Question

1.) Explain why the starting concentration of iron(III) ions exceeds the starting concentration of thiocyanate ions by two orders of magnitude. How is this important in terms of calculations?

2.) Using the reaction 2NH2CO2NH2(s)⇌2NH3(g)+CO2(g), determine the effect on equilibrium for the addition of the following below.

SnCl2

NH3

NaOH.

3.) Using Equation K = [C]c [D]d / [A]a[B]b, compare Kp to Kc for the reaction. Discuss the difference in the two equilibrium constants. What does this say about the equilibrium of the reaction? How do you know?

Answer 1

1. The starting concentration of iron(III) ions exceeds the starting concentration of thiocyanate ions because when the iron ions are in excess, it means that whole of the thiocyanate ions will react with iron ions. The thiocyanate will be limited and thus it will react completely to form the complex. In calculations, the concentration of the complex formed can be considered same as concentration of thiocyanate ions.

2. In the reaction, SSnCl2 is not present, so addition of SnCl2 will not affect the equilibrium.

Addition of NH3 will make the reverse reaction fast, because according to Le-Chatelier principle, the reaction moves in the direction in order to counteract the change which we have done. So, we have added NH3, which means that reaction will move in the direction in which NH3 will be consumed. Thus, the reaction will move in reverse direction.

Addition of NaOH will not affect the reaction.

3. Kp and Kc are both the equilibrium constants in terms of pressure and concentration respectively. The relation between the two is:

Kp = Kc(RT)^n

where R = gas constant

T = temperature

n = difference in the moles of product and reactant which are in gaseous form.

So, here n = (c+d)-(a+b)

Kp = Kc(RT)^[(c+d)-(a+b)]

This tells that if the value of K is more than 1, then the reaction is moving in forward direction but if it is less than 1, then it is moving in reverse direction. We can predict the progress of reaction from the value of K.