Investigate the factors affecting Ka of a weak acid.

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Name:        Jonathan Tam                Class:        12A

Chemistry Experiment Report

  • Planning (a), Planning (b), Data Collection, Data Processing and Presentation, Conclusion and Evaluation

Planning (a):        To investigate the factors affecting Ka of a weak acid

The acidity of an acid is determined by the dissociation of H+ ions in the acid, ie the higher it dissociates the more acidic. In other words, the concentration is not a factor affecting the acidity, but the degree of dissociation.

As the dissociation is at equilibrium for all acids, factors such as temperature should have an effect of shifting the equilibrium position, ie temperature affects the degree of dissociation.

e.g.                

If temperature increases I suspect [H+] will increase because the equilibrium will shift to the left (dissociation of ions is endothermic) as temperature increases. So temperature is one factor to investigate.

Since the formula of pH is , pH is affected by [HA], meaning the concentration of acid determines pH. So if an increase in temperature can increase [H+], then at the same time it can decrease [HA]. If [HA] decreases, Ka increases (Ka tells us the acidity). pKa decreases (since it is in a negative logarithm relationship). A simple graph can illustrate this hypothesis.

 


Also, organic acids contain a carbon chain and a H+ ion that will dissociate. If the length of the carbon chain increases, the dissociation of H+ might change.

e.g.                

                

                

I suspect that as the carbon chain increases in size, the level of dissociation decreases because it has become harder to dissociate the H+ as we go down the homologous series. This is because the chain is getting more electronegative as there are more and more electrons present. This also attracts the H+ ion to recombine and form the organic acid molecule again.

Since the formula of pH is , pH is affected by [HA], meaning the concentration of acid determines pH. So if an increase in carbon chain length can decrease [H+], then at the same time it can increase [HA]. If [HA] increases, Ka decreases (Ka tells us the acidity). pKa increases (since it is in a negative logarithm relationship). A simple graph can illustrate this hypothesis.

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We should be measuring pH for all cases because Ka (pKa) is related to pH, according to our definition of pH. ()


Planning (b):

Experiment for investigating temperature:


Experiment for investigating carbon chain:


Data Collection:

Acid used:                HCOOH

Concentration used:        0.05 mol dm-3

Volume used:                150ml

  • Mass of HCOOH used: 2.3g (Molar mass of HCOOH: 46)
  • Added into 1000 cm3 of distilled water

Data Processing and Presentation:

From our starting formula of pH:

 

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