potassium permanganate (${\mathrm{KMnO}}_4$) and hydrogen peroxide ($H_2{O}_2$). In biological systems, molecular oxygen ($O_2$) acts as the primary oxidizing agent in cellular respiration, driving the production of energy by accepting electrons.

What is a reducing agent?

A reducing agent, also known as a reductant, is a substance that donates electrons to another substance in a chemical reaction. By providing electrons, the reducing agent causes the other substance to be reduced, which means the oxidation state of the other substance decreases. As the reducing agent donates electrons, it itself is oxidized, which means it loses electrons and its oxidation state increases.

To remember this, you can think of the reducing agent as reducing the charge of another atom or molecule. A classic example of a reducing agent is hydrogen gas (H₂), which can donate electrons to other substances. In a redox (reduction-oxidation) reaction, the reducing agent and the oxidizing agent (the substance that accepts electrons) work together to transfer electrons. This electron transfer is fundamental to many chemical processes, including those that provide energy in biological systems and those that are used in industrial manufacturing.