SS2 Chemistry Lesson Note on Oxidizing and Reducing Agents

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OXIDIZING AND REDUCING AGENTS

1. Oxidizing agent: This is a substance which adds oxygen or removes hydrogen form a substance or a substance which accepts electron, i.e. electron acceptor. Consider the following reactions.
2. Oxidation in terms of addition of oxygen: oxidizing agent will add oxygen to a substance e.g.

C_(s) +2ZnO_(s)        →   CO_2(g) + 2Zn _(s)

ZnO  is the oxidising agent because it added oxygen to (i.e. oxidise) carbon to form CO₂

2. Oxidation in terms of removal of hydrogen: oxidizing agent will add oxygen to a substance e.g

2H₂S _(g) + O_{2 (g)}         →     2H₂O _(l) + 2S_(s) or

H₂S _(g) + Cl_{2 (g)}      →   2HCl _(g) + S_(s)

Oxygen and chlorine are the oxidizing agents because they removed hydrogen from H₂S to form sulphur

3. Oxidation in terms of loss of electrons: oxidizing agent will gain electrons e.g

2FeCl_2(s) + Cl_{2 (g)}          →        2FeCl_3(s)

Fe²⁺    → Fe³⁺    + 2e^–

Cl₂   + 2e^–    →  2Cl^–

The oxidizing agent is chlorine (Cl₂) because it gained electrons lost by Fe²⁺

Other means of identifying an oxidising agent is the substance.

1. Which is reduced
2. Whose oxidation number has decreased

1. Reducing agent: This is a substance which adds hydrogen to another substance or removes oxygen from that substance or this substance which donates election, i.e. an electron donor.

Consider the following reactions

1. Adds hydrogen e.g hydrogen sulphide reduces chloride to hydrogen chloride in

H₂S_(g) + Cl_2(g)      →       2HCl _(g) + 2S_(s)

2. Removes oxygen e.g. Carbon in

C_(s) + ZnO_(s)      →      CO_2(g) + 2Zn_(s)

3. Has its oxidation number increases e.g. in

Zn_(s) + CuSO_{4 (aq)}    →    ZnSO_{4 (a q)} + Cu_(s)

Zinc is the reducing agent because its oxidation number increased from zero in zinc to +2 in zinc tetraoxosulphate (vi) that is, it was oxidized.

The Table below give some common reducing agent and oxidizing agents.

OXIDIZING AGENTS

Oxygen	O2
Hydrogen peroxide	H2O2
Chlorine (and other halogen)	Cl2
Acidified potassium tetraoxomanganate (viii)	KMnO4
Acidified potassium heptaoxchromate (vi)	K2Cr2O7
Hot concentrated tetraoxsulphate (vi) acid	H2SO4
Concentrated trioxonitrate(v) acid	HNO3
Silver salt (and metals low in the series e.g cu)	Ag+

 

REDUCING AGENTS

Hydrogen	H2
Carbon	C
Potassium	KI
Hydrogen sulphide	H2S
Sulphur(iv) oxide	SO2
Iron(II) salt	SO2
Sodium (and other reactive metals)	Na
Ammonia	NH3

 

Generally, metals that are highly electro positive e.g. Na, K and Mg lose electrons easily and so are good reducing agents (they release their electrons to another thus reducing it). Non-metals that are highly electro negative e.g. O, Cl and F are good oxidizing agents (they easily accept electrons from another thus oxidizing it).

TEST FOR OXIDIZING AGENTS

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