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Kerala Syllabus Std 9 Chemistry Chapter 5 Chemical Kinetics Extra Questions and Answers
Question 1.
Reactions can be classified as Direct combination, decomposition, simple displacement, double decomposition, and Redox reactions. State which of the following types takes place in the reactions given below.
(i) Cl2 + 2KI → 2KCl + I2
(ii) 2Mg + O2 → 2MgO
(iii) AgNO3 + HCl → AgCl + HNO3
(iv) 4HNO3 → 4NO2 + 2H2O + O2
Answer:
(i) Cl2+ 2KI → 2KCl + I2: Simple displacement
(ii) 2Mg + O2 → 2MgO: Direct combination
(iii) AgNO3 + HCl → AgCl + HNO3: Double decomposition
(iv) 4HNO3 → 4NO2 + 2H2O + O2: Decomposition reaction
Question 2.
Give balanced equations for reactions involving the evolution of a gas in addition of dilute acid to:
(a) sodium sulphite
(b) calcium carbonate
Answer:
(a)Na2SO3 + H2SO4 [dil.] → Na2SO4 + H2O + SO2 [g]
Sulphur dioxide gas has evolved.
(b) CaCO3 + 2HCl [dil.] → CaCl2 + H2O + CO2 [g]
Carbon dioxide gas has evolved.
Question 3.
Define the following types of chemical changes or reactions with a suitable example of each.
(a) Direct combination reaction or synthesis
(b) Decomposition reaction
(c) Displacement reaction or substitution reaction
(d) Double decomposition reaction
Answer:
(a) A chemical reaction in which two or more elements or compounds react to form one new compound is called a Direct combination reaction or synthesis.
For example,
Hydrogen bums in the air to give water:
2H2 [g] + O2 [g] → 2H2O [1]
(b) A chemical reaction in which a chemical compound decomposes into two or more simpler
substances (elements and/or compounds) is called a Decomposition reaction.
For example,
Calcium carbonate, on heating, decomposes to Calcium Oxide and Carbon dioxide:
CaCO3 \(\xrightarrow{\Delta}\) CaO + CO2
(c) A chemical reaction in which an element or radical has replaced another element in a compound is known as a Displacement reaction or Substitution reaction.
For example,
Magnesium displaces Copper from Copper [II] sulphate solution:
Mg + CuSO4 → MgSO4 + Cu
(d) A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is known as a Double decomposition reaction.
For example,
Silver nitrate + Potassium chloride → Silver chloride + Potassium nitrate
AgNO3 + KCl → AgCl + KNO3
Question 4.
Define a displacement reaction with a suitable example. State how it is represented. Give a reason why zinc displaces hydrogen from dilute sulphuric acid, but copper does not.
Answer:
A chemical reaction that takes place when an element [or radical] has replaced another element in a compound is known as a Displacement reaction It is represented as :
X + YZ → Y + XZ
Eg: Magnesium + Copper [II] sulphate → Magnesium sulphate + Copper
Mg + CuS04 → MgS04 + Cu
As a more electropositive metal displaces a less electropositive metal from its aq. soln. therefore, zinc, being more electropositive, is placed above [H] in the activity series and displaces hydrogen from dilute sulphuric acid, whereas copper, being less electropositive, is placed below [H] in the electrochemical series and cannot displace hydrogen from sulphuric acid.
Question 5.
Differentiate between a decomposition reaction and a double decomposition reaction.
Answer:
| Decomposition reaction | Double decomposition reaction |
| A chemical reaction in which a compound decomposes to give two new elements / a new compound & an element / two new compounds is called a Decomposition reaction. | A chemical reaction in which both reactants [compounds] are decomposed to give two new compounds by exchanging their radicals is called a Double decomposition reaction. It is represented as XY + AB → XB + AY |
| For example: 2HgO → 2Hg + O2 | For example: CaCl2 + Na2CO3 → 2NaCl + CaCO3↓ |
Question 6.
