Two separate thermometers have to be used so that the experiment does not get tainted.
A different mass is used for each experiment so that the stoichiometry is the same as HCl acid.
The same equipment is used in each experiment so that the experiment is fair. A different polystyrene cup is used for each experiment.
Specific conditions
Two different thermometers are used so that the experiment is not contaminated with. By this I mean using one thermometer for one experiment and then transferring the same thermometer to the second experiment. This could affect the end result so I am using a different thermometer.
Two different polystyrene cups are used for the same above reason.
Safety
Wear goggles as we are using acid, so it won’t get into your eyes.
If you break any glass make sure you clean it up straight away as it could be a danger to you and others.
If you are cut by glass rinse the cut under water immediately as it could be dangerous especially if you are dealing with chemicals.
If there are any chemical spillages on the table tell the lab technicians, unless you know what it is don’t attempt to clean it up, and make sure people who are working around the spillage are aware of it.
Wear a lab coat as any chemicals could lead to damage on clothing.
Calculation
I am using 50cm³ of HCl acid so the stoichiometry has to be 1:1. To find the no of moles of HCl acid used, I divide the amount used by 1000 and times by the molarity
50 x 2 = 0.1 moles
1000
I have used 0.1 moles of HCl acid.
To get the no of moles of sodium hydrogen carbonate I need to find the Mr of NaHCO3. The Mr is 84.
I will now use the following formula to find the mass of NaHCO3.
Mass = Moles x Mr
- X 84
Mass = 8.4g
The mss of Sodium hydrogen carbonate I need is 8.4g.
I will now find the amount of Sodium Carbonate.
In this reaction I am using twice as much HCl acid so I will need to divide it by two top ensure I have the same no of moles in each reaction
100 x 2 = 0.2 moles
1000
0.2/ 2 = 0.1 moles.
The Mr of Na2CO3 is 106. I will need to divide this by 2, because the stoichoimetry is 1:2. So if I have divided HCl acid by 2 I will also need to divide the mass of sodium carbonate by 2.
106 x 0.1 = 10.6
10.6 = 5.3g
2
The mass of Sodium carbonate I need is 5.3g.
Method.
Measure out approx 8.4g of sodium hydrogen carbonate into a weighing boat. Then fill the 50cm³ measuring cylinder with HCl acid to the mark. Then pour the HCl acid into the polystyrene cup. Measure the temperature for a couple of minutes so you will get a steady temperature and then pour in some of the sodium hydrogen carbonate, a couple of seconds later add the rest of the sodium hydrogen carbonate. This will stop it from over filling so the reactants stay in the polystyrene cup. Then measure the temperature until the reaction stops and note down its lowest peak.
For the Sodium Carbonate experiment, you will need to measure out approx 5.3g of sodium carbonate, into a second weighing boat. You will then need to fill up the measuring cylinder up to the mark. Then pour it into the second polystyrene cup and add about half of the sodium carbonate then a few seconds later add the rest. Then measure
the temperature until the reaction stops and note down the highest peak.
Errors
There were some errors, which occurred in both experiments, which could have had an effect on the end result, these were as follows:
When I added the powder to the polystyrene cup some stayed in and it could have only been taken out using distilled water. So the amount I weighed out is not the same as the amount that reacted.
Also when I wanted to take a temperature reading down I had to pick up the thermometer so my body heat could have affected the end result. Also I used the thermometer as a stirrer.
When the powder was added a lot of heat was lost to the surrounding when the exothermic reaction occurred, and heat could have also been taken in during the endothermic reaction.
Improvements
The improvements I could have made in this experiment are as follows:
I could have used a mechanical stirrer to ensure that the reactants were mixed at the same speed. Also I could have used a colorimeter to show when the reaction ended by a colour change. I could have used a cup lid so during the endothermic reaction the heat stayed in the cup longer, and during the endothermic reaction it did not take heat in from the surroundings. I could have also used a different cup material, to see I would have an effect on the overall temperature change. I could have also repeated the experiment twice so I could see that if the experiment was right, by comparing the end points.
Results
NaHCO3
Mass of HCl -50g
Specific heat capacity- 4.2Jgˉ¹ Kˉ¹
ΔT- 10°C
ΔH= 4.2gˉ¹°Cˉ¹ x 50g x 10°C = 2100J
We then divide this by the no of moles used = 2100 = 21000J
0.1
21000 = 21KJ mol ˉ¹
1000
ΔH= - 21KJ mol ˉ¹
Na2CO3
Mass of HCl used – 50g
Specific heat capacity – 4.2gˉ¹°Cˉ¹
ΔT- 7°C
ΔH= 4.2gˉ¹°Cˉ¹ x 50g x 7°C = 1470J
We then divide by the no of moles used 1470 = 29400
0.05
29400 = 29.4KJ molˉ¹
1000
ΔH= - 29.4 KJ molˉ¹
It is a minus as it is a endothermic reaction.
2NaHCO3 = Na2CO3 + CO2 + H2O
In order to get the enthalpy change for the above equation I would need to multiply the sodium hydrogen carbonate reaction by 2 and multiply the sodium carbonate reaction by –1. and add the two numbers together.
Sodium hydrogen carbonate
ΔH= 21 x 2 = 42KJmolˉ¹
Sodium carbonate
ΔH= - 29.4 x –1 = 29.4KJ molˉ¹
42 + 29.4 = 71.4 KJ molˉ¹
The enthalpy change for the decomposition of sodium hydrogen carbonate is 71.4 KJ molˉ¹.
Bibliography
Chemistry 1- Brian Ratcliff, Helen Eccles, David Johnson. John Nicholson, John Raffan
Series Editor- Brian Ratcliff
www.yk.psu.edu
