An Introduction to Concentration
The concentration of the solution tells you how much solute has been dissolved in the solvent. Dilution is the addition of solvent, which decreases the concentration of the solute in the solution. The first measured concentration of a compound in a substance is the initial concentration. When both the reactants and the product are in a concentration that doesn't modify according to the time, it's stated to be in a state of equilibrium. During this state, the speed of forward reaction is the same as the rate of backward reaction. For a reaction, if you recognise the initial concentrations of the substances, you'll be able to calculate the equilibrium concentration. We will be focusing on how to calculate the initial concentration of a substance.
What is Concentration?
In Chemistry, concentration refers to the quantity of a substance in an outlined space. In chemistry, the concentration of a solution is the amount of a solute that is contained in a specific amount of solvent or solution. Knowing the concentration of solutes is vital in controlling the ratio of reactants for solution reactions.
Initial Concentration Formula
The formula below is employed in laboratories to calculate the final concentrations, volume which is unknown or the initial concentration of a solution.
${{C}_{1}}{{V}_{1}}={{C}_{2}}{{V}_{2}}$
${{C}_{1}}=\frac{{{C}_{2}}{{V}_{2}}}{{{V}_{1}}}$
Where, C1 = Initial concentration of solution
V1 = Initial volume of solution
C2 = Final concentration of solution
V2 = Final volume of solution
How to Find Initial Concentration?
We can also use integrated rate laws, and rate constants to find out how to calculate the initial concentrations or final concentrations.
The common integrated rate laws:
For a zero order reaction:
rate = k
The integrated rate law is:
$[A]$ = -kt + [Ao]
[Ao] = $[A]$ + kt
For first order reaction:
rate = k$[A]$
The integrated rate law is:
ln $[A]$ = -kt + ln [Ao]
ln [Ao] = ln $[A]$ + kt
Here, $[A]$ = final concentration or concentration of product.
[Ao] = Initial concentration or concentration of reactant.
k= is the rate constant and t is time taken for reaction to complete.
What is the Initial Concentration of HCl?
A sample problem is discussed below to find the concentration of HCl solution.
Q. If 100 ml of 0.5 M HCl solution is diluted with water to 1000 ml, find the new concentration or final concentration of the solution.
Ans. By using the solution dilution equation , ${{C}_{1}}{{V}_{1}}={{C}_{2}}{{V}_{2}}$
The initial concentration of HCl , C1 = 0,5 M
Initial volume ,V1=100 ml
Final concentration of HCl , C2 = to find
Final volume , V2 = 1000 ml.
${{C}_{2}}=\frac{{{C}_{1}}{{V}_{1}}}{{{V}_{2}}} \\$
${{C}_{2}}=\frac{0.5\times 100}{1000}=0.05M \\$
Therefore, the final concentration is 0.05 M.
Interesting Fact
To concentrate a solution, one should add more solute, or reduce the quantity of solvent.
The concentration of a solute is incredibly necessary in learning chemical reactions because it determines how usually molecules collide in solution and therefore indirectly determines the rates of reactions and also the conditions at equilibrium.
Conclusion
A dilution may be used to not only lower the concentration of the analyte being tested but also additionally to help minimise interferences from different compounds within the sample that would unnaturally change the analysis. In this article, the determination of initial concentration by using dilute equation or integrated rate law equation is explained with the assistance of a solved example.
FAQs on Initial Concentration
1. What is meant by initial concentration in chemistry?
In chemistry, the initial concentration refers to the concentration of a reactant or product at the very beginning of a chemical reaction, specifically at time t=0. It is a fundamental value used as the starting point for calculating reaction rates, determining equilibrium positions, and predicting the extent of a reaction as per the CBSE Class 11 and 12 syllabus.
2. What is the standard symbol for initial concentration in chemical equations?
The standard symbol for the initial concentration of a substance, for example, a reactant 'A', is [A]₀. The square brackets, [ ], denote the molar concentration (moles per litre), and the subscript '0' signifies that this is the concentration at time zero, before the reaction has started.
3. How does initial concentration differ from equilibrium concentration?
The key difference lies in the timing and state of the reaction:
- Initial Concentration ([A]₀): This is the concentration of a substance at the start of the reaction (t=0), before any products have formed or the reaction has begun to shift.
- Equilibrium Concentration ([A]eq): This is the concentration of the substance once the reaction has reached dynamic equilibrium, a state where the rate of the forward reaction equals the rate of the reverse reaction, and the net concentrations of reactants and products remain constant.
4. What are the most common units used to express initial concentration?
The most common unit for expressing initial concentration in the context of chemical kinetics and equilibrium is Molarity (M), which is defined as moles of solute per litre of solution (mol/L). Other units you might encounter include:
- Molality (m): Moles of solute per kilogram of solvent.
- Normality (N): Gram equivalent weight per litre of solution.
- Percentage Concentration: Such as % w/v (weight by volume) or % v/v (volume by volume).
5. Why is initial concentration a critical factor in determining the rate of a reaction?
The initial concentration is critical because the rate of a reaction is often directly dependent on the concentration of its reactants, as described by the rate law equation (Rate = k[A]ˣ[B]ʸ). By systematically varying the initial concentrations of reactants and measuring the corresponding initial reaction rates, chemists can determine the order of the reaction (the exponents x and y) and the value of the rate constant (k).
6. According to Le Chatelier's Principle, how does changing the initial concentration affect a reaction at equilibrium?
According to Le Chatelier's Principle, if a change of condition is applied to a system in equilibrium, the system will shift in a direction that counteracts the change. Therefore, if you increase the initial concentration of a reactant, the equilibrium will shift to the right to favour the formation of more products. Conversely, if you increase the initial concentration of a product, the equilibrium will shift to the left, favouring the formation of more reactants.
7. How is the concept of initial concentration applied to solid reactants in a reaction?
This is a special case. The concentration of a pure solid or a pure liquid is considered to be constant throughout a reaction because its density and molar mass do not change. Therefore, its active mass is taken as unity (1) and it is omitted from the rate law and equilibrium constant expressions. The initial amount of a solid reactant matters for stoichiometry, but its 'concentration' does not influence the reaction rate or equilibrium position.
8. What is the role of initial concentration in the integrated rate law for a first-order reaction?
In the integrated rate law for a first-order reaction, ln[A]t = -kt + ln[A]₀, the term [A]₀ represents the initial concentration of reactant A. This value is essential for calculating the concentration of the reactant, [A]t, at any given time 't' during the reaction. It is also fundamental for deriving and using the half-life formula (t₁/₂) for a first-order reaction, which is independent of the initial concentration.
