Calculating Enthalpy in a Calorimeter Experiment- PLEASE HELP!

Question

When 3g of Ammonium Chloride is added to 10ml of water in a calorimeter, the temperature over a five-minute period drops from 27.1 degrees celsius to 19.8 degrees celsius. The table of the results can be portrayed below:

Ammonium Chloride + Water Experiment 10ml
Time (min)	Trial 1 (°C)	Trial 2 (°C)	Trial 3 (°C)	Average (°C)
0min	21.4	21	21.4	21.26
1min	18.7	18.7	18.7	18.7
2min	18.9	18.7	18.5	18.7
3min	18.7	18.9	18.8	18.8
4min	18.7	19.1	18.9	18.9
5min	18.8	19.2	19	19

I was wondering what assumptions and scientific information can be made on why the reaction is endothermic and how to calculate the enthalpy of the reaction? Also, I was wondering if it's possible and necessary to do mole calculations based on the data. I have done the experiment also testing 50ml, and 100ml to see the effect of the amount of water on the reaction.

Answer 1

The assumptions to be made would be that no heat is lost from the calorimeter and that the density of the water is 1 g/ml.

You can tell the reaction is endothermic because the temperature of the water DECREASES, meaning heat is going from the surroundings into the reaction.

To calculate the enthalpy of the reaction we will use the equation...

q = mC∆T

q = heat = ?

m = mass of water = 10 ml x 1 g/ml = 10 g

C = specific heat of water = 4.184 J/gº

∆T = change in temperature = difference between average initial and final temperatures = 21.3 - 19.0º = 2.3º

q = (10 g)(4.184 J/gº(2.3º) = 96.2 Joules = enthalpy of reaction

If you want the molar enthalpy, then we convert 3 g AlCl3 to moles AlCl3...

molar mass AlCl3 = 133 g/mol

3 g x 1 mol/133 g = 0.0226 moles

Molar enthalpy = 96.2 J / 0.0226 moles = 4257 J/mol = 4 kJ/mol (1 sig. fig.)

Answer 2

Hello, Seta,

The reaction is considered endothermic because it absorbed energy from the water, as is evidenced by the drop in water temperature. The enthalpy can be calculated without converting to moles. It could be expressed in a per gram unit, such as J/gC. Tjhe Joules can be determined by using the specific heat of water and the temperature drop for the experiment. Specific heat for water is 4.184 J/gK. (It takes 4.184 Joules to raise the temperature of water by 1 degree K.).

Now we can calculate the amount of energy absorbed by the ammonium chloride dissolution. The temperature drop average 2.26C, which is also 2.26K since it is a difference. With 10g of water, we find that 94.56 Joules of heat was added to the water:

(10g)*(4.184J/gK)*(2.26K)= 94.56 Joules

This heat came from dissolving 3 grams of ammonium chloride in the water. So we could express this as 94.56J/3 grams, or 31.5 J/g for this solution reaction. No moles required in the calculation. :(

Bob