TN 10th Science Guide

10. Types of Chemical Reactions

I. Choose the correct answer.

1. H_2(g) + C_l2(g) → 2HCl_(g) is a

(a) Decomposition Reaction
(b) Combination Reaction
(c) Single Displacement Reaction
(d) Double Displacement Reaction.

2. Photolysis is a decomposition reaction caused by:

(a) heat            (b) electricity
(c) light           (d) mechanical energy

3 A reaction between carbon and oxygen is represented by C_(s) + O_2(g) → CO_2(g) + 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

(a) i and ii                    (b) i and iv
(c) i, ii and iii               (d) i, ii and iv.

4. The chemical equation
Na₂SO_4(aq) + BaCI_2(aq) → BaSO_4(s)↓ + 2NaCl_(aq) represents which of the following types of reaction?

(a) Neutralisation
(b) Combustion
(c) Precipitation
(d) Single displacement

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

(a) i, ii and iii
(b) i, ii and iv
(c) ii, iii and iv
(d) i, iii and iv.

6. A single displacement reaction is represented by
X_(s) + 2HCl_(aq) → 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.

(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)

7. Which of the following is not an “element + element → compound” type reaction?

a) C(s) + O2(g) → CO2(g)           

 b) 2K(s) + Br2(l) → 2KBr(s)

c) 2CO(g) + O2(g) → 2CO2(g)

d) 4Fe(s) + 3O2(g) → 2Fe2O3(s)

8. Which of the following represents a precipitation reaction?

a) A(s) + B(s) → C(s) + D(s)

                b) A(s) + B(aq) → C(aq) + D(l)

c) A(aq) + B(aq) → C(s) + D(aq)

d) A(aq) + B(s) → C(aq) + D(l)

9. The pH of a solution is 3. Its [OH^–] concentration is:

(a) 1 × 10^-3 M              (b) 3 M
(c) 1 × 10-11 M          (d) 11 M

10. Powdered CaCO₃ reacts more rapidly than flaky CaCO₃ because of ______.

(a) large surface area
(b) high pressure
(c) high concentration
(d) high temperature.

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.

5. The value of the ionic product of water at 25°C is 1 × 10^-14

6. The normal pH of human blood is 7.4

7. Electrolysis is type of Decomposition reaction reaction.

8. The number of products formed in a synthesis reaction is One.

9. Chemical volcano is an example for Decomposition reaction 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.

IV. True or False: (If false give the correct statement)

1. Silver metal can displace hydrogen gas from nitric acid.
       Answer:    False.
Correct statement: In the activity series, any metals that are below hydrogen will not react with HNO₃.

2. The pH of rainwater containing dissolved gases like SO₃, CO₂, NO₂ will be less than 7.
       Answer:    True.

3. At the equilibrium of a reversible reaction, the concentration of the reactants and the products will be equal.
     Answer:      False.
Correct statement: At equilibrium rate of the forward reaction is equal to the rate of backward reaction.

4. Periodical removal of one of the products of a reversible reaction increases the yield.
     Answer:      True.

5. On dipping a pH paper in a solution, it turns into yellow. Then the solution is basic.
      Answer:     True.

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.
Answer:
        KCl + AgNO₃ → AgCl ↓ + KNO₃.

2. Why does the reaction rate of a reaction increase on raising the temperature?

The rate of a reaction increases at higher temperature, because the heat to the reactants provide energy to breakup more bonds and speeds up the reaction.

3. Define a combination reaction. Give one example of an exothermic combination reaction.

A chemical reaction in which 2 or more reactants combine to form a single product, the reaction is known as combination reaction. Most of the combination reaction are exothermic because they involve formation of new bonds. For example,
         2 Mg_(s) + O_2(s) → 2 MgO_(s)

4. Differentiate reversible and irreversible reactions.
Answer:

Reversible reaction	Irreversible reaction
1. Reactions can be reversed	1. The reaction cannot be reversed
2. It proceeds in both directions	2. It is unidirectional
3. It attains equilibrium	3. Equilibrium is not attained
4. It is relatively slow	4. It is fast

VI. Answer in detail.

1. What is called thermolysis reactions?

A chemical reaction is a process in which old bond breaks up and new chemical bond get formed. Thermolysis chemical reactions is a special type of chemical reaction in which the reactant get decomposed by heat. For example,

In these reactions heat is supplied to break the bonds, so generally they are endothermic in nature.

2. Explain the types of double displacement reactions with examples.

When two compounds react, if their ions are interchanged, then the reaction is called double displacement reactions. There are two types of double displacement reactions. They are
(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. Because the insoluble compound, formed as one of the products, is a precipitate and hence the reaction is so called.

(ii) When aqueous solutions of potassium iodide and lead (II) nitrate are mixed, a double displacement reaction takes place between them.
   Pb(NO₃)_2(aq) + 2KI_(aq) → PbI_2(s)↓ + 2KNO_3(aq).

(iii) Potassium and lead displace or replace one other and form a yellow precipitate of lead (II) iodide.

