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Chemical Reactions and Equation – CBSE Class 10 Science – NCERT Solutions

Which of the given statements about the reaction below are incorrect?
a. Lead is getting reduced
b. Carbon dioxide is getting oxidised
c. Carbon is getting oxidised
d. Lead oxide is getting reduced
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
i) (a) and (b)
The above reaction is an example of a
(i). Combination reaction
(ii). Double displacement reaction
(iii). Decomposition reaction
(iv). Displacement reaction
(iv) Displacement reaction
What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
a. Hydrogen gas and iron chloride are produced
b. Chlorine gas and iron hydroxide are produced
c. No reaction takes place
d Iron salt and water are produced
Hydrogen gas and iron chloride are produced
What is a balanced chemical equation? Why should chemical equations be balanced?
An equation for a chemical reaction is an equation in which the number of atoms for each element in the reaction and the total charge are the same for both the reactants and the products. In other words, the mass and the charge are balanced on both sides of the reaction.
Translate the following statements into chemical equations and then balance them.
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 a precipitate of barium sulphate
d. Potassium metal reacts with water to give a potassium hydroxide and hydrogen gas
a. 3H2 + N2 —> 2NH3
b. 2H2S + 3O2 —> 2H2O + 2SO2
c. 3BaCl2 + Al2(SO4)3 —> 2AlCl3 + 3BaSO4
d. 2K + 2H2O —> 2KOH + H2
Balance the following chemical equations.
Write the balanced chemical equations for the following reactions:
Write the balanced chemical equation for the following and identify the type of reaction of each case.
a. 2KBr + BaI2 —> 2KI + BaBr2 — Double Displacement reaction
b. ZnCO3 —> ZnO + CO2 — Decomposition reaction
c. H2 + Cl2 —> 2HCl — Combination reaction
d. Mg + 2HCl –> MgCl2 + H2 — Displacement reaction
What is meant by exothermic and endothermic reactions? Give examples.
An exothermic reaction is one that releases heat.
If 1 mole of N2 molecule reacts with 1 mole of O2 molecule, the heat of 184 KJ has to be supplied to initiate the reaction to give 1 mole of NO molecule. This means that the bonds between N – N and O – O are so strong that they do not break easily. N2 has triple covalent bond between the two N atoms. O2 has a double covalent bond. Thus energy has to be put into the reaction to break the strong bonds. Thus the above reaction is a good example of an endothermic reaction.
Why is respiration considered an exothermic reaction?
During respiration, glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with the production of energy.
Glucose + oxygen —> carbon dioxide + water + energy
It is an exothermic reaction, because energy is produced during this process.
Why are decomposition reactions called the opposite of combination reactions? Write equations for decomposition reactions.
Essentially, decomposition reactions are the opposite of combination reactions. A compound decomposes (i.e., “splits-up”) into two or more compounds and/or elements. For example, mercury (II) oxide will, decompose into mercury metal and oxygen, upon heating:
2HgO —> 2Hg + O2
Since heat had to be added to make this reaction occur, it is an endothermic reaction. Most decomposition reactions are endothermic. Another example of decomposition reaction is the heating of calcium carbonate (sea shells, chalk):
Write one equation each for decomposition reactions in which energy is supplied in the form of heat, light or electricity?
When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide:
What is the difference between displacement and double displacement reactions? Write relevant equations for the above?
The difference between the displacement and double displacement reactions is that in a displacement reaction one element displaces another by virtue of it being more reactive whereas in a double displacement two anions and two cations switch places between two compounds respectively.
For example:
CuSO4 (aq) + Zn(s) —> ZnSO4 + Cu(s)
Blue copper sulphate solution reacting with solid zinc will give rise to colourless zinc sulphate solution and solid copper. Thus Zn displaces Cu in the salt form. Zn is more reactive than Cu.
An example of double displacement reaction is the reaction between sodium carbonate and calcium chloride, both in aqueous
Na2CO3 (aq) + CaCl2 (aq) —> CaCO3 (ppt) + 2NaCl(aq)
In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write down the reaction involved.

What do you mean by a precipitation reaction? Explain by giving examples:
A precipitation reaction is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. This insoluble compound is called a precipitate.

An example of a precipitation reaction
Aqueous silver nitrate (AgNO3) when added to a solution containing potassium chloride (KCl) precipitates a white solid, and silver chloride is observed.
AgNO3 (aq) + KCl(aq) —> AgCl(s) + KNO3(aq)
The silver chloride (AgCl) has formed a solid, which is observed as a precipitate.

A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
The shiny brown coloured element X is copper metal (Cu).When copper metal is heated in air, it forms a black coloured compound copper oxide. So, the black coloured compound is copper oxide or copper (II) oxide, CuO.

Why do we apply paint on iron articles?
Rust is a soft and porous substance, which gradually falls from the surface of an iron object, and then the iron below starts rusting. Thus, rusting of iron (or corrosion of iron) is a continuous process which, if not prevented in time, eats up the whole iron object. So, when we apply paint on iron articles it reduces the rusting of iron.

Oil and fat containing food items are flushed with nitrogen. Why?
Packaging fat and oil containing foods in nitrogen gas can prevent rancidity. When the fat and oil present in food materials gets oxidised (in air), their oxidation products have an unpleasant smell and taste. When it is surrounded by unreactive gas, nitrogen, there is no oxygen (of air) to cause its oxidation and make it rancid.

Explain the following terms with one example each.
a. Corrosion
b. Rancidity
a. Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical on their surface. Corrosion is caused mainly by the oxidation of metals by the oxygen of air.
Example: Rusting of iron is the most common form of corrosion. When an iron object is left in damp air for a considerable period of time, it gets covered with a red-brown flaky substance called ‘rust’. This is called rusting of iron.
b. The condition produced by aerial oxidation of fat and oil in food which is marked by an unpleasant smell and taste is called rancidity.
Example: Rancidity can be retarded by keeping food in a refrigerator.
The refrigerator has a low temperature inside it. When the food is kept in a refrigerator, the oxidation of fat and oil in it is slowed down due to low temperature. So, the development of rancidity due to oxidation is retarded.

Why should a magnesium ribbon be cleaned before burning in air?
To remove the oxide layer and facilitates rapid burning.

Write the balanced equation for the following chemical reactions.
i. Hydrogen + Chloride —> Hydrogen chloride
ii. Barium chloride + Aluminium sulphate —> Barium sulphate + Aluminium chloride
iii. Sodium + water —> Sodium hydroxide + Hydrogen
i. H2 + Cl2 —> 2HCl
ii. 3BaCl2 + Al2(SO4)3 —> 3BaSO4 + 2AlCl3
iii. 2Na + 2H2O —> 2NaOH + H2

Write a balanced chemical equation and state symbols for the following reactions.
a. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride
b. Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

A solution of a substance ‘X’ is used for white washing.
Name the substance ‘X’ and write its formula.
(i) Write the reaction of the substance ‘X; named in (ii) above with water

Why does the colour of copper sulphate solution change when an iron nail is dipped into it?
In this reaction, iron displaces copper from copper sulphate solution. The deep blue colour of copper sulphate solution fades due to the formation of light green solution of iron sulphate. A red-brown coating of copper metal is formed on the surface of the iron metal. This displacement reaction occurs because iron is more reactive than copper.
Identify the substances that are oxidised and the substances that are reduced in the following reactions.
In this reaction, copper oxide (CuO) gives the oxygen required for the oxidation of hydrogen; therefore, copper oxide is the oxidizing agent. Hydrogen is responsible for removing oxygen from copper oxide; therefore, hydrogen is the reducing agent here.

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