Oxidation-Reduction (Redox) Reactions

Caleb Thomas
7 min read
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Study Guide Overview
This study guide covers redox reactions, focusing on the transfer of electrons and changes in oxidation states. It explains oxidation (loss of electrons) and reduction (gain of electrons), and how to assign oxidation numbers. Balancing redox reactions in both acidic and basic solutions is detailed, along with practice problems and exam tips.
Redox Reactions: The Ultimate Guide
Hey there, future AP Chem master! ๐ Let's dive into redox reactions, the final boss of this unit. Remember, you've already conquered precipitation and acid-base reactions โ you got this! ๐ช
What are Redox Reactions?
Redox reactions, short for oxidation-reduction reactions, are all about the transfer of electrons between atoms, leading to changes in their oxidation states. Think of it as a game of electron hot potato! ๐ฅ
An oxidation state is like an atom's charge in a compound, showing how many electrons it has gained or lost compared to its neutral state. It's a number, positive or negative, that helps us track electron movement.
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Oxidation: When an atom loses electrons, its oxidation number increases. It's like going up a level! โฌ๏ธ
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Reduction: When an atom gains electrons, its oxidation number decreases. It's like going down a level! โฌ๏ธ
Electrons always travel from the oxidized species to the reduced species. Remember, oxidation and reduction always happen together โ you can't have one without the other!
Quick Recap:
- Redox reactions involve electron transfer.
- Oxidation is losing electrons (oxidation number increases).
- Reduction is gaining electrons (oxidation number decreases).
- The oxidized substance "donates" electrons to the reduced substance.
Writing out redox reactions shows us exactly how electrons are being transferred. Let's get into the nitty-gritty!
Mnemonic Time! Pick your favorite:
- OIL RIG: Oxidation Is Loss, Reduction Is Gain
- LEO says GER: Loss of Electrons = Oxidation, Gain of Electrons = Reduction
Assigning Oxidation Numbers
To figure out who's gaining and who's losing, we need to assign oxidation numbers. Here are the rules:
- Free Elements: (like Brโ, Na, Pโ) have an oxidation number of 0.
- Neutral Molecules: The oxidation numbers of all atoms sum to 0. For example, in IFโ, if iodine's oxidation number is x and fluorine's is y, then x + 6y = 0. 3. Monatomic Ions: The oxidation number equals the ion's charge (e.g., Naโบ is +1, Clโป is -1).
- Oxygen: Usually -2, except in peroxides (HโOโ, Oโโปยฒ) where it's -1.
- Hydrogen: Usually +1, except in metal hydrides (LiH, BaHโ) where it's -1.
- Fluorine: Always -1. Other halogens are usually -1 but can vary.
- Fractions: Oxidation numbers can be fractions, but it's rare (e.g., Oโโป is -ยฝ).
Redox Practice Problem
Redox reactions are everywhere! From batteries to rusting to cellular metabolism, they are super important. For the AP exam, you'll need to know how to balance redox equations, both in acidic and basic solutions. Let's break it down.
Balancing Redox in an Acidic Solution
Let's use this example: 2Mg (s) + Oโ (g) โ 2MgO (s)
Step 1: Assign Oxidation Numbers
- Mg and Oโ are neutral, so they have oxidation states of 0.
- In MgO, oxygen is usually -2, so magnesium must be +2 to balance the charges.
Step 2: Write Half-Reactions
A half-reaction shows either the oxidation or reduction process separately. Let's write them out using the oxidation number changes we just found:
- Oxidation: 2Mg โ 2Mgยฒโบ
- Reduction: Oโ โ 2Oยฒโป
Step 3: Balance Charge with Electrons
Remember, we need to conserve charge! โโ
- In the oxidation half-reaction, we need to add 4 electrons to the product side to balance the charge: 2Mg โ 2Mgยฒโบ + 4eโป
- In the reduction half-reaction, we need to add 4 electrons to the reactant side to balance the charge: Oโ + 4eโป โ 2Oยฒโป
Step 4: Combine Half-Reactions
Now, add the two half-reactions together:
2Mg + Oโ + 4eโป โ 2Mgยฒโบ + 2Oยฒโป + 4eโป
Notice that there are 4 electrons on both sides. Cancel them out:
2Mg + Oโ โ 2Mgยฒโบ + 2Oยฒโป
๐ You've balanced a redox reaction! ๐
Remember that in ionic bonds, electrons are completely transferred, while in non-ionic bonds, electrons are shared. Oxidation numbers help us track electron movement in both types of bonds.
