CHAPTER–9: (SHORT QUESTIONS AND ANSWERS)
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Topic No: 9.1 – REVERSIBLE REACTION AND DYNAMIC EQUILIBRIUM
Q1. Differentiate between products and reactants. (Conceptual) (12 times)
Ans:
Products and reactants:
In a chemical reaction the substances that combine are called reactants and the new substances formed are called products, for example when H₂ and O₂ (reactants) combine they form H₂O (product).
2H₂ + O₂ → 2H₂O
Q2. What is complete reaction? How is it represented? (Conceptual) (4 times)
Ans:
Complete reaction:
The reactions in which all the reactants have been converted into product.
They are represented by single arrow (→) between the reactants and products.
For example:
2H₂ + O₂ → 2H₂O
Q3. Define dynamic equilibrium? Write its two characteristics. (19 times)
Ans:
Dynamic equilibrium:
The state at which reaction does not stop, only the rate of forward and reverse reactions become equal to each other but take place in opposite direction. It is called dynamic equilibrium.
Characteristics of dynamic equilibrium:
(a) An equilibrium is achievable only in closed system.
(b) An equilibrium state is attainable from either way i.e. starting from reactant or from product.
Q4. Describe the difference between forward reaction and reverse reaction. / Macroscopic characteristics of forward and reverse reactions. (Conceptual) (35 times)
Forward Reaction:
(i) It is a reaction in which reactants react to form products.
(ii) It takes place from left to right.
(iii) At initial stage the rate of forward reaction is very fast.
(iv) It slows down gradually.
Reverse Reaction:
(i) It is a reaction in which products react to form reactants.
(ii) It takes place from right to left.
(iii) In the beginning the rate of reverse reaction is negligible.
(iv) It speeds up gradually.
Topic No: 9.2 – LAW OF MASS ACTION
Q5. Define reversible reaction. Give example. (4 times)
Ans:
Reaction in which the products can recombine to form reactants are called reversible reactions. These reactions never go to completion.
Example:
H₂ + I₂ ⇌ 2HI
Q6. What are irreversible reactions? Give a few characteristics of them? (17 times)
Ans:
The reaction in which products do not recombine to form reactants are called irreversible reactions.
Characteristics:
(i) They are suppose to complete.
(ii) They are represented by single arrow (→) between the reactants and products.
For example:
H₂ + O₂ → 2H₂O
Q7. Define Chemical Equilibrium State. (29 times)
Ans:
Chemical Equilibrium State:
When the rate of forward reaction takes place at the rate of reverse reaction, the composition of reaction mixture remains constant, it is called chemical equilibrium state.
Q8. Give the characteristics of reversible reaction. (26 times)
Ans:
They are represented by double arrow (⇌) between the reactants and products.
They never go to completion.
They proceed both ways i.e. forward as well as reverse direction.
Topic No: 9.3 – EQUILIBRIUM CONSTANT AND ITS UNITS
Q9. Write expression for the equilibrium constant for the reaction. (8 times)
Ans:
Equilibrium constant:
(i) N₂ + O₂ ⇌ 2NO
Rate of forward reaction = Rf = Kf [N₂][O₂]
Rate of reverse reaction = Rr = Kr [NO]²
Kc = [NO]² / [N₂][O₂]
(ii) PCl₃ + Cl₂ ⇌ PCl₅
Kc = [PCl₅] / [PCl₃][Cl₂]
Topic No: 9.4 – IMPORTANCE OF EQUILIBRIUM CONSTANT
Q10. Give importance of equilibrium constant.
Ans:
Importance of equilibrium constant:
Knowing the numerical value of equilibrium constant of a chemical reaction, following predictions can be made:
(i) Predicting direction of reaction.
(ii) Predicting extent of a reaction.
Q11. Explain what happens when Kc has large value and when Kc has small value. (5 times)
Ans:
Kc large value and Kc small value:
The large numerical value of Kc indicates that at equilibrium position the reaction mixture consists of almost all products and reactants are negligible.
2CO + O₂ ⇌ 2CO₂ Kc = 2.2 × 10²²
Small numerical value of Kc indicates that the equilibrium has established with a very small conversion of reactants to products.
