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Explain how an acid-base indicator, which is itself a weak acid, and may be represented by HX, functions - Leaving Cert Chemistry - Question (b) - 2006
Question (b)
Explain how an acid-base indicator, which is itself a weak acid, and may be represented by HX, functions. Draw a clearly labelled diagram of the titration curve you... show full transcript
Worked Solution & Example Answer:Explain how an acid-base indicator, which is itself a weak acid, and may be represented by HX, functions - Leaving Cert Chemistry - Question (b) - 2006
Step 1
Explain how an acid-base indicator, which is itself a weak acid, and may be represented by HX, functions.
Answer
An acid-base indicator is a weak acid that can exist in two forms: the protonated form (HX) and the deprotonated form (X⁻). The position of the equilibrium shifts depending on the pH of the solution.
In acidic conditions (low pH), the equilibrium lies to the left:
This shift results in a higher concentration of the undissociated form (HX), which typically displays a specific color, indicating the solution is acidic.
Conversely, in basic conditions (high pH), the equilibrium shifts to the right, leading to an increased concentration of the deprotonated form (X⁻). This form tends to have a different color compared to the undissociated form (HX), indicating a basic or alkaline environment.
Step 2
Draw a clearly labelled diagram of the titration curve.
Answer
The graph of the titration curve should illustrate pH on the y-axis, labeled from 0 to 14, and the volume of NaOH added on the x-axis, ranging from 0 to 50 cm³.
- The pH starts low (around 3) for ethanoic acid.
- As NaOH is added, the pH gradually increases until it sharply rises at the neutralization point (around 25 cm³ of NaOH).
- After the equivalence point, the pH increases rapidly, achieving a value near 14.
The graph should show a steep rise between pH 6.5 and 10.5 around the neutralization point at approximately 25 cm³.
Step 3
Explain with reference to your diagram why phenolphthalein is a suitable indicator for a titration of sodium hydroxide with ethanoic acid.
Answer
Phenolphthalein is a suitable indicator for this titration because it changes color in the pH range of approximately 8.3 to 10.0. This range is ideal for detecting the endpoint of the titration of a weak acid (ethanoic acid) and a strong base (sodium hydroxide).
In the initial acidic solution (pH < 8.3), phenolphthalein remains colorless. As NaOH is added, once the pH surpasses 8.3, phenolphthalein changes to a pink color, indicating that the solution has transitioned to a basic state. This dramatic color change occurs at a point when the solution is nearing neutralization, allowing for a clear and distinct endpoint.