Practice Exam 2 – 75 minute2

Chapters 5, 6, 13

#### Useful Information

Constants

1 atm = 760 mm Hg = 760 torr

R = 0.08206 L.atm/mol.K = 8.315 J/mol.K

NA = 6.022 x 1023 (Avagadro’s Number)

specific heat of water = 4.184 J/g.K

Van der Waal’s Equation

Ek = 1/2 mu2 = 3/2 kT per molecule

Ek = ½ Mu2 = 3/2 RT per mole

 = Rate of Effusion2     Rate of Effusion1

=

Kp = Kc(RT)Dn  where  Dn = mol product gas-mol reactant gas

Multiple Choice (2 points each)

_____1. The value of ∆E for a system that does 213 kJ of work on its surroundings and loses 79 kJ of heat is _______kJ.

A) +292           B) -292            C) +134           D) -134            E) -213

_____2. When NH4Cl(s) dissolves in water the solution formed shows a decrease in temperature (a “cold pack”).  This solution process is ____________ and has a ________ΔH.

A) endothermic, negative

B) endothermic, positive

C) exothermic, negative

D) exothermic, positive

E) cannot be determined from the information given

_____3. Given the thermochemical equation

2 SO2(g)   +   O2(g)      2 SO3(g)                  ∆H  =  -198 kJ

What is the enthalpy change, ∆H, for the decomposition of one mole of SO3 into SO2 and O2?

A) 198 kJ             B) -99 kJ        C)  99 kJ           D)  396 kJ          E)  -198 kJ

_____4. The standard enthalpy of formation, Df, for nitrogen dioxide is the enthalpy change for the reaction

A) N(g) + 2O(g) à NO2(g)

B) 1/2 N2(g) + O2(g) àNO2(g)

C) NO(g) + O(g) à NO2(g)

D) NO(g) + 1/2 O2(g) àNO2(g)

_____5. If the temperature of a particular sample of gas is tripled and its volume doubled, what will the new pressure be?

A) 2/3 Poriginal         B)1/6 Poriginal            C) 3/2 Poriginal       D)  6 Poriginal          E)  no change

_____6.Which set of conditions would give the least ideal gas conditions?

A) High temperature, high pressure

B) High temperature, low pressure

C) Low temperature, low pressure

D) Low temperature, high pressure

E) Does not matter;  all gases behave ideally.

_____7. What is the ratio of the rates of effusion of H2O(g) to D2O(g)? (molar mass D2O = 20.0 g/mol)

A) 0.900          B) 0.950          C) 1.05            D) 1.11

_____8. Consider a reaction 2 A(s) + B(g) Û 2 C(g). If 0.20 mol A, 0.30 mol B and 0.10 mol C are present in a 1.0 L flask at equilibrium, what is Kc?

A) 0.033          B) 0.83            C) 1.7              D) 30.              E) none of the above

_____9. This reaction occurs readily above 500 °C.

2HgO(s) Û2Hg(l) + O2(g)

What is the equilibrium constant expression for this reaction?

A) K = [Hg]2[O2] / [HgO]2
B) K = [Hg][O2] / [HgO]
C) K = [Hg]2[O2]
D) K = [O2]

_____10. Which reaction characteristics will be affected by a change in temperature?

1. value of equilibrium constant
2. equilibrium concentrations

A) 1 only
B) 2 only
C) 1 and 2
D) neither 1 nor 2

11. (6 pts) Equal masses of helium and neon are placed in separate containers of equal volume at the same temperature. Circle the correct answer for each part.

i.         Pressures of the gases.

a.   The pressure of helium is greater than the pressure of neon.

b.      The pressure of neon is greater than the pressure of helium.

c.       The pressures of the gases are the same.

ii.   Energies of the atoms

1. The average energy of helium atoms is greater than that of  neon atoms
2. The average energy of  neon atoms is greater than that of  helium atoms.
3. The average energy of  helium atoms is the same as that of  neon atoms.

iii.                  Speeds of the atoms.

a.   The average speed of  helium atoms is greater than that of  neon atoms

b.   The average speed of  neon atoms is greater than that of  helium atoms

c.   The average speed of  neon atoms is the same as that of  helium atoms

12. (7 pts) Given the two equilibrium below (I & II), what is the Keq for:

Zn3(PO4)2 (s)  +  12 OH (aq)   <–––>   3 Zn(OH)42– (aq)   +   2 PO43– (aq)       K = ???

(I)              3 Zn2+ (aq)   +  2 PO43– (aq)   <––––>    Zn3(PO4)2 (s)       KI = 1.1 x 1032

(II)            Zn2+ (aq)   +   4 OH (aq)   <––––>    Zn(OH)42– (aq)           KII = 4.6 x 1017

13. (7 pts) Given these values of DH°:

CS2(l) + 3O2(g) à CO2(g) + 2SO2(g)                DH° = - 1077 kJ

H2(g) + O2(g) à H2O2(l)                                    DH° = - 188 kJ

H2(g) + 1/2 O2(g) à H2O(l)                               DH° = - 286 kJ

What is the value of DH° for this reaction?

CS2(l) + 6H2O2(l) à CO2(g) + 6H2O(l) + 2SO2(g)

14. (10 pts) An alloy of aluminum and copper is analyzed for percentage composition. A 0.2052 g sample of the alloy is dissolved in sulfuric acid (copper does not react with sulfuric acid) and 229.5 mL of hydrogen gas are collected at 700. torr and 29 oC. The balanced equation is

2 Al + 3H2SO4 à Al2(SO4)3 + 3 H2

Calculate the percentage of aluminum in the alloy.

15. (7 pts) When 2.75 mol HI(g) are placed into a 1.00-L container at 25 °C and allowed to dissociate according to the equation:

2 HI (g) Û H2(g) + I2(g)

The final concentration of H2(g) is found to be 0.275 M. What is the Kc for the reaction?

16. (7 pts) At a certain temperature K = 6.3 x 107 for the reaction

2NO(g) + Cl2(g)  Û 2NOCl(g)

Calculate the concentration of NO at equilibrium if 2.0 moles of NO and 1.0 mole of Cl2 are placed in a 1.0 L flask.

17. (8 pts) Calculate the molar mass of  the compound if the 1.103 g sample occupies a volume of 582 mL at 765.5 torr and 25.00oC.

18. (10 pts) Ammonium carbamate, NH2COONH4, is a salt found in the blood and urine of mammals.  At 520 K,  KC is 1.58 x 10-8 for the following equilibrium:

NH2COONH4(s) Û 2NH3(g) + CO2(g)

If  7.80 g of NH2COONH4 is introduced into a 0.500 L evacuated container at 520 K, what will be the total pressure inside the container at equilibrium?

19. (8 pts) The enthalpy of neutralization, DH, of HCl (aq) by NaOH(aq) is -55.90 kJ/mol H2O produced.  If 50.00 mL of 1.05 M NaOH is added to 25.00 mL of 1.86 M HCl, with both solutions originally at 24.72 oC, what will be the final solution temperature?  Assume no heat is lost to the surrounding air and that the aqueous solution has a density of 1.02 g/mL and a specific heat of 3.98 J/g . oC.

20. (10 pts) For this reaction at equilibrium, how will the indicated changes affect the equilibrium?                      Fe3O4(s) + 4 H2(g) Û 3 Fe(s) + 4 H2O(g) DH > 0

A)

 Change to system at equilibrium Shift to re-establish equilibrium (left, right, or no change) Amount of Fe(s) present in the system increasing temperature adding H2 adding Fe(s) Decrease volume of container

B) How will increasing the temperature affect the value of the equilibrium constant, K,   for reaction?