Class 9 Chemistry: Chapter 03 Redox Reactions and Rate of Chemical Reactions - Questions and Answers
SCERT Solutions for Std IX Chemistry Chapterwise
Class 9 Chemistry Questions and Answers
Chapter 03 Redox Reactions and Rate of Chemical Reactions
* Law of conservation of mass.
In a chemical reaction, mass is neither created nor destroyed. This is the law of conservation of mass. That is, the total mass of reactants and the total mass of products in a chemical reaction will be equal.
This law was proposed by Antoine Lavoisier.
* Balancing of a chemical equation.
According to the Law of Conservation of mass, in a chemical reaction total mass of reactants will be equal to the total mass of reactants. If so the total number of atoms of each element in both sides of a chemical equation are also equal.
Equalising the number of atoms of each element in the reactant side and that on the product side is known as the balancing of the equation.
The equation of the reaction in respect of the burning of magnesium in the air is given below.
Mg + O₂ → MgO
In this equation, the number of oxygen atoms present on both sides is not equal. There are two oxygen atoms on the reactant side but only one oxygen atom on the product side. Hence this equation is not a balanced
equation.
When it is written as 2Mg + O₂ → 2MgO, the total number of oxygen atoms and magnesium atoms on both sides are equal and hence it is a balanced equation.
More examples are given in the table.
* Oxidation and Reduction.
It is by either transferring or sharing electrons, a chemical reaction takes place.
Oxidation is the process of loss of electrons and reduction is the process of gain of electrons.
Example: 1: Mg + Cl₂ →MgCl₂
In this reaction, two electrons are lost from the magnesium atom and the magnesium atom becomes a magnesium ion. And the chlorine atoms receive one electron each and becomes chloride ions.
Mg – 2e →Mg₂+ Cl + 1e → Cl-1
Hence in this reaction, magnesium is oxidised and chlorine is reduced.
The atom which loses an electron (oxidised atom) is the reducing agent and the atom which gains an electron (reduced atom) is an oxidising agent.
Therefore in the above reaction, magnesium is a reducing agent and chlorine is the oxidising agent.
Example: 2: 2Na + Cl₂ →2NaCl
Here Sodium is oxidised and hence it is a reducing agent. Since chlorine is reduced, it is the oxidising agent.
* Oxidation Number
The oxidation number is the charge assigned to the atoms due to the transferring or sharing of electrons during the formation of a molecule.
In the reaction, Mg + Cl₂ →MgCl₂, the Oxidation number of Magnesium is +2 and that of the chlorine is -1.
In the case of covalent molecules, the oxidation number is given by assuming the shared electrons are gained by more electronegative atom.
For example, H₂ + O₂ → 2H₂O
H₂O is a covalent compound. As oxygen is more electronegative than Hydrogen, it is assumed that Oxygen receives the electron and hydrogen losses its electron. And hence oxidation number of Hydrogen is +1 and that of Oxygen is -2.
The oxidation number in a molecule of an element is considered as zero as the bonded electrons are equally attracted towards each atom.
The sum of the oxidation numbers of all atoms in a molecule is zero.
See the Examples of Assigning oxidation number in a Chemical Equation.
Example:1. H₂ + Cl₂ →2HCl
Here the hydrogen and chlorine on the reactant side are the molecules of element and hence their oxidation numbers are zero. In HCl, as Cl is more electronegative than that of hydrogen, it is assumed that hydrogen gets
+1 charge by losing one electron and chlorine gets –1 charge by gaining one electron.
And the equation can be written by marking the oxidation number as shown below.
H2⁰ + Cl2⁰ →2H⁺¹Cl⁻¹
(Here hydrogen gets oxidised and chlorine gets reduced. So hydrogen is a reducing agent and chlorine is the oxidising agent. It is noted that the total oxidation number of the atoms in the HCl molecule is zero.)
Example:2. C⁰ + O2⁰ →C⁺⁴O2⁻²
(Carbon gets oxidise and oxygen gets reduce. Hence carbon is the reducing agent and Oxygen is an oxidising agent)
Example: 3. Zn⁰ + 2H⁺¹Cl⁻¹→Zn⁺²Cl₂⁻¹+ H₂⁰
The process in which the oxidation number increases is called oxidation. And the process in which the oxidation number decreases is called reduction.
This can be illustrated with the example given below.
C⁰ + O₂⁰ →C⁺⁴O₂⁻²
In this reaction, the oxidation number of carbon is increased from 0 to 4. Hence hence carbon undergoes oxidation. But oxidation number of oxygen decreases from zero to -2. So oxygen gets reduced.
