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Explain how an acid-base indicator, which is itself a weak acid, 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, may be represented by HX, functions. Draw a clearly labelled diagram of the titration curve you wou... show full transcript
Worked Solution & Example Answer:Explain how an acid-base indicator, which is itself a weak acid, 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, may be represented by HX, functions.
Answer
An acid-base indicator, such as HX, functions based on its ability to dissociate in response to changes in pH. This can be described by the equilibrium:
In low pH (acidic conditions), the equilibrium shifts to the left, favoring the undissociated form (HX), which imparts a specific color to the solution. Conversely, in high pH (basic conditions), the equilibrium shifts to the right, resulting in a greater concentration of dissociated ions (H⁺ and X⁻) and thus a different color. The position of the equilibrium determines the color change observed in the solution.
Step 2
Draw a clearly labelled diagram of the titration curve you would expect to obtain when 50 cm³ of a 0.1 M sodium hydroxide (NaOH) solution is added slowly to 25 cm³ of a 0.1 M ethanoic acid (CH₃COOH) solution.
Answer
The titration curve can be represented graphically with the following characteristics:
- The x-axis represents the volume of NaOH solution added, while the y-axis represents the pH.
- The curve shows a gradual increase in pH up to the equivalence point (around 25 cm³), where the pH rises steeply, indicating the neutralization of the acid by the base.
- After the equivalence point, the pH levels off indicating excess NaOH. Typically, the curve reaches around pH 14 at the end of the titration.
Here is a rough sketch of the titration curve:
pH
14 +
|
| .
10 + .
| .
7 +-------. .
| 25
| Volume of NaOH (cm³)
0 +----------------------------------
0 10 20 30
``
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 suitable for the titration of sodium hydroxide with ethanoic acid since it changes color in a pH range suitable for this titration. It has a transition range from approximately pH 8.3 to 10, which encompasses the steep increase in pH around the equivalence point of the titration (approximately pH 7). Thus, when phenolphthalein is added to the solution, it will transition from colorless (in acidic conditions) to pink (in basic conditions) as the solution is neutralized, indicating that the titration has reached its endpoint.