Chemistry, Level 3 – Midyear Exam – A – Problems



Chemistry, Level 3 – Final Exam Practice – Answers

1. B

2. C

3. O=S—O

4. B

5. A

6. B

7. A

8. B

9. A

10. A

11. B

12. B

13. B

14. C

15. B

16. D

17. C

18. A

19. C

20. C

21. B

22. D

23. A

24. D

25. A

26. D

27. B

28. D

29. B

30. B

31. B

32. B

33. A

34. D

35. [pic]

36. B

37. D

38. A

39. D

40. D

41. B

42. B

43. B

44. D

45. C

46. C

47. B

48. C

49. A and B

50. B

51. B

52. A

53. A

54. D

Free-Response Problems – Answers

1. Propane burns in air by the equation shown below.

C3H8(g) + 5 O2(g) ( 3 CO2(g) + 4 H2O(g) (H = -2043 kJ

a) Is this reaction endothermic or exothermic? Exothermic (ΔH < 0)

b) How much heat is produced if 16.0 grams of propane react completely.

[pic]

c) How much heat is produced if 15 L of oxygen (at STP) react completely?

[pic]

d) Use heats of formation to calculate the enthalpy ((H) for the reaction.

[pic]

2. A hollow, sealed plastic toy with a volume of 1.1 L is dropped off of a cruise ship where the temperature is 1.0 atm and the temperature is 37(C. It is heavy enough to sink below the waves, but down below the water reaches a depth where the pressure is 5.0 atm and the temperature 5(C. What is the new volume of the toy at this depth?

GIVEN: Combined gas law

V1=1.1 L V2=?

P1=1.0 atm P2 = 5.0 atm

T1=37°C+273=310 K T2 = 5.0°C+273= 278 K

[pic]

3. Calculate the density of COCl2 gas at 25(C and 1.15 atm.

[pic]

4. A balloon animal with 0.37 moles of gas in it is banished to the freezer. It reaches a temperature of -2 (C at 0.9 atm of pressure. What is the volume of the balloon animal in the freezer?

GIVEN: Ideal gas law

V=? P = 0.9 atm

n=0.37 moles T = -2°C+273=271 K

[pic]

5. 101.9 g of C12H24O11 are dissolved in 618 g of water (591 mL). Calculate the molarity, molality, and mole fraction of the solution.

[pic]

6. Determine the molarity, molality, and freezing point of a solution of 10.9 g of salt (sodium chloride) are dissolved in 3.25 kg of water. The Kf of water is 1.86(C/m.

Determing the molarity is impossible without the density of the solution.

[pic]

7. Ksp of PbSO4 is 1.3 x 10-8. What is the molar solubility (equilibrium concentrations of ions) of PbSO4?

PbSO4(s) ( Pb2+(aq) + SO42-(aq)

Ksp = [Pb2+][SO42-]=(x)(x)=x2=1.3 x 10-8

x =[Pb2+] = [SO42-] =1.1 x 10-4 M

8. For the reaction N2O4(g) ( 2 NO2(g) , Keq = .2. At a particular time, the following concentrations are measured: [N2O4] = 2.0 M, [NO2] =.2 M. Is this reaction at equilibrium? If not, in which direction will the reaction proceed?

[pic]

Since Q is less than Keq there are too many reactants and products will be formed.

9. Draw the Lewis structure for thionyl chloride, SOCl2. State the shape, bond angle, hybridization of the central atom, and whether the molecule is polar or nonpolar.

[pic]

This molecule is pyramidal (3 bonding regions and a lone pair), it is sp3 hybridized, and is polar. (). Likely, this is more difficult than a question that actually would be considered for the exam.

10. What does Le Chatelier’s Principle say about chemical equilibrium? Describe at lease three changes that can be made to a system and how Le Chatelier’s Principle applies. Do any of these changes affect the equilibrium constant? Which ones?

See section 16.3 (p. 548-555) in your textbook.

11. Calculate the pH, POH, and [OH-] for a .50 M HBr solution.

This is a strong acid, so [H3O+]=[acid] because it completely dissociates.

[pic]

12. Calculate the pH of a 1.0 M solution of H2CO3. Ka is 4.5 x 10-7.

This is a weak acid, so [H3O+]=square root of Ka x [acid] because it only partially dissociates and forms an equilibrium.

[pic]

13. Describe four tests that you could perform in order to distinguish an acid from a base. What would a positive test look like in each case?

|Test |Acid |Base |

|Reaction with CaCO3 |Bubbles |Nothing |

|Reaction with a metal |Bubbles |Nothing |

|Litmus (indicator) |Red |Blue |

|Phenolphthalein (indicator) |Clear |Pink |

|pH (paper, meter, etc.) |Less than 7 |Greater than 7 |

Taste and touch are never considered a good, safe idea with an unknown substance.

14. Describe the experimental technique of acid-base titration. In what situations and how is this technique used?

See section 19.3 (p. 636-643) in your textbook.

INTEGRATED PRACTICE PROBLEMS

15. Answer the questions below that relate to the five aqueous solutions at 25(C shown below.

[pic]

(a) Which solution has the highest boiling point? Explain.

Solution # 1 (Pb(NO3)2) has the highest boiling point. With 3 ions it has the largest van’t Hoff factor (i=3) and therefore since all have the same concentration it will elevate the boiling point the highest.

(b) Which solution has the highest pH? Explain.

Solution # 4 will have the highest pH. The H on the end of ethanol (C2H5OH) is slightly acidic and will lead to a slightly acidic solution. Solution # 5 will have a slightly basic pH, since C2H3O2- (acetate ion) is a weak base (the conjugate of acetic acid). This question is harder than any you’ll likely get on the exam.

(c) Identify a pair of the solutions that would produce a precipitate when mixed together. Write the formula of the precipitate.

Pb2+(aq) (solution # 1) + Cl-(aq) (solution # 2) ( PbCl2(s)

(d) Which solution would be the least effective conductor of electricity? Explain.

Solution # 4 is the most covalent …the least likely to dissociate into ions. That will make it the poorest conductor of electricity.

_______________________________________________________________________________________

16. Answer the following questions, which pertain to the binary compound SO2.

(a) Draw a complete Lewis electron-dot diagram for the SO2 molecule.

See multiple choice # 3.

(b) On the basis of the Lewis electron-dot diagram that you drew in part (a), predict the molecular geometry of the SO2 molecule.

bent or angular (from trigonal planar)

(c) On the basis of your Lewis electron-dot diagram(s) in part (c), identify the hybridization of the sulfur atom in the SO2 molecule.

sp2

The reaction between SO2(g) and O2(g) to form SO3(g) is represented below.

2 SO2(g) + O2(g) ( 2 SO3(g)

The reaction is exothermic. The reaction is slow at 25˚C; however, a catalyst will cause the reaction to proceed faster.

(c) Write the equilibrium-constant expression for the reaction shown.

[pic]

(d) If you have a container with 18.4 L of O2 at 25˚C and 2.3 atm, how many moles of O2 are in it?

[pic]

(e)Using the axes provided, draw the complete potential-energy diagram for both the catalyzed and uncatalyzed reactions. Clearly label the curve that represents the catalyzed reaction.

See Figure 22-21 on p. 742 in your textbook.

17. An experiment is performed where 24.5 g of an unknown nonvolatile, nonelectrolyte are dissolved in 100.0 g of water. The freezing point of the new solution is -8.9 ºC. Kf for water is 1.86 ºC.

a) Would you expect that this solute is polar or nonpolar? Explain your reasoning.

Polar. It dissolves in water in large quantities.

b) What is the molality of this solution?

[pic]

c) How many moles of the unknown were present in this solution?

[pic][pic]

d) What is the molar mass of the unknown?

[pic]

e) The Kb for water is 0.52 ºC. What would be the boiling point elevation for this solution?