Match the chemical reactions in List I with the appropriate answer in List II.
| List I | List II |
| AB → A + B | Endothermic reaction |
| X+Y– + A+B– → X+B– + A+Y– | Decomposition reaction |
| X + YZ → XZ + Y | Double displacement reaction |
| X + Y → XY – Δ (on heating) | Displacement reaction |
Answer:
| List I | List II |
| AB → A + B | Decomposition reaction |
| X+Y– + A+B– → X+B– + A+Y– | Double displacement reaction |
| X + YZ → XZ + Y | Displacement reaction |
| X + Y → XY – Δ (on heating) | (on heating) |
Question 7.
Analyse the chemical equations and answer the following questions.
NaOH + HCl → NaCl + H2O
(a) What is the salt formed in this reaction?
(b) What is the name of this type of reaction?
Answer:
(a) Sodium chloride
(b) Double displacement reaction
Question 8.
(a) What is the chemical, required to prepare hydrogen in the laboratory
(b) Write the balanced chemical equation of this reaction
(c) Which type of reaction is this
(Decomposition, Combination, Displacement, Double decomposition)
Answer:
(a) Dilute hydrochloric acid and Zinc
(b) Zn + 2HCl → ZnCl2 + H2
(c) Displacement reaction
Question 9.
A small quantity of sodium chloride solution is taken in a test tube. One drop of silver nitrate solution is added to this.
(a) What do you observe?
(b) NaCl + AgNO3 → X + NaNO3
Answer:
(a) A white curdy precipitate of silver chloride (AgCl) is formed
(b) Silver chloride (AgCl)
Question 10.
How does a physical change differ from a chemical change?
Answer:
Both physical and chemical changes involve a change in matter, but they differ in the way the matter is altered.
| Physical change | Chemical change |
| The composition remains the same | Composition Changes |
| No new substances are formed | New substances are formed with different properties |
| Reversible | Irreversible |
| It may involve energy transfer | Involves energy transfer |
| Example: Melting of ice into water | Example: Burning wood |
Question 11.
What are the methods usually adopted to make firewood burn faster?
Answer:
- Provide more air
- Split up into small pieces
- Make firewood dry
Question 12.
Describe an experiment to prove that the nature of the reactants affects the rate of a chemical reaction.
Answer:
Materials required for the experiment. Zn, Mg, dil. HCl and test tubes.
Procedure
Take an equal volume of dil. HCl in two test tubes. Add Zn to one and Mg of the same mass to the other. Hydrogen gas is produced in both test tubes. The rate of reaction is faster in the test tube containing Mg. This is because Mg is more reactive than Zn.
Question 13.
Why does the rate of reaction increase when concentration increases?
Answer:
As the concentration of reactants increases, the number of molecules per unit volume and the number of effective collisions increase. Consequently, the rate of reaction increases.
Question 14.
What is the relation between the rate of reaction and surface area? Write an experiment to prove it.
Answer:
Take an equal volume of dil. HC1 in two beakers. Add a small piece of marble into one and marble powder of equal mass into the other. The reaction rate is greater when marble powder is used. The rate of reaction increases when surface area increases.
Question 15.
Why does the rate of reaction increase when surface area increases?
Answer:
When solids are made into small pieces or powder, their surface area increases. As a result, the number of molecules undergoing effective collision also increases. Hence, the rate of reaction increases.
Question 16.
Write an experiment to prove the relation between temperature and rate of reaction.
Answer:
Materials required: Sodium thiosulphate, hydrochloric acid, water, boiling tube, spirit lamp – Procedure: Prepare a dilute solution of sodium thiosulphate in a beaker. Take equal volumes of this solution in two boiling tubes. Heat one boiling tube for some time. Add dilute hydrochloric acid in equal amounts in both boiling tubes.
Observation: Reaction is faster in a heated test tube.
Question 17.
What are catalysts?
Answer:
Catalysts are substances that alter the rate of chemical reactions without undergoing anypermanent chemical change.
Question 18.
The chemical reaction between marble and dilute HC1 is given.
CaCO3 + 2HCl → CaCl2 + H2O + X
(a) Which gas is formed here? How can you identify this gas?
(b) Suggest any two ways you would choose to increase the rate of this chemical reaction.
Answer:
(a) Carbon dioxide (CO2). Show a burning splinter in the gas. It will get extinguished.
