There are 2 moles of HCl (hydrochloric acid) in 1000cm³ so there are 0.1moles of HCl (hydrochloric acid) in 50.0cm³. So I will use 50.0cm³ of HCl (hydrochloric acid).
So in this experiment I would want to use 5.0 grams of CaCO3 (calcium carbonate) and 50.0cm³ of HCl (hydrochloric acid) but I want the HCl (hydrochloric acid) to be in excess so I would want 70.0cm³ of HCl (hydrochloric acid).
Method for Route 1
CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
Step 1 Measure out 70.0cm³ (an excess) of approximately 2 mol dmˉ ³ HCl (hydrochloric acid) into a polystyrene beaker using a 100cm³ measuring cylinder.
Step 2 Weigh out accurately a weighing container containing 5.0g of calcium carbonate, CaCO3 on a balance. Record the results in a table.
Step 3 Take the temperature of the acid using a 100˚C thermometer and record this value in a table. The thermometer should not be left unsupported or it may topple over and break.
Step 4 Add the weighed portion of CaCO3 to the HCl (hydrochloric acid) carefully. Immediately replace the lid of the polystyrene beaker. Then stir the mixture, then take the highest or lowest temperature and record this value in a table.
Step 5 Reweigh the weighing container on the balance and record this value in a table.
ROUTE 2
Diagram for Route 2
Apparatus Needed
The apparatus, equipment and chemicals that is needed for this experiment is:
- 100cm³ measuring cylinder
- 250cm³ beaker
- Polystyrene beaker and lid
- A weighing bottle
- Thermometer
Substance Needed
- Calcium Oxide (CaO)
- Hydrochloric acid-HCl (2.0 mol dmˉ ³)
The amount of substance I chose to use was 2.8grams of calcium carbonate because from the equation
CaO(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
The stoichiometry (stoichiometry is simply the molar ratio between any of the chemicals) of the equation which is only concerning the CaO (calcium oxide) and HCl (hydrochloric acid) is 1:2 ratio.
The Mr of CaO (calcium oxide) is 56 so I can say 1 mole of CaO (calcium oxide) is 56 grams but this amount is too big too use so I want to scale it down to 0.05moles which is 2.8 grams of CaO (calcium oxide) which is an appropriate quantity to use.
So because there is a 1:2 ratio between CaO (calcium oxide) and the HCl (hydrochloric acid) that means I want 0.1 moles of HCl (hydrochloric acid) so double the amount of CaO (calcium oxide).
There are 2 moles of HCl (hydrochloric acid) in 1000cm³ so there are 0.1moles of HCl (hydrochloric acid) in 50.0cm³. So I will use 50.0cm³ of HCl (hydrochloric acid).
So in this experiment I would want to use 2.8 grams of CaO (calcium oxide) and 50.0cm³ of HCl (hydrochloric acid) but I want the HCl (hydrochloric acid) to be in excess so I would want 70.0cm³ of HCl (hydrochloric acid).
Method for Route 2
CaO(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
Step 1 Measure out 70.0cm³ (an excess) of approximately 2 mol dmˉ ³ HCl (hydrochloric acid) into a polystyrene beaker using a 100cm³ measuring cylinder.
Step 2 Weigh out accurately a weighing container containing 2.8g of calcium oxide, CaO on a balance. Record the results in a table.
Step 3 Take the temperature of the acid using a 100˚C thermometer and record this value in a table. The thermometer should not be left unsupported or it may topple over and break.
Step 4 Add the weighed portion of CaO to the HCl (hydrochloric acid) carefully. Immediately replace the lid of the polystyrene beaker. Then stir the mixture, then take the highest or lowest temperature and record this value in a table.
Step 5 Reweigh the weighing container on the balance and record this value in a table.
Safety
Safety is very important and it is necessary to follow safety procedures. It is important to wear eye protection when working with any acid. Also long hair should be tied back. Be careful when handling glass ware and keep stools under bench so that they are not in the way.
The chemicals used are corrosive it is important to take extra care while working around them. Especially when handling with the hydrochloric acid (HCl) this is corrosive and it may cause burns when it in contact with skin. The vapour is very irritating to the respiratory system. The hazards of the chemical were taken from the hazard cards.
Accuracy
In order to reduce errors it is necessary to choose accurate and reliable equipment. That why using measuring cylinder would be a good choice. This instrument has an accuracy of ±0.05cm³, which is suitable for the experiment and should produce accurate and reliable results.
Calculations
Once I done the experiment I will use the results to calculate the enthalpy change for the decomposition of calcium carbonate (CaCO3). But first I will need to calculate the heat produced from calcium carbonate (CaCO3) reacting with hydrochloric acid (HCl) and calcium oxide (CaO) reacting with hydrochloric acid (HCl). To do this I need to use the formula:
Heat produced = mass of solution x change in temperature x specific heat capacity of
water
Once I work out the heat produced for calcium carbonate (CaCO3) and calcium oxide (CaO) I then need to calculate how many moles of calcium carbonate (CaCO3) and calcium oxide (CaO) were reacted and then determine the enthalpy change for 1 mole of calcium carbonate (CaCO3) and calcium oxide (CaO). By using the formula:
Moles = Mass
Mr
Then I can use what I just calculated to calculate the enthalpy change for the decomposition of calcium carbonate.