(iv) Neutralization reactions: When an acid reacts with the base to form a salt and water. It is called ‘neutralization reaction’ as both acid and base neutralize each other.

(v) Reaction of sodium hydroxide with hydrochloric acid is a typical neutralization reaction. Here, sodium replaces hydrogen from hydrochloric acid forming sodium chloride, a neutral soluble salt.
   NaOH_(aq) + HCl_(aq) → NaCl_(aq) + H₂O_(l)

3. Explain the factors influencing the rate of a reaction.

The factors influencing the rate of a reaction are,
(i) Nature of the reactants: The reaction of sodium with hydrochloric acid is faster than that with acetic acid. Do you know why? Hydrochloric acid is a stronger acid than acetic acid and thus more reactive. So, the nature of the reactants influences the reaction rate.

2Na_(s) + 2HCl_(aq) → 2NaCl_(aq) + H2_(g) (fast)

2Na_(s) + 2CH₃COOH_(aq) → 2CH₃COONa_(aq) + H_2(g) (slow)

(ii) The concentration of the reactants: Changing the number of reactants also increases the reaction rate. The amount of the substance present in a certain volume of the solution is called ‘concentration’. More the concentration, more particles per volume exist in it and hence faster the reaction. Granulated zinc reacts faster with 2M hydrochloric acid than 1M hydrochloric acid.

(iii) Temperature: Most of the reactions go faster at a higher temperature. Because adding heat to the reactants provides energy to break more bonds and thus speed up the reaction. Calcium carbonate reacts slowly with hydrochloric acid at room temperature. When the reaction mixture is heated the reaction rate increases.

(iv) Pressure: If the reactants are gases, increasing their pressure increases the reaction rate. This is because on increasing the pressure the reacting particles come closer and collide frequently.

(v) Catalyst: A catalyst is a substance which increases the reaction rate without being consumed in the reaction. In certain reactions, adding a substance as catalyst speeds up the reaction. For example, on heating potassium chlorate, it decomposes into potassium chloride and oxygen gas, but at a slower rate. If manganese dioxide is added, it increases the reaction rate.

(vi) The surface area of the reactants: When solid reactants are involved in a reaction, their powdered form reacts more readily. For example, powdered calcium carbonate reacts more readily with hydrochloric acid than marble chips. Because powdering of the reactants increases the surface area and more energy is available on the collision of the reactant particles. Thus, the reaction rate is increased. You will study more about reaction rate in your higher classes.

4. How does pH play an important role in everyday life?

Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change. Different body fluids have different pH values.
Eg: pH of blood is ranging from 7.35 to 7.45. Any increase or decrease in this value leads to diseases. The ideal pH for blood is 7.4.

pH in our digestive system : It is very interesting to note that our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach. During indigestion the stomach produces too much acid and this causes pain and irritation. pH of the stomach fluid is approximately 2.0.

pH changes as the cause of tooth decay : pH of the saliva normally ranges between 6.5 to 7.5. White enamel coating of our teeth is calcium phosphate, the hardest substance in our body. When the pH of the mouth saliva falls below 5.5, the enamel gets weathered. Toothpastes, which are generally basic ate used for cleaning the teeth that can neutralise the excess acid and prevent tooth decay.

pH of soil : 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.

pH of rain water : 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 chemical equilibrium? What are its characteristics?

Chemical equilibrium is the state for a reversible chemical reaction where the rate of forwarding direction is equally balanced by the rate of backward direction and the process seems like to be stopped.
Its characteristics are,

• The rate of the forward and backward reaction are equal in chemical equilibrium.
• The observable properties such as pressure, concentration, colour, density, viscosity, etc. of the system unchanged with time.
• In physical equilibrium, the volume of all phases remains 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.

            A solid compound 'A' is Calcium carbonate decomposes on heating into Calcium (B) oxide and a gas Carbon dioxide (O). On passing this Carbon dioxide (O) through water, it becomes audic because the formation of Carbonic acid.

compound molecular formula name	compound molecular formula name	compound molecular formula name
A CaCO3 calcium carbonate	A CaCO3 calcium carbonate	A CaCO3 calcium carbonate
B CaO calcium oxide	B CaO calcium oxide	B CaO calcium oxide
C CO2 carbon dioxide	C CO2 carbon dioxide	C CO2 carbon dioxide
compound molecular formula name	compound molecular formula name	compound molecular formula name

2. Can a nickel spatula be used to stir copper sulphate solution? Justify your answer.

No, a nickel spatula cannot be used to stir CuSO₄ solution, because Nickel will displace copper from CuSO₃ solution and Cu gets deposited on the Ni spatula.

VIII. Solve the following problems.

1. Lemon juice has a pH = 2, what is the concentration of H⁺ ions?
Solution:

2. Calculate the pH of 1.0 × 10^-4 molar solution of HNO₃?
Solution:

3. What is the pH of 1.0 × 10^-5 molar solution of KOH?

Solution:

KOH is a strong base and dissociates in its solution as,

4. The hydroxide ion concentration of a solution is 1 × 10^-11. What is the pH of the solution?
Solution: