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

2021
Views:

487

Pages:

2
Author:

Emilio Nash

1. (2 points) Write the chemical reaction for which ΔrH = 4ΔfH (H2O(l)) – 2ΔfH (CH3OH(g)) + 2ΔfH (CO2(g)). 2 CH3OH(g) + 3O2(g) → 2CO2(g)+ 4H2O(l) 2. (3points) The enthalpy of formation of NH3(g) is -45.9 kJ/mol, the H2 bond enthalpy is 436 kJ/mol, and the N2 bond enthalpy is 945 kJ/mol. Calculate the N-H bond enthalpy. ½ N2(g) + 3/2 H2(g)  NH3(g) 1 2 3 2 ×ΔbH(N≡N) + ×ΔbH(H-H)−3× ΔbH(N-H)= ΔfH (NH3(g)) 1 3 1 2 ΔbH(N-H)= × ( × 945𝑘𝐽 𝑚𝑜𝑙 + 3 2 × 436𝑘𝐽 𝑚𝑜𝑙 − (− 45.9𝑘𝐽 )) 𝑚𝑜𝑙 = 390.5 kJ/mol ΔbH(N-H) = 391 kJ/mol 3. Below is an enthalpy diagram for the following reaction: 2HNO(g) H2(g) + O2(g) + N2(g) 2H-N=O(g)  NN(g) a. ∆rH = -218 kJ/mol + H–H (g) +O=O(g) (2 points) Using HN0(g), H(g), O(g), N(g) and their stoichiometric coefficients, fill in the species for the horizontal lines. 1: 2H(g) + 2O(g) +2N(g) 2: H2(g) + O2(g) + N2(g) (3 points) For the labeled arrows on the diagram above, write the expression for the enthalpy change in terms of only bond enthalpies and enthalpies of formation. A: 2∆fH(HNO(g)) B: ∆bH(H-H)+ ∆bH(O=O )+∆bH(NN) B Enthalpy b. 1 C H2HNO (g2(g) ) 2(g) + O A D 2 C: −2 ∆bH(H-N) − 2∆bH(N=O) 1 1. (2 points) Write the chemical reaction for which ΔrH = 3ΔfH (H2O(l)) – ΔfH (C2H5OH(g)) + 2ΔfH (CO2(g)) C2H5OH(g) + 3O2(g) → 2CO2(g)+ 3H2O(l) 2. (3 points) The enthalpy of formation of H2O(g) is -241.8 kJ/mol, the H2 bond enthalpy is 440.0 kJ/mol, and the O2 bond enthalpy is 498.0 kJ/mol. Calculate the O-H bond enthalpy. ½ O2(g) + H2(g)  H2O(g) 1 2 ×ΔbH(O=O) + ΔbH(H-H)−2× ΔbH(O-H)= ΔfH (H2O(g)) 1 1 ΔbH(O-H)=2 × (2 × 498𝑘𝐽 𝑚𝑜𝑙 + 440𝑘𝐽 𝑚𝑜𝑙 − (− 241.8𝑘𝐽 ))=465.4 𝑚𝑜𝑙 kJ/mol ΔbH(O-H)= 465.4 kJ/mol 3. Diazene, N2H2, is a useful organic reagent. Below is an enthalpy diagram for the following reaction: N2H2(g)  N2(g) + H2(g) H–N=N–H(g) ∆rH = -200 kJ/mol ⟶ H–H (g) + NN(g) a. (2 points) Using N2(g), H2(g), N(g), H(g) and their stoichiometric coefficients, fill in the labels for the following horizontal lines. 1: 2H(g) +2N(g) 1 2: N2(g) + H2(g) b. (3 points) For the labeled arrows on the diagram above, write the expression for the enthalpy change in terms of only bond enthalpies and enthalpies of formation. N2H2(g) A: ∆fH(N2H2(g)) 2 B: ∆bH(H-H)+ ∆bH(NN) C: −2 ∆bH(H-N) − ∆bH(N=N) 2