S + O2 → SO2
In the given reaction, oxygen is added to sulfur to form sulfur dioxide. Hence sulfur gets oxidized to sulfur dioxide.
Zn + 2HCl → ZnCl2 + H2
While in this reaction, hydrochloric acid gets oxidized to zinc chloride.
"The addition of electronegative elements or removal of electropositive element is also termed as oxidation". According to the modern concept of oxidation, a reaction in which any element or atom donates a number of electrons and gets converted to a cation is termed as oxidation reaction. Hence the oxidation number increases in an oxidation reaction. Removal of electrons can take place from,
An atom and form cation: Na → Na+ + e-
A cation and form another cation with more charge: Sn 2+ → Sn4+ + 2e-
An anion and form atom: 2Cl- → Cl2
A complex anion: [Fe(CN)6]4- → [Fe(CN)6]3- + e-
Hence in oxidation process:
H2 + Cl2 → 2HCl (Addition of hydrogen to chlorine)
ZnO + H2 → Zn + H2O (Removal of oxygen from zinc oxide)
"A reaction corresponding to the addition of an electropositive element and the removal of an electronegative element is also termed as reduction".
According to the modern concept, a chemical reaction in which an element or ion accepts electrons to form anions is termed as reduction. For example,
The substance which gets reduced is known as the oxidizing agent and the substance which gets oxidized is termed as reducing agent.
If we have pure carbon and a complete combustion, carbon is converted into carbon dioxide. The reaction can be given as below.
C(s) + O2(g) → CO2(g)
Here, the oxidation number of carbon changes from 0 in C(s) to +4 in CO2. Simultaneously, oxygen is reduced from 0 in O2 (g) to -2 in CO2 (g).
C (s) + O2 (g) → CO2 (g)
Here, the oxidation number of carbon changes from zero in C(s) to +4 in CO2. Simultaneously, oxygen is reduced from zero in O2 (g) to -2 in CO2 (g).
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Let us take a simple example and try writing the redox equations for it. There are two main steps in writing redox equations. They are:
Copper metal reacts with aqueous silver nitrate solution to form aqueous Cu (II) ions in solution and solid silver metal.
Copper metal + Aqueous Silver (I) ions → aqueous Cu (II) ions + Silver metal
The half reaction for the oxidation process is the loss of two electrons from the more electro positive, copper metal.
Cu (s) → Cu 2+ (aq) + 2e-
In the corresponding reduction process, a silver ion gains a single electron to form silver metal.
Ag+ (aq) + e- → Ag (s)
Now, let us try balancing redox reactions. All the electrons lost in the oxidation half reaction must be accounted for by the redox half reactions. So, in order to balance redox reactions, two silver ions must be accepting the two electrons lost by the Cu to form two atoms of Ag(s).
So, the reduction equation becomes,
2 Ag+ (aq) + 2 e- → 2 Ag (s)
We get this by multiplying the reduction half reaction by 2.
Now, the final step is to add the two half equations:
Cu (s) → Cu 2+ (aq) + 2 e- oxidation half reaction
2 Ag + (aq) + 2 e- → 2 Ag (s) reduction half reaction
Cu (s) + 2 Ag + (aq) → Cu 2+ (aq) + 2 Ag (s) net balanced redox reaction
There are a number of steps involved in the redox reactions. They are as follows.
In oxidation reduction reactions or redox reactions the electrons are transferred from one species to another, the species which loses the electrons is oxidized while the one which gains electrons is reduced. There are numerous examples of redox reactions in our every day life. Some of them are corrosion reactions, reactions in batteries, and some are metabolic reactions in the body.
Oxidation is the loss of electrons by a species, leading to an increase in the oxidation number of one or more atoms.
Reduction is the gain of electrons by a species, leading to a reduction in the oxidation number of one or more atoms.
Oxidizing agent is the chemical species causing the oxidation. This species is reduced and can also be called the electron acceptor.
Reducing agent is the species causing the reduction. This species is oxidized and can be called the electron donor.
A Redox Reaction: Silver Coating Copper
Silver nitrate + copper → silver + copper nitrate
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Given below are some of the redox reaction examples along with their solutions.
The reaction between zinc and hydrochloric acid can be represented as below:
Zn (s) + 2 HCl (aq) → ZnCl2 (aq) + H2 (g)
The net ionic equation can be written as,
Zn (s) + 2H+ (aq) → Zn 2+ (aq) +H2 (g)
Now, let us determine the oxidation numbers.
Zn (s) and H 2 (g) are elements. So, their oxidation numbers are zero each. H+ has an oxidation number of +1 and Zn 2+ that of 2+.
Zn loses electrons to form Zn 2+. This is the oxidation reaction. H+ accepts electrons to form H2. This is the reduction reaction.
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)
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