Oxidation – Classical and Modern Definition of Oxidation, Oxidising Agents, Practice Problems and FAQ

Everyone would have had the terrible experience of eating stale butter. It would taste weird and has a bad smell.

Have you ever wondered why butter turns stale?

When food containing fats or oils comes into contact with sufficient oxygen or bacterial action or moisture, they are completely or incompletely auto-oxidised to form short-chain fats, ketones and aldehydes, and this process is called rancidification. In other words, the oxidation of fats and oils is what causes the unwanted smell and flavour of butter.

We now understand that rancidification is a result of oxidation, but what is oxidation?

This concept page gives a brief account of oxidation, so let’s get to know about oxidation without any further ado!

TABLE OF CONTENTS

Oxidation
Classical Idea of Oxidation
Modern Definition of Oxidation
Oxidising Agents or Oxidants
Difference between Oxidation and Reduction
Redox Reactions
Practice Problems
Frequently Asked Questions – FAQ

Oxidation

Early on, the term "oxidation" was used to describe the reaction of an element with oxygen. The definition was later modified to include all processes in which electrons are lost, regardless of whether oxygen was present after it was discovered that the element loses electrons when it is oxidised.

Oxidation is the process of addition of oxygen or an electronegative element or group or the removal of hydrogen or an electropositive element or group. Oxidation also refers to the loss of electrons from the element in consideration or the increase in the oxidation state of the element in consideration.

When you cut (or bruise) an apple, oxygen enters the harmed plant tissue. The polyphenol oxidase (PPO) enzymes in the chloroplasts quickly oxidise the phenolic compounds that are naturally present in apple tissues to o-quinones, which are colourless precursors to secondary products that are brown when oxygen is present in cells.

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Classical Idea of Oxidation

According to classical concepts, oxidation is a process in which oxygen or any other electronegative element is added, or hydrogen or another electropositive element is removed.

The process of an atom or ion losing one or more electrons is known as oxidation, according to the electronic notion.

Oxidation can be explained using four different cases.

Case 1: Addition of oxygen

$M g (s) + O_{2} (g) \to M g O (s) (o x i d a t i o n o f m a g n e s i u m : (0 \to + 2))$

Case 2: Removal of hydrogen

${2 H}_{2} O (l) \to 2 H_{2} (g) + O_{2} (g) (o x i d a t i o n o f o x y g e n : (- 2 \to 0))$

Case 3: Addition of an electronegative element

$H_{2} O (g) + F_{2} (g) \to 2 H F (g) + O_{2} (g) (o x i d a t i o n o f o x y g e n : (- 2 \to 0))$

Case 4: Removal of an electropositive element

${2 K I + H}_{2} O_{2} (l) \to I_{2} (g) + 2 K O H (g) (o x i d a t i o n o f i o d i n e : (- 1 \to 0))$

Modern Definition of Oxidation

Oxidation is any chemical reaction in which electrons are lost by an element. Actually, it signifies that the element whose oxidation state increases has undergone oxidation.

Iron metal is oxidised to produce iron oxide, also known as rust. It increases the oxidation state of iron.

Fe (s) is in 0 oxidation state. When it is oxidised, the oxidation state changes to (+3) in Fe2O3.xH2O (s). Hence, the oxidation state increases from 0 to +3.

$F e (s) + O_{2} (g) + x H_{2} O (l) \to F e_{2} O_{3} . x H_{2} O (s)$

Another example of an element reacting with oxygen is when magnesium metal and oxygen combine to form magnesium oxide. Mg (s) is in 0 oxidation state. When it is oxidised, the oxidation state changes to (+2) in MgO (s). Hence, the oxidation state increases from 0 to +2.

$M g (s) + O_{2} (g) \to M g O (s)$

Oxidising Agents or Oxidants

An oxidant or an oxidising agent is a compound that can oxidise others while also getting reducing itself during a chemical reaction. In other words, an oxidising agent is a reagent that gains electrons in a redox reaction.

$2 A l (s) + {F e}_{2} O_{3} (s) + h e a t \to 2 F e (s) + {A l}_{2} O_{3} (s)$

In the reaction given above, Fe in Fe2O3(s) is in +3 oxidation state and changes to 0 oxidation state in Fe(s). Thus, the oxidation state of Fe decreases, which implies that Fe2O3(s) is reduced to Fe(s). Hence, Fe2O3(s) is an oxidising agent.

Difference between Oxidation and Reduction

Oxidation	Reduction
Loss of electrons	Gain of electrons
Addition of Oxygen (O)	Removal of Oxygen (O)
Removal of Hydrogen (H)	Addition of Hydrogen (H)
Addition of an electronegative element	Removal of an electronegative element
Removal of an electropositive element	Addition of an electropositive element
Oxidation state increases	Oxidation state decreases

Redox Reactions

A chemical process known as a reduction-oxidation or redox reaction happens when both reduction and oxidation take place simultaneously. While the oxidised species loses electrons, the reduced species gains them.