* Method of determining the Oxidation number
Find out the oxidation number of manganese in the molecules of the compounds KMnO₄, MnO₂, Mn₂O₃ and Mn₂O₇ [Oxidation number of Oxygen is -2 and that of Potassium is +1]
i. Let 𝑥 be the oxidation number of manganese.
Since the sum of the oxidation numbers of all the atoms in a molecule is zero,
i. 1x1+𝑥 +4x-2 = 0 1+ 𝑥 - 8 = 0 Then 𝑥 = +7
ii. 𝑥+ 2x-2 = 0 𝑥 - 4 = 0 Then 𝑥 = +4
iii. 2x𝑥+3x-2= 0 2𝑥 -6= 0 Then 𝑥 = +3
iv. 2x𝑥+7x-2= 0 2𝑥 -14=0 Then 𝑥 = +7
* Redox reactions
The oxidation and reduction that takes place simultaneously in a reaction are known as the Redox reaction. In a redox reaction, oxidising agent undergoes reduction and the reducing agent undergoes oxidation.
Example: Mg + F₂ →MgF₂
In this reaction, Mg is Oxidised and Fluorine is reduced. That is, oxidation and reduction take place simultaneously. So it is a redox reaction.
* Threshold Energy.
It is the minimum kinetic energy of reactant molecules that should be possessed so as to take part in a chemical reaction.
* Factors influencing Rate of Chemical Reaction.
i. Nature of the reactants.
The nature of the reactants is one of the factors that influence the rate of the chemical reaction. We can understand this from the following experiment. Take an equal amount of dilute hydrochloric acid in two test tubes. Add a piece of magnesium into the first test tube and a piece of zinc into the second. A more vigorous reaction can be observed in the test tube in which magnesium is added. Mg + 2HCl →MgCl₂ + H₂
ii. Concentration
The concentration of reactants influences the rate of the chemical reaction.
Experiment: Take two pieces of magnesium ribbon of the same size in two separate test tubes. Add an equal volume of concentrated hydrochloric acid to one test tube and add the same volume of dilute hydrochloric acid to the
second one. The vigorous reaction can be seen in the test where concentrated HCl is taken.
According to modern theory, the effective collisions between reactant molecule is responsible for chemical reaction and to form new substances. When the concentration of reactants increases, the number of molecules per unit volume increases and hence the number of collisions increases. That is why the rate of reaction increases when the concentration of reactants increases.
iii. Surface area
In the cases of solid reactants, the rate of reaction will be improved if the reactants are crushed or powdered.
When the reactants are made into small pieces, their surface area increases. As a result the number of collisions between the reactant molecules also increases. That is, the rate of reaction increases when the
reactants are used in powdered form.
Experiment: Take an equal amount of dilute hydrochloric acid in two small beakers. Add a piece of marble into the first beaker. Take another piece of marble of the same size and put it into the second beaker after crushing it. A severe reaction can be observed in the beaker where the crushed marble is added.
CaCO₃ + 2HCl →CaCl₂ + CO₂ + H₂O
The faster burning of firewood when it is cut into small pieces is very familiar to us. When it is cut, the surface area of the firewood increases.
iv. Temperature
The rate of reaction increases when temperature increases.
Experiment: Take an equal volume of sodium thiosulphate solution in two boiling tubes. Heat one boiling tube for some time. Add dilute hydrochloric acid in an equal amount into both the boiling tubes. The faster precipitation can be seen in the boiling tube that was heated.
Na₂S₂O₃ + 2HCl →2NaCl + SO₂ + S + H₂O
When temperature increases the kinetic energy of the reactant molecules also increases. Then the number of reactant molecules possessing threshold energy increases. As a result, the number of effective collisions increases and thus rate of reaction also increases.
v. Presence of catalyst.
Catalysts are the substances that alter the rate of a chemical reaction without themselves undergoing any chemical change.
Manganese dioxide is a catalyst that increases the rate of decomposition of hydrogen peroxide. H₂O₂ → H₂O+O₂
Experiment: Take some hydrogen peroxide in a test tube and bring a glowing incense stick into the test tube.
Observe the burning of the stick. Now add some manganese dioxide into the test tube. Bring the incense stick once more. A flare-up can be seen then.
When manganese dioxide is added, more oxygen is released because of the fast decomposition of hydrogen peroxide. This is the reason for the flare-up of the incense stick.
The catalyst that increases the speed of reaction is called a positive catalyst. Manganese dioxide is a positive catalyst.
The catalyst that decreases s the rate of reaction is called a negative catalyst.
Phosphoric acid is a negative catalyst as it reduces the decomposition rate of hydrogen peroxide.
