Redox Reactions - Online Tutor, Practice Problems & Exam Prep

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.

Redox Reactions

Redox reactions, also known as oxidation reduction reactions, involve transference of an electron or electrons between reactants and now a memory tool that'll help us remember. What does it mean 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 2:00 to 4:00.

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 O maybe I go from +3 to -1, my oxidation number has decreased.

So just remember, Leo means lose electrons oxidation, I become more positive, my oxidation number goes up, GR 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 in 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 reducing agent is the element or compound that is oxidized.

So add this to your idea of Leo, the lion goes GRR. OK, 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're 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 been 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 a zinc ion. So here we have two moles of lithium solid reacting with one 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 says 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 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 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 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 one. We're going to say that its oxidation number went up, it increased, it went from zero to 1+. Remember, if your oxidation number increases, that means that you've been oxidized. So lithium solid has been oxidized.

Zinc starts off with two 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 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 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.