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Electrochemistry Unit
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Electrochemistry Unit

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Questions and Answers

How would you determine the standard electrode potential of the system Mg^2+|Mg?

By consulting the table of standard electrode potentials.

Can you store copper sulphate solutions in a zinc pot?

No

Consult the table of standard electrode potentials and suggest three substances that can oxidise ferrous ions under suitable conditions.

Fluorine, chlorine, bromine

Define electrochemistry.

<p>Electrochemistry is the study of production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous chemical transformations.</p> Signup and view all the answers

What is a galvanic cell also known as?

<p>Voltaic cell</p> Signup and view all the answers

The reduction half reaction occurs on the zinc electrode in a galvanic cell.

<p>False</p> Signup and view all the answers

The electrode potential is also known as _____ potential.

<p>electrode</p> Signup and view all the answers

What does a positive E⊖ value indicate for a redox couple?

<p>It is a weaker reducing agent than the H+/H2 couple.</p> Signup and view all the answers

What does a negative E⊖ value indicate for a redox couple?

<p>It is a stronger reducing agent than the H+/H2 couple.</p> Signup and view all the answers

What is the cell reaction for the Daniell cell?

<p>Ni(s) + 2Ag+(aq) →Ni2+(aq) + 2Ag(s)</p> Signup and view all the answers

What is the Nernst equation for a general electrochemical reaction?

<p>E(cell) = E(cell) - nF ln [A]a [B]b</p> Signup and view all the answers

What is the equilibrium constant equation in terms of standard potential for a reaction?

<p>ΔrG⊖ = -RT ln K</p> Signup and view all the answers

How can an important thermodynamic quantity be obtained from the measurement of cell potential?

<p>Standard Gibbs energy of the reaction</p> Signup and view all the answers

What is the equation to calculate the potential of a hydrogen electrode in a solution with pH 10?

<p>E = E⊖ + 0.0592 pH</p> Signup and view all the answers

What is the electrical resistance represented by in symbols and its unit of measurement?

<p>R, ohm (Ω)</p> Signup and view all the answers

What is the constant of proportionality in the relationship between resistance and area of cross section?

<p>ρ (rho)</p> Signup and view all the answers

Which term is used for the inverse of resistance?

<p>Conductance</p> Signup and view all the answers

What is the SI unit of conductance?

<p>Siemens (S)</p> Signup and view all the answers

Materials with very low conductivity are classified as conductors.

<p>False</p> Signup and view all the answers

What is the most familiar example of a primary battery?

<p>The dry cell (Leclanche cell).</p> Signup and view all the answers

What is the main difference between primary and secondary batteries?

<p>Primary batteries have reactions that occur only once and cannot be reused, while secondary batteries can be recharged and reused multiple times.</p> Signup and view all the answers

Which of the following components act as the anode in a dry cell?

<p>Zinc container</p> Signup and view all the answers

The space between the electrodes in a dry cell is filled by a moist paste of ammonium chloride (NH4Cl) and zinc chloride (ZnCl2), to facilitate the flow of _______________.

<p>ions</p> Signup and view all the answers

What is the symbol used to represent limiting molar conductivity?

<p>Λm</p> Signup and view all the answers

What does the Kohlrausch law of independent migration of ions state?

<p>The limiting molar conductivity of an electrolyte is the sum of the individual contributions of the anion and cation of the electrolyte.</p> Signup and view all the answers

What does the value 'A' represent in the equation Λm = Ë m° – A c^(1/2)?

<p>Constant for a given solvent and temperature</p> Signup and view all the answers

The conductivity of 0.001028 mol L^–1 acetic acid is ____. Calculate its dissociation constant if Λm° for acetic acid is 390.5 S cm^2 mol^–1.

<p>4.95 × 10^–5 S cm^–1</p> Signup and view all the answers

Why does the conductivity of a solution decrease with dilution?

<p>The conductivity of a solution decreases with dilution because there are fewer ions available to carry the charge in a more diluted solution.</p> Signup and view all the answers

How can the Λ ° value of water be determined?

<p>The Λ ° value of water can be determined by measuring its molar conductivity at a specific concentration.</p> Signup and view all the answers

Calculate the degree of dissociation and dissociation constant of 0.025 mol L–1 methanoic acid with given values of λ0(H +) and λ0(HCOO–).

<p>Degree of dissociation: 0.122, Dissociation constant: 1.34 x 10^-4</p> Signup and view all the answers

What are Faraday's two laws of electrolysis?

<p>First Law: Amount of chemical reaction is proportional to quantity of electricity. Second Law: Amounts of substances liberated are proportional to their chemical equivalent weights.</p> Signup and view all the answers

What is the quantity of electricity in coulombs needed to reduce 1 mol of Cr2O 72–?

<p>The quantity of electricity needed is 96,800 C.</p> Signup and view all the answers

What is the quantity called which is denoted by the symbol G* and depends on the distance between the electrodes and their area of cross-section?

<p>cell constant</p> Signup and view all the answers

What is the equation to calculate the cell constant G*?

<p>G* = l / A</p> Signup and view all the answers

What is the equation to calculate molar conductivity (Λm)?

<p>Λm = κ / c</p> Signup and view all the answers

What is the relationship between molar conductivity expressed in S cm² mol⁻¹ and S m² mol⁻¹?

<p>1 S m² mol⁻¹ = 10⁴ S cm² mol⁻¹</p> Signup and view all the answers

What is the equation to calculate resistivity (ρ) using resistance (R) and area (A)?

<p>ρ = R * A</p> Signup and view all the answers

Study Notes

Electrochemistry

  • Electrochemistry is the study of the production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous chemical reactions.

