Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chemistry 107

Chem 107

Hour Exam II
  Name
Fall 1996

(100 pts)
  Lab Section (circle): Tu Th

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(12) 1. Write Lewis Dot Structures for each of the following:

 a. Si

 

 

  b. CaO  

 c. H2CO

 

 

  d. HCN  


(12) 2. Consider H2CO & HCN (c & d above). What is:
 The steric number for: c. 3 (for carbon) d. 2
 The electronic geometry for: c. trigonal planar d. linear
 The molecular geometry for: c. trigonal planar d. linear



(4) 3. Would you expect H2CO (c above) to be polar? Explain. Yes. The oxygen is significantly more electronegative than the carbon (3.5 vs. 2.5) so we expect a polar bond between them.


(12) 4. Consider the reaction of hydrochloric acid and iron metal to give ferrous chloride (iron II chloride) in solution (as ions) and hydrogen gas as products.

a. Write a balanced, net ionic, equation for this reaction.


2 H+ + Fe Æ Fe2+ + H2 (g)


b. What is the oxidizing agent in your equation? Hydrogen ion.

c. Which species is oxidized? Iron.

d. What is the reductant? Iron.

(15) 5. Consider the chemical reaction diagram below:

 

a. Label the axis on this diagram. Answer on diagram in red.

b. Show on the diagram: reactants and products; activation energy (Ea), and the change in free energy (DG). Answer on diagram in purple.

c. Is this reaction exergonic or endergonic?

Exergonic (DG is -)

d. Is this reaction spontaneous? Explain briefly.

Yes - reaction is spontaneous if DG is negative (products are at lower energy than reactants, therefore more stable, therefore favored).


(10) 6. Two moles of NOCl gas are placed in a 5.00-liter container at 25°C. NOCl is known to decompose slightly to NO and Cl2gases. If the equilibrium constant for this reaction is 2.0 x 10-10 what are the concentrations of all species at 25°C? SHOW all work! NO CREDIT for lone answers!

Balanced equation: 2 NOCl Æ 2 NO + Cl2
Initially (before reaction): 2 mol/5.00 L = 0.400 M 0 0
Let x = Cl2 generated, then:
After reaction: (0.400 - 2x) M   2x M   x M

Keq = [NO]2[Cl2]/[NOCl]2 = [2x]2[x]/[0.400 - 2x]2

assume x << 0.400, then 0.400 - 2x @ 0.400, and

Keq = [2x]2[x]/[0.400]2 = 4x3/0.160 = 2.0 x 10-10;

x3 = (0.160/4)/(2.0 x 10-10) = 8.0 x 10-12

x = 2.0 x 10-4;

Assumption OK, x is smaller than error in 0.400 (its in the next [fourth] significant figure).

Substituting: [Cl2] = 2.0 x 10-4 M; [NO] = 4.0 x 10-4 M; [NOCl] = 0.400 M


(10) 7. Consider the cylinder of an engine in an automobile. Assume that the cylinder is filled with a gas at 756.0 mmHg of pressure at 30.0°C, and that it has a volume of 450.0 ml when the piston is all of the way down. The piston then compresses the gas to 40.0 ml and ignites the mixture, raising the temperature to 827°C. Assuming there are the same total number of moles of gas before and after reaction, calculate the new pressure, in atm, in the cylinder. SHOW all work! NO CREDIT for lone answers!

PV = nRT; PV/T = nR; Pf = Pi(Vi/Vf)(Tf/Ti)

    • Pi = 756.0 mmHg = 756.0 mmHg/760.0 mmHg/atm = 0.9947 atm
    • Pf = ?
    • Vi = 450.0 mL
    • Vf = 40.0 mL
    • Ti = 30.0 ° + 273.15 ° = 303.15 K
    • Tf = 827 ° + 273.15 ° = 1100.15 K

Pf = 0.9947 atm (450 mL/40 mL)(1100.2 K/303.2 K) = 40.61 atm

= 40.6 atm

(10) 8. Assume that you "empty" an aerosol spray can of paint in your garage. on the back of the can you note that it has a volume of 475 mL. If the temperature is 23°C and atmospheric pressure is measured to be 764 mmHg, how many moles of gas remain in the can? SHOW all work! NO CREDIT for lone answers!

PV = nRT

    • P = 764 mmHg = 764 mmHg/760.0 mmHg/atm = 1.005 atm
    • V = 475 mL/1000 mL/L = 0.475 L
    • R = 0.0821 L*atm/mol*K
    • T = 23 ° + 273.15 ° = 296.15 K

n = PV/RT = (1.005 atm)(0.477 L)/(0.0821 L*atm/mol*K)(296.2 K) = 1.963 x 10-2 mole

= 1.96 x 10-2 mole

(10) 9. Consider the reaction below:

2 NO(g)+ O2(g) Æ 2 NO2(g)+  H

a. Write the Mass Action Expression for this reaction.

Q = [NO2]2/[NO]2[O2]


b. What will happen to this reaction if the volume is increased?

It will shift left.


c. What will happen to the [O2] if the temperature is decreased?

[O2] will decrease (reactions shifts right).

(5) 10. Why do reaction rates increase with increasing temperature?

More molecules will have enough energy (KE) to get over the activation energy (Eact) barrier.

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Last modified 15 August 2005