In this lab we will be observing the qualitative changes in equilibrium by changing known concentrations of reactants: iron and thiocyanate ions, and finding its effect on the position of equilibrium. We will be using a colorimeter to find the absorbance of the solutions with known concentrations and we will plot a graph of absorbency vs. concentration which will allow us to quantitatively track the changes in equilibrium of a solution. I will be using iron nitrate and potassium thiocyanate in order to give us ironthiocyanate.
Fe3+(aq) + SCN-(aq) FeSCN2+
1. Variables Chart
The variable I will be manipulating is concentrations of the FSCN2+ (ironthiocyanate).
Position of equilibrium shown through equilibrium constant Kc.
Control Variables – the variables that must be kept constant during the experiment
How variables affect experiment
How the method compensates for experiment
Without the reactants including metal ions, the solution would be colourless and thus there would be no way of measuring absorbency and thus of finding concentration.
I will keep this constant by using metal ions (Fe3+) and using that same reactant throughout all of my trials.
Different colorimeters may have different absorbency ratings. As well using different cuvettes may change the absorbency readings.
I will use the same colorimeter and the same plastic cuvette for all trials.
Concentration of Reactants
Different concentrations of both reactants would give me an unkown product concentration. Then I will not be able to even make a curve of absorbancy vs concentration if I do not even know the concentration.
Use same concentration of both reactants. Since 1mol of reactants give us 1mol of product, then whatever concentration of reactants I have will tell me the concentration of products.
* 1 colorimeter
* 20 ml solution Fe(NO3)3
* 20 ml solution KSCN
* 10 ml distilled water
* 4 test tubes
* 1 test tube rack
1. Wash a plastic cuvette
2. Fill it with clean water, as this will be a reference point of absorbance for the colorimeter and measure the absorbance. Since it is clear it should be 0.
1. Label the test 1,2,3,4 and fill with the following amounts:
2. Prepare the thiocynate solution (Fe(SCN)2+(aq) by mixing 18 ml of 0.200 Fe(NO3)3 with 2 ml of 0.002 KSCN. This will give us the solution whose unknown concentration we want to find.
3. We fill the plastic cuvette with the solution in test tube 1 and find the absorbency. We write it down in the following table.