A: Pre-exponential factor (frequency factor).

$\log_{10} k = -\frac{E_a}{2.303R} \frac{1}{T} + \log_{10} A$.

For two temperatures: $\log_{10} \frac{k_2}{k_1} = \frac{E_a}{2.303R} \left(\frac{T_2 - T_1}{T_1 T_2}\right)$.

Effect of Catalyst:

Increases reaction rate without being consumed.
Provides an alternative reaction pathway with lower activation energy.

7. Elements of Groups 16, 17 and 18

7.1 Group 16 (Oxygen Family / Chalcogens)

Elements: O, S, Se, Te, Po.
Electronic Configuration: ns$^2$np$^4$.
Occurrence: Oxygen (most abundant), Sulfur (sulfates, sulfides).
Atomic Properties:
- Atomic/Ionic Radii: Increase down the group.
- Ionization Enthalpy: High, decreases down the group. Lower than Group 15 due to np$^4$ configuration.
- Electronegativity: Decreases down the group. Oxygen is highly electronegative.
- Electron Gain Enthalpy: Becomes less negative down the group. Oxygen has less negative than sulfur due to small size.
Physical Properties:
- Oxygen (gas), others (solids).
- O, S (nonmetals); Se, Te (metalloids); Po (metal).
- Melting/boiling points increase down the group.
- All exhibit allotropy (e.g., O$_2$, O$_3$; Rhombic and Monoclinic S).
Anomalous Behavior of Oxygen: Small size, high electronegativity, absence of d-orbitals.
- Diatomic (O$_2$) vs. polyatomic.
- Paramagnetic O$_2$.
- Oxidation states: -2, -1, +2 (in OF$_2$).
- Hydride (H$_2$O) is liquid.
- Max covalency 2 (rarely 4).
Chemical Properties:
- Oxidation States: -2, +2, +4, +6. Oxygen common -2 (except OF$_2$, peroxides). Stability of +6 decreases, +4 increases down the group.
- Reactivity towards H: Form H$_2$E. Acidic character increases H$_2$O < H$_2$S < H$_2$Se < H$_2$Te. Thermal stability decreases down the group.
- Reactivity towards O: Form EO$_2$ and EO$_3$. Acidic oxides.
- Reactivity towards Halogens: Form EX$_6$, EX$_4$, EX$_2$. Stability: fluorides > chlorides > bromides > iodides.
- Reactivity towards Metals: Form corresponding oxides/sulfides.
Oxoacids of Sulfur: H$_2$SO$_3$, H$_2$SO$_4$, H$_2$S$_2$O$_7$, etc.
Dioxygen (O$_2$):
- Preparation: Heating KClO$_3$, decomposition of metal oxides, H$_2$O$_2$; electrolysis of water; fractional distillation of liquid air.
- Properties: Colorless, odorless gas; sparingly soluble in water; paramagnetic.
- Reactions: With metals, nonmetals, some compounds (e.g., 2SO$_2$ + O$_2 \rightarrow$ 2SO$_3$).
Ozone (O$_3$): Allotrope of oxygen.
- Preparation: Silent electric discharge through O$_2$.
- Properties: Pale blue gas; characteristic smell; diamagnetic.
- Reactions: Powerful oxidizing agent (O$_3 \rightarrow$ O$_2$ + O); bleaching agent; reduces peroxides.
- Ozone Depletion: Caused by NO, CFCs.
Sulfur Dioxide (SO$_2$):
- Preparation: Burning S, Na$_2$SO$_3$ + H$_2$SO$_4$, roasting sulfides.
- Properties: Colorless gas, pungent smell; highly soluble in water (sulfurous ac