In my AP Chem class we are working on testing Hess's Law and conducted three reactions. Then enthalpy changes of the 1) NaOH+HCl and 2) NaOH+NHX4Cl to predict the enthalpy change of 3) HCl+NHX3.
I used the equation q=CpmΔT. We determined the q-values using a method given to us by our teacher. She told us to assume a denisty of 1.03 g/mL for all solutions, we then multiplied the density by 50 mL to determine the grams of each reactant present. That value is the m term. We used 4.18 J/(g ∘C) for the specific heat of water. The temperature change for the first reaction was 14.7 ∘C, the second was 1.2 ∘C, and the third was 9.8 ∘C.
The first reaction yielded a q value of 6328.938 J, the second yielded 516.648 J, and the third yielded 4219.292 J.
The next step would be to convert this q values into kJ/mol. I consulted my lab partner who did the following operation:
kJ/mol=6328.938 J×1 kJ/1000 J×x/0.2 mol
In his math, there was no numerator on the 0.2 mol term. The mol value he used came from there being 0.2 moles of the reactants present. I believe his math to be incorrect, but as my teacher is out sick and there are no other chemistry teachers present, I cannot determine the correct conversion. I believe it to be:
kJ/mol=6328.938 J×1 kJ/1000 J×6.02223/0.2 mol
Both of the conversions yield values with significant % error. Any assistance is appreciated.
