Nya Winter 08 Unit Test 2

John Abbott College 202-NYA S28 Winter 2008 CLASS TEST 2 (GREEN) 1. Phosphorous trifluoride and phosphorous pentafluoride are both known compounds. However nitrogen, which is in the same family as phosphorous, can only form nitrogen trifluoride. a. Why? (4 MARKS)

b. Draw all three compounds in three dimensions. Indicate the electronic and molecular structure for each one. Indicate the hybridization of the central atom. (6 MARKS)

c. Draw a partial orbital-box diagram for the nitrogen atom in nitrogen trifluoride, before and after hybridization. (3 MARKS)

2. a. Place the following ionic compounds in order of increasing lattice energy(2MARKS): i. MgO(s) ii. K2O(s) iii. SrO(s)

b. Explain your choice. (3 MARKS)

3. In three dimensions, draw two isomers of the compound C2H4O. Indicate the hybridization of the oxygen atom in each case. (6 MARKS)

4. Draw reasonable resonance structures for the thiocyanate ion NCS- . Which one is favoured? (4 MARKS)

5. In the lab, you are asked to determine the enthalpy of formation of strontium oxide. The following data table is provided. Heat of formation of liquid water Sublimation of strontium st 1 ionization energy of strontium 2nd ionization energy of strontium 1st electron affinity of oxygen Crystal lattice energy of SrO Bond energy of oxygen

-286 kJ/mol 164 kJ/mol 594 kJ/mol 1045 kJ/mol -141 kJ/mol -3320 kJ/mol 498 kJ/mol

Using calorimetry, the following data are obtained: SrO(aq) + 2HCl(aq) → SrCl2(aq) + H2O(l) ΔHf = 94 kJ/mol Sr(s) + 2HCl(aq) → SrCl2(aq) + H2(g) ΔHf = -400 kJ/mol

a. Use Hess’s Law to determine ΔHf for strontium oxide from its elements. (4 MARKS)

b. Construct a Born-Haber Cycle for the formation of strontium oxide and determine the second electron affinity for oxygen.(10 MARKS)

c. Is the second electron affinity for oxygen endothermic or exothermic? Explain. (3 MARKS)

6. Consider the following compounds: H2(g), HF(g) and F2(g). Which of these compounds is polar? Draw Lewis structure for the polar compounds and indicate the direction of the dipole moment. (5 MARKS)