[pic]

f) If the solute was actually an electrolyte, would the calculated molar mass be lower, higher, or the same? Explain your reasoning.

If the solution was an electrolyte, then i (van’t Hoff factor) would be greater than 1. This would mean that in (b) the equation would be ΔTf=iKfm. Therefore, our calculated molality would be too high. With the molality too high, our calculate # of moles (c) would also be too high and in (d) our molar mass would be too small.

______________________________ _________________________________________________________

|Reaction |Equation |∆H˚298 |∆S˚298 |∆G˚298 |

|X |C(s) + H2O(g)(CO(g) + H2(g) |+ 131 kJ mol-1 |+134 J mol-1 K-1 |+91 kJ mol-1 |

|Y |CO2(g) + H2(g) ( CO(g) + H2O(g) |+41 kJ mol-1 |+42 J mol-1 K-1 |+29 kJ mol-1 |

|Z |2 CO(g) ( C(s) + CO2(g) |? |? |? |

18. Answer the following questions using the information related to reactions X, Y, and Z in the table above.

(a) For reaction X, write the expression for the equilibrium constant, Keq.

[pic]

(b) For reaction Y at 298 K which is larger: the total bond energy of the reactants or the total bond energy of the products? Explain.

Since the reaction is endothermic (ΔH > 0), then the products have more total bond energy than the reactants do.

(c) If the pressure is increased on reaction Z, will the reaction shift to the right, left, or stay the same? Explain your reasoning.

If the pressure is increased on reaction Z, then the reaction will shift towards the left to create more products. This shift will occur because there are fewer gas particles on the product side of the equation and therefore more products will result in a reduction of the overall pressure.

(d) Is the following statement true or false? Justify your answer.

“On the basis of the data in the table, it can be predicted that reaction Y will occur more rapidly than reaction X will occur.”

False. This table has thermodynamic data. There is no way to determine kinetics (rate) information from thermo data.

(e) Consider reaction Z at 298 K.

(i) Is ∆S˚ for the reaction positive, negative, or zero? Justify your answer.

∆S˚ will be negative. Creating a solid and one gas particle from two gas particles results in an overall decrease in entropy.

(ii) Determine the value of ∆H˚ for the reaction.

Adding up the two equations using Hess’s law (and reversing the answer), we find that ∆H˚ is equal to

-172 kJ/mole.

(iii) A sealed glass reaction vessel contains only CO(g) and a small amount of C(s). If a reaction occurs and the temperature is held constant at 298 K, will the pressure in the reaction vessel increase, decrease, or remain the same over time? Explain.

Over time the pressure will decrease as some CO is converted to CO2.

______________________________ _________________________________________________________

19. Give a scientific explanation for the following observations. Use equations or diagrams if they are relevant.

(a) It takes longer to cook an egg until it is hard-boiled in Denver (altitude 1 mile above sea level) than it does in New York City (near sea level).

At higher altitude there is a lower atmospheric pressure. Since, by definition, the boiling point of a substance occurs when the vapor pressure of a liquid equals atmospheric pressure, then water boils at a lower temperature in Denver. So, in order to cook the egg fully it must be cooked for a longer period of time.

(b) Burn coal containing a significant amount of sulfur leads to acid rain.

The combustion of coal leads to the formation of SO3 as a combustion product. When SO3 reacts with water vapor in the atmosphere, H2SO4 is formed which falls as acid rain.

(c) Perspiring is a mechanism for cooling the body.

Perspiring cools the body since excess body heat is used to break the intermolecular forces (hydrogen bonds) in liquid water in order to make it a gas. This use of excess heat prevents the body from warning up too extensively.

e) The addition of antifreeze to water in a radiator decreases the likelihood that the liquid in the radiator will either freeze or boil.

Antifreeze is a nonvolatile, nonelectrolyte and increases the boiling point or decreases the freezing point when it is added to water. This occurs because in both cases it makes it more difficult for water molecules to organize—either into the solid structure of ice or in order to leave the solution.

-----------------------

..

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download