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Redox reactions are those chemical reactions in which both oxidation as well as reduction occur simultaneously. 

• Oxidation and reductions go hand in hand. 
• The substance which undergo reduction is called oxidising agent while the substance which undergo oxidation is called reducing agent. One can say that the substance that causes the oxidation of any substance in reaction is called the oxidizing agent while the substance that causes the reduction is called the reducing agent.
• Neither reduction nor oxidation occurs alone. Both of them occur simultaneously. Since both these reactions must occur at the same time they are often termed as "redox reactions". The oxidation or reduction portion of a redox reaction, including the electrons gained or lost can be determined by means of a Half-Reaction.

Reactions which involve change in oxidation number of the interacting species.

(c) A reducing agent is one that undergo reduction is incorrect.

The compound nitric oxide is formed by the combination of the elemental substances, nitrogen and oxygen; therefore, this is an example of combination redox reactions. 

The reaction involves the breaking down of lead nitrate into three components; Therefore, this is categorized under decomposition redox reaction.

K = +1, Cl = +7, O = -2

c) Cl^-   species acts as reducing agent.

Decomposition of silver bromide in the presence of light.

Rewrite the two equations in the reduction form thus.

Pb²⁺(aq) + 2eE^– → Pb(s); 

E° = -0.13V ...(i)

Ag⁺(aq) + e^– → Ag(s); 

E° = + 0.80V …(ii)

To obtain the equation for the cell reaction, multiply Eq. (ii) with 2 and subtract Eq. (i) from it, we have,

Pb(s) + 2Ag⁺(aq) → Pb²⁺(aq) + 2Ag(s); 

E°cell = + 0.80 - (-0.13) 

= + 0.93V

It is oxidation.

E°(Fe³⁺/Fe²⁺) is lower than that of E°(Br/B^–).

Therefore Fe²⁺ reduce Br₂ but Br^– cannot reduce Fe³⁺. Thus Fe³⁺ cannot oxidize Br^– to Br₂.

Let the Ox. no. of Cr in K₂CrO₄ be x.

2(+1) + x + 4(-2) = 0 

+2 + x - 8 = 0 

x = +6

No, as CuSO₄ reacts with Zn. 

(iii) Phosphorus is undergoing oxidation as well as reduction.

(iv) Hydrogen is undergoing neither oxidation nor reduction.

(i ,ii) The electrodes having negative electrode potentials are stronger reducing agents than H₂ gas and therefore, will act as anodes.

MnO₄ ^- possessMn in highest oxidation state (+7) But in MnO₄ ^2- ,Mn shows +6 oxidation state Hence MnO₄ ^2- undergoes disproportionation reaction but MnO₄ ^- does not.

F is most electronegative element and cannot possess three oxidation states. Hence it cannot undergo disproportionation reactions. 

Decomposition of hydrogen peroxide produces water and oxygen. O.N Increases as well as decreases. 

(i) Decomposition reaction 

(ii) Double decomposition reaction 

Loss of electron by an atom is called oxidation.

Reductant: Which increases its own ox. no. and decreases that of other in a chemical reaction.

Reduction is a process in which gain of electrons takes place.

In this reaction, Na is oxidised to Na⁺ and hydrogen is reduced from H₂ and H^- ion. Therefore it is a redox reaction.

2NaO + H₂O → 2Na⁺ H^-1(s)

The cell works on the principle of redox reactions . One of the reactant undergoes oxidation while another undergoes reduction.

c) A is true but R is false.

O.N of Cr in Cr₂O₇ ^2- = +6 

O.N of Cr in Cr₂O₇ ^2- = +6 

Since the oxidation number of Cr has neither decrease nor increased, the above reaction is not a redox reaction.

The correct answer is (iv) Ag⁺

(iv) Cu will reduce Br₂

Oxidising Agent 

The substance (atom, ions or molecules) that gain electrons and is thereby reduced to a low valency state is called an oxidising agent. 

When the reactant and products involved in a chemical change are written in molecular form in a chemical equation, it is termed as molecular equation.

Example: MnO₂ + 4HCl → MnCl₂ + 2H₂O +Cl₂ 

In above example the reactant and products have been written in molecular forms, thus it is a molecular equation

When the reactant and products involved in a chemical change are ionic compounds, these will be present in the form of ions in the solution. The chemical change is written in ionic forms in the chemical equation, it is termed as ionic equation. 

Example: MnO₂ + 4H⁺ + 4Cl^- → Mn²⁺+ 2Cl^- + 2H₂O +Cl₂ 

In the above example, the reactant and products have been written in ionic forms, thus the equation is termed as ionic equation.

A series of elements arranged in decreasing order of their standard electrode potential.

ON of P in PO4^3- is x . so x +(-2)x4 = -3; or, x = +5.

It maintains the electrical neutrality of the half cells.

Salt bridge is inverted U tube like structure that contain strong electrolyte (KCl. NH₄NO₃) in agar agar gel.

(i) To maintain electrical neutrality of the two solutions. 

(ii) To complete the circuit. 

(iii) It allows the flow of current.

It can behave as anode and cathode depending upon another electrode taken. 

(a) When it acts as anode reaction will be :- 

H₂ ---> 2H⁺ + 2e 

Representation will be :- Pt, H₂ (1atm.) / H⁺ ( 1M) 

(b) 2H⁺ + 2e^- -----> H₂ 

Representation will be :- / H⁺ ( 1M) / H₂ (1atm.), Pt

Fe₂O₃ ..............

The arrangement of elements in the order of increasing standard reduction potential is known as electrochemical series.

P₄ + NaOH ------> PH₃ + H₂PO₂^-

(i) At Anode :- Zn ------> Zn⁺² + 2e^- 

(ii) At Cathode :- Cu⁺² + 2e^- ------> Cu 

The current will flow from Cu electrode to Zn electrode .

(iv) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Fe⁺² / Fe and Sn⁺² / Sn

Cl₂(g) + 2 OH^– (aq) ---------> ClO^– (aq) + Cl^– (aq) + H₂O (l) 

 So, Household bleaching agent can be formed by dissolving chlorine in the base. 

Oxidation is a process in which loss of electrons take place.

Because F Can show only -1 oxidation state and not +ve O.S. due to highest electronegativity.