Electrochemistry: Identify Oxidation and Reduction Half-Reactions
Question
Identify each half-reaction below as either oxidation or reduction.
- Cr (s) ⟶ Cr3+ (aq) + 3e−
- Li+ (aq) + e− ⟶ Li (s)
Show/Hide Answer
- Oxidation
- Reduction
Refer to Section 9.1: Balancing Oxidation-Reduction Reactions (1).
Strategy Map
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Check out the strategy map.
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| Strategy Map Steps |
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1. Identify the oxidation numbers of the reactant and product species..
Show/Hide HintRecall oxidation numbers:
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2. Determine if electrons are being gained or lost.
Show/Hide HintRecall that the total charge must be the same between the reactant and product side of the chemical reaction equation:
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Solution
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a. Cr (s) ⟶ Cr3+ (aq) + 3e−
Chromium is becoming more positively charged and losing electrons; therefore, it is being oxidized.
Answer: Oxidization
b. Li+ (aq) + e− ⟶ Li (s)
Lithium is getting less positively charged and gaining electrons; therefore, it is being reduced.
Answer: Reduction
Guided Solution
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The guided solution below will give you the reasoning for each step to get your answer, with reminders and hints.
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| Guided Solution Ideas |
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This question is a theory type problem where you must identify what type of reaction you are provided with using your knowledge on redox reactions.
Show/Hide ResourceRefer to Section 9.1: Balancing Oxidation-Reduction Reactions (1). |
Recall the characteristics of an oxidation reaction
Show/Hide Think About This!Oxidation is a process where electrons are lost from the species. The product will have a more positive oxidation number compared to the reactant. When electrons are ‘lost,’ they appear on the product side of the half-reaction. Note: The reactant species that is oxidized acts as the ’reducing agent’. |
Recall the characteristics of a reduction reaction
Show/Hide Think About This!Reduction is a process where electrons are gained. The product will have a more negative oxidation number compared to the reactant. When electrons are ‘gained,’ they appear on the reactant side of the half-reaction. Note: The reactant species that is reduced acts as the ’reducing agent’. |
| Assess each provided reaction to determine if electrons are being lost or gained.
a. Cr (s) ⟶ Cr3+ (aq) + 3e− Chromium is becoming more positive and losing 3 electrons. This is an oxidation half reaction. Show/Hide Watch Out!Cr(s) is being oxidized. Cr(s) is acting as the reducing agent. b. Li+ (aq) + e− ⟶ Li (s) Lithium is becoming less positive and electron and gaining 1 electron. This is a reduction half-reaction. Show/Hide Watch Out!Li+(aq) is being reduced. Li+(aq) is acting as the oxidizing agent. |
| Complete Solution |
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| a. Cr (s) ⟶ Cr3+ (aq) + 3e−
Chromium is becoming more positively charged and losing electrons as products; therefore, it is being oxidized. This is an oxidation half-reaction. Answer: Oxidization |
| b. Li+ (aq) + e− ⟶ Li (s)
Lithium is becoming less positively charged and gaining an electron as a reactant; therefore, it is being reduced. This is a reduction half-reaction. Answer: Reduction |
Check Your Work
Each of these reactions are called half reactions. They occur in pairs which is why they are called redox reactions. In one reaction (the oxidation reaction), electrons are lost as products, and in another (the reduction reaction), electrons are gained as reactants.
Refer to Section 9.1: Balancing Oxidation-Reduction Reactions (1).
Does your answer make chemical sense?
Show/Hide Answer
It makes sense that electrons (each with -1 charge) will be added to the more positively charged side of the reaction. This will balance the total charge between the product and reactant sides of the reaction equation.
PASS Attribution
- LibreTexts TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) (2).
- Question 9.E.1 (b & d) from LibreTexts TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520) (3) is adapted under a CC BY-NC-SA 4.0 license.
- Question 9.E.1 (b & d) is adapted from question Q1 (b & d) from OpenStax Chemistry 2e (4), which is under a CC BY-NC-SA 4.0 license.
References
1. OpenStax. 9.1: Balancing Oxidation-Reduction Reactions. In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/09%3A_Electrochemistry/9.01%3A_Balancing_Oxidation-Reduction_Reactions.
2. Thompson Rivers University. TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2024. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520).
3. Thompson Rivers University. 9.E: Exercises on Electrochemistry. In TRU: Fundamentals and Principles of Chemistry (CHEM 1510 and CHEM 1520); LibreTexts, 2022. https://chem.libretexts.org/Courses/Thompson_Rivers_University/TRU%3A_Fundamentals_and_Principles_of_Chemistry_(CHEM_1510_and_CHEM_1520)/09%3A_Electrochemistry/9.E%3A_Exercises_on_Electrochemistry.
4. Flowers, P.; Theopold, K.; Langley, R.; Robinson, W. R. 17.1 Review of Redox Chemistry. In Chemistry 2e; OpenStax, 2019. https://openstax.org/books/chemistry-2e/pages/17-exercises.
