Process Of The Shift In Equilibrium Experiment: Law, Principle and Precaution

Chemical equilibrium is defined as the state at which there is no change in the concentration of reactants and products. Shifting equilibrium means an increase in the conversion rate of substances, predicting the change in the reaction in the first place.

Key terms: Reaction, Irreversible Reaction, Reversible Reaction, Equilibrium, Backward Reaction, Forward Reaction, Molar Concentration

Also Read: Homogenous Equilibrium

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Chemical Reaction

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Chemical reactions are mainly classified into reversible reactions and irreversible reactions. In the reversible reactions, the reaction can simultaneously proceed the same in the forward and in the backward direction, keeping the temperature and pressure under the same conditions. 

When a reaction attains dynamic equilibrium then the rate of the forward reaction becomes equal to the rate of the reverse reaction and the reaction becomes still.

Reversible Reaction & Irreversible Reaction

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Law Of Mass Action

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At a temperature T let us look at a simple reversible reaction:

A+B « C+D

We are applying the law of mass action, which states that the rate of forwarding reaction will be directly proportional to the concentration of both the reactants. 

Similarly, the rate of backward reaction will be directly proportional to the concentration of the products.

After applying the law of mass action, we will get

r₁= k₁ [A][B], r₂ = k₂ [C][D]

Here r₁= rate of forward reaction

r₂ = rate of backward reaction

k₁ = rate constant for the forward reaction

k₂ = rate constant for the backward reaction

[A], [B], [C], [D] = molar concentrations of A, B, C, D

At equilibrium, r₁= r₂

So, k₁ [A][B] = k₂ [C][D]

Þk₁ / k₂= [C][D]/[A][B]

Now, put k₁ / k₂ = K_c

K_c = [D] [C]/[A][B]

Here, K_c = equilibrium constant

The equilibrium constant does not depend on the initial concentration of the reactants. At constant temperature, it remains constant though it is a function of temperature.

Also Check: Chemical Reactions

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Le Chatelier Principle For Shift In Equilibrium

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According to Le Chatelier’s principle, the equilibrium condition of the system is disturbed when:

• Any change in concentration of one or more reactants occurs
• Changes in temperature occurs
• Change in pressure occurs

So, to maintain the equilibrium state a net reaction will take place in some direction.

The principle states that if a system at equilibrium is subjected to a change of pressure or temperature or number of moles of the component, there will be a tendency for a net reaction in the direction that reduces the effect of this change.

Also Check: Le Chatelier’s Principle, Henry’s Law, and Chemical Equilibrium

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Experiment To Study The Shift In Equilibrium Process

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Aim: To study the shift in equilibrium process by the reaction of ferric ion and thiocyanate ion through increasing the concentration of any one of these ions.

Theory

The change in intensity of color of the solution is the indicator of the equilibrium reaction between ferric chloride and potassium thiocyanate.

Chemical Equation

Fe³⁺ (aq) + SCN^– (aq) « [Fe (SCN)]²⁺ (aq)

(Blood red color)

The equilibrium constant for the reaction, K:

K= [[Fe(SCN)] ⁺² (aq)]]/ [Fe ⁺³ (aq)] [SCN^- (aq)] ( K= const. at constant temperature)

Materials Required For Experiment

• 2 beakers of 100ml each
• 1 beaker of 250 ml
• 6 boiling tubes
• 4 burettes
• 2 Glass droppers
• 1 test tube stand
• 1 glass rod
• 0.100g ferric chloride
• 0.100g potassium thiocyanate

Procedure Of Experiment

1. Take 100ml of water in a beaker and dissolve 0.100g of ferric chloride in it.
2. Now take 100 ml of water in another beaker and dissolve 0.100g of potassium thiocyanate in it.
3. Take 20 ml of ferric chloride solution and mix it with 20 ml of potassium thiocyanate solution. The solution will turn blood-red in color. Fill this solution in a burette.
4. Label 5 boiling tubes as A, B, C, D, and E. Remember they should be of the same size.
5. In each of the boiling tubes add 2.5ml of the blood-red solution from the burette
6. Now we have to make the total volume of solution in boiling tube A as 20 ml. So we will add 17.5ml of water to the boiling tube A.
7. Take three burettes and label them as A, B, and C.
8. In burette, A fills the ferric chloride solution. In the burette, B filled the potassium thiocyanate solution, and in the burette, C fill the water.
9. From burette A add 1.0ml, 2ml,3ml, and 4 ml of ferric chloride solution to boiling tubes B, C, D, E.
10. Now we have to make the total volume of solution in each boiling tube as 20 ml respectively. So, we will add 16.5 mL, 15.5 mL, 14.5 mL, and 13.5 mL of water to boiling tubes B, C, D, and E.
11. Now compare the color intensity of the solution in each boiling tube with reference to the color intensity of the solution in boiling tube A.
12. You can repeat the above experiment again by using different concentrations of potassium thiocyanate solution from burette B and note the color intensity of the solution with reference to the equilibrium solution in boiling tube A.
13. You can make a table like the below and record your observation in that. You may use different concentrations and amounts of potassium thiocyanate and ferric chloride solution and compare them with the reference solution.

Also Check: Reversible and Irreversible Processes

Observation Table

Table 1: Equilibrium shift on increasing the concentration of ferric ions

Observation Table

Table 2: Equilibrium shift on increasing the concentration of thiocyanate ions

Also Check: Shift in Equilibrium between Ferric Ions and Thiocyanate Ions Experiment

Result Of Experiment 

As we increase the concentration of thiocyanate ion (SCN ^- ) or ferric ion (Fe ⁺³ )the increase in the concentration of ferric thiocyanate [Fe(SCN)]²⁺ can be seen. To maintain the equilibrium and K as constant, the equilibrium shifts in the forward direction. 

This results in the increase of color intensity of blood-red color that has occurred due to the formation of ferric thiocyanate ion [Fe (SCN)]²⁺. As soon as the reaction attains equilibrium the intensity will remain constant.

Precautions 

1. The solution should be very dilute with ferric chloride and potassium thiocyanate.
2. For the comparison of the test tube and the reference test tube, they must be on the same plane and same angle.
3. Only distilled water must be used.
4. Handle ferric chloride and potassium thiocyanate solution with care.

Also Check: Chemistry

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Things To Remember

1. All the reactions must happen at constant temperature and pressure.
2. The total volume of solution in each test tube must be kept the same for comparison
3. When the rate of forwarding reaction becomes equal to the rate of backward reaction then the reaction is said to be in equilibrium
4. The chemical equation is dynamic in nature.
5. The reaction occurs in both directions but with the same speed.

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Important PYQ’s Based On Equilibrium

1. What is the equilibrium expression for the reaction... [JCECE - 2008]
2. Consider the water gas equilibrium reaction... [VITEEE-2015]
3. The equilibrium constant for the following reactions... [KEAM - 2016] 
4. What is the equilibrium expression for the reaction... [JCECE - 2008]