Unit 2: Molecular Interactions and Reactions - Answer Sheet B Review

Vocabulary flashcards covering VSEPR geometry, molecular shapes, intermolecular forces, gas laws, molar calculations, aqueous solutions, acidity, collision theory, and reaction rates.

$\text{BeCl}_2$ Geometry and Polarity

Electron-pair geometry is linear, molecular shape is linear, bond angle is $180^\circ$, and the molecule is non-polar because identical, oppositely directed bond dipoles cancel exactly.

$\text{NH}_3$ Geometry and Polarity

Electron-pair geometry is tetrahedral, molecular shape is trigonal pyramidal, bond angle is $\approx 107^\circ$, and the molecule is polar with a net dipole pointing toward the lone pair.

$\text{CO}_2$ Geometry and Polarity

Electron-pair geometry is linear, molecular shape is linear, bond angle is $180^\circ$, and the molecule is non-polar because two identical oppositely directed bond dipoles cancel.

$\text{H}_2\text{S}$ Geometry and Polarity

Electron-pair geometry is tetrahedral, molecular shape is bent / angular, bond angle is $\approx 92^\circ$, and the molecule is polar.

Intermolecular Forces in Methane ($\text{CH}_4$)

Only dispersion forces are present because it is a non-polar molecule with no permanent dipole.

Intermolecular Forces in Chloromethane ($\text{CH}_3\text{Cl}$)

Dipole-dipole forces exist due to the polar $\text{C--Cl}$ bond creating a permanent dipole, in addition to dispersion forces.

Intermolecular Forces in Methanol ($\text{CH}_3\text{OH}$)

Hydrogen bonding occurs due to the $\text{O--H}$ bond, in addition to dipole-dipole and dispersion forces.

Boiling Point Trend of $\text{CH}_4$, $\text{CH}_3\text{Cl}$, and $\text{CH}_3\text{OH}$

Increasing boiling point order: $\text{CH}_4\ (-161.5\,^\circ\text{C}) < \text{CH}_3\text{Cl}\ (-24.2\,^\circ\text{C}) < \text{CH}_3\text{OH}\ (64.7\,^\circ\text{C})$. Boiling point increases with the strength of the strongest intermolecular force present.

Retention Factor ($R_f$)

Calculated as $R_f = \text{distance travelled by spot} \div \text{distance travelled by solvent front}$. A higher $R_f$ value indicates stronger attraction to the mobile phase.

Charles's Law

States that gas volume increases linearly with temperature in $^\circ\text{C}$ at constant pressure, representing a direct linear relationship when temperature is expressed in kelvin ($V \propto T$ in $\text{K}$).

Absolute Zero (Temperature Extrapolation)

The theoretical temperature at which an ideal gas volume reaches zero ($V = 0$), extrapolated as approximately $-273\,^\circ\text{C}$ ($0\,\text{K}$).

Deviation of Real Gases at Low Temperatures

Real gases deviate from ideal behavior at low temperatures because intermolecular forces become significant relative to kinetic energy, and gas particles themselves occupy a fixed volume, causing liquefaction and solidification before reaching $V = 0$.

Ideal Gas Law Equation

Formula $PV = nRT$, relating pressure ($P$ in $\text{Pa}$), volume ($V$ in $\text{m}^3$), amount of substance ($n$ in $\text{mol}$), ideal gas constant ($R = 8.314\,\text{J\,K}^{-1}\text{mol}^{-1}$), and temperature ($T$ in $\text{K}$).

Molar Volume at STP

At Standard Temperature and Pressure (STP), $1\,\text{mol}$ of any ideal gas occupies a volume of $22.7\,\text{dm}^3$ (or $22.7\,\text{L}$).

Molarity ($C$)

The concentration of a solute in solution, given by $C = \frac{n}{V}$, expressed in units of $\text{mol\,dm}^{-3}$ (or $\text{mol\,L}^{-1}$).

Dilution Law

Formula $C_1V_1 = C_2V_2$, based on the principle that the total moles of solute ($n$) remain constant when solvent is added to dilute a solution.

Saturated Solution

A solution that holds the maximum possible mass of dissolved solute at a specific temperature, lying directly on its solubility curve.

Silver Chloride Precipitation

Mixing $\text{AgNO}_3(aq)$ and $\text{NaCl}(aq)$ produces a white precipitate of $\text{AgCl}(s)$. Net ionic equation: $\text{Ag}^+(aq) + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s)$.

Barium Sulfate Precipitation

Mixing $\text{BaCl}_2(aq)$ and $\text{Na}_2\text{SO}_4(aq)$ produces a white precipitate of $\text{BaSO}_4(s)$. Net ionic equation: $\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)$.

Lead(II) Iodide Precipitation

Mixing $\text{Pb(NO}_3)_2(aq)$ and $\text{KI}(aq)$ produces a bright yellow precipitate of $\text{PbI}_2(s)$. Net ionic equation: $\text{Pb}^{2+}(aq) + 2\text{I}^-(aq) \rightarrow \text{PbI}_2(s)$.

$\text{pH}$ Definition and Formula

A measure of hydrogen ion concentration defined by $\text{pH} = -\log_{10}([\text{H}^+])$, where $[\text{H}^+] = 10^{-\text{pH}}\,\text{mol\,dm}^{-3}$.

Strong Acid vs Weak Acid Dissociation

Strong acids ionise almost completely in water to yield high $[\text{H}^+]$ and low $\text{pH}$, whereas weak acids ionise only partially at equilibrium, leaving most molecules intact and resulting in lower $[\text{H}^+]$ and higher $\text{pH}$ at equal overall concentration.

Temperature Effect on Reaction Rate

Increasing temperature raises particle kinetic energy, increasing collision frequency and significantly boosting the fraction of collisions with energy $\ge E_a$, resulting in a faster reaction rate.

Surface Area Effect on Reaction Rate

Increasing solid surface area exposes more reactant particles for collisions, raising collision frequency per unit time without affecting collision energy.

Concentration Effect on Reaction Rate

Increasing solution concentration packs more reactant particles into a given volume, increasing the frequency of collisions per second and accelerating the reaction rate.

Maxwell--Boltzmann Distribution Curve under Catalysis

A catalyst lowers the activation energy ($E_a$) threshold, increasing the fraction of particles with sufficient energy to react, while leaving the underlying kinetic energy distribution curve unchanged.

Enthalpy Change ($\Delta H$)

Defined as $\Delta H = E(\text{products}) - E(\text{reactants})$. A catalyst does not alter $\Delta H$ because initial and final energy states remain identical.

Activation Energy ($E_a$)

The minimum energy required for colliding reactant particles to form products, equal to $E(\text{transition state}) - E(\text{reactants})$. A catalyst lowers $E_a$ by offering an alternative reaction pathway.

Reaction Progress Curve at Higher Temperature

At higher temperature, the curve rises more steeply due to a higher initial reaction rate and levels off sooner, but plateaus at the same final volume because the amount of limiting reactant is unchanged.