Question

Lab Experiment 8 Equilibrium II – Le Chatelier’s Principle

Part B ‐ Thiocyanate Complexes Given:

Fe3+  +  SCN‐   ⇌  FeSCN2+   

Ag+   +  SCN‐   ⇌ AgSCN

2. Describe the change observed when silver nitrate is added in step b(b. Add enough AgNO3 to cause a change. Swirl/stir to mix thoroughly) and explain why this change is consistent with Le Chatelier's principle.

3. Describe the change observed when sodium chloride is added in step c (c. Add enough NaCl to cause a change. Swirl/stir to mix thoroughly. Record observations. (Note – there are two different concentrations of NaCl available at the station.)) and explain why this change is consistent with Le Chatelier's principle.

4. Consider only the FeSCN2+ equilibrium, if a large excess of SCN‐ is added to the system the resulting equilibrium solution contains only a very small amount of Fe3+. Explain why this observation would be consistent with Le Chatelier's principle.

Answer 1

Le Chatelier's principle: If a system at equilibrium is subjected to stress (i.e. change in concentration or pressure or temperature) then the system will shift in a direction so as to nullify the stress.

Given equilibria:

Fe³⁺  +  SCN^‐   ⇌  FeSCN²⁺   

Ag⁺   +  SCN^‐   ⇌ AgSCN

2. When silver nitrate (i.e. AgNO₃) is added in step (b), i.e. formation of AgSCN, the concentration of Ag⁺ ions increases as AgNO₃ is a strong electrolyte and completely dissociates into Ag⁺ and NO₃^- as shown below.

AgNO₃    Ag⁺ + NO₃^-

According to Le Chatelier's principle, the equilibrium shifts in a direction to decrease the concentration of Ag⁺ ions, i.e. towards the right side (product side).

3. Step c is not shown here.

for e.g., let's consider the dissociation of AgCl in aqueous media as follows.

AgCl ⇌ Ag⁺   + Cl^‐

When sodium chloride (i.e. NaCl) to the above equilibrium, the concentration of Cl^‐ion increases as NaCl is a strong electrolyte and completely dissociates into Na⁺ and Cl^- as shown below.

NaCl Na⁺ + Cl^-

According to Le Chatelier's principle, the equilibrium shifts in a direction to decrease the concentration of Cl^- ions, i.e. towards the left side (reactant side).

4. If a large excess of SCN^‐ is added to the step (a), i.e. formation of AgSCN, there will be a high concentration of SCN^- ions and according to Le Chatelier's principle, the equilibrium shifts to decrease the concentration of SCN^‐ ions towards the right side (product side).