Question

The Common Ion Effect Reaction: Fe3+ + SCN <> Fe(SCN)2+ 1. After the addition of 0.1 M KSCN to test tube 2, what is the color of the solution? How does the addition of KSCN affect the equilibrium of the reaction above? How does this relate to the change, if any, in the color of the solution? Include equations for reactions in your explanation. Color: dark red After the addition of 0.1 M Fe(NO3)3 to test tube 2, what is the color of the solution? How does the addition of Fe(NO3)3 affect the equilibrium of the reaction above? How does this relate to the change, if any, in the color of the solution? Include equations for reactions in your explanation. Color:dark red 3.After the addition of 6 M NaOH to test tube 2, what is the color of the solution? How does the addition of NaOH affect the equilibrium of the reaction above? How does this relate to the change, if any, in the color of the solution? Include equations for reactions in your explanation. yelolw w/ red percipitate

Answer 1

• Addition of 0.1 M KSCN ,the concentration of reactant (i.e.
  SCN
  ions) increases which,according to Le-Chatelier's principle drive the reaction to RIGHT i.e the rate of Forward reaction will increase and the color of the solution intensifies.

$Fe^{3+}_{(aq)}+SCN^{-}_{(aq)}(excess)\rightleftharpoons Fe(SCN)^{2+}_{(aq)}$ ---1

• Addition of 0.1 M $Fe(NO_{3})_{3}$ increases the concentration of iron(III) (shown in the reaction below) in the medium which drives the reaction to RIGHT in accordance with Le-Chatelier's principle and the rate of Forward reaction increases with more intense color of Fe(II) thiocyanate.

$Fe(NO_{3})_{3}\rightleftharpoons Fe^{3+}_{(aq)}+3NO_{3}^{-}_{(aq)}$...2

$Fe^{3+}_{(aq)}(excess)+SCN^{-}_{(aq)}\rightleftharpoons Fe(SCN)^{2+}_{(aq)}$ ...1

• Introducing 6 M NaOH in the test tube causes the iron (III) to form yellow iron hydroxide precipitate

$Fe^{3+}_{(aq)}+3OH^{-}_{(aq)}\rightleftharpoons Fe(OH)_{3}_{(s)}$...3

due to which the supply of iron (III) in equilibrium 1 is hindered and to prevent this,$Fe(SCN)^{2+}$ dissociates back to give the reactants by driving the reaction to LEFT.Here ,the rate of forward reaction decreases and the dark red color fades slowly.