Strong and Weak Acids And Bases

Total Percent Error= 63.5%

Total Percent Uncertainty= 11.3%

Experiment #2- Conductivity

Table 4- Conductivity Observations

Experiment #3- Action of Weak and Strong Acids on Metals

Table 5 -3M Hydrochloric Acid

Table 6 -3M Ethanoic Acid

Experiment #4- Effect of Acid Concentration on Reaction Rate

Reaction

Mg + 2HCl →H2 (g) + MgCl2

Table 7- Effect of Acid Concentration on Reaction Rate of Magnesium

Experiment #5- Titration 

Theoretical Values

                      HCl (aq) + NaOH (aq)→ H2O (l) +    NaCl (aq)

Molarities        0.2M        0.2M

Volume           10ml         ?      

It is a one-one ratio so:

0.20molHCl= 0.20mol NaOH= 10.0ml NaOH theoretically is needed

       

                         CH3COOH (aq) + NaOH (aq) →  H2O (l) + Na (CH3COO) (aq)

Molarities           1.0M                  1.0M

Volume              10ml                    ?      

It is a one-one ratio so:

1.0molHCl= 1mol NaOH= 10.0ml NaOH theoretically is needed

Table 8- Titration of Acids (Experimental Values)

Experimental  Values

                      HCl (aq) + NaOH (aq)→ H2O (l) +    NaCl (aq)

Molarities        0.2M        0.2M

Volume           9.97ml         ?      

It is a one-one ratio so:

0.20molHCl= 0.20mol NaOH= 9.97ml NaOH was needed in experiment

       

                         CH3COOH (aq) + NaOH (aq) →  H2O (l) + Na (CH3COO) (aq)

Molarities           1.0M                  1.0M

Volume              11.27                    ?      

It is a one-one ratio so:

1.0molHCl= 1mol NaOH= 11.27 ml NaOH was needed in experiment

Percent Errors

| Theoretical – experimental | x 100 = percent error

             Theoretical

HCI and NaOH

| 10.00 –9.97 | x 100 = 0.3 % error

    10.00

 CH3COOH and NaOH

| 10.00 –11.27 | x 100 = 12.7 % error

    10.00

Total % Error= 13.0%

Percent Uncertainties

HCI and NaOH

0.04  x 100 = 0.40 % uncertainty

9.97

0.04  x 100 = 0.35 % uncertainty

11.27

Total % Uncertainty= 0.75%

Discussion

Experiment #1- Measurement of pH Values

        In this experiment we used a pH paper and a pH meter to determine the pH of several acids and bases with different concentrations. Color was used to determine the pH using the pH paper. By using graphs that was produced with the aid of the pH meter, we were also able to determine the pH values. Hydrochloric acid is a strong acid and therefore it did completely ionize. Ethanoic acid was a weak acid and therefore was only partially ionized. With an increase in concentration, the pH values became lower, or the solution became more acidic. For example, 0.01M HCl had an experimental pH value of about 2 with the pH paper, but 0.1M HCl had an experimental value of about 1 with the pH paper. This is exactly what should be expected, because with an increase in concentration of HCl, the concentration of hydrogen ions in the acid also increases. 0.1M Ethanoic acid had a pH of 3, so we can say that it was less acidic than the 0.1M hydrochloric acid. This is because ethanoic acid is a weaker acid while HCl is a strong acid with more hydrogen ions in its solution. The pH of an acid is defined as the ‘power of hydrogen’, so as the hydrogen ion concentration increases, the pH decreases. We can use this data to determine the exact value of the pH for HCl and ethanoic acid. From our results, we can determine that hydrochloric acid is a strong acid as it had a lower pH value as a dissolved substance, meaning there were many ions dissociated in the solution, resulting in more hydrogen ions. Ethanoic acid had a higher pH value, meaning there were less ions in the solution, less ions disassociated, fewer hydrogen ions, and therefore is a weaker acid. Our experimental and theoretical values were off, but these differences can be determined by human errors. The total % uncertainty for the pH paper was much greater than the total % error. Because the % uncertainty was larger than the % error, equipment uncertainties were more significant than systematic errors, while using the ph paper. Because the % error was larger than % uncertainty for ph meter, systematic errors were more significant than equipment errors. These % values make sense as the pH meter is obviously much more accurate than the pH paper.

   

        There were a few possible errors for this experiment. The errors of the pH paper were much more significant than human errors. To begin with, the pH paper turned different colors to indicate the pH level, but sometimes it was difficult to accurately match these colors with the legend. This also resulted in errors. It was also possible that as we tested the different acids, other substances may have touched and reacted with the pH paper. Human errors were much more significant in using the ph meter. Contamination between the acids could have occurred, when the pH meter was used to test the different acids. The graphs the ph meter and the computer produced may have also been inaccurate because the solution was not stirred enough with probe, while the computer was taking in the results. This would have led to an inaccurate reading of the pH.

        There are a few solutions to this experiment. The pH paper would be easier to use with a clearer legend, so that the colors could be better compared. Having a more accurate pH meter would have resulted in more accurate results. Also, for the pH meter the probe could be wiped more carefully after each use so contamination would not occur.

