Thermodynamics Question

Chemistry
Tutor: None Selected Time limit: 1 Day

I am working through an experiment where 3.016g of NH4NO3(s) was dissolved in 50g of H2O in a coffee cup calorimeter. I determined the qrxn = 19.655 kJ/mol (qsoln + qcup). I am now asked to calculate the change in enthalpy for the reaction in moles using deltaHrxn = Sigma deltaHf(products) - Sigma deltaHf(reactants) and the deltaHf NH4NO3 is -495 kJ/mol. I am stuck here, blanking out on how to proceed

Jun 7th, 2015

Thank you for the opportunity to help you with your question!

The qrxn u calculated is the enthalpy change or the heat lost when the solute dissolved .it is therefore the sigma delta H of the product.divide it by the mol weight 80 to get the delta H in kj/mol unit.


Please let me know if you need any clarification. I'm always happy to answer your questions.
Jun 7th, 2015

NH4NO3 (s)   →   NH4+ (aq) + NO3- (aq)


Jun 7th, 2015

To use the formula deltaHf of each of the ions on the right hand side should be given. Do you have that?

Jun 7th, 2015

To use the formula deltaHf of each of the ions on the right hand side should be given. Do you have that?

Jun 7th, 2015

Hi, the equation given is NH4NO3(s) = NH4NO3(aq) I know that it dissociates in ions but they don't give us deltaHf for anything other than the one I outlined. I guess this is why I am confused. All of the data from the experiment that I have been using is: Wgt of NH4NO3 = 3.016g, No of moles calculated to be 0.03768 mols. 50g of H2O. T(initial) 23.5C. T(final) 20.5C. Asked to calculate heat associated with qH2O (which I took to be qsoln) and worked that out to be qsoln = 53.016g x 4.184 J/g.C X (20.5-23.5) = -665.5J, then asked for heat released/absorbed by deltaHrxn - that is -qsoln = 665.6J. Then asked for change in enthalpy of reaction = 0.666kJ. Then they wanted the calorimeter introduced and gave it a C of 25J/C. Now my enthalpy was 19.655kJ/mol. Thats when I arrive at the question I posted, and the only information I have is what I have detailed.

Jun 7th, 2015

Standard enthalpies for NH4+ (aq) and NO3-(aq) are -132.8 kj/mol and -206.6kj/mol respectively.

So Delta Hrxn = -132.8-206.6-(-495)= 

Jun 7th, 2015
Jun 7th, 2015
Jun 7th, 2015

I understand where you are getting the values for the ions and if I do that calculation then the answer is 155.6. However the next question is a %error question between this value and the value I calculated of 19.665kJ/mol. Won't that be a large %error

Jun 7th, 2015

You can get the values for standard enthalpies of the ions from any standard textbook table.

Jun 7th, 2015

You can get the values for standard enthalpies of the ions from any standard textbook table.

Jun 7th, 2015

Go to the website and make sure all calculations are correct. It is shown step by step very nicely over there.

Jun 7th, 2015

Go to the website and make sure all calculations are correct. It is shown step by step very nicely over there.

Jun 7th, 2015

Thank you for your help

Jun 7th, 2015

Qrxn is the heat change for the reaction and it is the sum of the heat changes associated with soln and the cal.

Qcal = CX(20.5-23.5)= -25x3= -75 J= -0.075 kj

This amount of heat is lost by the cal.

Qsoln= -0.665 kj

This is the heat lost by the solution .

The sum of these two is the heat aborbedaborbed by the solute to dissolve I.e. qrxn.. divide by 80 to get deltaHf of reaction.

Jun 7th, 2015

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