Consider the chemical equilibrium of a soap that is present in its aqueous solution.
[tex]\begin{array}{c}
C_{17} H_{35} COONa(aq) \\
\text { sodium stearate (soap) }
\end{array} \Leftrightarrow C_{17} H_{35} COO^{-}(aq)+Na^{+}(aq)+\begin{array}{l}
\text { stearate ion }
\end{array}[/tex]
What will happen if a large quantity of NaCl is added to the aqueous solution?
A. The equilibrium will shift to the right, favoring the dissociation of soap in aqueous solution.
B. The equilibrium will shift to the left to such an extent that it leads to the precipitation of solid soap.
C. The equilibrium will not be affected by the addition of NaCl to the solution.
D. The equilibrium will be permanently destroyed by the addition of NaCl.
Answer (2)
Adding NaCl increases N a + concentration.
Le Chatelier's principle dictates the equilibrium shifts left.
This shift favors undissociated soap formation.
High NaCl concentration can precipitate solid soap: The equilibrium will shift to the left to such an extent that it leads to the precipitation of solid soap.
Explanation
Problem Analysis The problem describes the equilibrium of soap (sodium stearate) in an aqueous solution and asks what happens when a large quantity of NaCl is added. We need to use Le Chatelier's principle to determine the shift in equilibrium.
Effect of NaCl NaCl is a strong electrolyte that dissociates completely in water into N a + and C l − ions: N a Cl ( a q ) → N a + ( a q ) + C l − ( a q ) . The addition of NaCl increases the concentration of N a + ions in the soap solution.
Le Chatelier's Principle According to Le Chatelier's principle, if we add a product ( N a + ) to a system at equilibrium, the equilibrium will shift to consume the added product. In this case, the equilibrium will shift to the left, favoring the formation of undissociated soap ( C 17 H 35 COON a ( a q ) ).
Precipitation of Soap If a large enough quantity of NaCl is added, the equilibrium will shift significantly to the left, causing the concentration of undissociated soap to increase. This can lead to the precipitation of solid soap from the solution.
Final Answer Therefore, the correct answer is: The equilibrium will shift to the left to such an extent that it leads to the precipitation of solid soap.
Examples
Consider a scenario where you're trying to clean your clothes with soap in hard water, which contains dissolved minerals like calcium and magnesium ions. These ions act similarly to adding NaCl, causing the soap to precipitate out of solution and reducing its effectiveness. This is why detergents, which are less sensitive to hard water ions, are often preferred for laundry.
Adding NaCl increases the concentration of Na + ions in a soap solution, causing the equilibrium to shift left according to Le Chatelier's principle. This shift results in the formation of more undissociated soap, which can lead to precipitation. Therefore, the correct answer is B: The equilibrium will shift to the left to such an extent that it leads to the precipitation of solid soap.
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