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AP Chemistry FRQ Practice

1. Atomic Structure and Periodicity

Question:

The atomic radius of elements decreases as you move across a period from left to right on the periodic table. Explain this trend in terms of effective nuclear charge and electron shielding.

(a) Define effective nuclear charge and electron shielding.

(b) Explain why atomic radius decreases across a period using the concepts of effective nuclear charge and electron shielding.

(c) How does this trend compare to the atomic radius as you move down a group?

Response Guidelines:

  • Part (a):
    Effective nuclear charge is the net positive charge experienced by an electron in an atom. It accounts for both the total nuclear charge (the number of protons) and the shielding effect of inner electrons that reduce the attraction between the nucleus and outer electrons. Electron shielding refers to the effect that inner electrons have on reducing the effective nuclear charge experienced by outer electrons.
  • Part (b):
    As you move across a period from left to right, the number of protons in the nucleus increases, which increases the effective nuclear charge. However, the number of inner electrons stays the same, so the shielding effect does not significantly change. As a result, the increased effective nuclear charge pulls the outer electrons closer to the nucleus, causing the atomic radius to decrease.
  • Part (c):
    As you move down a group, the atomic radius increases because additional electron shells are added. These additional shells increase the distance between the nucleus and the outermost electrons, and the shielding effect also increases, which reduces the effective nuclear charge experienced by outer electrons. Despite the increase in nuclear charge, the outer electrons are less tightly held, resulting in a larger atomic radius.

2. Chemical Bonding

Question:

Consider the compound sodium chloride (NaCl), which is formed from the ionic bonding of sodium (Na) and chlorine (Cl).

(a) Explain the process of ionic bond formation between Na and Cl.

(b) What is the lattice energy of NaCl, and how does it relate to the stability of the compound?

(c) Predict the result of dissolving NaCl in water. What happens to the ions in solution?

Response Guidelines:

  • Part (a):
    In the formation of NaCl, sodium (Na) donates one electron to become a Na+ ion, while chlorine (Cl) accepts the electron to become a Cl- ion. This results in the formation of two oppositely charged ions. The electrostatic attraction between these oppositely charged ions forms the ionic bond that holds NaCl together.
  • Part (b):
    The lattice energy of NaCl is the energy released when one mole of NaCl forms from its gaseous ions. Lattice energy is a measure of the strength of the ionic bond and is directly related to the stability of the compound. A higher lattice energy means a more stable ionic compound. Factors that affect lattice energy include the charge on the ions and their sizes. NaCl has a relatively high lattice energy due to the small size of Na+ and Cl- and their high charge-to-size ratio.
  • Part (c):
    When NaCl is dissolved in water, the Na+ and Cl- ions separate due to the ion-dipole interactions between the ions and the polar water molecules. Water molecules surround the ions, stabilizing them in solution. Na+ ions are surrounded by the oxygen end of water molecules, while Cl- ions are surrounded by the hydrogen end of water molecules.

3. Thermochemistry

Question:

Consider the combustion of methane (CH4):

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

(a) Write the balanced chemical equation for the combustion of methane.

(b) The enthalpy of combustion of methane is -890.3 kJ/mol. What does this value indicate about the reaction?

(c) If 10.0 g of methane is burned, how much heat is released?

Response Guidelines:

  • Part (a):
    The balanced chemical equation for the combustion of methane is:
    CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
  • Part (b):
    The enthalpy of combustion of methane is -890.3 kJ/mol, which indicates that the reaction is exothermic. This means that heat is released when methane undergoes combustion. The negative sign indicates that energy is released to the surroundings.
  • Part (c):
    To calculate the heat released, first find the molar mass of methane (CH4):
    Molar mass of CH4 = 12.01 g/mol (C) + 4.00 g/mol (H) = 16.04 g/molNext, use the mass of methane burned (10.0 g) to find the number of moles:
    Moles of CH4 = 10.0 g / 16.04 g/mol = 0.623 molSince the enthalpy change is -890.3 kJ/mol, the heat released is:
    Heat released = 0.623 mol × -890.3 kJ/mol = -555.9 kJTherefore, 555.9 kJ of heat is released when 10.0 g of methane is burned.

Grading Rubric for AP Chemistry FRQs

1. Atomic Structure and Periodicity

  • Part (a):
    • 0-1 points: Incorrect or incomplete definition of effective nuclear charge and electron shielding.
    • 2 points: Correctly defines both effective nuclear charge and electron shielding.
  • Part (b):
    • 0-1 points: Incorrect or incomplete explanation of atomic radius trend across a period.
    • 2 points: Correctly explains why atomic radius decreases across a period using effective nuclear charge and electron shielding.
  • Part (c):
    • 0-1 points: Incorrect or incomplete explanation of atomic radius trend down a group.
    • 2 points: Correctly explains the trend in atomic radius as you move down a group.

2. Chemical Bonding

  • Part (a):
    • 0-1 points: Incorrect or incomplete explanation of ionic bond formation between Na and Cl.
    • 2 points: Correctly explains the ionic bond formation between Na and Cl.
  • Part (b):
    • 0-1 points: Incorrect or incomplete explanation of lattice energy.
    • 2 points: Correctly defines lattice energy and its relation to stability.
  • Part (c):
    • 0-1 points: Incorrect or incomplete explanation of what happens when NaCl dissolves in water.
    • 2 points: Correctly explains the dissociation of NaCl in water.

3. Thermochemistry

  • Part (a):
    • 0-1 points: Incorrect or incomplete balanced chemical equation for the combustion of methane.
    • 2 points: Correctly writes the balanced chemical equation.
  • Part (b):
    • 0-1 points: Incorrect or incomplete explanation of enthalpy of combustion.
    • 2 points: Correctly explains the meaning of the enthalpy of combustion value.
  • Part (c):
    • 0-1 points: Incorrect or incomplete calculation of heat released.
    • 2 points: Correctly calculates the heat released from burning 10.0 g of methane.

Sample Grading Breakdown (for one FRQ)

  • Atomic Structure and Periodicity: 6 points
  • Chemical Bonding: 6 points
  • Thermochemistry: 6 points
  • Total: 18 points (for all three FRQs combined)