Redox Reaction

Oxidation is a reaction where oxygen combines with an element/compound to form another substance

Oxidation – Gain of oxygen

When calcium burns in oxygen, the following reaction takes placecalcium + oxygen calcium oxide2Ca(s) + O2 2CaO(s)

Since calcium has gained oxygen. It has be oxidised. Hence this is oxidation.

Oxidation – Loss of Hydrogen

When ammonia is passed over heated copper(II) oxide, the following reaction takes place.

ammonia + copper(II) oxide nitrogen + copper + water vapour2NH3(g) + 3CuO(s) N2(g) + 3Cu(s) + 3H2O(g)

Since ammonia has lost hydrogen, it has been oxidised.

Oxidation – Loss of Electrons

Oxidation can still take place despite having no oxygen/hydrogen. When a substance loses electrons it is considered oxidation.

magnesium + chlorine magnesium chlorideMg(s) + Cl2(g) MgCl2(s)

What is Reduction?

Definition Of Reduction

• Reduction is a chemical reaction where it loses oxygen, gain hydrogen, gain electron and have a decrease in oxidation state by a molecule, atom or ion.

• A reactant that reduces another substance is a reducing agent.

• The reducing agent transfers electrons to another substance, it reduces others, and is thus itself oxidized. And, because it "donates" electrons, it is also called an electron donor.

Reduction-Loss Of Oxygen

When zinc burns in copper(II) oxide, the following reaction takes place.Zinc + copper oxide Zinc oxide + copperZn(s)+CuO(s) ZnO(s)+Cu(s)

Since Zinc has gain oxygen,it is oxidised. Copper(II) oxide loses oxygen,thus it is reduced.

Reduction-Gain Of Hydrogen

When nitrogen burns with hydrogen gas, the following reaction takes place.Nitrogen +Hydrogen AmmoniaN2 + 3H2 2NH3

Nitrogen gain hydrogen, thus it is being reduced.

Reduction-Gain Of Electron

When iron reacts with chloride acid, the following reaction takes place.Iron + chlorine Iron(III) + Chloride2Fe2++Cl2 2Fe3++2Cl-

Since iron loses electron, it is oxidised. Chlorine gains electron, thus it is reduced.

Common Reducing Agent

• Lithium aluminium hydride (LiAlH4)• Nascent (atomic) hydrogen• Sodium amalgam• Diborane• Sodium borohydride (NaBH4)• Compounds containing the Sn2+ ion, such as tin(II) chloride• Sulfite compounds• Hydrazine (Wolff-Kishner reduction)• Zinc-mercury amalgam (Zn(Hg)) (Clemmensen reduction)• Diisobutylaluminum hydride (DIBAL-H)• Lindlar catalyst• Oxalic acid (C2H2O4)• Formic acid (HCOOH)• Ascorbic acid (C6H8O6)• Phosphites, hypophosphites, and phosphorous acid• Dithiothreitol (DTT) – used in biochemistry labs to avoid S-S bonds• Compounds containing the Fe2+ ion, such as iron(II) sulfate• Carbon monoxide(CO)• Carbon (C)

Oxidation State

The oxidation state is the charge an atom of an element would have if it existed as an ion in a compound.

To work out the oxidation state of an atom, we apply the following rules:

Rule Example Oxidation state

1. The oxidation state of a free element is zero.

Cu, S, Cl2 0, 0, 0

2. The oxidation state of a simple ion is the same as the charge on the ion.

K+, Zn2+, Cl-, O2- +1, +2, -1, -2

3. The oxidation states of the atoms present in the formula of a compound add up to zero. The example shown here is CaCO3 .

Ca, C, 3O +2, +4, 3x(-2)=-6

All add up to zero.

4. The total of the oxidation states of the atoms in a polyatomic ion is equal to the charge on the ion. The example shown here is SO4 2- .

S, 4O +6, 4x(-2)=-6

All add up to -2.

Solution to question: Find the oxidation state of each element in ammonium sulfate.

(NH4)2SO4Since hydrogen in NH4 has an o.s of +1,Ammonium has a charge of 1.Let n be x2(x+4(1))=22x-2-8x=-3Hence nitrogen has an o.s of -3

Since oxygen in SO4 has an o.s of -2,Sulfate has a charge of -2.Let s be yy+4(-2)=-2y-8=-2y=6Hence sulfur has an o.s of 6.

Source

CHEMISTRY Matters textbook

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Done by

• Edwin• Kian Leong• Jeremy• Melvan