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 means oxidation (increasing oxidation number), while gaining electrons means reduction (decreasing oxidation number). The oxidizing agent is the substance that gets reduced, and the reducing agent is the one that gets oxidized. Understanding these concepts is crucial for identifying reactants and products in redox reactions, as well as their respective 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. So 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. So 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 plus 2 to plus 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 that your number is decreasing. So your oxidation number is going down. So maybe I go from plus 3 to minus 1. My oxidation number has decreased. So just remember, LEO means Lose Electrons Oxidation, I become more positive, my oxidation number goes up. GER means 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 says, consider the following reaction below when solid lithium reacts with the zinc ion. So 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? So if we take a look here, lithium solid is in its natural state. So lithium solid's oxidation number initially is 0. Zinc here is in its ionic form. It has its charge present. Remember, if we have the charge of the ion, then that can equal the oxidation number. So here this would be plus 2. Going to the other side, we have lithium ion. Again, it's in its ionic form. If the charge is present, then that charge is connected to the oxidation number, so it's plus 1. And then zinc is now in its natural state, so it's 0.

So when we're talking about what's been oxidized, what's been reduced, we look at the reactants. So we have lithium solid, and we have what? Zinc 2⁺. Lithium as a reactant goes from having an oxidation number of 0 to an oxidation number of plus 1. We are going to say that its oxidation number went up. It increased. It went from 0 to plus 1. Remember, if your oxidation number increases, that means that you've been oxidized. So lithium solid has been oxidized. Zinc. Zinc starts off with plus 2 as its oxidation number, and then as a product, it goes down to 0. We see that its oxidation number decreased. Remember, if your oxidation number decreased, you've been reduced. So the zinc 2⁺ ion has been reduced.

Remember, what's been oxidized and reduced is the reactant. We look on the product side just to see how our oxidation number has changed. But again, it is the reactants that are oxidized or reduced. In this case, it would give us lithium solid as being oxidized and zinc 2⁺ ion as being reduced.

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. Understanding these changes is crucial for identifying the reactants and products in redox reactions.

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

An oxidizing agent is a substance that gets reduced by gaining electrons, while a reducing agent is a substance that gets oxidized by losing electrons. These terms are opposites: the oxidizing agent facilitates oxidation by accepting electrons, and the reducing agent facilitates reduction by donating electrons. For example, in the reaction between hydrogen and oxygen to form water, oxygen acts as the oxidizing agent (it gains electrons) and hydrogen acts as the reducing agent (it loses electrons).

How do you determine the oxidation number in a redox reaction?

To determine the oxidation number in a redox reaction, follow these rules: (1) The oxidation number of an element in its natural state 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, and hydrogen is usually +1. (4) 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 track changes in oxidation numbers to identify which substances are oxidized and reduced.

What is the mnemonic 'LEO the lion goes GER' and how does it help in understanding redox reactions?

The mnemonic 'LEO the lion goes GER' helps remember the processes of oxidation and reduction in redox reactions. 'LEO' stands for 'Lose Electrons Oxidation,' meaning that when a substance loses electrons, it undergoes oxidation and its oxidation number increases. 'GER' stands for 'Gain Electrons Reduction,' meaning that when a substance gains electrons, it undergoes reduction and its oxidation number decreases. This mnemonic simplifies the understanding of electron transfer and changes in oxidation states in redox reactions.

Can you provide an example of a redox reaction and identify the oxidizing and reducing agents?

Consider the reaction between hydrogen and fluorine to form hydrogen fluoride: $H_2+F_2\to {\mathrm{HF}}_2$. In this reaction, hydrogen (H₂) is oxidized to H⁺ (loses electrons), and fluorine (F₂) is reduced to F^- (gains electrons). Therefore, hydrogen is the reducing agent (it donates electrons), and fluorine is the oxidizing agent (it accepts electrons).