The Mistake Bank
Class 12 - Chapter 2: Electrochemistry
Sparks fly when logic fails. Keep your potentials straight.
Conductivity vs Dilution
ConductivityScenario: What happens to Conductivity ($\kappa$) and Molar Conductivity ($\Lambda_m$) when you dilute a solution?
Student thinks: "Dilution makes ions move freely, so conductivity must increase."
Conclusion: "Both $\kappa$ and $\Lambda_m$ increase."
(Half right, half deadly!)
Kappa Drops, Lambda Pops!
Conductivity ($\kappa$): Decreases because the number of ions per unit volume decreases.
Molar Conductivity ($\Lambda_m$): Increases because the increase in volume dominates the decrease in $\kappa$.
Equilibrium Conditions
Nernst EquationScenario: When a cell reaches equilibrium, what is the value of $E^\circ_{cell}$?
Student thinks: "At equilibrium, the battery is dead. Voltage is zero."
Answer: "$E^\circ_{cell} = 0$"
(Wrong! The standard potential is a constant!)
$E_{cell}$ vs $E^\circ_{cell}$
At equilibrium, the potential difference ($E_{cell}$) becomes zero.
The Standard Potential ($E^\circ_{cell}$) remains constant for that specific reaction.
At equilibrium: $E_{cell} = 0$, but $E^\circ_{cell} = \frac{0.059}{n} \log K_c$.
Electrolysis of Aqueous NaCl
ElectrolysisScenario: What is formed at the Cathode during electrolysis of Aqueous NaCl?
Student sees $Na^+$ ions moving to Cathode.
Answer: "Sodium metal ($Na$)."
(Sodium reacts with water immediately/Reduction potential issue.)
Water Wins the Race!
The reduction potential of Water (to form $H_2$) is higher (less negative) than Sodium.
$$ 2H_2O + 2e^- \rightarrow H_2 + 2OH^- $$
Product: Hydrogen Gas ($H_2$).
(Sodium is only formed in Molten NaCl).
The "Anode is Positive" Myth
Cell TypesScenario: Determine the sign of the Anode in a Galvanic Cell vs. Electrolytic Cell.
Student memorizes: "Anode is Positive (P.A.N.I.C?)"
Applies this to Galvanic cells.
Answer: "Positive." (Wrong!)
Follow LOAN and Red Cat!
LOAN (Galvanic): Left Oxidation Anode Negative.
- Galvanic: Anode is Negative (Source of electrons).
- Electrolytic: Anode is Positive (Attracts anions).
Note: Oxidation always happens at Anode, regardless of sign.
Log K Calculation
Calculation ErrorScenario: Calculating Equilibrium Constant ($K_c$) from $E^\circ_{cell}$.
Student gets the formula correct: $\log K_c = \frac{n E^\circ}{0.059}$.
Calculates value, say 50.
Writes Answer: $K_c = 50$.
(Forgot the Antilog!)
Don't Stop at Log!
The formula gives you the logarithm of K.
If $\log K_c = 50$, then:
$$ K_c = 10^{50} $$
Always take the antilog as the final step.
Faraday's Law Valency
Quantitative ElectrolysisScenario: How much charge is needed to deposit 1 mole of Aluminum from $AlCl_3$?
Student thinks: "1 mole requires 1 Faraday."
Answer: 96500 C.
(Ignores the charge on the ion!)
Q = nF
Reaction: $Al^{3+} + 3e^- \rightarrow Al$.
It takes 3 moles of electrons to deposit 1 mole of Aluminum.
Charge = $3F = 3 \times 96500 \text{ C}$.
Confess Your Sins!
"Current flows from + to -, but electrons flow from - to +. Are you flowing against the logic?"
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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