Calculating Enthalpy, Entropy, and Molar 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.

Furthermore, I was wondering what the difference between enthalpy, molar enthalpy, and entropy is and how it can be useful for this experiment?

Answer 1

You state that the temperature drops from 27.1 degrees to 19.8 degrees, but the data show an average starting temperature of 21.3º, not 27.1º. At any rate, to answer your question, the assumptions made are that no heat is lost to the surroundings and that the density of water is 1 g/ml so you can convert mls to grams.

The reaction has to be endothermic because the temperature of the water DECREASES meaning heat is going from the water (surroundings) into the system (salt dissolving).

Enthalpy of reaction is calculated from q = mC∆T where q = heat, m = mass of water, C = specific heat of water and ∆T = change in temperature. And yes, it is possible to do mole calculates by converting 3 g ammonium chloride to moles and then dividing the value of q by the number of moles to obtain molar enthalpy. Is it necessary to do mole calculations, the answer is NO.

The only difference between enthalpy of reaction and molar enthalpy is as explained above, where in one case you simply calculate q (enthalpy of reaction) and in the other case you correct for the moles present (molar enthalpy).

Entropy is a completely different measurement and is related to enthalpy via Gibbs free energy and temperature.