= 50g
and then you should add the mass of the solid used
The specific heat capacity of HCl (aq) is 4.2 Jg-1k-1
Energy = mass of x Specific Heat x Temperature
ΔH1 Solution Capacity of Water Change
= (50+2.25) x 4.2 x 4
= 882.84 per 2.25g of CaCO3
Energy = mass of x Specific Heat x Temperature
ΔH2 Solution Capacity of Water Change
= (50+1.2) x 4.2 x 9
= 1935.36 per 1.2g of CaO
As there is heat being given off so the reactions are endothermic, this means that ΔH is a negative value. So…
ΔH1 = -882.84 per 2.25g of CaCO3
ΔH2 = -1935.36 per 1.2g of CaO
As it says above the values are only per a percentage of the solution so now we have to work it out per mole…
-882.84 per 2.25g of CaCO3
Moles = mass x RAM = 2.25 x CaCO3
CaCO3 = Ca – 40
C - 12
O3 – (3 x 16)
= 100
Moles = mass x RAM = 2.25 x CaCO3
= 2.25 x 100
= 225
-877.8 x 225 = -197.32 Kjmol-1
1000
ΔH1 = -197.32 Kjmol-1
-1935.36 per 1.2g of CaO
Moles = mass x RAM = 1.30 x CaO
CaO = Ca – 40
O – 16
= 56
Moles = mass x RAM = 1.30 x CaO
= 1.30 x 56
= 72.80
-1935.36 x 72.80 = -141.16 Kjmol-1
1000
ΔH2 = -141.16 Kjmol-1
So now have ΔH1 and ΔH2 we can work out ΔH3…
ΔH3 = ΔH1 - ΔH2
= (-197.32) – (-141.16)
= +56.16 Kjmol-1 to 4 significant figures.
This is an approximate value as the procedure was full of holes that would allow a less accurate result being taken.
I had to weigh out the calcium carbonate and the calcium oxide in a weighing bottle on electric scales. First it was the calcium carbonate. I
placed the bottle on the scales with the calcium carbonate in and took the combined weight. To find the weight of the calcium carbonate used I took the empty bottle and took that away from the initial figure. It would have been more accurate to have placed the weighing bottle on the scales and then zeroed them, as there would have been less chance of an error due to the weight of the bottle.
The method we were given told us to weight out the calcium carbonate and the weighing bottle to a combined weight of between 2.4 and 2.6g. and not a specific amount. This means that had I have conducted out the experiment three times to find an average, it could have been different each time. To ensure the highest accuracy the best thing to would have been to be given a specific amount, or to do as I did and go in the middle, as to little calcium carbonate and the heat change may have not been enough.
I made sure the scales were clean and were zeroed before placing the bottle on them, as any added debris would have affected the weight and my final calculation. I also used the same bottle for each part, and ensured they were both clean from any particles that could have affect the final weight.
The next part of the experiment was to pour 250 cm2 of HCl in to a measuring cylinder. This part also brought about inaccuracies to the experiment. First of all the graduations on the measuring cylinder are in multiples of two. This means that when using them to measure out the solution, you could only ever be at most 2ml out. This would have bought about problems in my calculation, as the temperature change would have been affected by the amount of substance there is. This also means that when working out the calculation you could only be as accurate as the apparatus you used. The best would have been to use a more accurate piece of equipment, which would have had smaller graduations, so the calculations could be more accurate.
When measuring out the HCl I was also taking the reading from the bottom of the meniscus, as this was more accurate.
You then had to pour the HCl into a beaker. I did so, being careful not to spill any. I then quickly submerged the thermometer. I did this because the thermometer would have been at room temperature and not that of my hydrochloric acid. This would have made the values I obtained for the rise in temperature more/less, depending on where the HCl had been kept. To make this part more accurate, it would have been better to let the thermometer to adjust to the temperature of the HCl over a period time.
You then added the calcium carbonate to the hydrochloric acid and waited until the two had stopped reacting and then take the final temperature. This is where there was the biggest flaw in the method. The whole point of the experiment is to work out how much heat was given out when the calcium carbonate and the hydrochloric acid combined to help work our the enthalpy change. But when doing this, the thermometer would not have been recording the highest possible amount of heat given out.
The beaker was open topped and so the majority of the heat would have escaped out of it. To combat this a lid could have been placed on top, and to have insulated the beaker so it didn’t escape through the sides as much either. A smaller beaker may have also helped, as the heat would have been more concentrated. To improve this part of the experiment further it would have been better to have a conical flask with a bung in. the bung should have two holes, one for the thermometer and the other to let the gas escape.
There was also another point while the reaction was taking place, and that was with the thermometer. We had to record the temperature once the reaction had stopped. I took this as the point at which it stopped bubbling, but it could have been that the thermometer could have still been adjusting to the temperature in the beaker. This is due to the amount of time it takes the mercury to react. The way to improve this would have been to use an electric thermometer, which does not need time for a liquid to expand, as the values recorded electronically.
We then had to repeat this with the calcium oxide, and that method saw the same pit falls. So with both values not as accurate as they should be the final amount will also be out. If I were to do this again I would do each three times, work out the enthalpy change three times and then find the average, to give a more accurate answer.
