Calorimetry: q NH4NO3 Dissolving + Exothermic/Endothermic?

[webz]

Homework Statement

20g of NH4NO3 (MM=80.05 g/mol) if dissolved in 125g of water in a coffee-cup colorimeter, the temperature falls from 23.5C to 13.4C. Determine 'q' for the dissolving of the compound. Is the process exothermic or endothermic?

Homework Equations

q(calorimeter)=C(calorimeter)*[Delta]T

The Attempt at a Solution

[Delta]T= -10.1
C(calorimeter)=~n(water)*C(water) = 125g H2O/18.016g/mol = 6.9382mol H2O
Ccal=~6.9382mol H2O* 75.291 J/mol C = 522.4 J/C
qcal=52.4J/C*-10.1C= -5.28kJ
q(calorimeter)= -q(chemical) = +5.28kJ


This answer makes sense, however, I don't understand why I am not taking into account the amount of moles NH4NO3. A larger amount of moles would make a larger deviation in T, which would change q.
Why are moles NH4NO3 not taken into account? Also, I'm using the heat capacity (C) of water, not of NH4NO3. The question is asking for 'q' for the dissolving compound. Is this correct?

 

[Borek]

To be precise you should take into account heat capacity of the solution - not water nor amonium nitrate. It is solution that changes temperature (well - this is more complicated - at first it is pure water that changes temperature, later it is solution, but it doesn't matter at the end). You are not given heat capacity of the solution, so you used heat capacity of water, which is the best approximation available to you.

You were asked to determine amount of heat needed for dissolving the compound - and looks like you did OK.

Number of moles will be required if the question asked for molar heat of dissolution - but it didn't.

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[Blueshift5]

I would like to clarify a few things. Firstly, the formula used for calorimetry is q = mCΔT, where q is the heat transferred, m is the mass of the substance, C is the specific heat capacity, and ΔT is the change in temperature. In this case, we are dealing with a coffee-cup calorimeter, so the heat transferred is equal to the heat absorbed by the water and the calorimeter.

In this experiment, we are dissolving 20g of NH4NO3 in 125g of water. The change in temperature is from 23.5C to 13.4C, which means that the water has lost heat to the surroundings. This indicates that the process is exothermic, as the heat is being released from the system.

To determine the heat transferred, we need to take into account the specific heat capacity of both NH4NO3 and water. The formula for this would be q = mNH4NO3*C(NH4NO3)*ΔT + mH2O*C(H2O)*ΔT. We can assume that the specific heat capacity of NH4NO3 is similar to that of water, so we can simplify the equation to q = (mNH4NO3 + mH2O)*C(H2O)*ΔT.

In this case, the mass of NH4NO3 is 20g and the mass of water is 125g, so the total mass is 145g. Plugging this into the equation, we get q = (20g + 125g)*4.18 J/gK*(-10.1C) = -5.24 kJ.

Therefore, the heat transferred for this process is -5.24 kJ, which means that the dissolving of NH4NO3 is an exothermic process. It is important to take into account the specific heat capacities of all substances involved in the reaction to get an accurate value for the heat transferred.