An Overview of Class 11 Chemistry Effect Of Change In Concentration On Chemical Equilibrium Experiment
The system reaches a condition of chemical equilibrium whenever the forward reaction rate equals the rate of the backward reaction. The system will react somehow when a chemical equilibrium state is abruptly interrupted until chemical equilibrium is restored. When a compound is added, a reaction absorbs it, reducing the reactants or products or reactant concentration added. The chemical reaction that occurs in the path that replaces the substance that is withdrawn relieves the products or reactants concentration eliminated. Thus, the concentration could change the reaction's value by either adding or removing the compound.
Table of Content
Aim
Apparatus Required
Theory
Procedure
Result
Aim
To study the effect of change in concentration of chemical equilibrium shift of ferric and thiocyanate ions.
Apparatus Required
Ferric Chloride
Potassium Chloride
Potassium Thiocyanate
Dropper
Test Tube Stands
Test Tubes
Test Tube Holder
Glass Rod
Beakers
Measuring Flasks and Cylinders
Theory
To reduce the impact of concentration variations, the chemical equilibrium mixture composition varies whenever the concentrations of the products or reactants in a chemical reaction at chemical equilibrium are modified.
Look at the example below to see how a change in concentration can affect equilibrium.
Fe3++SCN-→[Fe(SCN)2+]
The equation for the constants of chemical equilibrium for the reaction mentioned above is,
${K_c} = \dfrac{{\left[ {Fe{{\left( {SCN} \right)}^{2 + }}} \right]}}{{\left( {F{e^{3 + }}} \right)(SC{N^ - })}}$
Here Kc is the equilibrium constant, [Fe3+], [SCN-], and [Fe(SCN)2+], are the chemical equilibrium concentrations of the corresponding species.
The chemical equilibrium is disturbed whenever the concentrations of either of these species change, as well as the reaction quotient, persists below the equilibrium value. The ions connect in a way that makes the reaction quotient equivalent to the chemical equilibrium constant once more to restore equilibrium.
Procedure
In a 250mL clean and dry beaker add 10 mL of ferric chloride (FeCl3) using a measuring cylinder and glass rod.
Then add 10mL of potassium thiocyanate (KSCN) to the beaker containing FeCl3. The colour of the solution in the beaker will be blood-red.
Take 4 test tubes and label them as A, B, C, and D. In each of the 4 test tubes add 10mL of blood-red solution and keep them in the test tube stand.
In a test tube, A containing red blood solution, add 5mL of distilled water and keep it as a reference.
In test tube B add 5mL of FeCl3, in test tube C add 5mL of KSCN and in test tube D add 5mL of KCl.
Now examine the colours in test tubes B, C, as well as D against test tube A containing the red colour solution, which has been used as a reference. Record the observation.
Observations
Result
The results of the chemistry experiment on the effect of change in concentration of equilibrium verified that:-
The increase in reactant (Fe3+ or SCN-) concentration shifts the equilibrium towards the right.
The decrease in the reactant (Fe3+ or SCN-) concentration shifts the equilibrium towards the left.
Precautions
Handle chemicals and apparatus with caution.
Every apparatus should be rinsed with distilled water and dried before starting the experiment.
Utilize test tubes of nearly the same diameter.
Thiocyanate concentrations should be diluted.
A solution's colour intensity should indeed be evaluated by placing the reference and the solution side-by-side and subsequently viewing from the top.
After the completion of the experiment, wash your hands thoroughly.
Lab Manual Questions
1. Why are concentrated acids not used in this experiment?
Ans: When using concentrated acids, the reaction is extremely violent and the mixture gets hot. This speeds up the process much more and the liquid within may foam. The process is significantly under control when the acid is diluted.
2. Why were test tubes of the same diameter used in this experiment?
Ans: To obtain a precise measurement of the solutions that were introduced for reference for comparison study.
3. What is the aim of transferring solutions employing glass rods?
Ans: Solutions are poured using glass rods to prevent spillage. A glass rod placed against a beaker's pouring edge enables the solution to run down the glass rod and into the collecting vessel, as opposed to splashing beyond the lip.
4. What will happen if solid KCl is added to the system when it is in equilibrium?
Ans: If solid KCl is added, then the solubility gets decreased. The composition of the equilibrium is changed by the addition of a common ion; however, the ionization constant is unaffected.