Example: $H_{2} O (g) + F_{2} (g) \to 2 H F (g) + O_{2} (g)$

$R e d u c t i o n : F_{2} (0) \to H F (- 1);$

$O x i d a t i o n : H_{2} O (- 2) \to O_{2} (0)$

There is clearly a change in the oxidation states of O and F, which involves both oxidation and reduction. As a result, the ensuing reaction is a redox reaction.

Example: $Z n O (s) + H_{2} (g) \to Z n (s) + H_{2} O (a q)$

$R e d u c t i o n : Z n O (+ 2) \to Z n (0);$

$O x i d a t i o n : H_{2} (0) \to H_{2} O (+ 1)$

There is clearly a change in the oxidation states of H and Zn, which involves both oxidation and reduction. As a result, the ensuing reaction is a redox reaction.

Practice Problems

1. Which of the following statements about oxidation is correct?

Oxidation of an element occurs when the oxidation state decreases.
Oxidation of an element occurs when the oxidation state increases.
Oxidation of an element occurs when it gains electrons.
Oxidation of an element occurs when there is the addition of oxygen only.

Answer: B

Solution: The addition of oxygen is not a necessary condition for an oxidation process despite its name. When an element loses its electrons, its oxidation state increases. Oxidation of an element occurs when there is the addition of oxygen or any other electronegative element and only on the addition of oxygen.

Thus, the statements given in options A, C and D are incorrect, but the statement given in option B is correct.

So, option B is the correct answer.

2. According to classical or previous concepts, oxidation is a process

in which oxygen is removed
in which hydrogen is added
in which electropositive element is added
in which an electronegative element is added

Answer: D

Solution: In accordance with traditional or earlier conceptions, oxidation is a reaction in which hydrogen or another electropositive element is removed and oxygen or another electronegative element is added.

For example, O in H2O has -2 oxidation state, when F2 being electronegative element is added, the oxidation state of O increases from -2 to 0 in O2 and oxidation of oxygen takes place.

$H_{2} O (g) + F_{2} (g) \to 2 H F (g) + O_{2} (g)$

So, option D is the correct answer.

3. Which of the following elements is oxidised in the given reaction?

${Z n C O}_{3} (s) \to {Z n O (s) + C O}_{2} (g)$

Zn
C
O
None of these

Answer: D

Solution: In the given reaction, ${Z n C O}_{3} (s) \to {Z n O (s) + C O}_{2} (g)$

Zn has +2 oxidation state in both ZnCO3 and ZnO. Hence, there is no gain or loss of electrons.

O has -2 oxidation state in ZnCO3, CO2and ZnO. Hence, there is no gain or loss of electrons.

C has +4 oxidation state in both ZnCO3 and CO2. Hence, there is no gain or loss of electrons.

If there is no gain or loss of electrons in a reaction, neither reduction nor oxidation has taken place.

So, option D is the correct answer.

4. In the given reaction, which of the following elements gets oxidised?

${2 M n O}_{4}^{-} + 10 I^{-} + 16 H^{+} \to 2 {M n}^{2 +} + 5 I_{2} + 8 H_{2} O$

Mn
I
O
None of these

Answer: B

Solution: In the given reaction, ${2 M n O}_{4}^{-} + 10 I^{-} + 16 H^{+} \to {2 M n}^{2 +} + 5 I_{2} + 8 H_{2} O$

Mn has +7 oxidation state in MnO4- and +2 oxidation state in Mn2+. Hence, there is decrease in oxidation state from +7 to +2 . Therefore, it has gained electrons, or in other words, it undergoes reduction.

O has -2 oxidation state in MnO4-and H2O. Hence, there is no gain or loss of electrons, or in other words, it undergoes neither oxidation nor reduction.

I has -1 oxidation state in I- and 0 oxidation state in I2. Therefore, it has lost electrons, or in other words, it undergoes oxidation. Thus, there is an increase in the oxidation state from -1 to 0.

Oxidation takes place when there is an increase in the oxidation state. Hence, in this reaction, I- is oxidised.

So, option B is the correct answer.

Frequently Asked Questions – FAQ

1. What practical use does oxidation have?
Answer: Oxidation occurs frequently in daily life processes like breathing, combustion, and plant photosynthesis. Oxidation leads to the rancidification of fats and the rotting of fruits.

2. What would happen if oxidation didn't occur naturally?
Answer: If oxidation does not happen naturally, all-natural metabolic processes like breathing and photosynthesis, life would not exist as oxidation is significant in these reactions.

3. Can oxidation happen in the absence of reduction?
Answer: Without concurrent reduction, oxidation cannot occur. The electrons lost by the element undergoing oxidation must be gained or accepted by another element. Therefore oxidation cannot happen in the absence of reduction.

4. Does oxidation have any effect on the colour of a species?
Answer: Yes, oxidation has an effect on the colour of species. For example, when transition metal is oxidised, the oxidation state of transition metals increases and the amount of splitting of the d orbitals also varies. Hence, it alters the colour of the light absorbed and, as a result, the colour of species changes.