There is a specific catalyst for each chemical reaction. In the manufacture of sulphuric acid, vanadium pentoxide and in the manufacture of ammonia, iron is used as a positive catalyst.
* PRACTICE QUESTIONS& ANSWERS
1. One gram hydrogen and 8 g oxygen are reacted to form water.
a. What will be the mass of water obtained?
b. Name the law that helped you to answer the question.
Answer:
a. 9 g.
b. Law of conservation of mass.
2. Name the proponent of the law of conservation of mass.
Answer: Antoine Lavoisier.
3. Balance the following equations.
a. Na + Cl₂ →NaCl
b. H₂O₂ → H₂O + O₂
c. CO₂ + C →CO
d. Zn + HCl →H₂ + ZnCl₂
e. Al + O₂ →Al₂O₃
Answer:
a. 2Na + Cl₂ →2NaCl
b. 2H₂O₂ → 2H₂O + O₂
c. CO₂ + C →2CO
d. Zn + 2HCl →H₂ + ZnCl₂
e. 4Al + 3O₂ →2Al₂O₃
4. "In a balanced chemical equation, the number of molecules on the reactant side and that in the product side will be equal.” Comment to this statement.
Answer: The statement is wrong. In a balanced equation, the number of atoms on either side will be equal. But the number of molecules may not be equal.
For Example,2H₂O₂ → 2H₂O + O₂
It is a balanced equation. On its left side, there are only two molecules. But on the right side, there are three molecules.
5. See the given chemical equation. Mg + Cl₂ →MgCl₂
(Electronic configuration Mg – 2,8,2 Cl – 2,8,7 )
a. Insert the oxidation numbers on the respective atoms in the equation.
b. Find out the element whose oxidation number gets increased.
c. Identify the oxidised element and reduced element.
d. Identify the oxidising agent and reducing agent.
Answer:
a. Mg⁰ + Cl₂⁰ →Mg⁺²Cl₂⁻¹
b. Oxidation number of magnesium increases from zero to +2.
c. Magnesium is oxidised and chlorine is reduced.
d. Magnesium – Reducing agent. Chlorine - Oxidising agent.
6. Calculate the oxidation number of sulphur in sulphur trioxide (SO₃)
Answer: 𝑥+ 3x(-2) = 0 𝑥 = 6
oxidation number of sulphur in this molecule is +6
7. i. Ca⁺²C⁺⁴O3⁻² →Ca⁺²O⁻² + C⁺⁴O2⁻²
ii. 2N2⁰ + O2⁰ → 2N⁺²O⁻²
Two equations are given.
a. From the equations find out the one in which oxidation and reduction take place.
b. Which is the redox reaction?
Answer:
a. ii. 2N2⁰ + O2⁰ → 2N⁺²O⁻²
b. Since there is oxidation and reduction in the second equation, it is the redox reaction.
8. When the solid reactants are powdered, the rate of reaction increases. Why?
Answer: When they are crushed or powdered their surface areas increase. As a result the number of collisions increases and hence the rate of reaction also increases.
9. Unfolded paper flares up severe than crumpled paper. Explain the reason.
Answer: When the paper is unfolded, its surface area increases. As a result, the collision with the oxygen molecules increases which lead to an increase in the rate of reaction and burns the paper into ashes at the earliest.
10. Hydrogen peroxide (H₂O₂) is a compound that undergoes decomposition.
a. Write down the balanced equation of the decomposition.
b. Name a substance that reduces the decomposition rate of hydrogen peroxide.
c. What is called such substance?
Answer:
a. 2H₂O₂ → 2H₂O + O₂
b. Phosphoric acid.
c. Negative catalyst.
11. Hydrogen can be made available by the decomposition of Hydrogen Peroxide (H₂O₂). Suggest a method to produce more oxygen by this method.
Answer: Add manganese dioxide, a positive catalyst, to hydrogen peroxide.
12. It is put some zinc granules into a test tube containing dilute hydrochloric acid.
a. Write down the balanced chemical equation of the reaction that takes place here.
b. Identify the gas that evolves in the reaction.
c. Suggest two methods to make available this gas in excess.
Answer:
a. Zn + 2HCl →H₂ + ZnCl₂
b. Hydrogen.
c. i. Add concentrated acid. ii. heat up the reactants.
13. When it is blown into a fireplace, the fire flares up. Why?
Answer: In combustion, oxygen is one of the reactants. When it is blown, the presence of oxygen increases. That is why the fire flares up.
14. What is a positive catalyst? Give two examples.
Answer: The catalysts that increase the rate of reaction is called a positive catalyst.
Manganese dioxide is a positive catalyst in the decomposition of hydrogen peroxide.
