SYDNEY CHEM1111 - Chemical Bonding, Thermochemistry & Chemical Equilibrium Flashcards

Question 1

What is the molecular geometry and bond angle of $SF_6$ according to VSEPR theory?

Accepted Answer

Octahedral geometry with $90^\circ$ bond angles. The central sulfur atom undergoes $sp^3d^2$ hybridization with 6 bonding pairs and 0 lone pairs.

Question 2

What is the molecular geometry of $H_2O$ and why does its bond angle deviate from $109.5^\circ$?

Accepted Answer

Bent geometry with an approximate bond angle of $104.5^\circ$. The angle is smaller than the ideal $109.5^\circ$ tetrahedral angle due to non-bonding lone pair repulsions exerting greater repulsive force than bonding electron pairs.

Question 3

Define $sp^2$ hybridization in terms of atomic orbital combination and geometry.

Accepted Answer

$sp^2$ hybridization occurs by mixing one $s$ orbital and two $p$ orbitals, forming three equivalent $sp^2$ hybrid orbitals arranged in a trigonal planar geometry with $120^\circ$ bond angles.

Question 4

What is the key difference between a $\sigma$ (sigma) bond and a $\pi$ (pi) bond?

Accepted Answer

A $\sigma$ bond is formed by head-on (end-to-end) overlap of orbitals along the internuclear axis. A $\pi$ bond is formed by side-to-side overlap of parallel $p$ orbitals above and below the internuclear axis.

Question 5

How is Bond Order calculated using Molecular Orbital (MO) Theory?

Accepted Answer

$	ext{Bond Order} = \frac{1}{2} (N_b - N_a)$, where $N_b$ is the number of bonding electrons and $N_a$ is the number of antibonding electrons.

Question 6

What is the standard enthalpy of formation ($\Delta H_f^\circ$)?

Accepted Answer

The enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at $1	ext{ atm}$ and $298.15	ext{ K}$. By definition, $\Delta H_f^\circ = 0	ext{ kJ/mol}$ for pure elements in their standard state.

Question 7

State Hess's Law of Constant Heat Summation.

Accepted Answer

If a reaction takes place in several steps, the overall reaction enthalpy change equals the sum of the enthalpy changes of individual steps: $\Delta H_{rxn}^\circ = \sum n\Delta H_f^\circ(	ext{products}) - \sum m\Delta H_f^\circ(	ext{reactants})$.

Question 8

What formula calculates heat absorbed or released in a calorimetry experiment?

Accepted Answer

$q = m c \Delta T$, where $q$ is heat energy (J), $m$ is mass (g), $c$ is specific heat capacity ($	ext{J}\cdot	ext{g}^{-1}\cdot	ext{K}^{-1}$), and $\Delta T = T_{final} - T_{initial}$ (K or $^\circ	ext{C}$).

Question 9

What is the Second Law of Thermodynamics in terms of universe entropy?

Accepted Answer

For any spontaneous process, the total entropy of the universe increases: $\Delta S_{univ} = \Delta S_{sys} + \Delta S_{surr} > 0$.

Question 10

State the Gibbs Free Energy equation and the criterion for spontaneity.

Accepted Answer

$\Delta G = \Delta H - T\Delta S$. A reaction is spontaneous when $\Delta G < 0$, non-spontaneous when $\Delta G > 0$, and at equilibrium when $\Delta G = 0$.

Question 11

Under what conditions is a reaction spontaneous if $\Delta H > 0$ and $\Delta S > 0$?

Accepted Answer

The reaction is spontaneous only at high temperatures, specifically when $T > \frac{\Delta H}{\Delta S}$, making the $-T\Delta S$ term sufficiently negative to yield $\Delta G < 0$.

Question 12

Write the general equilibrium constant expression ($K_{eq}$) for $aA + bB \rightleftharpoons cC + dD$.

Accepted Answer

$K_{eq} = \frac{[C]^c [D]^d}{[A]^a [B]^b}$, where brackets indicate equilibrium molar concentrations. Pure solids and pure liquids are omitted ($activity = 1$).

Question 13

What is the relationship between $K_p$ and $K_c$ for gas-phase reactions?

Accepted Answer

$K_p = K_c (RT)^{\Delta n_{gas}}$, where $R$ is the gas constant ($0.08206	ext{ L}\cdot	ext{atm}\cdot	ext{mol}^{-1}\cdot	ext{K}^{-1}$), $T$ is absolute temperature in Kelvin, and $\Delta n_{gas} = n_{	ext{gas, products}} - n_{	ext{gas, reactants}}$.

Question 14

How does comparing the Reaction Quotient ($Q$) to $K_{eq}$ predict reaction direction?

Accepted Answer

If $Q < K$, the reaction shifts **right** (forward) to form products; if $Q > K$, it shifts **left** (reverse) to form reactants; if $Q = K$, the reaction is at dynamic equilibrium.

Question 15

State Le Chatelier's Principle.

Accepted Answer

If a chemical system at dynamic equilibrium is subjected to a change in concentration, temperature, volume, or pressure, the system shifts its equilibrium position to counteract the change.

Question 16

How does an increase in temperature affect $K_{eq}$ for an endothermic reaction ($\Delta H > 0$)?

Accepted Answer

For an endothermic reaction, heat acts like a reactant. Increasing temperature shifts equilibrium to the right (towards products), thereby increasing $K_{eq}$.

Question 17

What effect does adding an inert gas at constant volume have on chemical equilibrium?

Accepted Answer

It has **no effect** on the equilibrium position or $K_{eq}$ because it does not alter the partial pressures or concentrations of the reacting species.

Question 18

How are standard free energy change ($\Delta G^\circ$) and the equilibrium constant ($K$) related?

Accepted Answer

$\Delta G^\circ = -RT \ln K$, where $R = 8.314	ext{ J}\cdot	ext{mol}^{-1}\cdot	ext{K}^{-1}$ and $T$ is temperature in Kelvin.

Question 19

What is the molecular geometry of $SF_4$ (Sulfur Tetrafluoride)?

Accepted Answer

See-saw geometry. The central sulfur atom has 5 electron pairs (4 bonding, 1 lone pair; $sp^3d$ hybridization), with the lone pair located in an equatorial position to minimize repulsion.

Question 20

What is the fundamental difference between $\Delta G$ and $\Delta G^\circ$?

Accepted Answer

$\Delta G^\circ$ is the free energy change under standard state conditions ($1	ext{ M}$, $1	ext{ atm}$, $298	ext{ K}$). $\Delta G$ is the free energy change under actual, non-standard conditions, calculated as $\Delta G = \Delta G^\circ + RT \ln Q$.

SYDNEY CHEM1111 - Chemical Bonding, Thermochemistry & Chemical Equilibrium

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