Titration Lab Report

CHEMISTRY LAB

Titration Curves of Strong and Weak Acids and Bases

Processing the Data:

Questions:

1. Examine the time data for each of the Trials 1-4. In which trial(s) did the indicator change color at about the same time as the large increase in pH occurred at the equivalence point? In which trial(s) was there a significant difference in these two times?

In all the 4 trials, the time taken for color change and the time taken for a large increase in pH was the same, leaving no significant difference between the two values.

2. Phenolphthalein changes from clear to red at a pH value of about 9. According to your results, with which combination(s) of strong or weak acids and bases can phenolphthalein be used to determine the equivalence point?

The combination of a Strong Acid and Base will give us the equivalence point: there will be a color change of phenolphthalein at pH 9. It is also observed that the reaction between a Weak Acid and Strong Base can be used to obtain a pH of 9.

3. On each of the four printed graphs, draw a horizontal line from a pH value of 9 on the vertical axis to its intersection with the titration curve. In which trial(s) does this line intersect the nearly vertical region of the curve? In which trial(s) does this line miss the nearly vertical region of the curve?

For Trials 1 and 3, the horizontal line from pH 9 intersects the S curve. For Trials 2 and 4, because the equivalence point is lower than pH9, they nearly miss the vertical region of the curve.

4. Compare your answers to Questions 1 and 3. By examining a titration curve, how can you decide which acid-base indicator to use to find the equivalence point?

After examining S Titration Curves, it is observed that color change is visible around the equivalence point, which can either be seen above or below the point of color change.

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5. Methyl red is an acid-base indicator that changes color at a pH value of about 5. From what you learned in this lab, methyl red could be used to determine the equivalence point of what combination of strong or weak acids and bases?

The reaction between a strong acid and a weak base shows color change at around pH 5. Since this reaction has a low equivalence point

6. Of the four titration curves, which combination of strong or weak acids and bases had the longest vertical region of the equivalence point? The shortest?

Longest vertical region of equivalence point was noticed in the reaction between weak acid and strong base while shortest was seen in the reaction between strong acid and weak base.

7. The acid-base reaction between HCl and NaOH produces a solution with a pH of 7 at the equivalence point (NaCl + H2O). Why does an acid-base indicator that changes color at pH 5 or 9 work just as well for this reaction as one that changes color at pH 7?

Even though it is noticed that the equivalence point for this reaction is at pH 7, the acid-base indicator will work and a color change will be noticed because in terms of the time difference, they’re very much similar and the reaction will not be affected by a change that occurs in the vertical region.

8. In general, how does the shape of a curve with a weak specie (NH3 or HC2H3O2) differ from the shape of a curve with a strong specie (NaOH or HCl)?

As seen from the graphs, the shape of a weak specie has a shorter vertical range whereas that of a strong specie has a higher slope and more curves are seen; resulting into longer vertical range.

9. Complete each of the equations in the table.

DATA TABLE

EVALUATION:

There were a number of sources of error that may have affected the accuracy of our lab.

The reliability of data and its accuracy are dependent on the apparatus we used, and its accuracy. Readings were taken as accurately as possible, using the apparatus at hand. For example, when filling up burettes with water, we made sure that the bottom of the meniscus touched the calibrated line. Also, by looking at this point from several angles we tried to reduce the error of parallax. However, even whilst using the equipment we used, there’s uncertainty present which is as follows:

pH probe: ±

Burette:± 0.05 ml

Measuring cylinder: ±0.1 ml

Also, the phenolphthalein that was used to titrate was found not to be very effective. It took longer than expected for the color change and thus ph to occur. Because of our time constraint, we could not wait until the phenolphthalein changed the color of the solution. After trial 1, we started adding a couple of more drops of the indicator to account for the slow process. This however may have altered our results and there was an understatement of time taken for color change.

The change of color occurred to quickly as well, it was not very easy to determine where exactly color changed occurred. It was due to this that our results are not very effective.

CONCLUSION:

It was observed that an S shaped curved is generated for titration reactions. The only difference is when acids and bases of different strengths react. Strong acid and base reactions brought about a steeper curve, hence a bigger vertical region where color change occurs. A reaction between weaker acids and bases brought about a lesser steep curve and a smaller vertical region. Strong acid and base curves thus need more time for color change. In this experiment, the strong acid and base reaction took about 98 seconds. A weak acid and base took much lesser time, resulting in color change after only 39 seconds. Also, I can now be able to identify titration curves and what reactions they belong to. The use of an indicator and how it works best for certain reactions was also seen. I now understand why certain indicators are better, due to their pH values which are responsible for bringing about color change and thus equivalence points.

Peer Reviews

Here's what a star student thought of this essay

skatealexia

6th of Mar 2012

Quality of writing

Grammar, punctuation and spelling are all fine. Formula are all correct. Not too sure about the type of front used that alters the A, as this would not be the correct format for an exam board.

Level of analysis

The analysis of the results by the questions gives a good idea of the theory that has been grasped correctly by the candidate. The evaluation could include more, such as calculating percentage error, and explaining a few more of the errors that may have gone wrong in the experiment. The conclusion is adequate as a picture of the results.

Response to question

A good piece of coursework for the investigation and level. Not in an essay format but more an outlay of how questions should be answered for a practical exam. The answer to question 1 is adequate and correct. Does not provide the start data, so quite hard sometimes to assess whether or not the answer is right without having prior scientific knowledge. The candidate in question 2 has the theory that a strong acid and base can give a PH of around 9, but the answer of a weak acid and strong base would have been closer to the mark. The graphs are all correctly drawn and the conclusions from them are also correct. The candidate's answers to all the other questions are adequate. Equations are fine, and uses the correct formulae. An introduction explaining the experiment would have been more useful.

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