OR
Show a glass rod dipped in clear lime water into the gas. The lime water will turn milky
(b) 1) Use powdered marble
2) Use concentrated HCl
3) Heat the mixture.
Question 19.
Sulphur pieces do not react with cold, concentrated nitric acid. But sulphur powder reacts.
(a) Explain the reason why the rate of chemical reaction is increased here.
(b) Suppose you want to increase the rate of reaction again. Which way would you choose? Give reason
Answer:
(a) When surface area increases, the rate of effective collision increases. So, the rate of reaction increases.
(b) Heat the mixture.
When temperature increases, more molecules will get threshold energy. So rate of effective collision and rate of reaction increases.
Question 20.
Small amounts of phosphoric acid are usually added to hydrogen peroxide to prevent its decomposition.
a. What is the function of phosphoric acid here?
b. By which name are these types of substances known?
c. Which substance would you add to increase the rate of decomposition of hydrogen peroxide?
Answer:
(a) Slow down the rate of decomposition of hydrogen peroxide
(b) Negative catalyst
(c) Manganese dioxide
Question 21.
The experiments conducted by two students are given below.
Experiment 1:
2 mL of sodium thiosulphate solution is taken in a test tube and heated, and 2 mL of diluted HC1 solution is added.
Experiment 2:
2 mL of sodium thiosulphate solution is taken in a test tube, and to it, 2 mL of diluted HC1 solution is added.
(a) In which experiment is the precipitate formed quickly? Justify your answer.
(b) Write the balanced equation for the reaction.
Answer:
(a) In a heated test tube. (Experiment 1) When temperature increases, more molecules get threshold
energy. So, the rate of effective collision increases, and the rate of reaction increases.
(b) Na2S2O3 + 2HCl → 2NaCl + SO3 + S
Question 22.
Some materials available in the laboratory are given below. Magnesium ribbon, marble powder, marble pieces, dilute HCl, concentrated HCl.
a. Which materials will you choose for the preparation of more CO2 in less time?
b. Write the balanced chemical equation of the reaction.
Answer:
(a) Marble powder and concentrated HCl
(b) CaCO2 + 2HCl → CaCl2 + H2O + CO2
Question 23.
What are the factors that influence the speed of chemical reactions?
Answer:
- Nature of the reactants
- Concentration of the reactants
- Total surface area
- Temperature
- Presence of catalyst
Question 24.
Find a reason for the following.
a. Splitted-up logs burn fast.
b. It is said that drinking water along with food is not good.
c. Certain medicines are kept in refrigerators.
Answer:
(a) When logs are split up, the total surface area increases. So, the number of collisions per unit area increases, and the speed of chemical reaction (burning) increases.
(b) Intake of water will dilute the digestive enzymes and hydrochloric acid produced in the stomach which help the digestive process. So it is better to avoid drinking water half an hour before and half an hour after food.
(c) A rise in temperature may cause the decomposition of certain chemicals. Such substances are kept in refrigerators.
Question 25.
Match the following suitably.
| Activity | Catalyst |
| To reduce the decomposition of hydrogen peroxide | Manganese dioxide |
| Industrial production of sulphuric acid | Phosphoric acid |
| Industrial production of ammonia | Vanadium pentoxide |
| To speed up the decomposition of potassium permanganate | Iron |
Answer:
| Activity | Catalyst |
| To reduce the decomposition of hydrogen peroxide | Phosphoric acid |
| Industrial production of sulphuric acid | Vanadium pentoxide |
| Industrial production of ammonia | Iron |
| To speed up the decomposition of potassium permanganate | Manganese dioxide |
Question 26.
When, eggshell is added to Hydrochloric acid, a gas is produced
(a) Identify this gas
(b) Write the chemical equation of this reaction.
(c) If we add powdered eggshell, what happens to the rate of the above chemical reaction? Why?
Answer:
(a) Carbon dioxide (CO2)
(b) CaCO3 + 2HCl → CaCl2 + H2O + CO2
(c) The reaction becomes faster. When the solid reactants are made into small pieces or-powders, their surface area increases. More reactant molecules come in contact and take part in collision, resulting in an increase in the number of effective collisions. Thus, the speed of the reaction increases.