Question 1

Which of the following best explains why the lattice energy of NaF is greater than that of NaCl?

Accepted Answer

Fluoride ions are smaller than chloride ions, resulting in stronger electrostatic attraction in NaF.

Question 2

Which statement correctly describes the relationship between bond order and bond length in diatomic molecules?

Accepted Answer

As bond order increases, bond length decreases.

Question 3

A student mixes 50.0 g of water at 25.0°C with 50.0 g of water at 75.0°C in a coffee-cup calorimeter. Assuming no heat loss, what is the final temperature of the mixture? (Specific heat of water $$c = 4.18 \ \mathrm{J/g\cdot K})$$

Accepted Answer

$$50.0\ ^\circ\mathrm{C}$$

Question 4

Which of the following molecules has the largest dipole moment?

Accepted Answer

HF

Question 5

Which of the following is true regarding the enthalpy change ($$\Delta H) $$for an exothermic reaction?

Accepted Answer

$$\Delta H is $$negative because energy is released to the surroundings.

Question 6

Which of the following best describes the electron domain geometry and molecular geometry of $$\mathrm{NH_3$}$$?

Accepted Answer

Electron domain geometry: tetrahedral; molecular geometry: trigonal pyramidal

Question 7

Which of the following statements about resonance structures is correct?

Accepted Answer

Resonance structures represent delocalized electrons and contribute to the true structure of the molecule.

Question 8

Calculate the enthalpy change ($$\Delta H) $$for the reaction: $$\mathrm{C_2H_4(g) + H_2$(g) \rightarrow C_2H_6(g)}$$, given the following bond enthalpies: $$\mathrm{C=C}$$: $$614\ \mathrm{kJ/mol}$$, $$\mathrm{C-C}$$: $$348\ \mathrm{kJ/mol}$$, $$\mathrm{C-H}$$: $$413\ \mathrm{kJ/mol}$$, $$\mathrm{H-H}$$: $$436\ \mathrm{kJ/mol}.$$

Accepted Answer

$$-124\ \mathrm{kJ}$$

Question 9

Which of the following best explains why $$\mathrm{CO_2$} is a $$nonpolar molecule despite having polar bonds?

Accepted Answer

The molecule is linear, so the dipoles cancel each other out.

Question 10

Which of the following processes is endothermic?

Accepted Answer

Evaporation of water

Question 11

A reaction has $$\Delta H = +150\ \mathrm{kJ}$$ and $$\Delta S = +200\ \mathrm{J/K}. At $$what temperature will the reaction become spontaneous? (Assume $$\Delta G = \Delta H - T\Delta S)$$

Accepted Answer

$$T \u003e 750\ \mathrm{K}$$

Question 12

Which of the following best describes the difference between a sigma ($$\sigma) $$bond and a pi ($$\pi) $$bond?

Accepted Answer

$$\sigma$$ bonds result from head-on overlap of orbitals, while $$\pi$$ bonds result from side-to-side overlap.

Question 13

Which of the following molecules violates the octet rule?

Accepted Answer

$$\mathrm{BF_3$}$$

Question 14

Which of the following is the best explanation for why ionic compounds tend to have high melting points?

Accepted Answer

Strong electrostatic forces between oppositely charged ions require a large amount of energy to break.

Question 15

Which of the following statements about bond enthalpy is correct?

Accepted Answer

Bond enthalpy is the energy required to break one mole of a bond in the gas phase.

Question 16

Which of the following is the correct formula for calculating the heat absorbed by a solution in a calorimeter?

Accepted Answer

$$q = m c \Delta T$$

Question 17

Which of the following best describes the molecular geometry of $$\mathrm{SF_4$}$$?

Accepted Answer

Seesaw

Question 18

Which of the following is true about the relationship between enthalpy of formation and stability of a compound?

Accepted Answer

A more negative enthalpy of formation indicates a more stable compound.

Question 19

Which of the following is the best description of a spontaneous process?

Accepted Answer

A process that occurs without outside intervention under given conditions.

Review for Exam III: Chemical Bonding and Thermochemistry

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