CBSE Class 10 Science HOTs Chemical Reactions and Equations

Question. What happens when a chemical reaction occurs?
Answer : When a chemical reaction occurs:
i. New substances are formed by breaking and making of new bonds.
ii. Some gases may evolve during the reaction.
iii. Change in smell.
iv. Change in colour.
v. Change in state (liquid, solid or gaseous)
vi. Change in temperature.

Question. What happens when hydrogen combines with oxygen in the presence of an electric current?
Answer : When hydrogen reacts with oxygen in the presence of an electric current, formation of water takes place which conducts electricity.
At the cathode, the electrons and the hydrogen ions combine with oxygen to make water.
The reaction takes place is given as:
2H₂ + O₂→ 2H₂O

Question. Give an example of a double displacement reaction.
Answer : The reaction in which there is an exchange of ions between the reactant takes place is called double displacement reaction.

For example:
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Sodium Barium Barium Sodium sulphate chloride sulphate chloride
In the above reaction, the exchange of ions (SO₄^2- and Ba²⁺) takes place. The white precipitate of BaSO₄ is formed by the reaction of SO₄^2- and Ba²⁺. The other product formed NaCl remains in the solution.

Question. “We need to balance a skeletal chemical equation.” Give reason to justify this statement.
Answer : Balanced chemical equation: A chemical equation in which the number of atoms of reactants are equal to the number of atoms of products is called a balanced equation.
We need to balance a skeletal chemical equation because:
i. According to the law of conservation of mass, mass (or atoms) are neither created nor destroyed in chemical reactions.
ii. It means the total mass of the products formed in chemical reaction must be equal to the mass of reactants consumed.

Question. Giving an example list two information which make a chemical equation more useful (informative).
Answer : Chemical equations can be made more informative by the following characteristics:
Physical state
There are three different states which are solid, liquid and gaseous. If the substance exists in solid state, it is represented by (s).
If the substance exists in aqueous state, it is represented by (aq).
If the substance exists in gaseous state, it is represented by (g).
For example:

Evolution of gas
If the gas is liberated in a reaction, it is indicated by an arrow upward and its physical state is shown by (g).
For example:
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑

Question. Write a balanced chemical equation for process of photosynthesis.
Answer : Photosynthesis reaction: CO₂ + H₂O → C₆H₁₂O6 + O₂ + H₂O
Balanced equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
Step 1: Write the unbalanced equation
CO₂ + H₂O → C₆H₁₂O₆ + O₂ + H₂O
Step 2: Compare the number of atoms of reactants with the number of atoms of products.

Step 3: Now, first we consider the element having unequal no. of atoms on both sides. Thus, first let us consider carbon atom. If we multiply 6 in the reactant (in CO₂), we will get the equal number of atoms as in product (in C₆H₁₂O₆)

Step 4: Write the resulting equation:
6CO₂ + H₂O → C₆H₁₂O₆ + O₂ + H₂O
Step 5: Now check whether the equation is balanced or not by comparing the atoms

We find that the equation is not balanced yet. As the number of
Hydrogen and oxygen atoms are unequal on the two sides.
First balance the hydrogen atom.
Step 6: If we multiply 12 in the reactant (in H₂O) and 6 in the product (in H₂O), we will get the equal number of hydrogen atoms both sides.

Step 7: Write the resulting equation:
6CO₂ + 12H₂O → C₆H₁₂O₆ + O₂ + 6H₂O
Step 8: Now check whether the equation is balanced or not by comparing the atoms.

We find that the equation is not balanced yet. As the number of oxygen atoms are unequal on the two sides
Step 9: Now, we consider oxygen atoms. If we multiply 6 in the product (in O₂), we will get the equal number of atoms as in reactants (in 6CO₂ and 12H₂O)

Step 10: Write the resulting equation:
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
We find that the equation is balanced now.
Step 11: Write down the final balanced equation:
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Question. When do desert plants take up carbon dioxide and perform photosynthesis?
Answer : In deserts, the weather is very hot and the amount of rainfall is very low. Hence, if desert plants take CO₂ during day, large amount of water can be lost. To avoid this water loss, some of the desert plants take CO₂ during night.

Question. Zinc liberated hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?
Answer :

As you can observe in the above series that zinc is more reactive than the hydrogen whereas copper is less reactive than hydrogen.
More reactive the metal is, more easily it will displace hydrogen atoms. Hence,
i. When zinc reacts with hydrochloric acid, it liberates hydrogen gas (hydrogen is displaced)
Zn + HCl → ZnCl₂ + H₂
ii. On the other hand, when copper reacts with hydrochloric acid, it is unable to displace the hydrogen gas. As a result, no hydrogen gas is liberated.
Cu + HCl → No reaction

Question. Identify the reducing agent in the following reactions:
(a) 4NH₃ + 5O₂→ 4NO + 6H₂O
(b) H₂O + F₂→ HF + HOF
(c) Fe₂O₃ + 3CO → 2Fe + 3CO₂
(d) 2H₂ + O₂→ 2H₂O
Answer : Reducing agent: A reducing agent is an element that loses electrons.
The reducing agent means to loose electrons; it is said to have been oxidized.
The element which undergoes oxidation (gets oxidized) is called reducing agent.
a) NH₃ is reducing agent
4NH₃ + 5O₂→ 4NO + 6H₂O
In the given reaction, nitrogen is oxidized to NO by gaining oxygen atom. Thus, NH₃ is reducing agent as it undergoes oxidation.
b) F₂ is reducing agent
H₂O + F₂→ HF + HOF
In the given reaction, F₂ is oxidized to HOF by gaining hydrogen atom. Thus, F₂ is reducing agent as it undergoes oxidation.
(c) CO is reducing agent
Fe₂O₃ + 3CO → 2Fe + 3CO₂
In the given reaction, CO is oxidized to CO₂ by gaining oxygen atom. Thus, CO is reducing agent as it undergoes oxidation.
(d) H₂ is reducing agent
2H₂ + O₂→ 2H₂O
In the given reaction, 2H₂ is oxidized to 2H₂O by gaining oxygen atom. Thus, 2H₂ is reducing agent as it undergoes oxidation.

Question. You might have noted that when copper powder is heated in a china dish, the surface of copper powder becomes coated with a black colour substance.
(a) How has this black coloured substance formed? (b) What is that black substance? (c) Write the chemical equation of the reaction that takes place.
Answer : (a) When copper powder is heated in the presence of air (oxygen), cooper reacts with oxygen to form copper oxide. The copper oxide formed is black in colour. The black colour is formed because the oxidation of copper takes place.
(b) The black substance formed is copper oxide.
(c) The chemical equation of the reaction that takes place is given below:
2Cu + O₂→ 2CuO

Question. Identify the oxidising agent (oxidant) in the following reactions:
(a) Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O
(b) 2Mg + O₂→ 2MgO
(c) CuSO₄ + Zn → Cu + ZnSO₄
(d) V2O₅ + 5Ca → 2V + 5CaO
(e) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
(f) CuO + H₂→ Cu + H₂O
Answer : Oxidizing agent: An oxidizing agent is an element that gains electrons. Since, the oxidizing agent means to gain electrons; it is said to have been reduced.
The element which undergoes reduction (gets reduced) is called oxidizing agent.
(a) Pb₃O₄ is reducing agent
Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O
In the given reaction, Pb₃O₄ loses oxygen and reduces to PbCl₂
Thus, Pb₃O₄ is an oxidizing agent as it undergoes reduction.
(b) O₂ is oxidizing agent
2Mg + O₂→ 2MgO
In the given reaction, O₂ is reduced by losing oxygen atoms. Thus, O₂ is an oxidizing agent as it undergoes reduction.
(c) CuSO₄ is oxidizing agent
CuSO₄ + Zn → Cu + ZnSO₄
In the given reaction, CuSO₄ is reduced to Cu by losing oxygen atoms. Thus, CuSO₄ is an oxidizing agent as it undergoes reduction.
(d) V₂O₅ is oxidizing agent
V₂O₅ + 5Ca → 2V + 5CaO
In the given reaction, V₂O₅ is reduced to V by losing oxygen atoms. Thus, V₂O₅ is an oxidizing agent as it undergoes reduction.
(e) H₂O is oxidizing agent
3Fe + 4H₂O → Fe₃O₄ + 4H₂
In the given reaction, H2O is reduced to H₂ by losing oxygen atoms. Thus, H₂O is an oxidizing agent as it undergoes reduction.
(f) CuO is oxidizing agent
CuO + H₂→ Cu + H₂O
In the given reaction, Cu is reduced to Cu by losing oxygen atom. Thus, CuO is an oxidizing agent as it undergoes reduction.

Question. Write the balanced chemical equation for the following equations for the following reaction and identify the type of reaction in each case.
(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.
(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.
(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in presence of conc. H₂SO₄.
(d) Ethene is burnt in presence of oxygen to form carbon dioxide, water and releases heat and light.
Answer : (a) When nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas. The following reaction takes place

The above reaction is a type of combination reaction.
Combination reaction is reaction in which two reactants combined together to form a product.
(b) When sodium hydroxide solution is treated with acetic acid to form sodium acetate and water. The following reaction takes place:
NaOH + CH₃COOH → CH₃COONa + H₂O
The above reaction is a type of neutralization reaction/ double displacement reaction.
A neutralization reaction is a reaction in which base (NaOH) and acid (CH₃COOH) react together to form a salt (CH₃COONa) and water.
(c) When ethanol is warmed with ethanoic acid to form ethyl acetate in presence of conc. H₂SO₄. The following reaction takes place:

An esterification reaction is a reaction in which alcohol (C2H5OH) and Carboxylic acid (CH3COOH) react together to form a sweet-smelling substance called ester (CH3COOC2H5)
(d) When ethene is burnt in presence of oxygen to form carbon dioxide, water and releases heat and light. The following reaction takes place:
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O + heat + light
The above reaction is a type of combustion reaction.
Combustion reaction is a reaction which takes place in the presence of oxygen to form carbon dioxide.

Question. Complete the missing components/variables given as x and y in the following reactions:
(a) Pb(NO₃)2(aq) + 2Kl(aq) → PbI₂(x) + 2KNO₃(y)
(b) Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + x
(c) Zn(s) + H₂SO₄(aq) → ZnSO₄(x) + H₂(y)

Answer : (a) Pb(NO3)₂(aq) + 2Kl(aq) → PbI₂(s) + 2KNO₃(aq)
Hence, x is (s) and y is (aq) (s) means PbI₂ exists in solid state and (y) means that KNO3 exists in aqueous/liquid state.
(b) Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Hence, x is Ag(s) In the above reaction, Ag gets replaced by Cu.
(c) Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
Hence, x is (aq) and y is (g) (aq) means copper sulphate formed exists in aqueous state. (g) Means hydrogen gas is liberating.

Hence x is heat
When CaCO₃ is heated in the presence of oxygen, it forms calcium oxide (CaO) and carbon dioxide.

Question. Identify the type of chemical reaction taking place in each of the following:
(a) Barium chloride solution is mixed with a copper sulphate solution and a white precipitate is observed.
(b) On heating copper powder in the air in a china dish, the surface of copper powder turns black.
(c) On heating, green coloured ferrous sulphate crystals, reddish brown solid is left and a smell of a gas having an odour of burning sulphate is experienced.
(d) Iron nails when left dipped in blue copper sulphate solution become brownish in colour and the blue colour of copper sulphate fades away.
(e) Quick lime reacts vigorously with water releasing a large amount of heat.
Answer : (a) Double displacement and precipitation reaction
When barium chloride solution is mixed with copper sulphate solution and a white precipitate is observed.
In double displacement reaction, a precipitate is formed which settles down at the bottom. Any reaction that produces a precipitate is called a precipitation reaction.
BaCl₂ + CuSO₄ → BaSO₄ (↓) + NaCl White ppt.
Note: A precipitate is a substance which is insoluble in water.
(b) Oxidation reaction
On heating copper powder in air in a china dish, the surface of copper powder turns black because copper undergoes oxidation in the presence of air.
2Cu +O₂→ 2CuO Black
(c) Thermal decomposition
On heating green coloured ferrous sulphate crystals, reddish brown solid is left and smell of a gas having odour of burning sulphate is experienced.

In the above reaction, ferrous sulphate (FeSO4) decomposes to ferric oxide (Fe2O3)
(d) Displacement reaction
Iron nails when left dipped in blue copper sulphate solution become brownish in colour and the blue colour of copper sulphate fades away.
Fe + CuSO₄ → FeSO₄ + Cu
As we know that displacement reaction is a reaction in which one element displaces other element from its compound and takes its place there in.
In the above reaction Fe replaces Cu and form a new compound.
(e) Exothermic reaction
Quick lime reacts vigorously with water releasing a large amount of heat:
CaO + H₂O → Ca(OH)₂ + heat
In the above reaction, the heat is released. Hence, it is an exothermic reaction.
Exothermic reaction is a reaction in which heat is released when reactants changes into products.

Question. Seema likes sugar apple which is a multipurpose, an all-rounder fruit. It can be used by athletes as high calorie fruit for high energy. The powdered of its seeds is an effective pesticide in agriculture and horticulture and also used to remove head lice. Its leaves have anti-diabetic properties. The alcoholic extract of its leaves and stems can treat tumours. This extract is an antidepressant. Fruits are sweet and increase the haemoglobin, cooling and act as sedative. Seeds are insecticides. Its roots are powerful purgatives and used in dysentery.
(a) Why is sugar apple used by athletes?
(b) Why should we use it as a pesticides and insecticides instead of chemicals? Which value is associated with its use?
(c) Which metal is present in haemoglobin?
Name one more fruit which is rich in iron?
(d) Why should people take fruits rich in iron?
(e) What values are possessed by Seema who uses this multipurpose fruit instead of junk food?
Answer : (a) Sugar is used by athletes because:
i. It is a multipurpose, an all-rounder fruit.
ii. It can be used by athletes as high calorie fruit for high energy.
iii. It increases the flow of blood in the whole body.
iv. It increases haemoglobin too.
(b) We should use sugar apple as a pesticides and insecticides instead of chemicals because:
i. Chemicals can harm plants and can stop their growth.
ii. On the other hand, the powdered seeds of sugar apple do not cause any harm to plants as they are purely natural.
(c) Iron is present in our haemoglobin. Iron is an important element for the formation of blood in the body. The lack of iron in body can cause too many problems. The other fruits which is rich in iron is banana.
(d) People take fruits rich in iron because iron increases the production of blood in the body. It increases the haemoglobin which keeps us fit from top to bottom
(e) Seema is health-conscious, aware and knowledgeable too.