Concepts of oxidation and reduction
Oxidation states
The hypothetical oxidation state of a nucleus is equal to the charge it would have if all its bonds with other nuclei were completely ionic. A compound's oxidation state is the atomic charge it would have if all bonds were ionic with every atom in it. A hypothetical charge given by an atom if it is bonding between atoms of different elements is perfect is called the electronic charge. Various oxidation statescan be represented by numbers, either positive, negative, or zero. There are elements in which the oxygen content is a fraction of its average, such as the iron in magnetite (Fe3O4) which has an oxygen content of 8/3. Among the highest known oxidation states of chemical elements are those of ruthenium, xenon, osmium, iridium, and hassium; the lowest known oxidation state is that of some carbon-based elements.Oxidation involves losing electrons when an atom's oxidation state increases because of a chemical reaction; reduction involves gaining electrons when its oxidation state decreases due to a chemical reaction.
The Oxidation States: A General Guide
- In a free element (an element that isn't combined), the oxidation state is zero.
- Ions with a single atom's net charge have the same oxidation state as their net charge. Using Cl– as an example, an oxidation state of -1 is indicated.
- When hydrogen and oxygen are present in most compounds, their oxidation states are +1 and -2, respectively. An active metal's hydroxide (such as LiH) always has a negative oxidation state. Peroxides (such as H2O2) and superoxide (such as KO) have oxidation states of -1 and -1/2, respectively.
- A neutral molecule must have exactly zero oxidation state algebraic sum for all its atoms. Atoms of an ion must have an oxidation state equal to the algebraic sum of their charges.
Assessing Oxidative Stress
On the periodic table, the group number of most elements indicates their oxidation state.The oxidation state of nitrogen (a group V element) is usually -3, while that of boron (a group III element) is normally +3. The oxidation state of a substance can change, so this approach should only be seen as a guide; for example, transition metals vary widely in their oxidation states and rarely follow set formulas.
It is effective for the charge of a molecule or polyatomic ion to equal the sum of all the oxidation states of all the atoms in the molecule or polyatomic ion. By knowing the common oxidation state of all of the elements, we can determine which element within a molecule or ion has the corresponding oxidation state. Sulfite ions (SO32-) are assumed to possess a total charge of 2, and each oxygen atom is assumed to have an oxidation rate of 20%. Oxygen contributes to sulfite because of its three oxygen atoms for the total charge of 3×−2=−6. Thus, for sulfur to have a charge of 2 on sulfite, it must have an oxidation state of +4: (+4 –6 = -2).
Charge on an atom should not be confused with oxidation state (and it often is, when it comes to polyatomic ions), as they may be different. NH4+ contains a nitrogen atom that has a 1+ charge, whereas the formal oxidation state of nitrogen is -3, which is the same as ammonia. It is true, however, that the formal charge on the N atom does not change between ammonium and ammonia.
Types of redox reactions
Compound reactions may take one of five forms: combination, decomposition, displacement, combustion, or disproportionation.We are constantly exposed to redox reactions. Redox reactions underlie much of our technological advances, including fires and laptop batteries. When the state of oxidation of the reactant changes, it is called a redox reaction (reduction-oxidation), this occurs because electrons move between species during such reactions. There are two kinds of redox reactions: Simple processes involving combustion (for instance, burning carbon in oxygen to release CO2), or more complex ones, such as the oxidation of glucose (C6H12O6) in the human body through electron transfer.
Electrons are transferred between reduction and oxidation in a redox reaction. Here are some descriptions of these processes:
- An atom, molecule, or ion that undergoes oxidation loses electrons or increases its oxidation state.
- Molecules, atoms, or ions gain electrons or decrease their oxidation state when they reduce.
In redox reactions, one species is oxidized, which leads to another being reduced. Keep this in mind when learning about the redox reactions that include combinations, decompositions, displacements, combustions, and disproportions.
Combination
A chemical compound is created when elements are combined through a reaction. During reduction and oxidation, both processes occur simultaneously.A general equation would be: A + B → AB
Example – 2H2 + O2 → 2H2O
It follows that the reaction is effective when the product has the same oxidation state as the reactant: 0 + 0 x (2) (+1) + (-2) The oxidation states of H2 and O2 in this equation are identical since both are the molecular forms of their respective elements. Oxygen and hydrogen are oxidized to -2 for oxygen and +1 for hydrogen, resulting in H2O.
Decomposition
Compounds undergo degradation via decomposition reactions, i.e., they are decomposed into their constituent elements, the opposite of combination reactions.A general equation would be: AB → A + B
Example - 2 H2O → 2 H2 + O2
Calculation for equation would be:(2) (+1) + (-2) = 0 → 0 + 0
Hydrogen and oxygen, which are both neutral, are formed when water is "decomposed" in this equation. According to the same calculation as before, after decomposing H2O, oxygen is oxidized to 0, whereas hydrogen is reduced to 0, resulting in a total oxidation state of 0 for H2O.
Displacement
Compounds and elements are replaced during displacement reactions, which are also known as replacement reactions. The reactions may occur in either a single or double replacement configuration.A general equation would be: (Single displacement) - A + BC → AB + CA
A single replacement reaction involves substituting a reactant with another product element.
Example - Cl2 + 2 NaBr → 2 NaCl + Br2
Calculation would be: 0 + [(+1) + (-1) = 0] →→ [(+1) + (-1) = 0] + 0
As a result of this reduction, Cl replaces Br, while Br is oxidized.
A general equation for double displacement would be: AB + CD → AD + CB
In a double replacement reaction, two elements of the reactants are exchanged with two of the products. Reactions of this type resemble single replacement reactions.
Example: Fe2O3 + 6 HCl → 2 FeCl3 + 3 H2O
As well as Fe and H, O and Cl are also changing roles in this equation.
Combustion
Oxygen and organic fuel are required for combustion reactions.A general equation for combustion reaction would be:
CxHy + (x+y/4) O2 → xCO2+ y/2 (H2O)
Disproportion
The oxidation and reduction of substances are both possible in some redox reactions. The term disproportionation refers to simultaneous reactions. One example of this reaction is the one that occurs when hydrogen peroxide, H2O2, is poured into a wound. This reaction might appear simple at first glance since hydrogen peroxide decomposes to produce oxygen and water:2 H2O2(aq) → 2 H2O(l) + O2(g)
Using oxygen in its various oxidation states is key to this reaction, however. Oxygen is present in both products and the reactant. Although, Oxygen in H2O2 is in oxidation state -1 because it is in state -1. Hydrogen is reduced in H2O because its oxidation state is -2. However, when applied to O2, its oxidation state is zero, and therefore, it has already been oxidized! This reaction has reduced and oxidized oxygen simultaneously, making it a disproportionation reaction. This reaction generally takes the following form:
2A → A’ + A”
Applications of redox reactions
On food
Food industry titrations are widely used analytical techniques. Manufacturers of food can determine the amount of a reactive ingredient in a sample with this method. It can be used to determine how much salt or sugar is in a product, or the concentration of Vitamin C or E in a product that affects its color.On dentistry
A whitening agent which contains hydrogen peroxide or carbamide peroxide is effective in lightening teeth. The chemical composition of carbamide peroxide and hydrogen peroxide is highly similar; when applied to the teeth, carbamide peroxide turns into hydrogen peroxide. A tooth whitening procedure involves oxidizing hydrogen peroxide, which breaks the bonds of the chromogen molecules. Stains are caused by accumulations of chromogens, so when they are scattered, their color appears lighter.
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