Viva Questions
1. What is chemical equilibrium?
Ans: Chemical equilibrium is the condition that occurs in a chemical process when the forward reaction rate equals the backward reaction rate.
2. What happens when inert gas is added during chemical equilibrium?
Ans: Introduction of inert gas has no effect on concentration on chemical equilibrium.
3. What are the types of chemical equilibrium?
Ans: Homogeneous and heterogeneous equilibrium.
4. What are the factors affecting chemical equilibrium?
Ans: Concentration, temperature, and pressure of the system affect the chemical equilibrium.
5. Does the consistency of colour intensity imply that equilibrium is dynamic?
Ans: No, since even if the process completely halted at equilibrium, the colour would remain constant.
6. Define forward reaction.
Ans: A forward reaction is one wherein the reactants are solely transformed into products.
7. Define backward reaction.
Ans: Backward reactions are those wherein the products are transformed back into the reactants.
8. What are the industrial applications of chemical equilibrium?
Ans: Haber’s and contacts process.
9. What complex is produced when a thiocyanate ion is added to a ferric salt solution?
Ans: [Fe (SCN)2+]- Thiocyanato (III) ion
10. What is the chemical formula of the complex ion when HCl is added to a solution comprising [Co (H2O)6]2+ ions?
Ans: [CoCl4]2-
Practical-Based Questions
Which of the following statements is false?
The chemical equilibrium is affected by the catalyst
The chemical equilibrium is not affected by the catalyst
A catalyst speeds up the reaction
A catalyst speeds up both forward and backward reaction
Ans: The chemical equilibrium is affected by the catalyst.
In a chemical equilibrium, what happens to the composition of the mixture when the reactant or product concentration changes?
The composition of the mixture does not change
The composition of the mixture changes
The composition of the mixture remains constant
None of the three options
Ans: The composition of the mixture changes while the product or reactant concentration changes.
Why does the concentration of colour intensity increase?
Due to an increase in the concentration of SCN-
Due to the decrease in the concentration of SCN-
Maintaining the concentration of solution without any increase or decrease
None of the three options
Ans: Due to the increase in the concentration of SCN- the concentration of colour intensity increases.
What happens when KCl is added?
The concentration of Cl- increases
The concentration of Cl- decreases
The concentration of K+ increases
The concentration of K+ decreases
Ans: When KCl is added, the concentration of K+ increases.
What is the colour of [Fe (SCN)2+] complex?
Pink
Blue
Red
Blood-red
Ans: The colour of [Fe (SCN)2+] is blood-red.
What influences potassium and thiocyanate ions to combine to generate additional potassium thiocyanate, which causes the chemical equilibrium to move too backwards?
Increase in SCN- ion concentration
Increase in K+ ion concentration
Decrease in K+ ion concentration
None of the three options
Ans: Increase in K+ ion concentration.
What happens when thiocyanate ion concentration decreases?
The reaction remains constant
The reaction shifts toward the left
The reaction shifts towards the right
None of the three options
Ans: The reaction shifts towards the left when the thiocyanate ion concentration decreases.
How does distilled water act in chemical laboratories?
Indicator
Solvent
Universal solvent
None of the three options
Ans: Distilled water acts as a universal solvent in chemical laboratories.
What happens if the chemical equilibrium reaction mixture' temperature is raised?
Equilibrium remains constant
Equilibrium shifts towards an exothermic direction
Equilibrium shifts towards an endothermic direction
None of the three options
Ans: The equilibrium shifts towards the endothermic direction if the temperature rises.
What happens when a potassium ferrocyanide solution is combined with a ferric salt solution?
Blue colour deepens
Changes to blue colour
Red colour deepens
No colour change
Ans: The red colour deepens when a potassium ferrocyanide solution is combined with a ferric salt solution.
Conclusion
This chemistry experiment on the effect of change in concentration on chemical equilibrium infers that the Fe3+ or SCN- ion reactant concentration changes cause the equilibrium to shift towards the right (forward reaction), while changes in either of the reactant's concentrations cause the equilibrium to shift towards the left (backward reaction).
As the ferric ions' concentration rises, more thiocyanate ions combine with ferric ions to form more [Fe (SCN)]2+ complex, and red colour intensity increases. As a result, raising Fe3+ ions concentration causes the aforementioned equilibrium to change.
