Chemistry Exam Review

Lattice Enthalpy

• Lattice Enthalpy Definition: The amount of energy released when gaseous ions combine to form a solid ionic compound.

• Comparison of Ionic Compounds: Lithium chloride ($LiCl$) likely has a greater lattice enthalpy compared to sodium chloride ($NaCl$) because:

  • Lithium ion ($Li^+$) is smaller than sodium ion ($Na^+$), leading to a stronger attraction between ions due to proximity, which results in higher lattice energy.

  • The chloride ion ($Cl^-$) size is the same in both compounds, hence the metal ion's size plays a critical role.

Dissolution of Ionic Compounds

• Dissolving Process: When an ionic compound dissolves in water, it undergoes an endothermic process because:

  • The ionic bonds must break, which requires energy (endothermic).

  • The energy released from ion-dipole interactions between ions and water must exceed the energy needed to break ionic bonds for the overall reaction to become exothermic.

• Type of Interactions:

  • Solute-solute interactions (ionic bonds in solid) and solvent-solvent interactions (hydrogen bonds in water) must be broken to allow dissolution.

  • Ion-dipole interactions are formed as ions interact with water molecules.

Temperature Effects on Gas Behavior

• Motion of CO2 Molecules:

  • Increasing temperature increases the kinetic energy of gas molecules, resulting in more vigorous motion and more energetic collisions of the molecules with the walls of the container, thus increasing gas pressure.

• Pressure Calculation: Using the ideal gas law: $P1V1/T1 = P2V2/T2$

  • Given initial pressure ($P1 = 0.7 ext{ atm}$) and initial temperature ($T1 = 299 ext{ K}$), we can calculate the final pressure at a new temperature ($T_2 = 425 ext{ K}$).

  • The calculated pressure ($P_2$) at $425 ext{ K}$ is approximately $0.99 ext{ atm}$.

Ideal vs. Non-ideal Gases

• Explanation of Pressure Deviations:

  • Ideal gas law assumes molecules do not attract one another and have negligible volume.

  • Actual CO2 gas pressure is lower than predicted due to intermolecular forces between gas molecules which cause deviations from ideal behavior.

  • Dispersion forces in CO2 help explain the attractive forces that cause lower-than-predicted pressures.

Net Ionic Equations

• Complete Ionic Equation Example:

  • For the reaction of a strong acid with aqueous ions:
    2e{(s)} + 6HBr{(aq)}
    ightarrow 2e^{3+}{(aq)} + 6Br^{-}{(aq)} + 3H_2(g)

  • Only aqueous species dissociate; solids and gases remain intact.

• Example Net Ionic Equation:

  • The net ionic equation is derived by removing spectator ions:
    2e{(s)} + 6H^+{(aq)}
    ightarrow 2e^{3+}{(aq)} + 3H2(g)

Collecting Gases and Equilibrium Concepts

• Gas Measurement:

  • Collection of a gas generated in a chemical reaction can be measured volume-wise and should be recorded to a certain precision (e.g., hundredths place).

• Using Water Displacement:

  • If total pressure is known and vapor pressure of water at a certain temperature is known, the pressure of the gas can be derived:
    P{total} = P{H2} + P{H_2O}

  • Therefore:
    P{H2} = P{total} - P{H_2O}

Buffer Solutions and pKa Calculation

• Buffer Preparation:

  • To prepare a buffer solution containing carbonate and bicarbonate, the key is maintaining an equilibrium established by:

  • Knowing $pKa$ which can be derived from $pKb$ (e.g., for sodium carbonate).

  • Calculate $pKa$ using $pKb$ values:
    pKa = 14 - pKb

Exam Preparation Tips

• General Advice:

  • Focus on understanding and applying the core principles from the curriculum rather than memorizing.

  • Practice calculations and equilibrium concepts as they often recur in exams.

  • Pay attention to specific wording in questions, especially when asked to agree or disagree with given statements and justify with calculations/explanations.

• Test Structure Overview:

  • Expect a combination of multiple choice and free response questions; many do not count towards the final score but are included for statistical purposes.

• Time Management:

  • Watch the clock during the exam; plan to answer quicker to allow time for review.

  • Use given paper judiciously and ensure completeness in answering each question.