Consider the following system at equilibrium.
[tex]CaCO_3(s) \Leftrightarrow Ca^{2+}(aq)+CO_3^{2-}(aq)[/tex]
The addition of which compound will cause a shift in equilibrium because of a common ion effect?
A. [tex]CCl _4[/tex]
B. [tex]CO _2[/tex]
C. [tex]CuSO _4[/tex]
D. [tex]Na _2 CO _3[/tex]
Answer (2)
The common ion effect shifts equilibrium when a soluble salt containing an ion already in the equilibrium is added.
N a 2 C O 3 dissolves into N a + and C O 3 2 − ions, introducing a common ion ( C O 3 2 − ).
The equilibrium shifts left, favoring C a C O 3 ( s ) formation, to counteract the increased C O 3 2 − concentration.
Therefore, the addition of N a 2 C O 3 causes a shift due to the common ion effect. N a 2 C O 3
Explanation
Understanding the Problem The problem describes a system at equilibrium: C a C O 3 ( s ) ⇔ C a 2 + ( a q ) + C O 3 2 − ( a q ) . We need to identify which of the given compounds will cause a shift in equilibrium due to the common ion effect. The common ion effect occurs when a soluble salt containing an ion already present in the equilibrium is added, shifting the equilibrium according to Le Chatelier's principle.
Analyzing the Options Let's analyze each option:
CC l 4 : Carbon tetrachloride is a nonpolar solvent and does not contain either C a 2 + or C O 3 2 − ions. Therefore, it will not cause a common ion effect.
C O 2 : Carbon dioxide dissolves in water to form carbonic acid ( H 2 C O 3 ), which can further dissociate into bicarbonate ( H C O 3 − ) and carbonate ( C O 3 2 − ). While it can introduce C O 3 2 − ions, the primary effect is through changing the pH of the solution.
C u S O 4 : Copper(II) sulfate dissolves to form C u 2 + and S O 4 2 − ions. It does not contain either C a 2 + or C O 3 2 − ions, so it will not cause a common ion effect.
N a 2 C O 3 : Sodium carbonate dissolves to form N a + and C O 3 2 − ions. It contains the C O 3 2 − ion, which is already present in the equilibrium. Therefore, it will cause a common ion effect.
Determining the Shift in Equilibrium The addition of N a 2 C O 3 will increase the concentration of C O 3 2 − ions in the solution. According to Le Chatelier's principle, the equilibrium will shift to the left, favoring the formation of C a C O 3 ( s ) , to counteract the increase in C O 3 2 − concentration.
Conclusion Therefore, the addition of N a 2 C O 3 will cause a shift in equilibrium due to the common ion effect.
Examples
The common ion effect is crucial in various applications, such as controlling the solubility of sparingly soluble salts in industrial processes. For instance, in water treatment, adding a common ion can help precipitate unwanted minerals, making the water safer for consumption. Similarly, in pharmaceutical formulations, understanding the common ion effect helps in controlling the dissolution rate of drugs, ensuring optimal therapeutic efficacy. By manipulating ion concentrations, we can fine-tune chemical reactions and processes to achieve desired outcomes.
The compound that will cause a shift in equilibrium due to the common ion effect is N a 2 C O 3 , as it increases the concentration of the common ion C O 3 2 − . This results in the equilibrium shifting to the left, favoring the formation of solid C a C O 3 . Therefore, the chosen option is N a 2 C O 3 .
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