1.  Define and give an example of ANY THREE of the following

(A) a Weak Electrolyte: a compound that is partially ionized in solution
                                         example: any weak acid or base, HF, HCH₃COO, etc
(B) an Oxidizing Agent: a substance that reacts by gaining electrons (from another substance)
                                      example: any metal cation (preferably low in the Activity Series) or a non metal such as O₂, Cl₂ etc
(C) a State Function: a property of a system, the value of which depends only upon the state or condition of the system (and not how it came to be in that state).
example: H, E, etc
(D) an Endothermic Process : a process in which the system gains heat (or enthalpy) from the surroundings.
example: melting of a solid, evaporation of a liquid, H₂ ==> 2 H, etc
2 pts per definition, 2 pts for a specific example

2.  Write complete, balanced equations in net ionic form for the reactions that occur when

 

(A) powdered aluminum metal is added to a solution of cupric nitrate

2 Al + 3 Cu²⁺ ==> 2 Al³⁺ + 3 Cu          DF = oxidation-reduction

(B) a solution of potassium carbonate is added to one of lead(II) nitrate

CO₃^2- + Pb²⁺ ==> Pb CO₃                   DF = precipitation (metathesis)

(C) silver carbonate is mixed with dilute hydrochloric acid

Ag₂CO₃ + 2 H⁺ + 2 Cl^- ==> 2 AgCl + H₂O + CO₂   DF = precipitation/formation of weak or                 nonelectrolyte/formation of a gas
In each case, state what is the driving force for the reaction.

eqn: 2 pts;  balancing: 1 pt;    DF; 1 pt

3. Sketch an energy level diagram for the hydrogen atom according to the Bohr model, showing the ground state and several excited states, together with the corresponding values of quantum number n.
Sketch:  any reasonable facsimile of Figure 6.15 in Text  (4 pts)
What is the significance (meaning) of n in this model?
indicates distance of electon from nucleus Or size of allowed electron orbit (2 pts)
How does this model explain the appearance of four sharp lines (the Balmer Series) in the visible region of the emission spectrum of atomic hydrogen?
Electron jumping from orbit of higher to lower n-value. Energy difference is emitted as a photon (E = hn). For Balmer lines transitions are to n = 2 level. (4 pts)

4. A gas reaction carried out in a cylinder under constant pressure releases 126 kJ heat to the surroundings and does 48 kJ of work in moving the piston against the external atmospheric pressure. What are the values of H and E for this process?

DE = q + w     DH = q_P = -126 kJ
DE = -126 + (-48) = -172 kJ                                                       6 pts

5. In the laboratory 6.67g of Sr(NO₃)₂ is dissolved in enough water to form 0.750 L. A 0.100-L sample is withdrawn from this stock solution and titrated with a 0.0460 M solution of Na₂CrO₄. What volume of Na₂CrO₄ solution is required to precipitate all the Sr²⁺(aq) as SrCrO₄? [Homework Problem 4.56]

molwt  Sr(NO₃)₂ = 87.6+2(14+48) = 211.6
6.67g Sr(NO₃)₂ = 6.67/211.6 = 0.0315 mol
molarity of Sr(NO₃)₂ is 0.0315/0.750 = 0.0420 M
0.100 L contains 0.0420 x 0.100 = 0.00420 mol Sr(NO₃)₂
Sr(NO₃)₂ + Na₂CrO₄ ==> SrCrO₄ + 2 NaNO₃
Since equation shows equal numbers of moles required
0.00420 mol Na₂CrO₄ needed
0.0460 x V = 0.00420
V = 0.00420/0.0460 = 0.0913 L = 91.3 mL                                                    10 pts

6. It has been suggested that, in the stratosphere, hydroxyl radicals (HO) react with chlorine molecules according to the following equation,

 
HO(g) + Cl₂g) ==> HOCl(g) + Cl(g)
 

Use Hess's Law to calculate DH for this reaction given the following data
 

Cl₂(g) ==> 2 Cl(g)      DH = 242 kJ                                                  (1)
 

H₂O₂ (g) ==> 2 HO(g)     DH = 134 kJ                                            (2)
 

H₂O₂ g) + 2Cl (g) ==> 2HOCl (g)     DH = -209 kJ                         (3)

(1/2 eq 3)    1/2 H₂O₂ + Cl ==> HOCl                DH = -104.5 kJ
(-1/2 eq 2)         HO ==>  1/2 H₂O₂                    DH = -67 kJ
(eq 1)            Cl₂ ==> 2 Cl                                    DH = 242 kJ
Sum:       1/2 H₂O₂ + Cl + HO + Cl₂ ==> HOCl + 1/2 H₂O₂ + 2 Cl

     DH = -104.5 -67 + 242 = +70.5 kJ                                       6 pts
 

7. Mercury vapor street lights emit the atomic spectrum of mercury. One of the lines in the blue region of this spectrum has a wavelength of 435.8 nm.
(A) What is the frequency of this line?

n = c/l = 3.00 x 10⁸/435.8 x 10^-9 = 6.88 x 10¹⁴ s^-1

(B) What is the energy (in kilojoules) of a mole of 435.8-nm photons?

