Q.1: What are irreversible and reversible reactions?
Ans: Irreversible reactions:
“Those chemical reactions which proceed in forwarding direction only are called irreversible reactions.” OR
Those chemical reactions in which reactants are changed into products only and products are not changed to reactants are called irreversible reactions.”
Characteristics of irreversible reactions:
Irreversible reactions have the following characteristics:
i.These reactions proceed in one direction only.
ii.They are represented by a single arrow sign (——► )
iii.No equilibrium state is established in irreversible reactions.
iv.These reactions go to completion.
“Those chemical reactions which proceed both in forwarding and backwards directions are called reversible reactions.” OR
“Those chemical reactions in which the reactants react to form the products and the products react to form the reactants back are called reversible reactions.”
Characteristics of reversible reactions:
A reversible reaction has the following characteristics:
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These reactions proceed in two directions i.e. forward and backwards
These reactions never go to completion i.e. the reactants are not fully converted into products.
question 2: What is meant by chemical equilibrium/ Dynamic equilibrium? Explain with examples.
ANS: Chemical equilibrium/Dynamic equilibrium:History:Chemical equilibrium (Dynamic equilibrium) was first discovered by a French chemist Claude Louis Berthollet in 1803.
Definition: “The state of a reversible chemical reaction in which the rate of forwarding reaction becomes equal to the rate of the reverse reaction is called chemical equilibrium/Dynamic equilibrium.”
“The state of a reversible chemical reaction at which the reactants are continuously changing to products and the products are continuously changing to the reactants back but the concentrations of reactants and products remain constant is called chemical equilibrium/Dynamic equilibrium.
”Explanation: Consider a general chemical reaction in which reactant A reacts with reactant B in a gaseous state, in a closed container, to ‘form products Cand D.
A(g) + B (g) <———-> C (g) + D (g)
The detail about this general reversible reaction is given below;
i. Concentrations at the start:
At the start of the reaction, the concentrations of A and B are maximum while that of C and D are zero.
ii. Concentrations after some time:
With time, the concentrations of C and D gradually increase while that of A and B gradually decreases.
iii. Variation of the rate of forwarding and reverse reactions:
Change in concentrations of reactants or products per unit time is called the rate of reaction.
In the beginning, the rate of forwarding the reaction is maximum while the rate of the reverse reaction is almost zero but as the time passes and sufficient amounts of C and Dare formed then the rate of backward reaction increases while that of forwarding reaction decreases until the rate of forwarding reaction becomes equal to the rate of the reverse reaction. Thus a state of. dynamic equilibrium is established.
iv. Concentration at equilibrium state:
The concentrations of reactants or products remain constant at equilibrium state. These concentrations are called equilibrium concentrations.
FIGURE: Graph showing Reversible reaction and equilibrium state
i. Change of a liquid to gas in a closed container:
Consider a closed container which is partially filled with a liquid at a given temperature. At the start, evaporation starts and the vapour molecules are collected at the liquid surface.
At the passage of time, the collected gas molecules over the liquid surface convert to liquid back(condensation starts). In the beginning rate of evaporation is faster than the rate of condensation but with time the rate of