AP Chem

Semester 1 Replacement Answers

A

  1. D
  2. A
  3. E
  4. A/C
  5. C
  6. C
  7. C
  8. D
  9. E
  10. A
  11. D
  12. C
  13. C
  14. C
  15. B
  16. A
  17. E
  18. C
  19. A
  20. A
  21. A
  22. E
  23. E
  24. C
  25. A
  26. B
  27. D
  28. A
  29. C
  30. C
  31. B
  32. D
  33. C
  34. B
  35. A
  36. C
  37. E
  38. D
  39. B
  40. B
  41. A
  42. C
  43. A
  44. A
  45. A
  46. C
  47. D
  48. E
  49. C
  50. C

Fr1:

  1. 156g/mol
  2. 5.43L
  3. C10H8N2
  4. -6240J/mol
  5. Less
  6. -3120KJ/mol
  7. -1775KJ/mol

2. Explain each statement using chemical principles.

A. Iodine is a solid at room temperature but chlorine is a gas

Answer: Both have dispersion as their only force but iodine is more polarizable. Iodine is more polarizable because it is larger and the electrons are held weakly compared to chlorine.

B. Fluorine is more reactive than oxygen.

Fluorine has a weak single bond compared to oxygen’s stronger single bond so it is easier to get fluorine to react in the beginning of a reaction. Once the reaction starts the bonds made with fluorine are strong and so lots of heat is released making the reaction go faster.

C. Water is a liquid at room temperature but ammonia (NH3) is a gas.

The dominant force in both substances is hydrogen bonding. The H-bonding in water is stronger than the H-bonding in ammonia because oxygen is more electronegative than nitrogen. With stronger intermolecular forces, water has a higher boiling point.

D. Sodium chloride is soluble in water but insoluble in gasoline (assume gasoline is C8H18).

In order for salt to dissolve it needs to have a solvent that can separate positive and negative charges. Polar solvents can do this. The negative end of a polar molecule surrounds the cations and the positive end surrounds the anions. Water is polar while gasoline is not.

E. The aluminum ion is smaller than the aluminum atom.

Both have the same number of protons but the Al3+ ion has one less level of shielding. Zeffective is higher for the ion so it is smaller.

F. The difference in energy between the second and third ionization energy of magnesium is much larger than the difference between the first and the second.

Taking the second involves taking a valence electron. The third electron is a core electron with one less level of shielding and a higher Zeffective so more work must be done to remove the electron from the ion.

G. Equimolar solutions of sucrose and sodium chloride have different melting points.

The van’t Hoff factors are different because sucrose is a covalent compound that stays together when dissolved while sodium chloride is ionic and splits up into a sodium ion and a chloride ion giving two moles of particles in solution per mole of compound. By the way, this is an aqueous solution so don’t compare the melting points of sucrose and sodium chloride.

H. Hydrogen chloride gas deviates more from ideal behavior than helium gas.

Ideal gases have no forces of attraction for each other. The dominant (only) intermolecular force for helium is dispersion which is very weak in helium as there are only two electron which are held in 1s which is the least polarizable orbital. HCl gas has dipole-dipole forces which are stronger than the weak dispersion forces in helium.

3. A chemist has samples of methanol CH3OH, methanethiol CH3SH, ethane C2H6, and ethyne C2H2.

A. Draw a Lewis structure for one of the substances above.

B. Which substance is the most soluble in water? Explain your answer.

Methanol as both it and water have H-bonding. Methanethiol has dipole-dipole forces and the two hydrocarbons have dispersion only.

C. Draw a Lewis structure for thioethanoic acid. This molecule contains two carbons, an oxygen, a sulfur and four hydrogens. One of the hydrogens is bonded to the sulfur.

D. Give the geometry around the two carbons in thioethanoic acid.

Hey, AP test, did you mean electronic or molecular geometry? Since I’m a Torrey Pines student I know both. In both atoms there are no lone pairs so the electronic and molecular geometry is the same. The carbon with 3 hydrogens is tetrahedral and the carbon with the double bond to oxygen is trigonal planar.

E. A student says that ethane has a higher boiling point than ethyne because there are more hydrogen-carbon bonds to break. Are they right or wrong? Justify your answer.

Answer: This student is wrong. Boiling involves disruption of intermolecular forces for molecular covalent compounds like ethane and ethyne. The force for both molecules is dispersion. Dispersion forces are proportional to molar mass because polarizability is proportional to molar mass. Breaking covalent bonds would be involved in the decomposition of the compounds but not in boiling.