EXPERIMENT 5 - OBSERVATIONS :- (WRITE IN LHS OF THE RECORD)
EXPT NO.5 CONTENT BASED EXPERIMENT (Write in RHS OF THE RECORD)
AIM
To study the shift of equilibrium between ferric ions and thiocyanate ions by changing the concentration of either of them.
THEORY
(Le chatelier s principle)
When a system in equilibrium is suddenly disturbed by changing the concentration ,Pressure, Temperature etc it will respond in some way until equilibrium is re-established.
The equilibrium reaction and equilibrium constant can be represented as
where [Fe(SCN)]2+, [Fe3+] and [SCN–] are the equilibrium concentrations of the respective species while K is the equilibrium constant. For a particular reaction the value of K is constant at a particular temperature.
When concentration of any species involved in the equilibrium is disturbed, the concentration quotient,
remains no longer equal to K. In order to re-establish the equilibrium, the ions interact in such a way so that the concentration quotient again becomes equal to the equilibrium constant K.
(a) Effect of increasing concentration of ferric ions.
When ferric chloride solution is added, the increase in concentration of ferric ions results in more of thiocyanate ions combining with ferric ions to give more of [Fe(SCN)]2+ complex and therefore, the colour intensity of red-solution increases.
Thus, increase in concentration of Fe3+ ions shifts the above equilibrium in the forward direction because according to Le chatelier s principle, when we increase the concentration of ferric ions the system will try to reduce the concentration by undergoing reaction in such a way that the added ions are used up and its concentration decreases.
RHS OF THE RECORD
(b) Effect of increasing concentration of thiocyanate ions. Since thiocyanate ion is in the denominator in the equilibrium law equation, the addition of more and more of thiocyanate results in more of ferric ions reacting with thiocyanate ions to give more of [Fe(SCN)]2+ complex. Hence, the colour intensity of red-solution increases. Thus, increase in the concentration of SCN– ions shifts the above equilibrium in the forward direction according to Le chatelier s principle
(c) Effect of adding oxalate ion solution
Fe 3+ + C2 O4 2- → [Fe(C2 O4 )3 ] 3- Ferric oxalate complex
The equilibrium can be shifted in the opposite direction by adding reagents that remove Fe3+ or SCN – ions. For example, oxalate ions of oxalic acid (H2C2O4 ), reacts with Fe3+ ions to form the stable complex ion [Fe(C2O4 ) 3 ] 3–, thus decreasing the concentration of free Fe3+(aq). In accordance with the Le Chatelier’s principle, the concentration stress of removed Fe3+ is relieved by dissociation of [Fe(SCN)]2+ to replenish the Fe 3+ ions. Because the concentration of [Fe(SCN)]2+ decreases, the intensity of red colour decreases.
PROCEDURE :- (RHS)
1. Take a 250ml beaker thoroughly washed and clean.
2. Put 10ml of 0.1M FeCl3 solution in it by using a measuring cylinder.
3. Put 10ml of 0.1M KSCN solution in it by using a measuring cylinder.
4. Deep red colour is obtained due to complex formation [Fe(SCN)]2+(aq).
5. Dilute the above deep red solution by adding 50ml of distilled water.
6. Take four test tubes and label them as A, B, C and D
Add 5ml of the deep red solution to each of the four test tubes.
7. Arrange the test tubes in a test tube stand
8. A 5 ml of water B 5 ml of 0.1 M FeCl3 solution
C 5 ml of 0.1 M KSCN solution D 5 ml of 0.1 M H2C2O4 solution
9. Shake the test tubes well
10. Now compare the intensity of the colours in test tubes B, C and D with the red colour in test tube A taken as reference tube.
11. The intensity of the red colour corresponds to concentration of complex [Fe(SCN)]2+(aq) and if the concentration of this ion increases , the colour intensity will also increase.
RESULT :- RHS
Increase in concentration of either of the reactants (Fe3+ ions or SCN– ions) shifts the equilibrium ‘ in the forward direction (towards right), on the other hand decrease in concentration of any of the reactants shifts the equilibrium in the backward direction (towards left).
