Book Back Questions with Answers - Types of Chemical Reactions
I. Choose the correct answer.
1. \(H_{2(g)} + Cl_{2(g)} \rightarrow 2HCl_{(g)}\) is a
2. Photolysis is a decomposition reaction caused by ___________
3. A reaction between carbon and oxygen is represented by \(C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)} + \text{Heat}\). In which of the type(s), the above reaction can be classified?
(i) Combination Reaction
(ii) Combustion Reaction
(iii) Decomposition Reaction
(iv) Irreversible Reaction
4. The chemical equation \(Na_2SO_{4(aq)} + BaCl_{2(aq)} \rightarrow BaSO_{4(s)}\downarrow + 2NaCl_{(aq)}\) represents which of the following types of reaction?
5. Which of the following statements are correct about a chemical equilibrium?
(i) It is dynamic in nature
(ii) The rate of the forward and backward reactions are equal at equilibrium
(iii) Irreversible reactions do not attain chemical equilibrium
(iv) The concentration of reactants and products may be different
6. A single displacement reaction is represented by \(X_{(s)} + 2HCl_{(aq)} \rightarrow XCl_{2(aq)} + H_{2(g)}\). Which of the following(s) could be X.
(i) Zn (ii) Ag (iii) Cu (iv) Mg.
Choose the best pair.
7. Which of the following is not an “element + element → compound” type reaction?
8. Which of the following represents a precipitation reaction?
9. The pH of a solution is 3. Its \([OH^-]\) concentration is
10. Powdered \(CaCO_3\) reacts more rapidly than flaky \(CaCO_3\) because of ___________.
II. Fill in the blanks
1. A reaction between an acid and a base is called neutralization.
2. When lithium metal is placed in hydrochloric acid, hydrogen gas is evolved.
3. The equilibrium attained during the melting of ice is known as physical equilibrium.
4. The pH of a fruit juice is 5.6. If you add slaked lime to this juice, its pH Increases (increse/decrese)
5. The value of ionic product of water at 25ºC is \(1.00 \times 10^{-14}\).
6. The normal pH of human blood is 7.35 - 7.45.
7. Electrolysis is type of decomposition reaction.
8. The number of products formed in a synthesis reaction is one.
9. Chemical volcano is an example for decomposition type of reaction.
10. The ion formed by dissolution of \(H^+\) in water is called Hydronium ion.
III. Match the following
Identify the types of reaction
Answer:
1. Single Displacement
2. Combination
3. Neutralization
4. Thermal decomposition
IV. True or False: (If false give the correct statement)
1. Silver metal can displace hydrogen gas from nitric acid.
False
2. The pH of rain water containing dissolved gases like \(SO_3, CO_2, NO_2\) will be less than 7.
True
3. At the equilibrium of a reversible reaction, the concentration of the reactants and the products will be equal.
False
4. Periodical removal of one of the products of a reversible reaction increases the yield.
True
5. On dipping a pH paper in a solution, it turns into yellow. Then the solution is basic.
False
V. Short answer questions:
1. When an aqueous solution of potassium chloride is added to an aqueous solution of silver nitrate, a white precipitate is formed. Give the chemical equation of this reaction.
2. Why does the reaction rate of a reaction increase on raising the temperature?
3. Define combination reaction. Give one example for an exothermic combination reaction.
A combination reaction is a reaction in which two or more reactants combine to form a compound. It is otherwise called synthesis reaction or composition reaction.
Example: On burning magnesium in air, it combines with oxygen to form magnesium oxide. \(2Mg_{(s)} + O_{2(g)} \rightarrow 2MgO_{(s)}\)
4. Differentiate reversible and irreversible reactions
| Reversible Reaction | Irreversible reaction |
|---|---|
| i. It can be reversed under suitable conditions | i. It cannot be reversed |
| ii. Both forward and backward reactions take place simultaneously | ii. It is unidirectional. It proceeds only in forward direction |
| iii. It attains equilibrium | iii. Equilibrium is not attained |
| iv. The reactants cannot be converted completely into products | iv. The reactants can be completely converted into products |
| v. It is relatively slow | v. It is fast |
VI. Answer in detail
1. What are called thermolysis reactions?
In this type of reaction, the reactant is decomposed by applying heat.
As the molecule is dissociated by absorption of heat, it is otherwise called ‘Thermolysis’.
\(2HgO_{(s)} \xrightarrow{\text{Heat}} 2Hg_{(l)} + O_{2(g)}\)
2. Explain the types of double displacement reactions with examples.
There are two major classes of double displacement reactions. They are
(i) Precipitation Reactions
(ii) Neutralization Reactions
(i) Precipitation Reactions: When aqueous solutions of two compounds are mixed, if they react to form an insoluble compound and a soluble compound, then it is called precipitation reaction.
\(Pb(NO_3)_{2(aq)} + 2KI_{(aq)} \rightarrow PbI_{2(s)} + 2KNO_{3(aq)}\)
(ii) Neutralization Reactions: Acid reacts with the base to form a salt and water. It is called neutralization reaction as both acid and base neutralise each other.
Acid + Base → Salt + Water
\(HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)}\)
3. Explain the factors influencing the rate of a reaction.
Important factors that affect rate of a reaction are:
- (i) Nature of the reactants
- (ii) Concentration of the reactants
- (iii) Temperature
- (iv) Catalyst
- (v) Pressure
- (vi) Surface area of the reactants
(i) Nature of the reactants: The reaction of sodium with hydrochloric acid is faster than that with acetic acid. Hydrochloric acid is stronger acid than acetic acid and thus more reactive. So the nature of the reactants influence reaction rate.