Experiment #2- Conductivity

        In this experiment we used the light bulbs to measure the conductivity of different acids and bases with different concentrations. The conductivity of the HCl varied with the concentration because the concentration of an acid effects its ionization. The higher the concentration, the more of the acid is dissociated and there are more ions available for conductivity. For example, in this lab, 0.001M HCl was a faint conductor, but 1.0M HCl was a very strong conductor. This is because there were more hydrogen ions in the 1.0M solution as it was more dissociated, so it conducted better than the 0.001 solution. The conductivity value of the ethanoic acid was moderate. This means that there are enough hydrogen ions in the solution to moderately conduct electricity, but not enough to be a strong conductor. This indicates that ethanoic acid isn’t a very strong or a very weak acid because it ionizes moderately, and is therefore partially ionized. Systematic errors would most likely be more significant than equipment uncertainties in this experiment.

One main error that may have occurred is inaccurate interpretation of the brightness of the light. If we had used quantitative data instead of qualitative data, we would be able to have more accurate results.

Experiment #3- Action of Weak and Strong Acids on Metals

        In this experiment we compared the reaction rates of the two acids (hydrochloric acid and ethanoic acid) when they each reacted with Mg,Zn,Cu.

Magnesium reacted very quickly with both acids and a definite difference in reaction rates wasn’t observed. There was a slight bubbling observed, which represents the hydrogen gas formed in the reaction. Zinc reacted quickly with HCl but hardly reacted with the ethanoic acid. Copper had slow reactions with HCl, but didn’t react at all with the ethanoic acid. HCl is a stronger acid than ethanoic acid, so its reactions went further to completion during our observation period than did the reaction of ethanoic acid with the three metals. This is because there are more disassociated ions for the metal to react with and result in a reaction. Therefore it can be said that strong acids react much faster with metals than weaker acids. . Systematic errors would most likely be more significant than equipment uncertainties in this experiment.                                                                                    

 If we’d had more time for this lab, we could have observed the reaction rates of the acids and metals for a longer time. This would have better enabled us to observe their reactions. Also, if we had used quantitative data instead of qualitative data, we would be able to have more accurate results.

Experiment #4- Effect of Acid Concentration on Reaction Rate

In this lab we observed the effect of acid concentration on reaction rate. Hydrochloric acid was the acid that was used and a small strip of magnesium was used with the HCI to observe the effect of concentration. . Systematic errors would most likely be more significant than equipment uncertainties in this experiment.

Higher concentration of HCI led to a faster reaction rate with Magnesium. For example, 1.0M HCl reacted with the magnesium fast, while 0.01M HCl reacted with the magnesium slowly. The higher concentration HCl has more HCl molecules to react with the magnesium, so the reaction occurs much faster.

 If we’d had more time for this lab, we could have observed the reaction rates of the acids and metals for a longer time. This would have better enabled us to observe their reactions. Also, if we had used quantitative data instead of qualitative data, we would be able to have more accurate results.

Experiment #5- Titration 

Titration allows us to find the concentration or volume of an unknown amount of unknown acid or base by using another base or acid whose volume you already know. So by slowly adding the known of acid/base to the unknown base/acid, it will eventually be neutralized. When it's neutralized, you can do some calculations to find the unknown concentration/ strength. In this experiment NaOH was the unknown base that we were trying to find the volume of. In this experiment HCI was the acid in which we already knew the concentration and volume of. Because the % error was larger than % uncertainty, systematic uncertainties were more significant than system errors.

The main error that it was very easy to add too much NaOH after the reaction has already neutralized. Also, the solutions may contain impurities. The burette for example, may have been contaminated with some other solution other than the one being tested. We could have misread the volume measurement.  Additionally, while we were measuring the NaOH and the acids, air bubbles at the tip would affect the volume making us uncertain whether we had the right volume or not.  

These errors would likely disappear with practice over time. Taking readings at eye level to avoid parallax errors would also improve measurement errors. Other human errors could have been adding too much of too little of a solution, which would effect the reactions.

Conclusion

In this lab we learned that:

The properties of weak bases are:

• Low conductivity
• Less basic (lower pH values than strong bases)

Weak bases will dissociate less, and therefore will have fewer ions to conduct heat/electricity. They will be less basic because there won’t be as many OH- ions in the solution and pOH values will be high, resulting in low pH values.

The properties of strong bases are:

• Very high conductivity
• Very basic (high pH values than weak bases)

Strong bases will dissociate more, so there will be more OH- ions for conductivity, and to decrease the pOH values, which in turn will increase the pH values. This will make the solution very basic.

The properties of weak acids are:

• Low conductivity
• Less acidic (higher pH values than strong acids)

As with weak bases, weak acids will ionize less, so there will be fewer hydrogen ions for conductivity, and this will increase the pH values, making the solution less acidic.

The properties of strong acids are:

• High conductivity
• Very acidic (low pH values than weak acids)

Strong acids will ionize more, so the increase in hydrogen ions will improve conductivity and lower the pH value, making the solution more acidic. 

The properties of dilutions of strong acids are:

• The more dilute solutions of strong acids will be weaker conductors and will have higher pH values. T

• There will be fewer molecules to ionize, resulting in fewer hydrogen ions. As mentioned above, fewer hydrogen ions will result in the solution being a weaker conductor, and in being less acidic (higher pH value)