1. Introduction
Redox in short means reduction-oxidation. In such a reaction, these two processes take place simultaneously. There can be no oxidation, without the corresponding reduction and vice versa.
Browning of apples
In the presence of oxygen, the enzyme polyphenol oxidase (phenolase) catalyses the conversion of plant phenolic compounds into brown melanins.
Enzyme controlled browning can be slowed down by the presence of sulfite ions, ascorbic acid (vitamin C) and ethanoic acid (vinegar).
Flatulence
Sulfur-containing organic compounds are to blame for the variety of odours produced in the digestive system. The main component, hydrogen sulfide, is formed by the reduction of sulfates by sulfate-reducing bacteria in the gut.
Sulfate, which is poorly absorbed in the small bowel, is naturally present in cruciferous vegetables (cabbage, broccoli) and nuts and as an additive in bread and beer. Another major dietary source is sulfur-containing amino acids (methionine, cysteine) present in protein.
2. Gain/Loss of Oxygen
The Statue of Liberty is an icon of freedom and of the United States. The Statue's green colour is a result of oxidation by atmospheric oxygen. We will learn more about corrosion in the chapter of Metals. Meanwhile, read on to learn more from the sign on the right!
Blue Bottle
An alkaline solution of glucose acts as a reducing agent and reduces added methylene blue from a blue to a colourless form. Shaking the solution raises the concentration of oxygen in the mixture and this oxidises the methylene blue back to its blue form. When the dissolved oxygen has been consumed, the methylene blue is slowly reduced back to its colourless form by the remaining glucose, and the cycle can be repeated many times by further shaking.
3. Gain/Loss of Hydrogen
The Liver in Detoxification
In humans, the enzyme alcohol dehydrogenase (ADH) catalyses the redox reaction involving the breakdown of alcohol in the body, which would otherwise be toxic.
In this reaction, ethanol is oxidised (it loses hydrogen to form acetaldehyde) and a coenzyme called nicotinamide adenine dinucleotide is reduced (it gains hydrogen).
4. Gain/Loss of Electrons
Photochromic Lenses
It is very uncomfortable on the eyes to move rapidly in and out of intense sunlight and shadow, even if we are wearing sunglasses. Our eyes cannot adjust quickly enough to sudden changes of light intensity, unless we are wearing sunglasses made from photochromic lenses, which use chemical changes to compensate for changes in brightness.
Lens glass is made photochromic by adding tiny amounts of silver chloride and copper(I) chloride to the molten glass as it cools, which traps the crystals within the structure of the glass.
The overall ionic equation is:
Lighter Darker
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Using gain/loss of electrons, state the species oxidised and reduced.
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Write the half-equations for the reaction.
5. Changes in Oxidation State
Iridium forms compound in +9 oxidation state
Until now, the highest oxidation state any element was shown to exist at was +8.
An international team of scientists has made a compound containing iridium in the +9 oxidation state – something that has been predicted by theoretical models but never formed experimentally before.
Question: The +9 oxidation state of iridium was recorded in the [IrOₓ]⁺ ion. Calculate the value of x.
6. Oxidising and Reducing Agents
Antioxidants: Beyond the Hype
Your body generates free radicals as the inevitable byproducts of turning food into energy. These free radicals 'steal' electrons (by behaving as oxidising agents) from nearby molecules (such as DNA or proteins), causing damage to them.
When these reactive free species are in abundance, there will be an imbalance in the redox reaction favoring oxidation, which results in oxidative stress. Longevity of a species as well as the ability of individual organisms to withstand oxidative stress and inflammation can thus be critically linked to the efficacy of its antioxidant system. An antioxidant (better known as a reducing agent in Chemistry) is a molecule that inhibits the oxidation of other molecules.
Redox Reactions 3: Oxidising vs. Reducing Agents
In this video, we will learn what oxidising and reducing agents are and explore how we can use chemical tests to identify them. We will then consolidate what we have learnt about redox reactions and focus on phrasing of answers for redox questions.
Download the worksheet that accompanies this video so you can follow along!
Tick Tock, The Vitamin C Clock
Clock reactions represent a unique class of chemical reactions. In this particular reaction, two colorless solutions are mixed and nothing happens at the first moment. However, after a few seconds the solution suddenly changes its color to dark blue. This is a so called clock mechanism: we cannot observe any visual changes immediately after reactants are mixed, but color changing takes place suddenly after some time – this time is known as induction period.
Questions
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Which is the oxidising and reducing reagents in this experiment?
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State the change in oxidation state of iodine in this experiment.
7. Advanced Concepts in Redox Reactions
Redox Reactions 7: Using Quantitative Data to Determine Oxidation States
In this video, we will look at examples of chemical calculation questions that involve redox reactions, and see how the quantitative data can be used to determine the change in oxidation state in a substance.
Download the worksheet that accompanies this video so you can follow along!