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Chemical Equilibrium
The following reaction occurs when hydrogen and iodine gases react to generate hydrogen iodide:
H2(g)+I2(g)⇌2HI(g)
H2(g)+I2(g)=2HI is the forward reaction (g)
2HI(g)=H2(g)+I2(g)is the Reverse reaction
As no HI is present at first, only the forward reaction happens. Once some HI has been produced, it starts to break down into H2 and I2. The forward reaction rate gradually declines, while the reverse reaction rate increases. The rate at which H2 and I2 combine to form HI eventually equals the rate at which 2HI decomposes into H2 and I2. The reaction has reached a condition of equilibrium when the forward and reverse response rates have equaled each other. The state of a system in which the rate of the forward reaction equals the rate of the reverse reaction is known as chemical equilibrium.
In a reaction, equilibrium is reached when: H2(g)+I2(g)⇌H(g)+I2(g)⇌2HI(g).
Whether the reaction starts with all reactants and no products, all products and no reactants, or part of both, chemical equilibrium can be achieved. For two distinct reactions, the concentrations of H2, I2, and HI are shown in the graph below. Only H2 and I2 are present at the start of the reaction.At first, there is no HI. The concentrations of H2 and I2 .Steadily fall as the reaction approaches equilibrium, while the concentration of HI gradually increases. Equilibrium is reached when the curve flattens out and the concentrations all equalise.
Types of Equilibrium and Conditions for Equilibrium
It’s tempting to believe that once equilibrium is attained, the response will come to a halt. Equilibrium in chemistry is a dynamic process. Even after equilibrium has been attained, forward and reverse reactions persist. For a reaction that is at equilibrium, however, the relative concentrations of reactants and products do not vary since the reaction rates are the same.
1.The system must be closed, which means that no chemicals can enter or exit it.
2.Equilibrium is a dynamic process where both forward and reverse reactions are occurring, even if we don’t always see them.
3.The forward and backward reactions must have the same rate.
4.There is no requirement that the amount of reactants and products be equal.
5.The amounts of reactants and products will, however, remain constant after equilibrium is achieved.
The term “equilibrium” is used in this idea to describe the balance between reactants and products in a chemical reaction. Phase equilibrium and solution equilibrium are two more types of equilibrium. When a substance is in the middle of two states, it is said to be in phase equilibrium. When the rate of evaporation equals the rate of condensation, for example, a stoppered flask of water achieves equilibrium. When a solid substance is in a saturated solution, it reaches a solution equilibrium. The rate of dissolution equals the rate of recrystallization at this stage. Despite the fact that these are all different sorts of transformations, the majority of the equilibrium laws apply to any situation in which a reversible process happens.
Constant of Equilibrium
Consider the reversible process in which the reactants aA and bB combine to generate the products cC and dD. The lowercase letters denote the coefficients of each substance in the equilibrium depicted below.
aA+bB⇌cC+dD
aA+bB⇌cC+dD
As we’ve seen, the rates of forward and reverse reactions are the same at equilibrium, therefore all of the chemicals’ concentrations are constant. Given this, it stands to reason that at equilibrium, a ratio of concentration for any given reaction will keep a constant value. The equilibrium constant (Keq) is the ratio of the mathematical product of a reaction’s products to the mathematical product of the reaction’s reactant concentrations. In the balanced chemical equation, each concentration is raised to the power of its coefficient. The equilibrium constant expression for the general reaction above is as follows:
Keq=[C]c[D]d/[A]a[B]b
The concentrations of each chemical are expressed in molarity units (mol/L), as shown by the square brackets around the formula.
For any reaction, the value of the equilibrium constant can only be discovered through experiment. The location of equilibrium for a given reaction does not depend on the starting concentrations, as described above, and hence the value of the equilibrium constant is truly constant. It is, however, dependent on the reaction’s temperature. Because equilibrium is defined as a state in which the rates of forward and backward reactions are equal, this is the case. When the temperature changes, the reaction rates fluctuate as well, changing the equilibrium constant. The temperature should be stated for each reaction in which a Keq is given.
1.When Keq is greater than 1, the numerator is greater than the denominator, favouring the products, which means their concentration is greater than the reactants’.
2.As the denominator (reactants) is larger than the numerator, the reactants are favoured if Keq is smaller than 1. (products).
3.When Keq equals 1, the reactant and product concentrations are essentially equivalent.
Conclusion
We conclude that any chemical reaction is in principle reversible, and chemical equilibrium is the result of the balance between the rates of the forward reaction (reactants → products) and the reverse reaction (reactants ← products).
