What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

What happens at the anode and cathode of a galvanic cell?

Oxidation occurs at the anode (negative terminal) and reduction at the cathode (positive terminal).

Why is the standard hydrogen electrode used as a reference?

Its electrode potential is assigned exactly zero, so all other potentials are measured relative to it.

Write the Nernst equation for a cell at 298 K.

\(E_{cell}=E^{\circ}_{cell}-\dfrac{0.059}{n}\log Q\), where \(Q\) is the reaction quotient.

How are \(\Delta G^{\circ}\) and \(E^{\circ}_{cell}\) related?

\(\Delta G^{\circ}=-nFE^{\circ}_{cell}\); a positive cell potential corresponds to a negative (spontaneous) free energy.

How does molar conductivity vary with dilution?

It increases on dilution because the degree of dissociation and ionic mobility effectively increase.

State Kohlrausch's law of independent migration of ions.

At infinite dilution each ion contributes independently, so \(\Lambda_m^{\circ}=\lambda^{\circ}_++\lambda^{\circ}_-\).

State Faraday's first law of electrolysis.

The amount of substance liberated at an electrode is directly proportional to the quantity of electricity passed through the cell.

What value of charge equals one Faraday?

One Faraday is about 96500 C, the charge carried by one mole of electrons.

Why is the lead storage battery called a secondary cell?

It is rechargeable; the cell reaction can be reversed by passing current in the opposite direction.

What is the electrochemical basis of the rusting of iron?

Iron acts as anode and gets oxidised while oxygen is reduced at cathodic sites, forming hydrated iron(III) oxide (rust).

Electrochemistry

Class 12 Chemistry · Chemistry Part I · Chapter 2

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CBSE Class 12 · Chemistry

Electrochemistry: 9 marks, biggest unit

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Summary

Electrochemistry studies the relationship between chemical energy and electrical energy. In a galvanic (voltaic) cell a spontaneous redox reaction produces electricity; oxidation occurs at the anode and reduction at the cathode. Each electrode has a standard electrode potential, measured against the standard hydrogen electrode (assigned zero), and the standard cell potential is \(E^{\circ}_{cell}=E^{\circ}_{cathode}-E^{\circ}_{anode}\). The Nernst equation gives the potential under non-standard conditions, \(E=E^{\circ}-\dfrac{RT}{nF}\ln Q\), and at equilibrium relates to the equilibrium constant and to free energy through \(\Delta G^{\circ}=-nFE^{\circ}_{cell}\). Ionic conduction is described by conductivity and molar conductivity \(\Lambda_m\), which increases on dilution; for strong electrolytes it follows Debye-Huckel-Onsager behaviour and Kohlrausch's law of independent migration of ions lets us find limiting molar conductivities. Electrolysis uses electrical energy to drive non-spontaneous reactions, with the amounts deposited governed by Faraday's laws. Practical cells include the dry cell, mercury cell, lead storage battery, nickel-cadmium cell and fuel cells, while corrosion of iron is an electrochemical process that can be prevented by coating or sacrificial protection.

Galvanic cells and electrode potentialsStandard hydrogen electrode and EMF seriesNernst equation and equilibriumConductivity and molar conductivityKohlrausch's law and electrolysisBatteries, fuel cells and corrosion

Key terms

Galvanic cell

A device that converts the chemical energy of a spontaneous redox reaction into electrical energy.

Standard electrode potential

The potential of an electrode relative to the standard hydrogen electrode under standard conditions.

Nernst equation

Relates electrode or cell potential to concentration, \(E=E^{\circ}-\dfrac{RT}{nF}\ln Q\).

Molar conductivity

Conductance of all ions from one mole of electrolyte, \(\Lambda_m\), which rises on dilution.

Kohlrausch's law

Limiting molar conductivity is the sum of independent contributions of the cation and anion.

Faraday's laws

The mass of substance deposited at an electrode is proportional to the charge passed.

Extra questions & answers

Try one from this chapter

In a galvanic cell, oxidation occurs at the:

Flashcards & interactive practice