Consider the following chemical reaction of bromothymol blue indicator. It appears yellow in undissociated form and blue in its dissociated aqueous solution.
[tex]\underset{\substack{HC_2 H_3 O_2(a q) \\ \text { yellow }}}{\stackrel{+}{C_2} H_3 O_2^{-}(a q)} \text { blue }[/tex]
What will be the color of the solution if a large amount of [tex]H _2 CO _3[/tex] is added?
A. The solution will remain yellow.
B. The solution will turn blue.
C. The solution will turn pink.
D. The solution will turn green.
Answer (2)
Adding H 2 C O 3 increases H + concentration.
Equilibrium shifts to the left, favoring the undissociated form.
The undissociated form is yellow.
Therefore, the solution turns yellow. T h eso l u t i o n w i ll re main ye ll o w .
Explanation
Understanding the Reaction We are given a chemical reaction of bromothymol blue indicator, which appears yellow in its undissociated form ( H C 2 H 3 O 2 ( a q ) ) and blue in its dissociated form ( C 2 H 3 O 2 − ( a q ) ). The equilibrium is represented as:
Equilibrium Representation H C 2 H 3 O 2 ( a q ) ⇌ C 2 H 3 O 2 − ( a q ) + H + ( a q )
Adding Carbonic Acid We need to determine the color of the solution after adding a large amount of H 2 C O 3 . H 2 C O 3 is a diprotic acid, which will dissociate in water to produce H + ions:
Dissociation of Carbonic Acid H 2 C O 3 ( a q ) ⇌ H + ( a q ) + H C O 3 − ( a q ) and H C O 3 − ( a q ) ⇌ H + ( a q ) + C O 3 2 − ( a q )
Le Chatelier's Principle The addition of H 2 C O 3 will increase the concentration of H + ions in the solution. According to Le Chatelier's principle, increasing the concentration of H + will shift the equilibrium of the bromothymol blue indicator towards the left, favoring the formation of the undissociated form ( H C 2 H 3 O 2 ).
Determining the Color Since the undissociated form is yellow, the solution will turn yellow.
Examples
This principle is used in various applications, such as controlling the pH of solutions in chemical experiments or understanding how the acidity of rainwater affects the environment. For example, in environmental science, understanding how the addition of acidic pollutants affects the equilibrium of natural water systems is crucial for assessing and mitigating environmental damage. Similarly, in biology, maintaining the correct pH in cell cultures is essential for optimal cell growth and function.
The addition of carbonic acid ( H 2 C O 3 ) increases the concentration of hydrogen ions ( H + ) in the solution. This causes the equilibrium of bromothymol blue to shift toward the undissociated yellow form. Therefore, the solution will remain yellow, which corresponds to option A.
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