University:
Boston University
Course:
CAS CH 102 | General Chemistry 2
Academic year:

2020
Views:

125

Pages:

4
Author:

Lorelai Knox

TP “In my lecture I was told that change in enthalpy is products minus reactants, but my discussion leader told me that it is reactants minus products. I’ve also looked in another chemistry book and it say products minus reactants. Which is right?” 25% 25% 25% 25% 1. 2. 3. 4. products minus reactants reactants minus products both neither • Using ΔfH o’s to compute any ΔrH 2 H2(g) + O2(g)  2 H2O(g) Using ΔfH o’s to compute any ΔrH Sketch the enthalpy diagram for this reaction, labelling then enthalpy change as Δ1 H . 7 1 2 H2(g) + O2(g)  2 H2O(g) 2 H2(g) + O2(g)  2 H2O(g) Add “2 H2O l ” to your diagram. Add an arrow to your enthalpy diagram for 2 H2 g and label the arrow Δ2H. 2 H2(g) + O2(g)  2 H2O(g) TP Based on your diagram, Δ3H is … Add an arrow to your enthalpy diagram from 2 H2O l to 2 H2O g , and label the arrow Δ3H. 14% 14% 14% 14% 14% 14% 14% Since Δ1H Δ2 H Δ3 H Δ 1 H – Δ2 H Δ3 H , … 1. 2. 3. 4. 5. 6. 7. O2 g  2 H2O l , ΔfH H2O, g 2 ΔfH H2O, g ΔvapH H2O 2 ΔvapH H2O –ΔvapH H2O –2 ΔvapH H2O Something else 2 2 H2(g) + O2(g)  2 H2O(g) 2 H2(g) + O2(g)  2 H2O(g) Use your diagram to write the expression for ΔvapH H2O in terms of Δ1H and Δ2H. Use your diagram to write the expression for ΔvapH H2O in terms of the enthalpies of formation of liquid and gaseous water. ΔvapH H2O ½ Δ1 H Δ2 H 2 H2(g) + O2(g)  2 H2O(g) The enthalpy of formation of gaseous water is –241.8 kJ and that of liquid water is –285.8 kJ. Evaluate ΔvapH H2O . ΔvapH H2O … ΔfH H2O, g ΔfH H2O, l –241.8 kJ – –285.8 kJ 44.0 kJ ΔvapH H2O ΔfH H2O, g ΔfH H2O, l Using ∆fH˚’s to compute ∆rH 4 NH3 g 4 NH3 g  2 N2 g 6 H2 g 5 O2 g  5 O2 g 2 N2 g  4 NO g 2 O2 g 3 O2 g  6 H2O g 6 H2 g 5 O2 g  4 NO g ΔH 1 ΔH 2 ΔH 3 ΔH 4 6 H2O g 4 ΔHf NH3, g 0 4 ΔHf NO, g 6 ΔHf H2O, g Hess’s law: ΔrH ΔH1 ΔH2 ΔH3 ΔH4 4 ΔHf NO, g 6 ΔHf H2O, g 4 ΔHf NH3, g Note: Elements in standard states are absent 3 Using ∆fH˚’s to compute any ∆rH The key feature of standard enthalpies of formation is that, the enthalpy change of any reaction, ΔrH, can be computed using them, as … Δr H Sum ΔfH products – Sum ΔfH reactants TP “In my lecture I was told that change in enthalpy is products minus reactants, but my discussion leader told me that it is reactants minus products. I’ve also looked in another chemistry book and it say products minus reactants. Which is right?” 25% 25% 25% 25% 1. 2. 3. 4. products minus reactants reactants minus products both neither 4