Q: Which of the following is the correct expression for the enthalpy change of a reaction?

$\Delta H = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants})$

Question 1

Which of the following statements best describes the concept of electronegativity and its role in determining bond polarity?

Accepted Answer

Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond, and a large difference in electronegativity between two atoms results in a polar bond.

Question 2

Which of the following reactions is a correct example of a redox reaction?

Accepted Answer

$\mathrm{2H_2O_2 (aq) \rightarrow 2H_2O (l) + O_2 (g)}$

Question 3

A sample of 2.50 g of propane gas ($\mathrm{C_3H_8}$) is stored in a 250 mL aerosol can at $25^\circ$C. What is the pressure inside the can? (R = 0.0821 L·atm·mol$^{-1}$·K$^{-1}$, Molar mass of $\mathrm{C_3H_8}$ = 44.1 g/mol)

Accepted Answer

$\mathrm{P = 2.48\ atm}$

Question 4

Which of the following best explains why $\mathrm{H_2O}$ is a liquid at room temperature while $\mathrm{H_2S}$ is a gas, despite both having similar molecular weights?

Accepted Answer

$\mathrm{H_2O}$ forms hydrogen bonds, which are much stronger than the dipole-dipole interactions in $\mathrm{H_2S}$.

Question 5

Which of the following molecules would you expect to have the highest boiling point?

Accepted Answer

$\mathrm{CH_3CH_2OH}$

Question 6

Given the reaction $\mathrm{CaCO_3 (s) \rightarrow CaO (s) + CO_2 (g)}$, which of the following statements is correct?

Accepted Answer

This is a decomposition reaction and is endothermic.

Question 7

Which of the following best describes the difference between a strong electrolyte and a weak electrolyte?

Accepted Answer

A strong electrolyte completely dissociates into ions in solution, while a weak electrolyte only partially dissociates.

Question 8

Which of the following sets of quantum numbers is NOT allowed for an electron in an atom?

Accepted Answer

$n = 2,\ l = 2,\ m_l = 0,\ m_s = +\frac{1}{2}$

Question 9

Which of the following best describes the hybridization of the central carbon atom in $\mathrm{CO_2}$?

Accepted Answer

$sp$

Question 10

A cube of silver metal measures 1.000 cm on each edge. The density of silver is 10.5 g/cm$^3$. How many atoms are in the cube? (Molar mass of Ag = 107.9 g/mol, $N_A = 6.022 	imes 10^{23}$ mol$^{-1}$)

Accepted Answer

$5.86 	imes 10^{22}$ atoms

Question 11

Which of the following is the correct balanced equation for the synthesis of nitric acid from ammonia and oxygen?

Accepted Answer

$\mathrm{4NH_3 + 8O_2 \rightarrow 4HNO_3 + 4H_2O}$

Question 12

Which of the following ions has the largest radius?

Accepted Answer

$\mathrm{N^{3-}}$

Question 13

Which of the following molecules contains both ionic and covalent bonds?

Accepted Answer

$\mathrm{NaNO_3}$

Question 14

Which of the following is the correct expression for the enthalpy change of a reaction?

Accepted Answer

$\Delta H = \sum \Delta H_f^\circ (	ext{products}) - \sum \Delta H_f^\circ (	ext{reactants})$

Question 15

Which of the following best describes the effect of increasing pressure above the critical pressure of $\mathrm{CO_2}$ at a constant temperature above the critical temperature?

Accepted Answer

$\mathrm{CO_2}$ cannot be liquefied regardless of the pressure applied.

Question 16

Which of the following is the correct formula for calculating the work done by a gas during an isothermal, reversible expansion?

Accepted Answer

$w = -nRT \ln \left( \frac{V_f}{V_i} \right)$

Question 17

Which of the following best describes the concept of formal charge in a Lewis structure?

Accepted Answer

Formal charge is a bookkeeping tool to estimate the charge distribution in a molecule, calculated as: $	ext{Formal charge} = 	ext{valence electrons} - 	ext{nonbonding electrons} - \frac{1}{2}(	ext{bonding electrons})$

Question 18

Which of the following molecules has a trigonal planar geometry?

Accepted Answer

$\mathrm{BF_3}$

Question 19

Which of the following is the correct relationship between $K_p$ and $K_c$ for the reaction $aA(g) + bB(g) \rightleftharpoons cC(g) + dD(g)$?

Accepted Answer

$K_p = K_c (RT)^{\Delta n}$, where $\Delta n = (c + d) - (a + b)$

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