REDOX (GCSE)
KS4 National Curriculum Statement(s) covered
- Reduction and oxidation in terms of loss or gain of oxygen
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Oxidation and reduction, often referred to collectively as redox reactions, are fundamental concepts in chemistry that describe the changes substances undergo during chemical reactions. These processes involve the transfer of oxygen or electrons between substances. The study of redox reactions has a long history, dating back to the discovery of combustion and early experiments with metals and gases.
Gain/loss of Oxygen
Traditionally, oxidation and reduction were defined in terms of the addition or removal of oxygen. These terms were first coined in the 18th century by Antoine Lavoisier, a French chemist. Lavoisier's work revolutionised the understanding of chemical reactions and laid the foundation for modern chemical nomenclature.
Oxidation:
- Oxidation is the process of adding oxygen to a substance.
- For example, when magnesium reacts with oxygen to form magnesium oxide, magnesium is oxidised:
magnesium + oxygen → magnesium oxide
2Mg + O₂ → 2MgO - In this reaction, magnesium (Mg) combines with oxygen (O₂) to form magnesium oxide (MgO).
Reduction:
- Reduction is the process of removing oxygen from a substance.
- For example, when copper(Ⅱ) oxide reacts with hydrogen to form copper and water, copper(Ⅱ) oxide is reduced:
copper(Ⅱ) oxide + hydrogen → copper + water
CuO + H₂ → Cu + H₂O - In this reaction, copper(Ⅱ) oxide (CuO) loses oxygen and forms copper (Cu).
Gain/loss of Electrons
In a broader and more comprehensive sense, oxidation and reduction are defined in terms of the loss or gain of electrons. This shift towards an electron-based understanding of redox reactions occurred in the late 19th and early 20th centuries, as the nature of the atom and the role of electrons in chemical bonding became clearer.
The discovery of the electron by J.J. Thomson in 1897, and the further development of atomic theory, provided a deeper understanding of how atoms interact. It became evident that chemical reactions, including oxidation and reduction, are driven by the transfer of electrons between atoms and molecules. This electron transfer is what fundamentally changes the chemical properties of the substances involved.
Oxidation:
- Oxidation is the loss of electrons.
- For example, in the reaction between zinc and copper(Ⅱ) sulfate, zinc is oxidised as it loses electrons:
zinc + copper(Ⅱ) sulfate → zinc sulfate + copper
Zn + CuSO₄ → ZnSO₄ + Cu
Reduction:
- Reduction is the gain of electrons
- For example, in the reaction between zinc and copper(Ⅱ) sulfate, copper is reduced as it gains electrons:
zinc + copper(Ⅱ) sulfate → zinc sulfate + copper
Zn + CuSO₄ → ZnSO₄ + Cu
Reducing and Oxidising Agents
In redox reactions, substances that cause oxidation and reduction are known as oxidising agents and reducing agents, respectively.
Oxidising agent:
- An oxidising agent is a substance that causes oxidation by accepting electrons from another substance. It gets reduced in the process.
- For example, in the reaction between copper(Ⅱ) oxide and hydrogen:
CuO + H₂ → Cu + H₂O
Copper(Ⅱ) oxide (CuO) acts as the oxidising agent because it accepts electrons from hydrogen.
Reduction:
- A reducing agent is a substance that causes reduction by transfering electrons to another substance. It gets oxidised in the process.
- In the same reaction, hydrogen (H₂) acts as the reducing agent because it transfers electrons to copper(Ⅱ) oxide.
Half Equations
To better understand the electron transfer in redox reactions, chemists use half equations. Half equations separately show the oxidation and reduction processes, making it easier to see how electrons are transferred between reactants.
Oxidation half equation:
- This shows the substance that loses electrons.
- For example, the oxidation of zinc:
Zn → Zn²⁺ + 2e⁻ - Here, zinc (Zn) loses two electrons (2e⁻) to form zinc ions (Zn²⁺).
Reduction half equation:
- This shows the substance that gains electrons.
- For example, the reduction of copper(Ⅱ) ions:
Cu²⁺ + 2e⁻ → Cu - Here, copper(Ⅱ) ions (Cu²⁺) gain two electrons (2e⁻) to form copper metal (Cu).
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Did you know?
- Plants use redox reactions in photosynthesis to convert carbon dioxide and water into glucose and oxygen, providing energy for growth and releasing oxygen into the atmosphere.
- Respiration is a series of redox reactions. Glucose is oxidised to produce carbon dioxide, water, and energy, which cells use to perform various functions.
- Many household bleaches and disinfectants work through redox reactions, where the active ingredient oxidises and kills bacteria and other pathogens.
- Batteries operate based on redox reactions. In a typical battery, the anode undergoes oxidation, and the cathode undergoes reduction, generating electricity.
Why do we care?
- Knowing about the gain and loss of oxygen in reactions helps in processes like rust prevention and combustion in engines.
- Understanding the gain and loss of electrons in redox reactions explains how batteries work, providing power for everyday devices.
- Identifying reducing and oxidising agents is crucial for safety in chemical handling and in applications like disinfecting water with chlorine.
- Writing half equations helps us understand processes in electroplating, where a thin layer of metal is coated on objects for durability and aesthetics.
Key information
- Oxidation is the gain of oxygen, or loss of electrons.
- Reduction is the loss of oxygen, or gain of electrons.
- Oxidising agents cause oxidation by accepting electrons and being reduced.
- Reducing agents cause reduction by losing electrons and being oxidised.
- Exothermic reactions release energy (heat) to the surroundings, and endothermic reactions absorb energy (heat) from the surroundings.
- Half equations show the loss/gain of electrons to chemical species.