Chemistry 65.100 A, V
Midterm Test #2
November 3, 2000
A periodic table is on the back of this sheet. Please take the question
sheet with you.
Part A. Answer each question with a few sentences or equations where necessary. (5 Marks each)
1. What is the oxidation number of the Cr atoms in the dichromate ion, Cr2O7-2?
2. How can phosphorus form compounds such as PH5, which clearly has more than
an octet of electrons around the P atom?
3. Will the reaction 2 H2(g) + O2(g) ¾
2 H2O(g) have a positive or a negative value of DS?
Why?
4. When a molecule of O2 is ionized to O2+, the bond
length decreases. What can we say about the molecular orbital from which the electron was
removed?
5. What type of hybrid orbitals does boron use in the molecule BF3?
6. Reaction A has DH = -100 kJ mol-1. Reaction B has
DH = -200 kJ mol-1. From this information alone,
what can we say about the rates of these two reactions?
Part B. Answer each question. The best two will be used to calculate your mark (20 marks each).
1. Using VSEPR theory, predict the shapes of the following species. (Advice: don't guess - follow the rules.) Elecronegativities of the atoms are: As 2.0; H 2.1; Cl 3.0; Br 2.8; C 2.5
(a) AsH2+
(b) AsCl5
(c) CBr4
(d) BrCl2-
2. Balance the following REDOX reaction in a basic solution:
Cr2O7-2(aq) + Bi+3(aq) ® Cr+3(aq) + BiO3-(aq)
3. Calculate DH (in kJ) for the reaction:
2 N2(g) + 5 O2(g) ® 2 N2O5(g)
given the following data:
N2O5(g) + H2O(l) ® 2 HNO3(l) DH = -77 kJ
1/2 N2(g) + 3/2 O2(g) + 1/2 H2(g) ® HNO3(l) DH = -174 kJ
H2(g) + 1/2 O2(g) ® H2O(l) DH = -286 kJ