Redox Reactions - Online Tutor, Practice Problems & Exam Prep

Redox reactions, or oxidation-reduction reactions, involve the transfer of electrons between substances. The mnemonic "LEO the lion goes GER" helps remember that losing electrons indicates oxidation (increasing oxidation number), while gaining electrons signifies reduction (decreasing oxidation number). The oxidizing agent is the substance that is reduced, and the reducing agent is the one that is oxidized. Understanding these concepts is crucial for identifying reactants in redox reactions and their roles as oxidizing or reducing agents.

Redox Reactions

Redox reactions, also known as oxidation-reduction reactions, involve the transference of an electron or electrons between reactants. And now a memory tool that'll help us remember what it means to be oxidized versus reduced is that LEO the lion goes GER. So LEO stands for "lose electrons oxidation" and GER stands for "gain electrons reduction." Let's go back and think about this. "Lose electrons oxidation." Electrons are negatively charged. If I'm losing something that's negative, then my life, me as a substance, becomes more positive. So, losing the negative to become more positive means you're becoming more positive as you lose negative electrons. But let's talk about this in terms of oxidation numbers. I'm becoming more positive, that means my oxidation number is increasing. Maybe I go from +2 to +4.

Now, if you're gaining negative things in your life, which we all don't want to happen, you're going to become more negative. So, you become more negative in charge. What does that mean in terms of oxidation number? Well, if you are becoming more negative, that means your number is decreasing. So, your oxidation number is going down. Maybe I go from +3 to -1. My oxidation number has decreased. So just remember, "LEO: lose electrons oxidation," I become more positive, my oxidation number goes up. "GER: gain electrons reduction," I become more negative, my oxidation number decreases. We're going to remember this when we take a look at redox reactions.

Redox Reactions

Now usually accompanied with the terms of reduction and oxidation are the ideas of the reducing agent and the oxidizing agent. But what you need to realize here is that reduction and oxidation are the opposites of oxidizing and reducing agents. We're going to say the oxidizing agent is just the element or compound that is reduced, and then the reducing agent is the element or compound that is oxidized. So add this to your idea of Leo the lion goes ger. Okay? So if you've been oxidized, you're the reducing agent. If you've been reduced, you're the oxidizing agent. I know it can be a little bit weird, but just remember they are opposites of one another. If we can remember that, then we'll be able to solve redox reaction questions that ask us to identify what's been oxidized, what's been reduced, what's the oxidizing agent, and what's the reducing agent.

Redox Reactions Example 1

In this example question, it considers the following reaction where solid lithium reacts with the zinc ion. Here we have 2 moles of lithium solid reacting with 1 mole of zinc ion in its aqueous state, and it produces 2 moles of lithium ion in its aqueous state plus zinc solid. The question asks which reactant is undergoing oxidation and which reactant is undergoing reduction. If we examine closely, lithium solid is in its natural state, so its oxidation number initially is 0. Zinc, in this case, is in its ionic form and has a charge present. If the ion has a charge, then that charge equals the oxidation number, which in this instance would be +2.

On the other side of the reaction, we have lithium ion, again in its ionic form; if the charge is present, then that charge is tied to the oxidation number, making it +1. Zinc is now in its natural state, so its oxidation number is 0. Observing the changes, lithium as a reactant moves from having an oxidation number of 0 to an oxidation number of +1. Since its oxidation number increased, lithium solid has been oxidized. Conversely, zinc starts off with +2 as its oxidation number and ends up at 0 as a product. As its oxidation number decreased, zinc 2+ ion has been reduced. By tracking the changes in oxidation numbers, we identify that in this reaction, lithium solid has been oxidized and zinc 2+ ion has been reduced, focusing mainly on the reactants to indicate these changes.

2 Li (s) + Zn 2+ (aq) ⇌ 2 Li + (aq) + Zn(s)

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

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

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

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

Here’s what students ask on this topic:

What is a redox reaction and how does it work?

A redox reaction, short for oxidation-reduction reaction, involves the transfer of electrons between two substances. In these reactions, one substance loses electrons (oxidation) and another gains electrons (reduction). The mnemonic 'LEO the lion goes GER' helps remember this: LEO stands for 'Lose Electrons Oxidation' and GER stands for 'Gain Electrons Reduction'. When a substance is oxidized, its oxidation number increases, and when it is reduced, its oxidation number decreases. For example, in the reaction between hydrogen and oxygen to form water, hydrogen is oxidized (loses electrons) and oxygen is reduced (gains electrons).

What is the difference between an oxidizing agent and a reducing agent?

An oxidizing agent is a substance that gains electrons and is reduced in a redox reaction. Conversely, a reducing agent is a substance that loses electrons and is oxidized. Essentially, the oxidizing agent facilitates the oxidation of another substance by accepting its electrons, while the reducing agent facilitates the reduction of another substance by donating its electrons. For example, in the reaction between hydrogen and fluorine to form hydrogen fluoride, fluorine acts as the oxidizing agent (it gains electrons and is reduced), and hydrogen acts as the reducing agent (it loses electrons and is oxidized).

How do you determine the oxidation number of an element in a compound?

To determine the oxidation number of an element in a compound, follow these rules: (1) The oxidation number of an atom in its elemental form is 0. (2) For monoatomic ions, the oxidation number is equal to the ion's charge. (3) Oxygen usually has an oxidation number of -2, except in peroxides where it is -1. (4) Hydrogen usually has an oxidation number of +1, except when bonded to metals in hydrides where it is -1. (5) The sum of oxidation numbers in a neutral compound is 0, and in a polyatomic ion, it equals the ion's charge. By applying these rules, you can systematically determine the oxidation numbers of elements in a compound.

What is the significance of the mnemonic 'LEO the lion goes GER' in redox reactions?

The mnemonic 'LEO the lion goes GER' is significant in redox reactions as it helps students remember the processes of oxidation and reduction. 'LEO' stands for 'Lose Electrons Oxidation', indicating that when a substance loses electrons, it undergoes oxidation and its oxidation number increases. 'GER' stands for 'Gain Electrons Reduction', indicating that when a substance gains electrons, it undergoes reduction and its oxidation number decreases. This mnemonic simplifies the understanding of electron transfer in redox reactions, making it easier to identify which substances are oxidized and which are reduced.

How can you identify the substance that is oxidized and the substance that is reduced in a redox reaction?

To identify the substance that is oxidized and the substance that is reduced in a redox reaction, follow these steps: (1) Assign oxidation numbers to all elements in the reactants and products. (2) Determine which elements have changes in their oxidation numbers. (3) The element whose oxidation number increases is oxidized (loses electrons), and the element whose oxidation number decreases is reduced (gains electrons). For example, in the reaction between zinc and copper(II) sulfate, Zn + CuSO₄ → ZnSO₄ + Cu, zinc is oxidized (its oxidation number increases from 0 to +2), and copper is reduced (its oxidation number decreases from +2 to 0).