FAQs on Class 11 Chemistry Effect Of Change In Concentration On Chemical Equilibrium Experiment
1. State Le Chatelier's principle. For the CBSE Class 11 exam, how would you apply it to predict the shift in equilibrium for the reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) if the concentration of Cl₂(g) is increased?
Le Chatelier's principle states that if a change of condition (like concentration, temperature, or pressure) is applied to a system in equilibrium, the system will shift in a direction that counteracts the change. For the reaction PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), if the concentration of Cl₂ is increased, the equilibrium will shift to the left (backward direction). This shift consumes the added Cl₂ along with PCl₃ to form more PCl₅, thereby nullifying the effect of the change and re-establishing equilibrium. This is an expected question type for 2 or 3 marks.
2. Explain why chemical equilibrium is considered a dynamic process. What is the key implication for the reaction rates, which is an important concept for the 2025-26 board exams?
Chemical equilibrium is described as dynamic because the reactions do not stop once equilibrium is reached. Instead, the forward reaction (reactants to products) and the reverse reaction (products to reactants) continue to occur simultaneously. The key implication, which is a frequently tested concept, is that at equilibrium, the rate of the forward reaction becomes exactly equal to the rate of the reverse reaction. This results in no net change in the concentrations of reactants and products.
3. For the synthesis of ammonia, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), how does increasing the concentration of N₂ shift the equilibrium? Explain why removing NH₃ is a crucial industrial strategy, often appearing in application-based questions.
According to Le Chatelier's principle, increasing the concentration of a reactant like N₂ will shift the equilibrium position to the right to favour the forward reaction. This consumes the added N₂ and produces more NH₃. Removing NH₃ as it is formed is a crucial industrial strategy for the following reasons:
- It decreases the concentration of the product.
- This forces the equilibrium to continuously shift to the right to replenish the product.
- This maximises the overall yield of ammonia, making the process economically viable. This is a classic example for application-based questions worth 3 marks.
4. Why does the equilibrium constant, Kc, not change when you add more reactants to a system at equilibrium, even though the reaction shifts to form more products?
This is a key conceptual point. The equilibrium constant, Kc, is a ratio of the concentrations of products to reactants, each raised to the power of their stoichiometric coefficient, specifically *at equilibrium*. When you add more reactants, you temporarily disturb this ratio. The system then shifts to the right, increasing product concentration and decreasing reactant concentration, until the ratio once again becomes equal to the fixed value of Kc. Therefore, Kc itself does not change because it is dependent only on temperature, not concentration.
5. When writing an equilibrium constant expression (Kc), why are the concentrations of pure solids and liquids omitted? Provide an example for a heterogeneous equilibrium, as this is an important concept for exams.
The concentrations of pure solids and pure liquids are considered to be constant. Their concentration (defined as moles per unit volume) does not change significantly during a reaction. These constant values are incorporated into the equilibrium constant, Kc. Therefore, they are omitted from the expression for simplification.
Example of a heterogeneous equilibrium:
For the decomposition of calcium carbonate: CaCO₃(s) ⇌ CaO(s) + CO₂(g)
The equilibrium constant expression is written as Kc = [CO₂], because the concentrations of the solids CaCO₃ and CaO are constant.
6. What is the role of the Reaction Quotient (Qc) in determining the direction of a reversible reaction? How might a 3-mark exam question test this concept?
The Reaction Quotient, Qc, is a value calculated using the same expression as Kc, but for a reaction mixture that is not necessarily at equilibrium. Its role is to predict the direction in which the reaction will shift to reach equilibrium. An important 3-mark question might give initial concentrations and the value of Kc, requiring you to:
- Calculate Qc using the given concentrations.
- Compare Qc with Kc:
- If Qc < Kc, the reaction will proceed in the forward direction (to the right).
- If Qc > Kc, the reaction will proceed in the reverse direction (to the left).
- If Qc = Kc, the system is already at equilibrium.
7. State the law of mass action. For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), what is the expected shift in equilibrium if the concentration of O₂ is decreased?
The law of mass action states that the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants, with each concentration term raised to the power of its stoichiometric coefficient in the balanced chemical equation. For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), if the concentration of O₂ is decreased, the equilibrium will shift to the left (in the backward direction). This shift occurs to counteract the change by producing more reactants, including O₂, until a new equilibrium is established.