Results from Route 1
CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
The heat absorbed or evolved during this reaction is calculated by using the formula:
Heat produced = mass of solution x change in temperature x specific heat capacity of
water
(The specific heat capacity of solution = 4.2 J gˉ1 Kˉ1)
Heat produced = [(70 + 5.0) × 3.8 × 4.2] = 1197Joules
Moles of calcium carbonate (CaCO3) = Mass = 5.0
Mr 100
= 0.05 moles of calcium carbonate (CaCO3)
So to determine the enthalpy change for 1 mole of calcium carbonate (CaCO3)
1197 = 23940J / 23.94KJ
0.05
The answer will be negative because it is an exothermic experiment.
Answer: ˉ23.9KJ to 3 s.f
Results from Route 2
CaO(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)
The heat absorbed or evolved during this reaction is calculated by using the formula:
Heat produced = mass of solution x change in temperature x specific heat capacity of
water
(The specific heat capacity of solution = 4.2 J gˉ1 Kˉ1)
Heat produced = [(70 + 2.8) × 27.7 × 4.2] = 8469.552Joules
Moles of calcium oxide (CaO) = Mass = 2.8
Mr 56
= 0.05 moles of calcium oxide (CaO)
So to determine the enthalpy change for 1 mole of calcium oxide (CaO)
8469.552 = 169391.04J / 169.39104KJ
0.05
The answer will be negative because it is an exothermic experiment.
Answer: ˉ169.4KJ to 3 s.f
Enthalpy change for decomposition of calcium carbonate
So from the calculation above I can now work out the change in heat produced from calcium carbonate (CaCO3) to calcium oxide (CaO). In order to do this Hess’s Law needs to be used because it provides a method for the indirect determination of the enthalpy changes.
Hess’s Law is an extension of the First Law of Thermodynamics.
Hess’s Law states that, if a reaction can take place by move than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
Hess’ Law cycle to illustrate the enthalpy values.
CaCO3(s) CaO(s) + CO2(g)
∆H?
CaCO3(s) CaO(s) + CO2(g)
R2 + 2HCl acid
+ 2HCl acid R1 ˉ169.39104KJ
ˉ23.94KJ
CaCl2(aq) + CO2(g) + H2O(l)
Route 1 = Route 2
ˉ23.94 = ˉ169.39104 + ∆H
∆H = ˉ23.94 + 169.39104
∆H = +145.45104
Answer: +145.45 KJ/mol to 2 d.p.
Percentage error: 572 = 100%
145.45104 ?
? = 100 × 145.45104 = 25.4
572
100 – 25.4 = 74.6%
74.6% inaccurate
Errors
- When some acid was poured into the beaker the beaker may not have been fully clean so there could be contamination with other solutions
- When measuring the hydrochloric acid for the experiment using a measuring cylinder the meniscus may not have been exactly on the line which could have resulted in slightly higher or lower amount.
- Not all the hydrochloric acid may have run into the polystyrene beaker this would have resulted in a slightly smaller volume.
- When reading measurements there could have been human errors but I try to read as accurately as I could.
- The main problem would be the amount of heat loss into the air and into the polystyrene beaker and thermometer. The main heat loss was due to the fact when replacing the lid straight after putting the calcium carbonate or calcium oxide in. So not all the heat went into the solution the heat was being absorbed by other objects.
- When I rinsed the equipment I use to clean it for the next experiment calcium oxide with hydrochloric acid I did not wait for it to be dry before I carried out the experiment this may have resulted in a slight change of concentration of the solution.
- When adding hydrochloric acid into the polystyrene beaker there would have been tiny splashes where the solution gets onto the wall of the polystyrene beaker so not all the solution is being use. This might have a slight effect on the temperature.
- The parallax error although I tried to avoid this but still the error could of crept in.
From my calculations I can see that my value of change in heat is about 4 times smaller (+145.45KJ/mol) than the actual value which is 572KJ/mol. This could be because:
- The experiment was not done under standard conditions of temperature and pressure (298K/100Pa) so that meant my value would be different from the actual values for this experiment.
- The main different in value would be due to heat loss cause so much heat is loss when replace the lid to the polystyrene beaker after the calcium oxide and calcium carbonate put in. Also through the actual polystyrene beaker itself, the thermometer and the hole where the thermometer is put through.
- Not all the heat goes into the solution some would go into the thermometer, the polystyrene beaker so the heat is being absorbed by other objects instead of the solution.
- The concentration of the calcium oxide or calcium carbonate may not have been exactly right.
- Not all of the calcium oxide reacted in the second experiment cause there was still calcium oxide in the beaker at the end. That meant that the enthalpy value I got for calcium oxide is wrong.
- Because there were so much errors in both experiments that meant the final change in heat for the decomposition of calcium carbonate was very inaccurate (74.6%) due to all the factor mentioned above.
Improvements
- If equipment rinsed wait till it’s dry before using it.
- Use a container which will not absorbed as much heat as the polystyrene beaker does
- Maybe conduct the experiment in a close container where not so much heat can escape into the air.
- Or extend the experiment by doing exactly the same method but timing it so by recording down the temperature for every 30 seconds. This could be done by computer which would reduce the error of reading the thermometer wrongly.
Bibliography
A-level chemistry by E.N.Bamsden
Chemistry 1 by OCR
Internet
