HA(aq) + H20 (l) H30+(aq) + A-(aq)
We can simplify this to
HA(aq) H+(aq) + A-(aq)
By leaving out the water, which is present in excess.
In weak acids the tendency to donate H+ is weaker and the reaction with water is incomplete. Some H+ (aq) ions are formed but there is still some unreacted acid in solution. If we represent the weak acid as HA, its reaction with water can be represented as
HA(aq) Á H+(aq) + A-(aq)
Method
We choose to change the type of acid and keep the alkali the same. We choose the alkali to be sodium hydroxide after some preliminary work. The five different types of acids were hydrochloric acid, sulphuric acid, nitric acid, acetic acid, and phosphoric acid. We took six beakers and in three of them put 25 ml of sodium hydroxide and in the other three put 25 ml of one type of acid. We then put some universal indicator in each beaker and recorded the pH. Then we put a thermometer in one of the beakers with sodium hydroxide in it and added 25 ml of acid to it. Then we observed the thermometer until the maximum the temperature was reached and noted down the result and also noted down the new pH. We then did this two more times for the same acid, after which we changed the acid and did the same for the new acid. We did this for each of the five acids, with two repeats for each acid. Throughout the experiment we wore safety spectacles.
Results
Analysis
We can see from the results that the lower the pH the greater the change in temperature, as I said in my prediction. We can use this formula to find the energy conversion:
Energy conversion = mass x 4.2 x temperature change
The mass for our experiment is 50g, so we can use this bar chart to show the results:
So this agrees with my prediction, because the stronger acids have more energy given out when bonds are made in the solution than the weaker acids have.
Evaluation
The accuracy off the readings was not very good in this experiment. To find the maximum temperature during the reaction, you had to watch the thermometer and estimate the result. It would be impossible to record to over one decimal place; even one decimal place is a slight estimate. Also it was hard to know when the maximum temperature would arise. You had to watch until the temperature started going down and the record the maximum temperature.
Even though the results would not wholly accurate, the results came out as expected and agree perfectly with scientific theory so think this is evidence enough to make a firm conclusion that the lower the pH of an acid, the greater the energy conversion in a neutralisation reaction.
If I were to do a further investigation it would be to find out if the same applies to alkalis in a neutralisation reaction. So, if the alkali in the reaction has a higher pH, the energy conversation is greater. To do this I would do the experiment the same, except I would choose an acid to keep the same though preliminary work and vary the alkali, instead of the acid. My prediction would be that as the pH of the alkali increased, the energy conversation would increase.