Galvanic Cells

  • A galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy.
  • The Gibbs energy of the spontaneous redox reaction is converted into electrical work, which can be used to run a motor or other electrical gadgets.
  • The Daniell cell is a type of galvanic cell that consists of a zinc rod and a copper rod dipped in solutions of their respective salts.

Electrochemical Cell vs. Electrolytic Cell

  • An electrochemical cell (galvanic cell) converts chemical energy into electrical energy.
  • An electrolytic cell uses electrical energy to carry out non-spontaneous chemical reactions.

Galvanic Cell Construction

  • A galvanic cell consists of two half-cells or redox couples, each consisting of a metallic electrode dipped into an electrolyte.
  • The two half-cells are connected by a metallic wire through a voltmeter and a switch externally.
  • The electrolytes of the two half-cells are connected internally through a salt bridge.

Electrode Potential

  • The potential difference between the electrode and the electrolyte is called the electrode potential.
  • The electrode potential is the potential of a half-cell when all the species involved in the half-cell are at unity concentration.
  • Standard electrode potentials are now called standard reduction potentials.

Cell Potential

  • The potential difference between the two electrodes of a galvanic cell is called the cell potential.
  • The cell potential is measured in volts and is the difference between the electrode potentials of the cathode and anode.

Standard Hydrogen Electrode

  • The standard hydrogen electrode is a half-cell that consists of a platinum electrode coated with platinum black, dipped in an acidic solution and pure hydrogen gas is bubbled through it.
  • The concentration of both the reduced and oxidized forms of hydrogen is maintained at unity.
  • The standard hydrogen electrode is assigned a zero potential at all temperatures.

Measurement of Electrode Potential

  • The potential of individual half-cell cannot be measured.
  • The difference between the two half-cell potentials can be measured, which gives the emf of the cell.
  • The emf of the cell can be used to determine the standard electrode potential of a half-cell.### Electrochemistry
  • Platinum (Pt) is used in half-cells, such as:
    • Hydrogen electrode: Pt(s)|H2(g)|H+(aq) with half-cell reaction: H+(aq) + e- → ½H2(g)
    • Bromine electrode: Pt(s)|Br2(aq)|Br-(aq) with half-cell reaction: ½Br2(aq) + e- → Br-(aq)

Standard Electrode Potentials

  • The standard electrode potentials are important and can be used to extract useful information.
  • The values of standard electrode potentials for some selected half-cell reduction reactions are given in Table 3.1.
  • If the standard electrode potential of an electrode is greater than zero, its reduced form is more stable compared to hydrogen gas.
  • If the standard electrode potential is negative, hydrogen gas is more stable than the reduced form of the species.

Electrochemical Cells

  • Electrochemical cells are used to determine the pH of solutions, solubility product, equilibrium constant, and other thermodynamic properties.
  • They are also used for potentiometric titrations.

Nernst Equation

  • The Nernst equation is used to calculate the electrode potential at any concentration measured with respect to the standard hydrogen electrode.
  • The equation is: E(cell) = E°(cell) – (RT/nF) ln[Mn+]/[M]
  • Where E(cell) is the electrode potential, E°(cell) is the standard electrode potential, R is the gas constant, T is the temperature in Kelvin, n is the number of electrons, F is the Faraday constant, [Mn+] is the concentration of the species, and [M] is the concentration of the solid.

Daniell Cell

  • In the Daniell cell, the electrode potential for any given concentration of Cu and Zn2+ ions can be calculated using the Nernst equation.
  • The cell potential, E(cell), is the difference between the electrode potential of the cathode and the anode.
  • The cell potential depends on the concentration of both Cu2+ and Zn2+ ions, and increases with an increase in the concentration of Cu2+ ions and a decrease in the concentration of Zn2+ ions.### Nernst Equation and Electrochemical Cells
  • The Nernst equation is a fundamental concept in electrochemistry, relating the standard potential of a cell to the equilibrium constant of a reaction.
  • The Nernst equation is given by: E(cell) = E°(cell) - (RT/nF) ln Q, where E(cell) is the cell potential, E°(cell) is the standard cell potential, R is the gas constant, T is the temperature in Kelvin, n is the number of electrons transferred, F is the Faraday constant, and Q is the reaction quotient.

Electrochemical Cells and Gibbs Energy

  • The Gibbs energy of a reaction is related to the cell potential by: ΔrG = -nFE(cell), where ΔrG is the Gibbs energy of the reaction, n is the number of electrons transferred, F is the Faraday constant, and E(cell) is the cell potential.
  • The standard Gibbs energy of a reaction is related to the standard cell potential by: ΔrG° = -nFE°(cell).

Conductance of Electrolytic Solutions

  • Electrical conductance through electrolytic solutions is called electrolytic or ionic conductance.
  • The conductivity of electrolytic solutions depends on:
    • The nature of the electrolyte added
    • The size of the ions produced and their solvation
    • The nature of the solvent and its viscosity
    • The concentration of the electrolyte
    • The temperature (increases with increasing temperature)
  • Materials can be classified into conductors, insulators, and semiconductors based on their conductivity.

Measurement of Conductivity of Ionic Solutions

  • The conductivity of ionic solutions can be measured using a specially designed vessel called a conductivity cell.
  • The conductivity cell is used to overcome the problems of passing direct current through ionic solutions, which can change the composition of the solution.
  • Alternating current (AC) is used to measure the conductivity of ionic solutions to prevent electrochemical reactions.

Equilibrium Constant and Standard Cell Potential

  • The equilibrium constant of a reaction is related to the standard cell potential by: E°(cell) = (RT/nF) ln K, where E°(cell) is the standard cell potential, R is the gas constant, T is the temperature in Kelvin, n is the number of electrons transferred, F is the Faraday constant, and K is the equilibrium constant.

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