Balancing Redox in a Basic Solution
Balancing in a basic solution is similar, but with an extra step at the end. You'll need to neutralize the Hโบ ions with OHโป ions to form water (HโO), and then add the same amount of OHโป to the other side of the equation.
Steps for Balancing Redox (General)
โฌ๏ธ Here's a summary of the steps:
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Assign oxidation numbers and identify what's being oxidized and reduced.
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Write the oxidation and reduction half-reactions.
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Balance elements (except O and H).
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Balance oxygen atoms by adding HโO molecules.
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Balance hydrogen atoms by adding Hโบ ions.
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Balance the charge by adding electrons.
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Combine the half-reactions, canceling out any common species.
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For basic solutions: Neutralize Hโบ ions with OHโป to form HโO, and add the same number of OHโป to the other side.
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Double-check that atoms and charges are balanced.
Practice these steps repeatedly! The more you do it, the more natural it will become. Focus on understanding the logic behind each step, not just memorizing the process.
Final Exam Focus
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High-Priority Topics: Balancing redox reactions in both acidic and basic solutions, assigning oxidation numbers, understanding the difference between oxidation and reduction.
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Common Question Types: Multiple-choice questions on identifying oxidizing and reducing agents, free-response questions on balancing complex redox reactions, and short-answer questions involving oxidation state calculations.
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Time Management: Don't spend too long on one question. If you're stuck, move on and come back to it later. Make sure you have enough time to attempt every question.
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Common Pitfalls: Forgetting to balance charges, mixing up oxidation and reduction, and not applying the correct rules for assigning oxidation numbers.
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Strategies: Practice with a variety of examples, double-check your work, and don't be afraid to ask for help when you need it.
A common mistake is forgetting to balance the charges correctly by adding electrons on the right side of the equation. Always double check the charges on both sides of the equation.
Practice Question
Practice Questions
Multiple Choice Questions
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In the following reaction, which element is reduced?
(A) Mn (B) Cl (C) H (D) O
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What is the oxidation number of sulfur in the sulfate ion, SOโยฒโป?
(A) +2 (B) +4 (C) +6 (D) -2
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Which of the following is the correct half-reaction for the oxidation of Feยฒโบ to Feยณโบ?
(A) Feยฒโบ โ Feยณโบ + eโป (B) Feยณโบ + eโป โ Feยฒโบ (C) Feยฒโบ + eโป โ Feยณโบ (D) Feยณโบ โ Feยฒโบ + eโป
Free Response Question
Balance the following redox reaction in a basic solution:
Scoring Breakdown:
Points | Description |
---|---|
1 | Correctly assigning oxidation numbers to all species. |
2 | Writing the correct half-reactions for oxidation and reduction. |
2 | Balancing the half-reactions for mass and charge. |
1 | Combining the half-reactions and canceling out electrons. |
2 | Balancing the reaction in a basic solution by adding OHโป and forming HโO. |
1 | Final balanced equation with correct coefficients and states. |
Answers:
Multiple Choice
- (A) Mn
- (C) +6
- (A) Feยฒโบ โ Feยณโบ + eโป
Free Response
- Oxidation Numbers: Cr in CrโOโยฒโป is +6; Iโป is -1; Crยณโบ is +3; Iโ is 0. 2. Half-Reactions:
- Reduction: CrโOโยฒโป โ 2Crยณโบ
- Oxidation: 2Iโป โ Iโ
- Balanced Half-Reactions:
- Reduction: CrโOโยฒโป + 14Hโบ + 6eโป โ 2Crยณโบ + 7HโO
- Oxidation: 2Iโป โ Iโ + 2eโป
- Combined Reaction: CrโOโยฒโป + 14Hโบ + 6eโป + 6Iโป โ 2Crยณโบ + 7HโO + 3Iโ + 6eโป CrโOโยฒโป + 14Hโบ + 6Iโป โ 2Crยณโบ + 7HโO + 3Iโ
- Basic Solution: CrโOโยฒโป + 14Hโบ + 14OHโป + 6Iโป โ 2Crยณโบ + 7HโO + 14OHโป + 3Iโ CrโOโยฒโป + 7HโO + 6Iโป โ 2Crยณโบ + 14OHโป + 3Iโ
- Final Balanced Equation: CrโOโยฒโป(aq) + 7HโO(l) + 6Iโป(aq) โ 2Crยณโบ(aq) + 14OHโป(aq) + 3Iโ(s)
You've got this! Keep practicing, and you'll be a redox master in no time. Remember to stay confident and focused on test day! ๐

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Question 1 of 11
What is the main process that defines a redox reaction? ๐ง
The transfer of protons
The transfer of neutrons
The transfer of electrons
The transfer of energy