Pattern 1: First-Order Half-Life Calculations (Most Frequent)

NEET gives k and asks for t_{1}/_{2}, or gives % completion and asks for time.

Shortcut: 50% = 1t_{1}/{2}, 75% = 2t{1}/{2}, 87.5% = 3t{1}/_{2}

Use: $t = n \times t_{1/2}$ when [A] = [A]_{0} × (1/2)^n

Pattern 2: Arrhenius Equation Ea Determination

Given k at two temperatures, find Ea. Always:

1. Convert Celsius to Kelvin
2. Calculate 1/$T_{1}$ − 1/$T_{2}$ ($T_{1}$ < $T_{2}$, so this is positive)
3. Apply: log(k_{2}/k_{1}) = (Ea/2.303R)(1/$T_{1}$ − 1/$T_{2}$)

Common error: Using $T_{1}$ > $T_{2}$ in the formula → get negative Ea (wrong).

Pattern 3: Order vs Molecularity MCQs

Always remember:

• Molecularity is NEVER zero or fractional
• Order IS experimental and CAN be zero or fractional
• For complex reactions: order ≠ molecularity (in general)

Trap Question 1: Doubling Concentration

"First-order reaction t_{1}/{2} = 100 s. Zero-order reaction t{1}/{2} = 100 s at [A]{0} = 0.2 M. If [A]_{0} doubled to 0.4 M..."

• First-order t_{1}/_{2}: still 100 s (independent of concentration)
• Zero-order t_{1}/{2}: 200 s (directly proportional to [A]{0})

Trap Question 2: Fractional Order Units

"rate = k[A]^(1/2)[B]^(3/2)" → overall order = 1/2 + 3/2 = 2 → units = L $mol^{-1}$ $s^{-1}$

Trap: Students see 1/2 and 3/2 and think units must have fractional powers. Wrong — add orders first.

Trap Question 3: Catalyst and K

"A catalyst increases K" → FALSE. A catalyst does NOT change K (equilibrium constant). It only changes rate.

Trap Question 4: Molecularity of Complex Reaction

For overall reaction A + B → C + D, molecularity is NOT 2. Molecularity applies only to elementary reactions (individual mechanism steps), not overall reactions.