\(2Na_{(s)} + 2HCl_{(aq)} \rightarrow 2NaCl_{(aq)} + H_{2(g)}\) (fast)
(ii) Concentration of the reactants: Changing the amount of the reactants also increase the reaction rate. More the concentration, more particles per volume exist in it and hence faster the reaction.
(iii) Temperature: Increase in temperature provides energy to break more bonds and thus speed up the reaction.
(iv) Pressure: If the reactants are gases, increasing their pressure increases the reaction rate. Since on increasing pressure the reacting particles come closer and collide frequently.
(v) Catalyst: A catalyst is a substance which increase reaction rate without being consumed in the reaction. In certain reactions, adding a substance as catalyst speeds up the reaction.
(vi) Surface area of the reactants: When solid reactants are involved in a reaction, their powdered form reacts more readily. Because powdering of the reactants increases the surface area and more energy is available on collision of reactant particles. Thus the reaction rate is increased.
4. How does pH play an important role in everyday life?
- (i) Living organisms can survive only in a narrow range of pH change. Different body fluids have different pH values.
- (ii) Our stomach produces hydrochloric acid which helps in the digestion of food without harming the stomach. During indigestion, if the stomach produces too much acid and this causes pain and irritation. pH of the stomach fluid is approximately 2.0.
- (iii) pH of the saliva normally ranges between 6.5 to 7.5.
- (iv) When the pH of the mouth saliva falls below 5.5, the enamel gets weathered.
- (v) Toothpastes, which are generally basic are used for cleaning the teeth that can neutralise the excess acid and prevent tooth decay.
- (vi) In agriculture, the pH of the soil is very important. Citrus fruits require slightly alkaline soil, while rice requires acidic soil and sugarcane requires neutral soil.
- (vii) The pH of rain water is approximately 7, which means that it is neutral and also represents its high purity. If the atmospheric air is polluted with oxide gases of sulphur and nitrogen, they get dissolved in the rain water and make its pH less than 7. Thus, if the pH of rain water is less than 7, then it is called acid rain. When acid rain flows into the rivers it lowers the pH of the river water also. The survival of aquatic life in such rivers becomes difficult.
5. What is a chemical equilibrium? What are its characteristics?
Chemical Equilibrium: It is state of a reversible chemical reaction in which no change in the amount of reactants and products takes place. At equilibrium,
Rate of forward reaction = Rate of backward reaction
Characteristics of equilibrium :
- (i) In chemical equilibrium, the rates of forward and backward reactions are equal.
- (ii) The observable properties such as pressure, concentration, color, density, viscosity etc., of the system remain unchanged with time.
- (iii) The chemical equilibrium is a dynamic equilibrium, because both the forward and backward reactions continue to occur even though it appears static externally.
- (iv) In physical equilibrium, the volume of all the phases remain constant.
VII. HOT questions
1. A solid compound ‘A’ decomposes on heating into ‘B’ and a gas ‘C’. On passing the gas ‘C’ through water, it becomes acidic. Identify A, B and C.
\(CaCO_3 \rightarrow CaO + CO_2\)
\(CO_2 + H_2O \rightarrow H_2CO_3\) (Carbonic acid)
\(CaCO_{3[A:Calcium \ carbonate]} \rightarrow CaO_{[B:Calcium \ oxide]} + CO_{2[C:Carbon \ di \ oxide]}\)
\(CO_2 + H_2O \rightarrow H_2CO_3\) (Carbonic acid)
A: \(CaCO_3\) : Calcium carbonate
B: \(CaO\) : Calcium oxide
C: \(CO_2\) : Carbon di oxide
2. Can a nickel spatula be used to stir copper sulphate solution? Justify your answer.
VIII. Solve the following problems
1. Lemon juice has a pH 2, what is the concentration of \(H^+\) ions?
Solution
pH of lemon juice = 2
\([H^+] = ?\)
\(pH = -\log_{10}[H^+]\)
\(\log_{10}[H^+] = -2\)
\([H^+] = 10^{-2}\)
= 0.01 mole \(litre^{-1}\)
2. Calculate the pH of \(1.0 \times 10^{-4}\) molar solution of \(HNO_3\).
Solution
\([H^+] = 1.0 \times 10^{-4}\)
\(pH = -\log_{10}[H^+]\)
\( = -\log_{10}[1 \times 10^{-4}]\)
\(pH = -(\log_{10}1 - 4 \log_{10}10)\)
\(= 0 + 4 \times \log_{10}10\)
\(= 0 + 4 \times 1\)
\(= 4\)
pH = 4
3. What is the pH of \(1.0 \times 10^{-5}\) molar solution of KOH?
Solution:
\(KOH_{(aq)} \leftrightarrow K^+_{(aq)} + OH^-_{(aq)}\)
One mole of KOH would give one mole of \(OH^-\) ions.
\([OH^-] = 1 \times 10^{-5} \text{ mol } litre^{-1}\)
\(pOH = -\log_{10}[OH^-] = -\log_{10}[10^{-5}]\)
\(= - (-5 \times \log_{10}10) = - (-5) = 5\)
\(pH + pOH = 14\)
\(pH = 14 - pOH = 14 - 5 = 9\)
4. The hydroxide ion concentration of a solution is \(1 \times 10^{-11} M\). What is the pH of the solution?
Solution:
\([OH^-] = 1 \times 10^{-11} M\)
\(pOH = -\log_{10}[OH^-] = -\log_{10}[10^{-11}]\)
\(= - (-11 \times \log_{10}10) = - (-11) = 11\)
\(pH + pOH = 14\)
\(pH = 14 - pOH = 14 - 11 = 3\)
