Chemical Reactions and Equations NCERT Solution, study material, CBSE Notes

NCERT Solution: Chemical Reactions and Equations

A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms solution which turns red litmus blue. Identify X and also write the chemical reactions involved

Calcium oxide

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Write a balanced chemical equation for each of the following reactions and also classify them.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

Ans. Pb(CH₃COO)₂ + HCl → PbCl₂ + CH₃COOH

This is a Double Displacement reaction

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

Ans. 2Na + 2C₂H₅ONa + H₂

Ans. 2Fe₂O₃ + 3CO → 2Fe + 3CO₂

This is decomposition reaction.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid Sulphur and liquid water.
Ans.
2H₂O + O₂ -> 2S + 2H₂O

This is a Redox reaction.

Why do we store silver chloride in dark coloured bottles?

Silver chloride on exposure to sunlight may decompose as per the following reaction.

2 AgCl → 2Ag + Cl₂

Therefore, it is stored in dark coloured bottles.

Balance the following chemical equations and identify the type of chemical reaction

A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y

Ans. X is MgO Y is Mg₃N₂

(b) Write a balanced chemical equation, when X is dissolved in water.

Ans. MgO + H₂O → Mg(OH)₂

Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Zinc is above hydrogen whereas copper is below hydrogen in the activity series of metals. That is why zinc displaces hydrogen from dilute hydrochloric acid, while copper does not.

Zn + HCl → ZnCl₂ +H₂

Cu + HCl → No reaction

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
Ans. Metals such as silver when attacked by substances around it such as moisture, acids, gases etc, are said to corrode and this phenomenon is called corrosion.
(b) Name the black substance formed and give its chemical formula.
Ans. The black substance is formed because silver (Ag) reacts with H₂S present in air. It forms thin black coating of silver sulphide (Ag₂S)

On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed.

(a) Write a balanced chemical equation of the reaction.
Ans. Balanced chemical equation
2Cu(NO₃)₂(s) → 2CuO +O₂ + 4NO₂(g)

(b) Identity the brown gas X evolved.
Ans. The brown gas X evolved is nitrogen dioxide (NO₂).

(c) Identity the type of reaction.
Ans. This is a decomposition reaction

(d) What could be the pH range of aqueous solution of the gas X?
Ans. Nitrogen dioxide dissolves in water to form acidic solution because it is an oxide of non-metal. Therefore, pH of this solution is less than 7.