E = hn = 6.63 x 10^-34 x 6.88 x 10¹⁴ x 6.02 x 10²³ = 2.75 x 10⁵ J = 275 kJ/mol

(C) The "Work Function" of a metal is the energy that must be supplied to eject electrons from the surface. When potassium metal is irradiated by 435.8-nm photons the ejected electrons ("photoelectric effect") are found to have an average maximum kinetic energy of 53 kJ/mol. Calculate the work function of potassium in kJ/mol.

275 = work function + k.e. of electrons/mol

= work function + 53 kJ

work function = 275-53 =222 kJ/mol

8. (A) Sketch a p-orbital for the hydrogen atom. What is the value of the quantum number l, for this orbital, and what property of the electron does it determine?

Sketch = some reasonable facsimile of Figure 6.22(b) of Textbook.   (3 pts)

l = 1; (1 pt)        measures angular momentum of the electron  (2 pts)

(B) Use your sketch to explain what you understand by "electron density" and "node".

ED:  probability of finding the electgron at a defined position. The ED is high inside the orbital shown.  (2 pts)

Node: a region in space where the electron density is zero. For a p-orbital there is a node at the origin of the coordinate system.      (2 pts)

9. When 0.150 g of liquid octane, C₈H₁₈ (), was burned in a calorimeter (of negligible heat capacity) containing 1.500 kg water, the temperature of the water rose from 25.246 to 26.386C.
(A) Calculate the Standard Enthalpy of Combustion of octane.

molwt octane = 8(12)+18(1) = 114
0.150 g = 0.150/114 = 0.00132 mol octane
heat gained by the water = 1500 x 4.184 x (26.386-25.246 = 7155 J = 7.155 kJ
heat liberated by 1 mol octane = 7.155/0.00132 = 5420 kJ

DH of combustion = -5420 kJ/mol      (5 pts)
(B) From your answer to part (a) and the enthalpies of formation of CO₂ (g) and H₂O (), determine the Standard Enthalpy of Formation of octane.

C₈H₁₈ + 25/2 O₂ ==> 8 CO₂ + 9 H₂O

DH = -5420 = 8(-394) + 9(-286) - DH_f octane
 DH_f octane = -3152-2574+5420 = -306 kJ/mol      (5 pts)

10.  Write chemical equations, balanced in net ionic form, for the following aqueous reactions. In case of no reaction, write "NR" for the right hand side.
2 pts each (-1 for not balanced)

(A) H₂ + Pt(NO₃)₂ ==>
H₂ + Pt²⁺ ==> 2 H⁺  + Pt

(B) Cu + 4HNO₃ ==> Cu(NO₃)₂ + 2NO₂ + 2H₂O

Cu + 4 H⁺ + 2 NO₃^-  ==> Cu²⁺ + 2 NO₂ + 2 H₂O

(C) Cr(OH)₃ + H₂SO₄ ==>

Cr(OH)₃ + 3 H⁺ ==> Cr³⁺ + 3 H₂O

(D) Hg + Cr₂(SO₄)₃ ==>

Hg + 2 Cr³⁺ + 2 SO₄^2- ==> NR

(E) MnSO₄ + Ca(CH₃CO₂)₂ ==>

SO₄^2- + Ca²⁺ ==> CaSO₄

(F) 2KMnO₄ + 5CaCl₂ + 8H₂SO₄ ==> K₂SO₄ + 2MnSO4 + 5CaSO4 + 5Cl₂ + 8H₂O

2 MnO₄^- + 5 Ca²⁺  + 10 Cl^-- + 16 H⁺ + 5 SO₄^2- ==> 2 Mn²⁺ + 5 CaSO₄ + 5 Cl₂ + 8 H₂O

11. World Series Bonus. Let us note that Major League pitchers can throw a baseball at speeds up to 40 m/s (ca 95 mph). What wavelength is associated with a 140-g baseball at these speeds?

[Remember that 1 joule = 1 kg m² s^-2 ] {2 + 3%}

l = h/mv = 6.63 x 10^-34 x 0.140 x 40 = 1.2 x 10^-34 m
 

Avogadro's Number 6.02 x 10²³

Planck's Constant 6.36 x 10^-34 J-s  (This was a typo for 6.63 x 10^-34 - we accepted answers based on both values!)

Specific heat of water 4.184 J/g-K

de Broglie relationship = h/mv

Rydberg constant, R_H 2.18 x 10^-18 J
 

Standard Enthalpies of Formation (H_f) in kJ/mol:

H₂O () -286

CO₂ (g) -394