2NH₃ ⇌ N₂ + 3H₂ Kc = 3.0 × 10⁻⁹
Q12. How can direction of reaction be predicted?
(Conceptual) (13 times)
Ans:
Direction of a reaction at a particular moment can be predicted by inserting the concentration of the reactants and products at that particular moment in the equilibrium expression.
OR
By comparing reaction quotient (Qc) with equilibrium constant (Kc). The direction of reaction can be predicted.
i. If Qc < Kc, reaction goes from left to right i.e. in forward direction to attain equilibrium.
ii. If Qc > Kc, the reaction goes from right to left i.e. in reverse direction to attain equilibrium.
iii. If Qc = Kc, forward and reverse reactions take place at equal rates i.e., equilibrium has been attained.
Q13. How can you know that a reaction has achieved an equilibrium state?
(Exercise) (8 times)
Ans:
When the rate of forward and reverse reactions keep on taking place at the same rate but in opposite direction. In this way equilibrium established.
Q14. Why reversible reaction never complete?
(Conceptual) (8 times)
Ans:
Reversible reaction never completes because products recombine to form reactants again and these reactions occur in both directions i.e. forward as well as reverse. Equilibrium established before their completion.
Q15. What is static equilibrium, explain with an example?
(26 times)
Ans:
Static equilibrium:
When a reaction ceases to proceed is called static equilibrium.
For example:
A building remains standing rather than falling down because all the forces acting on it are balanced.
Q16. Define the Law of Mass Action. / What is relationship between active mass and rate of reaction?
(25 times)
Ans:
Law of Mass Action:
Guldberg & Waage in 1869 put forward this law. According to this law
“The rate at which a substance reacts is directly proportional to its active mass and the rate of a reaction is directly proportional to the product of the active masses of the reacting substances”.
Q17. How is dynamic equilibrium established?
(Conceptual) (10 times)
Ans:
In reversible reactions, dynamic equilibrium is established before the completion of reaction. When reaction does not stop, the rate of forward and reverse reaction take place at equal rate but in opposite direction then equilibrium is established.
Q18. Why at equilibrium state reaction does not stop?
(Conceptual) (11 times)
Ans:
At equilibrium state reaction does not stop, because forward and reverse reactions keep on taking place at the same rate but in opposite direction.
Q19. Why is equilibrium state attainable from either way?
(Conceptual) (3 times)
Ans:
If we add products the products decompose to form reactants to attain equilibrium. If we add reactants they react to form products to attain equilibrium.
Q20. What is relationship between active mass and rate of reaction?
(Conceptual) (17 times)
Ans:
Relation of active mass and rate of reaction:
The rate of reaction is directly proportional to the active mass and the rate of reaction is directly proportional to the product of active masses of the reacting substances. Generally active mass is considered as the molar concentration having units of mol dm⁻³ expressed as square brackets [ ].
🔹 LONG / NUMERICAL TYPE QUESTIONS
Q22. Drive equilibrium constant expression for the synthesis of ammonia from nitrogen and hydrogen and its unit.
(Conceptual) (12 times)
Ans:
Q23. Write equilibrium constant expression for the reaction.
(7 times)
(i)
[H_2 + I_2 \rightleftharpoons 2HI]
Rate of forward reaction = Rf = Kf [H₂][I₂]
Rate of reverse reaction = Rr = Kr [HI]²
[K_c = \frac{[HI]^2}{[H_2][I_2]}]
(ii)
[CO + 3H_2 \rightleftharpoons CH_4 + H_2O]
Rate of forward reaction = Rf = Kf [CO][H₂]³
Rate of reverse reaction = Rr = Kr [CH₄][H₂O]
[K_c = \frac{[CH_4][H_2O]}{[CO][H_2]^3}]
Q24. Define Equilibrium Constant.
(29 times)
Ans:
Equilibrium Constant:
Equilibrium constant is a ratio of the product of concentration of products raised to the power of coefficient to the product of concentration of reactants raised to the power of coefficient as expressed in the balanced chemical equation.
[K_c = \frac{Product\ of\ concentration\ of\ products^{coefficients}}{Product\ of\ concentration\ of\ reactants^{coefficients}}]
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