3. Chemical Reactions and Equations

To prevent rusting, a layer of zinc metal is applied on iron sheets.

Reason: Zinc is more reactive than iron. The zinc layer keeps air and moisture away from the iron, so the iron does not rust. This process is called galvanizing.

The conversion of ferrous sulphate to ferric sulphate is oxidation reaction.

Reason: In this change, Fe²⁺ → Fe³⁺ + e⁻. The ferrous ion loses one electron and its positive charge increases by one unit. Losing electrons is oxidation.

When electric current is passed through acidulated water electrolysis of water takes place.

Reason: Water decomposes into hydrogen and oxygen by means of electrical energy. Decomposition by electricity is called electrolysis.

Addition of an aqueous solution of ZnSO₄ to an aqueous solution of BaCl₂ forms a white precipitate is an example of double displacement reaction.

Reason: The ions of the two reactants are exchanged, and a white precipitate of barium sulphate is formed.

A reaction in which oxidation and reduction take place simultaneously is called a redox reaction.

Example: When hydrogen gas is passed over black copper oxide, reddish copper is formed.

• CuO loses oxygen, so copper oxide is reduced.
• H₂ gains oxygen to form water, so hydrogen is oxidised.
• H₂ is the reductant and CuO is the oxidant.

Redox reaction = Reduction + Oxidation.

Hydrogen peroxide decomposes slowly at room temperature. The rate can be increased by:

1. Adding a catalyst: adding manganese dioxide (MnO₂) powder makes it decompose rapidly.
2. Increasing the temperature: heating it increases the rate.
3. Increasing the concentration: concentrated hydrogen peroxide decomposes faster.

• Reactants: The substances which take part in a chemical reaction are called reactants.
• Products: The new substances formed as a result of a chemical reaction are called products.

In the first reaction, coal (carbon) and oxygen are reactants, and carbon dioxide is the product. In the second, copper sulphate and zinc are reactants, and zinc sulphate and copper are products.

1. Oxidation: The reaction in which a reactant combines with oxygen or loses hydrogen is called oxidation.

2. Reduction: The reaction in which a reactant gains hydrogen or loses oxygen is called reduction.

3. Redox: Oxidation and reduction occur together. In CuO + H₂ → Cu + H₂O, CuO is reduced and H₂ is oxidised.

Similarity: In both events heat is given out, so the temperature rises. Both are exothermic. Both form a basic solution.

Difference:

A chemical reaction in which heat is absorbed from the surroundings, or has to be supplied continuously from outside, is called an endothermic reaction.

When two or more reactants combine in a reaction to form a single product, it is called a combination reaction.

A chemical equation in which the number of atoms of each element in the reactants is the same as in the products is called a balanced equation. It follows the law of conservation of mass.

The reaction in which the place of the ion of a less reactive element in a compound is taken by a more reactive element by formation of its own ions is called a displacement reaction.

Zinc is more reactive than copper, so it displaces copper from copper sulphate.

1. On heating, limestone (calcium carbonate) decomposes and gives carbon dioxide gas.
2. When this CO₂ is passed through lime water, it forms calcium carbonate, which is insoluble in water.
3. This white insoluble calcium carbonate makes the lime water milky.

1. Shahabad tile contains calcium carbonate, which reacts with HCl and gives CO₂.
2. The rate of a reaction depends on the size of the particles of the reactants. Smaller the particles, higher the rate.
3. Powder has very small particles, so a larger surface comes in contact with the acid. Hence the powder reacts and disappears rapidly, while the pieces take time.

1. Dilution of concentrated sulphuric acid with water is a highly exothermic process. A very large amount of heat is liberated.
2. If water is poured into the acid, the water evaporates instantly and the acid may splash out, causing an accident.
3. When a small quantity of acid is added to water at a time with stirring, only a small amount of heat is liberated at a time, and it spreads evenly. So the dilution is safe.

1. When oil is left in contact with air for a long time, it undergoes air oxidation.
2. It becomes rancid: it gets a foul smell and its taste changes.
3. An air tight container keeps air (oxygen) away from the oil, so oxidation slows down and rancidity is prevented.

The picture shows the rusting of iron. Rusting is an electrochemical reaction. Different regions of the iron surface under the water drop act as the anode and the cathode.

1. At the anode: iron is oxidised to Fe²⁺ ions.

2. At the cathode: the electrons travel through the iron to the cathode, where oxygen is reduced.

3. Formation of rust: Fe²⁺ ions move through the water, are oxidised further to Fe³⁺ ions, and form a reddish hydrated oxide. This is rust, which collects on the surface.

The formula of rust is Fe₂O₃·xH₂O. Both air and water are necessary for rusting, and salt makes it faster.

Oxidation: Fe — iron loses 2 electrons (Fe → Fe²⁺ + 2e⁻).
Reduction: S — sulphur gains 2 electrons (S + 2e⁻ → S²⁻).

Reduction: Ag₂O — it loses oxygen and forms silver (Ag⁺ + e⁻ → Ag).
Oxidation: the oxide ions of the same Ag₂O lose electrons and form oxygen gas (2O²⁻ → O₂ + 4e⁻).

Oxidation: Mg — magnesium combines with oxygen.
Reduction: O₂ — oxygen gains the electrons lost by magnesium.

Reduction: NiO — nickel oxide loses oxygen.
Oxidation: H₂ — hydrogen gains oxygen and forms water.

Exothermic reaction — heat is given out.

Endothermic reaction — heat has to be supplied (Δ).

Exothermic reaction — heat is given out.

Endothermic reaction — heat has to be supplied (Δ).

Sodium hydroxide exchanges ions with many salt solutions and forms an insoluble metal hydroxide precipitate. A sample chart: