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Kerala Syllabus Std 9 Chemistry Chapter 4 Redox Reactions Extra Questions and Answers
Question 1.
State the law of conservation of mass.
Answer:
In a chemical reaction, the total mass of the reactants will be equal to the total mass of the products.
Question 2.
What do you mean by balancing of chemical equations?
Answer:
Balancing a chemical equation is the method of equalising the number of the same type of atoms in both the reactants and the products.
Question 3.
Why the total mass remains unchanged in a chemical reaction?
Answer:
When reactants combine in a specific mass proportion to form products, the atoms in them only undergo a rearrangement. Thus, there will not be any change in the total number of atoms or the total mass.
Question 4.
Consider the reaction between Zinc (Zn) and Hydrochloric Acid. (HCl) to form Zinc Chloride (ZnCl2) and Hydrogen gas (H2). Write a balanced chemical equation for this reaction.
Answer:
The balanced chemical equation for the reaction is:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Explanation:
- Zn(s) represents Zinc metal (solid).
- HCl(aq) represents Hydrochloric acid (aqueous solution).
- ZnCl2(aq) represents Zinc chloride (aqueous solution).
- H2(g) represents Hydrogen gas.
Balancing:
Step 1: Zn + HCl → ZnCl2 + H2
Reactants
Zn – 1
H – 1
Cl – 1
Products:
Zn – 1
H – 2
Cl – 2
Number of hydrogen atoms are not the same. Also, the number of Chlorine atoms is not the same.
Step 2: Zn + 2HCl → ZnCl2 + H2
Now, the number of both atoms is equal on the reactant and products. The equation is balanced.
Question 5.
Explain oxidation, reduction, oxidising agent and reducing agent.
Answer:
- The process involving loss of electrons in a chemical reaction is called oxidation.
- The process involving gaining of electrons in a chemical reaction is called reduction.
- The species that helps oxidation in a chemical reaction is the Oxidising agent. The oxidising agent gets reduced in a chemical reaction.
- The species that helps reduction is the reducing agent. The reducing agent gets oxidised in a chemical reaction.
Question 6.
What do you mean by oxidation number in a chemical reaction?
Answer:
If all the chemical bonds in a compound are considered to be ionic, the charge formed on each atom is considered as its oxidation number or oxidation state.
Question 7.
Write any three examples of redox reactions.
Answer:
- Glucose molecules decompose and release energy during cellular respiration.
- Formation of oxide coating on the surface of metals.
- Combustion of fuels.
- Decomposition of organic substances in the presence of oxygen.
- Production of electricity in electrochemical cells.
Question 8.
Analyse the chemical equation given below and answer the questions.
a) Which atom is getting oxidised?
b) Which atom is getting reduced? Why?
c) Which is the oxidizing agent?
d) Which is the reducing agent?
Answer:
a) Chlorine (Cl)
b) Manganese (Mn) Oxidation number decreases from +4 to +2.
c) Hydrogen chloride (HCl)
d) Manganese dioxide (MnO2)
Question 9.
From the given chemical equation identify the oxidising agent and the reducing agent by writing the chemical equation of oxidation and reduction.
i. Mg + F2 → MgF2
ii. 2Na + Cl2 → 2NaCl
Answer:
i. Mg → Mg2+ + 2e– (Oxidation)
F + 1e– → F– (Reduction)
Here magnesium is the reducing agent and fluorine is the oxidising agent.
ii. Na → Na+ + le– (Oxidation)
Cl + le– → Cl– (Reduction)
Here sodium is the reducing agent and chlorine is the oxidising agent.
Question 10.
What is the relation between the total mass of the reactants and the total mass of the products in a chemical reaction?
Answer:
Total mass of the reactants = Total mass of the products
Question 11.
Who stated the law of conservation of mass?
Answer:
Antoine Lavoisier
Question 12.
4g Hydrogen reacts with 32g of oxygen to form 36g water. Which is the law related to this?
Answer:
Law of Conservation of mass
Question 13.
Find out the relationship and fill up
Laws of electrolysis : Michael Faraday
Law of Conservation of mass : ……………………..
Answer:
Antoine Lavoisier
Question 14.
a) What are the chemicals required to prepare hydrogen in laboratory?
b) Write the balanced chemical equation of this reaction.
Answer:
a) Dilute hydrochloric acid and Zinc
b) Zn + 2HCl → ZnCl2 + H2
Question 15.
Balance the chemical reactions given below.
a) SO2 + O2 → SO3
b) H2O2 → H2O + O2
Answer:
a) 2SO2 + O2 → 2SO3
b) 2H2O2 → 2H2O + O2
Question 16.
The chemical equation of a reaction is given below
Zn + xHCl → ZnCl2 + H2
a) Find the value of ‘x’ and then balance the chemical equation.
b) Which are the reactants of this reaction?
Answer:
a) According to Law of conservation of mass, Total mass of the reactants will be equal to the total mass of the products.
Here in this reaction,
Zn + x HCl → ZnCl2 + H2
65.38 + x(1 + 35.5) = 65.38 + (2 × 35.5) + 2
65.38 + x + 35.5x = 65.38 + 71 + 2
65.38 + 36.5x = 138.38
36.5x = 73
x = \(\frac{73}{36.5}\) = 2
Balanced Chemical equation: Zn + 2HCl → ZnCl2 + H2
b) Reactants: Zinc (Zn) and Hydrogen Chloride (HCl)
Question 17.
Mg + 2HCl → MgCl2 + H2
24 g Mg completely reacts with 73 g HCl to form 95 g MgCl2 and ‘x’ g H2
a) Find the value of x.
b) To which law is it related?
Answer:
a) Mg + 2HCl → MgCl2 + H2
24g + 73g → 95g + xg
97 g → 95 g + x g
xg = 97g – 95g
x = 2g
b) Law of conservation of mass.
Question 18.
Check whether this chemical reaction obeys the law of conservation of mass.
N2 + H2 → NH3
(Hint H = 1 u, N = 14 u)
Answer:
Total mass of the reactants = (14 × 2) + (1 × 2)
= 28 + 2
= 30
Total mass of the products = (14 × 1) + (1 × 3)
= 14 + 3
= 17
The total mass of the reactants is not equal to the total mass of the products. So, this chemical reaction does not obey the law of conservation of mass.
Question 19.
Certain chemical equations are given below. From these find out the unbalanced chemical equations and balance them.
a) 2H2 + O2 → 2H2O
b) N2 + H2 → NH3
c) C + O2 → CO2
d) SO2 + O2 → SO3
Answer:
Unbalanced chemical equations: b), d)
Balanced chemical equation:
b) N2 + 3H2 → 2NH3
d) 2SO2 + O2 → 2SO3
Question 20.
Some chemical equations are given below.
i) 2Mg + O2 → 2MgO
ii) SO2 + O2 → SO3
iii) 2H2O2 → 2H2O + O2
a) Which of these are balanced equations?
b) Balance the unbalanced equation.
Answer:
i) and iii)
In i) and iii), the number of the same type of atoms in the reactants as well as the products are the same now.
ii) 2SO2 + O2 → 2SO3
Question 21.
Given below is a table showing the mass of the reactants and the products when hydrogen combines with chlorine to form hydrogen chloride in two situations.
Situation Mass of reac tants Mass of product
Write down the total mass of the reactants and also the total mass of product in the above experiment.
a) Situation 1: ……………………….
b) Situation 2: ……………………….
c) Check whether this reaction obeys law of conservation of mass.
Answer:
a) Total mass of reactants – 73 g
Total mass of products – 73 g
b) Total mass of reactants – 146 g
Total mass of products – 146 g
c) According to law of conservation of mass, the total mass of the reactants will be equal to total mass of the products.
Here in both cases total mass of the reactants = total mass of the products. Hence this reaction obeys law of conservation of mass in both cases.
Question 22.
An unbalanced chemical equation is given
N2 + H2 → NH3
a) Which are the reactants in this reaction?
b) Balance the above chemical equation.
c) What is the total number of product molecules in the balanced chemical equation.
d) Identify the compound in this equation?
Answer:
a) Nitrogen, Hydrogen
b) N2 + 3H2 → 2NH3
c) 2 molecules (2 Ammonia)
d) Ammonia (NH3)
Question 23.
Analyse the following equation fill up the blanks.
The oxidized atom is ……………………
Answer:
Zinc
Question 24.
In most of the compounds the oxidation number of Oxygen is ……………………..
(+1, -1, +2, -2)
Answer:
-2
Question 25.
Reduction refers to reaction involving …………………… of electrons.
Answer:
Gain
Question 26.
Give an example Tor redox reaction.
Answer:
Burning of Magnesium.
Question 27.
Which is the oxidising agent in this reaction?
Answer:
Chlorine.
(In a chemical reaction the process involving gaining of electrons is called reduction and the – species that helps in oxidation is oxidizing agent).
Question 28.
Oxidation refers to reaction involving
a) Loss of proton
b) Gain of electron
c) Loss of electron
d) Gain of proton
Answer:
c) Loss of electron.
Question 29.
Analyse the chemical equation given below and find out oxidizing and reducing agent.
Answer:
Oxidising agent – Cl (Oxidation number is decreasing. So, it gets reduced)
Reducing agent – Na (Oxidation number is increasing. So, it gets oxidized.)
Question 30.
Find out the oxidation number of phosphorous in H3PO4.
(Hint: Oxidation number H = +1 O + -2)
Answer:
(+1 × 3) + P + (-2 × 4) = 0
+ 3 + P + -8 = 0
P + -5 = 0
P = +5
Question 31.
Calculate the oxidation number of ‘S’ in H2SO4.
Answer:
Oxidation number of hydrogen = +1
Oxidation number of oxygen – 2
The sum of the oxidation numbers of all the elements in a compound is zero, Hence
(2 × +1) + 5 + (4 × -2) = 0
2 + 5 + -8 = 0
5 – 6 = 0
5 = +6
Oxidation number of S in H2SO4 is +6.
Question 32.
The equation of a chemical reaction is given.
Mg + F2 → MgF2
(Hint: Electronic configuration of Mg = 2, 8, 2 F = 2, 7)
a) Which atom loses electron in this reaction?
b) Which atom acts as oxidising agent here?
Answer:
a) In the formation of MgF2, Mg donates its 2 electrons to the fluorine atoms. So, the atom which loses electron is Magnesium (Mg).
b) Fluorine (F2) .
In a chemical reaction the process involving gaining of electrons is called reduction and the species that helps in oxidation is oxidizing agent.
Question 33.
Analyse the equation given below and answer the following questions
Mg + F2 → MgF2
[Hint: – Electronic Configuration Na = 2, 8, 1; Cl = 2, 8, 7]
a) Which atom gets oxidised here?
b) Write the equation for reduction in this reaction?
Answer:
a) Sodium (Na)
b) Cl2 + 2e– → 2Cl–
Question 34.
Analyse the statements and correct the wrong statements. If any
a) In elemental state, oxidation number of molecules is zero.
b) Generally, metals are oxidizing agents.
c) Reduction is the process of losing electrons.
Answer:
a) Statement is correct.
b) Wrong. Generally, metals are reducing agents OR Generally, non metals are oxidizing agents.
c) Wrong. Reduction is the process of gaining electrons. OR Oxidation is the process of losing electrons.
Question 35.
Analyse the chemical equation and answer the following questions.
[Hint: x and y are the oxidation numbers of Zn]
a) Find x and y
b) Oxidation number of which atom is increased?
c) Which is the reducing agent in this reaction?
Answer:
a) x = 0 (Zn in elemental state.)
ZnCl2 :
y + (2 × -1) = 0
y – 2 = 0
y = +2
b) Zn (from 0 to +2)
c) Zinc(Zn).
Question 36.
Q
a) Find the Oxidation state of Fe in FeCl2.
b) Is this a redox reaction? Justify your answer.
Answer:
a) FeCl2
x + (2 × -2) = 0
x – 4 = 0
x = +4
The oxidation number of Fe in FeCl2 is +4.
b) Yes, it is a redox reaction.
The oxidation number of increases from 0 to +4. So there happens the oxidation.
The oxidation number of hydrogen decreases from +1 to 0. So there happens the reduction. As both reduction and oxidation take place simultaneously, the above given reaction is a reduction reaction.
Question 37.
Match suitably those given in column A with those in column B.
| A | B |
| Oxidising agent | Losses electrons |
| Reducing agent | Oxidation number increases |
| Oxidation | Oxidation number decreases |
| Reduction | Gains electron |
Answer:
| A | B |
| Oxidising agent | Gains electron |
| Reducing agent | Losses electrons |
| Oxidation | Oxidation number increases |
| Reduction | Oxidation number decreases |
Question 38.
Explain the following
a) Balancing chemical equations
b) Oxidising agent
c) Redox reaction
d) Law of conservation of mass
Answer:
a) Balancing a chemical equation is the method of equalising the number of the same type of atoms in both the reactants and the products. The equation thus obtained is known as a balanced chemical equation.
b) Oxidising agent – The species that helps oxidation in a chemical reaction is the Oxidising agent.
c) Redox reactions – The reactions in which oxidation and reduction take place simultaneously are called redox reactions.
d) Law of conservation of mass – In a chemical reaction, the total mass of the reactants will be equal to the total mass of the products.
Question 39.
The oxidation number of certain atoms are given below.
O = -2, H = +1, K = +1
Find out the oxidation number of the atoms underlined in the compounds given below.
a) H2CO3
b) KNO3
Answer:
a) H2CO3
(+1 × 2) + C + (-2 × 3) = 0
+ 2 + C + -6 = 0
C + -4 = 0
C = +4
+4
b) KNO3
+ 1 + N + (-2 × 3) = 0
+ 1 + N + -6 = 0
N + -5 = 0
N = +5
Question 40.
Analyse the given equation and complete the table (Hint: Atomic number Mg = 12 0 = 17)
Mg + Cl2 → MgCl2
| a) Oxidised atom ………………..
b) Oxidising agent ………………… c) Equation of reduction ……………….. d) Equation of oxidation …………………. |
Answer:
a) Mg
b) Cl
c) Cl + le– → Cl–
d) Mg → Mg2+ + 2e–