The Mistake Bank
Class 12 - Chapter 3: Chemical Kinetics
Time waits for no one, especially during a rate of reaction problem.
Order vs Stoichiometry
Rate LawScenario: For reaction $2A + B \rightarrow C$, what is the Order?
Student sums up the stoichiometric coefficients:
$$ \text{Order} = 2 + 1 = 3 $$
(You cannot determine Order just by looking!)
Order is Experimental!
Unless it is specified as an Elementary Reaction, you cannot predict the order from the balanced equation.
It could be 0, 1, 2, or even fractional. It must be given experimentally.
Units of Rate Constant (k)
UnitsScenario: Calculate $k$ for a Second Order reaction. Units are crucial.
Student blindly uses the First Order unit:
$$ \text{Unit} = s^{-1} \text{ or } min^{-1} $$
(This only works for Order = 1.)
Use the General Formula!
$$ \text{Unit} = (mol \cdot L^{-1})^{1-n} \cdot s^{-1} $$
For Second Order ($n=2$):
$$ (mol \cdot L^{-1})^{-1} \cdot s^{-1} = L \cdot mol^{-1} \cdot s^{-1} $$
Half-Life of Zero Order
Half-LifeScenario: For a Zero Order reaction, concentration is doubled. What happens to $t_{1/2}$?
Student recalls: "Half-life is independent of concentration."
Answer: "It remains constant."
(Confusing it with First Order!)
Zero Order Depends on Initial Conc!
$$ t_{1/2} = \frac{[R]_0}{2k} $$
For Zero Order, half-life is directly proportional to initial concentration.
If conc doubles, $t_{1/2}$ doubles.
Catalyst Function
Activation EnergyScenario: A catalyst is added. Does $\Delta G$ (Gibbs Energy) of the reaction change?
Student thinks: "Catalyst makes reaction easier, so it becomes more spontaneous."
Answer: "$\Delta G$ becomes more negative."
Catalyst Affects Path, Not Destinaton!
A catalyst lowers the Activation Energy ($E_a$).
It does NOT alter thermodynamics parameters like $\Delta G$, $\Delta H$, or the Equilibrium Constant ($K$).
Molecularity vs Order
Complex ReactionsScenario: Reaction has Order = 1.5. What is its Molecularity?
Student assumes Molecularity = Order.
Answer: "1.5"
(Impossible! You cannot have 1.5 molecules colliding.)
Molecularity Must Be an Integer!
Molecularity is the number of colliding species in an elementary step (1, 2, or 3).
It cannot be zero or fractional.
Note: Molecularity has no meaning for complex reactions, only for elementary steps.
Arrhenius Slope
Arrhenius EquationScenario: Plot $\ln k$ vs $1/T$. What is the slope?
Student forgets the sign or the logarithm base.
Answer: "Slope = $E_a / R$"
(Watch the negative sign!)
Negative Slope!
Equation: $\ln k = \ln A - \frac{E_a}{RT}$
Plotting $y = \ln k$ vs $x = 1/T$:
$$ \text{Slope} = -\frac{E_a}{R} $$
(If plotting $\log_{10} k$, slope is $-\frac{E_a}{2.303R}$)
Confess Your Sins!
"Kinetics determines how fast you get the answer, but Accuracy determines if you get marks."
Did one of these catch you? Or do you have a different horror story from your last exam?
Scroll down to the comments section below and tell us:
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