Iron is a positive catalyst in the manufacture of ammonia.
15. Complete the second pair according to the first.
a. Ammonia: Catalyst - Iron; Sulphuric acid: Catalyst - .......
b. Decomposition of Hydrogen peroxide- Positive catalyst: Manganese dioxide; Decomposition of Hydrogen peroxide- Negative catalyst: .......
Answer:
a. Vanadium pentoxide
b. Phosphoric acid.
16. Complete the statements.
a. The oxidised atom is ...... (oxidising agent/reducing agent)
b. The atom whose oxidation number decreases undergoes .. (Oxidation/reduction)
c. When temperature increases threshold energy ....... (increases/decreases/ doesn't change)
d. The proponent of the law of conservation of mass is .....
e. The number of ...... on both side of a balanced chemical equation will be equal. (atoms/molecules)
f. The atom that loses electron undergoes ....... (oxidation/reduction)
g. The oxidation number of atoms in a molecule of the element is .........
Answer:
a. Reducing agent.
b. Reduction
c. doesn't change.
d. Antoine Lavoisier.
e. Atoms
f. Oxidation.
g. zero.
17. Catalysts are substances that alter the rate of chemical reactions
How do positive catalysts and negative catalysts differ from each other? Explain by citing examples
Answer:
Positive catalysts increase the rate of reaction while negative catalysts decrease the rate.
MnO₂ increases the rate of decomposition of hydrogen peroxide. Hence it is a positive catalyst. Phosphoric acid is a negative catalyst because it decreases the rate of decomposition of hydrogen peroxide.
18. Equal volumes of sodium thiosulphate solution is taken in two boiling tubes. Heat one boiling tube for some time. Add dilute hydrochloric acid in equal amounts in both the boiling tubes. Sulphur precipitates faster in the boiling tube which is heated.
a) Write down the chemical equation of the above reaction.
b) Name the factor that affects the rate of reaction in this experiment.
c) Why does the rate of reaction increase here?
Answer:
a) Na₂S₂O₃ + 2HCl → 2NaCl + H₂O +SO₂ +S
b) Temperature
c)When temperature increase, the number of molecules with threshold energy increases. As a result the number of effective collisions increases and thus the rate of reaction also increases.
19. Answer the following.
a) Describe an experiment to prove that the rate of reaction increases with an increase in temperature.
b) Why does the rate of reaction increase with an increase in temperature?
Answer:
a) For describing the reaction between sodium thiosulphate solution and HCl
b) When temperature increases, the number of molecules with threshold energy increases. As a result the number of effective collisions increases and thus the rate of reaction also increases.
20. Analyse the figure and answer the question given below.
Answer:
In the second beaker where powdered marble is taken.
The surface area of the reactant increases when marble is powdered.
21.Consider the reaction
Zn + 2HCl → ZnCl₂+ H₂
Which of the following methods can be used to increase the rate of reaction?
a)Decrease the temperature
b)Concentration of HCl is increased.
c)Powdered zinc is used.
d)Concentration of HCl is decreased.
Answer: b and c
22. Choose the oxidation reactions from the following.
a) Zn → Zn⁺² +2e-
b) Cu²⁺ +2e- → Cu
c) Mg → Mg²⁺ +2e-
d) Zn²⁺+ 2e- → Zn
Answer: a & c
23. Zn⁰ + Cu⁺²SO₄⁻² → Zn⁺²SO₄⁻² + Cu⁰
Does the chemical equation given above represent a redox reaction?
Give reason
Answer:
Yes
Zn gets oxidised to ZnSO₄
CuSO₄ gets reduced to Cu. Here oxidation and reduction take place simultaneously.
24. Examine the given equations and answer the following questions.
Zn⁰ + 2H⁺¹Cl⁻¹ → Zn+2Cl₂⁻¹ + H₂0
Are the statements given below correct? Justify your answer.
a) Zn gets oxidised
b) HCl is the oxidising agent.
Answer:
Both statements are correct.
The oxidation number of Zn increases from 0 to +2. Hence Zn gets oxidised.
The oxidation state of H in HCl reduces from +1 to 0. Hence it is the oxidising agent.
25. Fe + 2HCl → FeCl₂ + H₂
a) Find the oxidation state of Fe in FeCl₂
b) Identify the oxidising agent and reducing agent in the given equation.
Answer:
a) +2
b) oxidising agent HCl/H+
reducing agent Fe
26. 2Na + Cl₂ → 2NaCl
Analyse the equation and fill up the table given below
27. The chemical equation given below represents the reaction between hydrogen and oxygen-producing water.
H₂ + O₂ → H₂O
Is the given equation balanced? Give reason.
