1) Explain the difference between equivalence point and end point... Explain how

Question

1) Explain the difference between equivalence point and end point... Explain how the pH at the equivalence point varies as the strength of the acid increases.... Remember that acid strength depends upon Ka not upon analytical concentration. Your answer should be based upon the proton transfer reaction that is important at the equivalence point.

2) Explain the difference in change of pH with volume of titrant added near the equivalence point as compared to a point earlier in the titration. (i.e. How does the slope of the titration curve differ?)

3) why the initial pH is not the same for all the acids while it is nearly the same when the titration has been carried a few mL past the equivalence point. Your answer should include appropriate proton transfer NIE(s).

4) What is the difference in change of pH with volume of titrant added near the equivalence point as compared to a point earlier in the titration. (i.e. How does the slope of the titration curve differ?)

>> please, I need a clear explanation ... THANKS

Explanation / Answer

This virtually identically worded question was submitted by another ( user) . Here is the answer I posted for the other question:

Let us start by sorting out a few definitions:

Equivalence point: this is when exactly equal stoichiometric quantities of acid and base have been mixed and reacted together.

Endpoint: this refers to the situation where the indicator changes colour.

Now consider what happens with four different combinations of acid/ base titrations:

Phenolphthalein is going to be used as the indicator - this indicator is accepted to change colour at pH 8.3 - If added to an acid solution, it is colourless, and when a base is added, when the pH reaches 8.3, it starts to go pink.

1) strong acid / strong base: titrating 25.00mL of 0.100M HCl with 0.100M NaOH

Equation: HCl + NaOH ? NaCl + H2O.

Here the product NaCl is neutral, and the equivalence point will be at pH = 7.00 when exactly 25.00mL of the NaOH solution has been added.

But because the base is strong, during the titration, the pH of the solution will be found to change from the acid pH approx 4.0 to basic pH 9.0 by the addition of 1 drop of NaOH solution. This range spans the endpoint of the phenolphthalein indicator, and as a result it is a suitable indicator for this titration.

So there is your answer: phenolphthalein

1) suitable for strong acid / strong base: weak acid / strong base

2) unsuitable for weak base / strong acid or weak base / weak acid.

2)Comparing equivalence point with midpoint. It is apparent by studying a graph that the equivalence point is distinctly different from the midpoint. On the curve, the equivalence point is located where the graph is most steep. There is a fast and abrupt change of pH around this point, which can be observed by the color change the takes place during titration

)

4)

In this technique, two samples are used.The one with the known concentration is called titrant.The another one whose concentration is unknown is called as analyte.

The titrant is added through calibrated burette, which is marked for volume measurement.

The analyte is kept in a container like a flask or beaker under the burette.

The equivalence point is that point at which the moles of the titrant and analyte are equal.

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