Chemical Reactions and Equations

Magnesium is very reactive metal. When stored it reacts with oxygen to form a layer magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be exposed into air.

(i) H₂ + Cl₂ → 2HCl

(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃

(iii) 2Na + 2H₂O → 2NaOH + H₂

(i) Barium chloride + Sodium sulphate → Barium sulphate + Sodium chloride

(ii) Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water

The substance ‘X’ is calcium oxide. Its chemical formula is CaO. Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

Water (H2O) contains two parts hydrogen and one part oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other.

When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour. Therefore, the blue colour of copper sulphate solution fades and green colour appears.

An example of a double displacement reaction: Sodium carbonate reacts with calcium chloride to form calcium carbonate (precipitate) and sodium chloride.

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.

(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced. (ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).

(i) (a) and (b)

In the reaction 2PbO + C → 2Pb + CO₂: Lead (Pb) is being reduced (gains electrons, PbO → Pb). Carbon dioxide is not being oxidised — it is a product formed by oxidation of carbon. Carbon is being oxidised (C → CO₂, loses electrons). Lead oxide is being reduced. Therefore statements (a) "Lead is getting reduced" and (b) "Carbon dioxide is getting oxidised" are incorrect.

(d) The given reaction is an example of a displacement reaction.

(a) Hydrogen gas and iron chloride are produced. When dilute hydrochloric acid is added to iron filings, iron (II) chloride and hydrogen gas are produced.

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation. The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

(a) Hydrogen gas combines with nitrogen to form ammonia:

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide:

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and barium sulphate precipitate:

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas:

(a)

(b)

(c)

(d)

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

(a) Potassium bromide + Barium iodide → Potassium iodide + Barium bromide

This is a Double Displacement Reaction because the ions of two compounds exchange their positions to form new compounds.

(b) Zinc carbonate → Zinc oxide + Carbon dioxide

This is a Decomposition Reaction because a single compound breaks down into two simpler substances.

(c) Hydrogen + Chlorine → Hydrogen chloride

This is a Combination Reaction because two substances combine to form a single product.

(d) Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen

This is a Displacement Reaction because magnesium displaces hydrogen from hydrochloric acid.

Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions. Example: Mixture of sodium and chlorine to yield table salt In other words, combination reactions are exothermic. Reactions that absorb energy or require energy in order to proceed are called endothermic reactions. For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

In a decomposition reaction, a single compound breaks down into two or more simpler substances. These reactions require an input of energy. In a combination reaction, two or more substances combine to form a single new substance with the release of energy. Thus, decomposition is the reverse of combination.

Decomposition reaction:

Combination reaction:

Equations for decomposition by different energy forms:

(a) Thermal decomposition (heat):

(b) Decomposition by light:

(c) Decomposition by electricity (electrolysis):

In a displacement reaction, a more reactive element displaces a less reactive element from its compound (salt solution). Only one ion/element is replaced.

In a double displacement reaction, two compounds exchange their ions with each other to form two new compounds. Both cations and anions switch partners.

In the refining of silver, copper displaces silver from silver nitrate solution:

Copper is more reactive than silver and hence displaces silver from its solution. The recovered silver is used in making jewellery and other items.

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

Example 1: When sodium carbonate reacts with calcium chloride, calcium carbonate (white precipitate) is formed.

Example 2: When sodium sulphate reacts with barium chloride, barium sulphate (white precipitate) is formed.

(a) Oxidation is the gain of oxygen (or loss of hydrogen).

Examples:

(b) Reduction is the loss of oxygen (or gain of hydrogen).

Examples:

’X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented their presence is essential for rusting to take place.

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc. For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide. This hydrated iron oxide is rust. (b) Rancidity: The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity. For example, the taste and smell of butter changes when kept for long. Rancidity can be avoided by: 1. Storing food in air tight containers 2. Storing food in refrigerators 3. Adding antioxidants 4. Storing food in an environment of nitrogen