Answer:
No. The total number of atoms of each element in the molecules present on the reactant side and that on the product side are not equal.
28. From the statements given below, find the wrong one. Rewrite it correctly.
a) In a chemical reaction mass is neither created nor destroyed.
b) The total mass of the products will be greater than the total mass of the reactants.
c) In a chemical reaction, the total number of atoms of each element on the reactant side and that on the product side are equal.
Answer:
The wrong statement is (b).
The total mass of the products will be equal to the total mass of the reactants.
29. Name the catalyst which reduces the rate of decomposition of hydrogen peroxide?
Answer: Phosphoric acid
30. Take two test tubes A and B. Take equal volumes of sodium thiosulphate solution in both test tubes. Heat test tube A. Add equal volumes of dilute hydrochloric acid in both test tubes.
a. In which test tube the reaction is faster?
b. Which is the factor that affects the rate of reaction here?.
c. Explain why the rate of reaction is faster in this test tube
Answer:
a. In test tube A
b. Temperature
c. When temperature increases, the number of molecules with threshold energy increases. As a result the number of effective collisions increases and thus the rate of reaction also increases.
31. Take 5ml hydrogen peroxide (H₂O₂ ) solution in a test tube. Add some MnO₂ into it.
a. Which is the gas evolved? How will you identify it?
b.What is the function of MnO2 in this reaction?
Answer:
a. Oxygen(O₂). Introduce a glowing match stick into the test tube. Flaring up of the glowing match stick shows the presence of oxygen.
b) MnO₂ increases the rate of reaction. Hence it acts as a positive catalyst.
32. Which one of the following does not affect the rate of reaction?
(temperature, pressure, colour of reactants, concentration)
Answer: colour of reactants
33. Take 5ml of hydrogen peroxide (H₂O₂) solution in a test tube. Add some manganese dioxide (MnO₂) into it.
a. Which is the gas evolved?
b. What is the function of MnO₂ in this reaction?
c. Which are the substances remaining in the test tube when the reaction is over?
Answer:
a. O₂
b. Positive catalyst
c. H₂O and MnO₂
34. Analyse the figure given below and answer the following questions.
b. Which is the factor that influences the rate of reaction in this case?
c. Write down the balanced chemical equation of the reaction taking place here.
Answer:
a. Test tube 1
b. concentration of reactants
c. Mg + 2HCl → MgCl₂ + H₂
35. Zn⁰ + Cu²⁺(SO4)2²⁻ → Zn²⁺(SO4)2²⁻+ Cu⁰
Analyse the given equation and answer the following questions.
a. Which element gets oxidised?
b.Which element /ion gets reduced?
c. Which is the oxidising agent?
d. Which is the reducing agent?
Answer:
a. Zn
b.Element /ion get reduced Cu / Cu²⁺
c. Oxidising agent Cu(SO4)2 / Cu²⁺
d. Reducing agent Zn
36. Examine the given reaction
Fe + 2HCl → FeCl₂ + H₂
Does this reaction represent a redox reaction? Justify your answer
Answer:
Yes.
Fe changes to Fe 2+ ion by the removal of 2 electrons.
Fe → Fe ²⁺ + 2e⁻ (oxidation)
H⁺ ions change to H₂ by accepting electrons
2H⁺ + 2e⁻ → H₂ (reduction)
The given reaction represents a redox reaction because both oxidation and reduction take place simultaneously.
37. Hydrogen peroxide decomposes to give oxygen.
A student wrote the chemical equation of this reaction as follows.
H₂O₂ → H₂O + O
a) Which are the reactants and products in this reaction?.
b) Are the molecular formulae of reactants and products written here correct? If not rewrite them correctly.
c) write the balanced chemical equation for this reaction.
Answer:
a.Reactant – hydrogen peroxide
Products- water, oxygen
b. Molecular formula of oxygen given in this equation is wrong. The correct molecular formula of oxygen is O₂
c. 2H₂O₂ → 2H₂O + O₂
Chemistry Textbook (pdf) - Click here
👉 Quiz
PSC Solved Question Papers ---> Click here
PSC TODAY's EXAM RESULTS ---> Click here
PSC EXAM PROGRAMME -> Click here
CURRENT AFFAIRS QUESTIONS -> Click here
PSC Degree Level Questions & Answers - Click here
PSC 10th, +2 Level Questions & Answers - Click here
PSC SHORTLISTS -> Click here
PSC RANK LISTS -> Click here
 Chapter 03 REDOX REACTIONS AND RATE OF CHEMICAL REACTIONS){kind=link}
0 Comments