 LaShaw today's principle states that if you have a reaction to equilibrium and you apply a stress to that equilibrium, the reaction will shift in the direction in order to relieve the stress. We will look at a series of different reaction equilibria, we'll apply a stress to those equilibria and see in which direction they shift. In the reaction between iron plus 3 ion and thiocyanate ion, if we add some iron plus 3 in the form of iron 3 nitrate and to the iron 3 nitrate add some potassium thiocyanate, the two react to form a reddish-orange complex ion. We fill two test tubes with the reaction mixture. We're going to keep one test tube as a control so that we can see what happens to the color. To the other tube we're going to add some 0.1 molar iron nitrate to increase the concentration of the iron. We see that there is an increase in the dark red color, this indicates an increase in the concentration of the complex ion. This also indicates that the reaction has shifted in the forward direction. As you add reactant to an equilibrium, the reaction shifts in the direction to use up the added reactant, in this case the forward direction. Now we'll add 0.1 molar potassium thiocyanate to the second tube. This increases the concentration of the thiocyanate ion. We see that the reaction mixture turns a darker red color indicating an increase in the formation of the complex ion. We've added the other reactant, the thiocyanate, the reaction shifts in the forward direction to use up the added reactant. We will now add some 1 molar sodium hydroxide solution. We notice that the red color decreases significantly. This indicates that the reaction equilibrium has shifted in the reverse direction. The hydroxide ion precipitates the iron ion effectively removing it from the solution. The equilibrium shifts in the reverse direction to try and increase the concentration of the iron. We will now add solid potassium chloride to the solution. We notice the decrease in color indicating that the reaction has shifted in the reverse direction. The chloride ion forms a complex ion with the iron effectively tying up the iron concentration. The equilibrium shifts in the reverse direction to try and replace the iron that has been removed. We will now add 0.1 molar silver nitrate to the reaction mixture. We notice the disappearance of the red color which indicates that the equilibrium has shifted in the reverse direction. We also see the formation of a small amount of white solid. The silver ion reacts with the thiocyanate ion to form slightly soluble silver thiocyanate which is the white solid. The silver ion removes thiocyanate from solution. The equilibrium shifts in the reverse direction to try and replace the thiocyanate ion that has been removed. We will now add some 0.1 molar mercury 2 nitrate. The mercury plus 2 ion reacts with the thiocyanate ion to form a new complex ion. This effectively reduces the concentration of thiocyanate in the solution. The equilibrium shifts in the reverse direction to try and replace the removed thiocyanate.