Name………………….Student number…………………..
CH107 COLLEGE CHEMISTRY FINAL EXAMINATION, FALL 2011
Wednesday, 21 December 2011, 09.00-10.30. Attempt all 10 questions.
1. Write down Lewis diagrams, with formal charges on all atoms, for the isomeric molecules cyanic acid (HOCN) and fulminic acid (HCNO). State which molecule is likely to be the more stable, on the basis of formal charge. (4 points)
2. Use the VSEPR theory to predict the geometry of the CH3- anion: sketch the molecule, showing all electron pairs on C, and state its shape. (4 points)
Sketch Shape: pyramidal
(or trigonal pyramidal)
(1 point)
(3 points)
3. (a) List the following in order of their rates of effusion/diffusion: (NOCl), phosgene (COCl2), sulfuryl chloride (SO2 Cl2), carbon suboxide (C3O2), and carbonyl fluoride (COF2). (2 points)
1. (fastest) 2. 3. 4. 5. (slowest)
NOCl COF2 C3O2 COCl2 SO2Cl2 (2 points)
(b) Consider the reaction, 5 NaN3(aq) + NaNO3(aq) ® 3 Na2O(aq) + 8 N2(g), that is conducted in a sealed container with a constant volume of 2350.0 mL. How many moles of N2 can be produced from the consumption of 25.0 mL of 0.826 M NaN3 with an excess of NaNO3? Calculate the pressure exerted by the nitrogen gas at a temperature of 300 K. Show working. (4 points) [R = 0.0821 L atm/mol K]
No. moles NaN3 = 0.025 L x 0.826 mol/L = 0.0207 mol. No. moles N2 = 0.0207 mol NaN3 x (8 mol N2/5 mol NaN3) = 0.0330 mol N2.
PV = nRT;
P x 2.35 L = 0.0330 mol x 0.0821 L atm/mol K x 300 K; P = 0.346 atm
(6 points). Can give partial points
Allow any similar method that gives the correct answer. Can give partial points
4. Determine whether the following statements are true or false. Write T or F in the boxes adjacent to the questions. (10 x 1points)
(a) Air fresheners can permeate an entire room because gases mix thoroughly.
(b) As the kinetic energy of a gas decreases under constant pressure conditions, the volume that the gas occupies must increase.
(c) At a fixed temperature, the average speed of N2O molecules is greater than that of CO2 molecules.
(d) Nonpolar molecules exhibit only London forces between the particles.
(e) Network solids generally have high melting points because the particles in the solid are held in a lattice structure with covalent bonds rather than intermolecular forces.
(f) The melting of a solid is an exothermic process.
(g) Insoluble ionic compounds are those in which the ion-ion forces between the ions are weaker than the ion-dipole forces between the ions and water molecules.
(h) Acid rain is caused by the reaction between water and nonmetal oxides. T
(i) In the unbalanced equation, BrO3- + N2H4 → Br- + N2, the nitrogen in N2H4 is being oxidized.
(j) In a voltaic cell, the net redox equation is Ni(s) + Pb2+(aq) → Ni2+(aq) + Pb(s) and is spontaneous. The Ni(s) will act as the cathode.
5. (a) State which attractive forces are likely to predominate in associations amongst
particles of the following substances. (4 points)
(i) Cl2 (liquid) (ii) SO2 (liquid) (iii) KH (solid) (iv) CuSO4.5H2O (solid)
(i) (ii) (iii) (iv)
London Dipole-dipole Ionic Ionic and
hydrogen (4 points)
bonding
(b) Acetic acid (CH3COOH) forms a dimer ([CH3COOH]2) in organic solvents. Draw a suitable structure for the dimer and state which intermolecule force is mainly responsible for holding the monomers together in the dimer. (3 points)
(1 point)
6. A certain element, that forms divalent cations, has a melting point of –39.0 oC and a boiling point of 357 oC. The specific heat of the liquid is 0.139 J/g oC. The heat of fusion and heat of vaporization are 11.5 J/g and 29.5 J/g, respectively.
(a) Identify the element. (1 point)
(b) Calculate the supply of heat needed (in J) to convert 156 g of the solid element,
at its melting point, to vapor, at its boiling point. (4 points)
Can give partial points
7. (a) Calculate the mass of potassium hydroxide (KOH) contained in 25.3 mL of a 0.0102 M (mol L-1) solution. Show working. (3 points)
[molar masses (g mol-1): H = 1.008, O = 15.999, K = 39.098]
Number of moles = 25.3 x 0.0102 mol
1000
Since mass (g) = mol x molar mass (g/mol), and molar mass = 56.105 g/mol,
mass = 25.3 x 0.0102 x 56.105 = 0.0145 g (3 points)
1000
Allow similar reasoning that leads to correct answer. Can give partial points
(b) If the density of the above solution is 1.010 g/mL, calculate the concentration of KOH in parts per million (ppm). Show working. (3 points)
As (a). Can give partial points
8. Write equations to represent the ionization of sulfuric acid in water. (4 points)
9. A buffer solution is prepared by mixing equal concentrations of KNO2(aq) and
HNO2(aq). Write the two chemical equations that illustrate how the buffer system
removes small amounts of added OH- (aq) and H3O+ (aq). (4 points)
(2 x 2 points)
H+(aq) can be used in place of H3O+(aq)
10. (a) Although the inert gases are generally unreactive, Kr and Xe will form compounds in the right conditions. Three reactions of inert gas compounds are shown below, (i) – (iii).
(i) 3XeF4 + 6H2O ® XeO3 + Xe + 12HF + 3/2O2
(ii) XeF6 + 3H2O ® XeO3 + 6HF
(iii) KrF2 + AsF5 ® KrF+ AsF6-
Classify reactions (A) – (C) as redox or acid-base reactions. (3 points)
(b) Sulfides react with aqueous nitric acid according to the unbalanced ionic equation below.
S2-(aq) + NO3-(aq) ® SO42-(aq) + NO(g)
Write balanced ionic oxidation and reduction half equations and hence a fully balanced ionic redox equation for this reaction. (7 points)
(a) (i) Redox (ii) Acid-base (iii) Acid-base (3 points)
(b) (In this reaction sulfide (S2-) is oxidized to sulfate (SO42-) and nitrate (NO3-) is reduced to nitric oxide (NO) in aqueous acidic solution.)
Oxidation half equation: S2-(aq) + 4H2O(l) ® SO42-(aq) + 8H+(aq) + 8e-
Reduction half equation: NO3-(aq) + 4H+(aq) + 3e- ® NO(g) + 2H2O(l)
Full ionic redox equation:
3S2-(aq) + 8H+(aq) + 8NO3-(aq) ® 3SO42-(aq) + 4H2O(l) + 8NO(g)
(2 + 2 + 3 points)