Electrochemistry Concepts

Redox Reactions

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.
• Oxidation raises the oxidation number.
• Reduction lowers the oxidation number.
• S-block, d-block, and some p-block elements tend to undergo oxidation.
• P-block elements (further to the right) tend to undergo reduction.

Electrochemical Cells

• Electrochemical cells utilize redox reactions.
• Electron transfer creates an electrical current between electrodes.
• A potential difference is produced which can be measured by a voltmeter.
• Most cells contain two solutions with metal electrodes and a salt bridge.
• A salt bridge contains unreactive ions that balance charge without interfering.
• Each electrochemical cell contains two half-cells.
• Each half-cell has a cell potential, indicating its reaction tendency (oxidation or reduction).

Cell Potentials (E°)

• Cell potentials are measured against the Standard Hydrogen Electrode (SHE) under standard conditions.
• SHE serves as a reference with a standard electrode potential of zero.
• Standard conditions include 1.0 mol dm-3 concentration, 298K temperature, and 100 kPa pressure.
• SHE consists of hydrochloric acid solution, hydrogen gas, and platinum electrodes.
• Platinum electrodes are metallic (conductive) but inert (non-interfering).
• Positive potentials indicate the substance is easily reduced and gains electrons.
• Negative potentials indicate the substance is easily oxidized and loses electrons.

Conventional Cell Representation

• Half-cells with the most negative potential are placed on the left.
• The most oxidized species from each half-cell goes next to the salt bridge.
• A double line represents the salt bridge.
• State symbols are included.

Calculating Cell Emf

• Emf is calculated as the potential of the right side minus the potential of the left side.
• The overall cell potential can be remembered as the most positive potential minus the most negative potential.
• A positive overall cell potential indicates a spontaneous, favorable reaction.
• The more positive the better the reaction.
• Cell emf is calculated for electrochemical cells with: different metals/non-metals, or two half cells with the same element but in different oxidation states
• For example, a Fe2+ half cell and a Fe3+ half cell can be combined to make up an electrochemical cell.

Effects of Concentration and Pressure

• Changing conditions affects the emf value.
• Increasing solution concentration makes the cell emf more positive.
• Fewer electrons are produced as concentration is increased.
• Increasing the pressure of the cell makes the cell emf more negative.
• More electrons are produced as pressure is increased.

Ecell and Entropy

• Standard emf is proportional to both ln(K) and the total entropy change.
• A positive Ecell value corresponds to a positive entropy change.

Limitations

• There are limitations to calculating a standard cell potential using the SHE and using the calculated value to determine reaction feasibility.
• Cell emf indicates thermodynamic feasibility, but not the reaction's kinetics.
• Standard cell potential requires standard conditions throughout which may be difficult.

Oxidizing and Reducing Agents

• Standard electrode potentials can be referred to as standard reduction potentials.
• They can be organized into an electrochemical series.
• Very positive electrode potentials indicate better oxidizing agents.
• The agent will oxidize species with a more negative potential.
• Very negative electrode potentials indicate better reducing agents.
• The agent will reduce species with a less negative potential.

Disproportionation

• Species are both oxidized and reduced.
• The oxidation number increases and decreases.
• If the overall Ecell value is positive, disproportionation is feasible.

Commercial Cells

• Electrochemical cells are a source of commercial energy
• They can be non-rechargeable, rechargeable, or fuel cells.

Fuel Cells

• A type of cell that generates electrical current without needing to be recharged
• A fuel undergoes combustion in oxygen to generate voltage.
• The most common type is a hydrogen fuel cell, which uses a continuous supply of hydrogen generating a continuous current.
• Hydrogen-rich compounds like methane are other common fuels.

Hydrogen Fuel Cells

• The reaction in a hydrogen fuel cell produces water as the only waste product.
• Environmentally friendly.
• The downsides include high flammability of its primary component, high production cost, meaning they are not yet commonly used.
• The hydrogen fuel cell can be carried out with either an acidic or an alkaline electrolyte.
• The overall equation in both systems is the same: H₂ + ½O₂ ⇌ H₂O
• The H+ half equations are:
• Anode H₂ ⇌ 2H+ + 2e-
• Cathode ½O₂ + 2H+ + 2e- ⇒ H₂O
• Overall H2 + ½O2 ⇒ H₂O
• In an alkaline electrolyte, half equations are as follows:
• Anode H₂ + 2OH ⇒ 2H₂O + 2e
• Cathod O₂ + 2H₂O + 4e ⇒ 4OH
• Overall H2 + ½O2 ⇒ H₂O

Redox Titration Calculations

• Method for balancing half equations:
• Balance all atoms except for oxygen and hydrogen.
• Add H₂O to balance oxygens (if needed).
• Add H+ ions to balance hydrogens (if needed).
• Add e to balance charges.

Reactions Between Ions and Potassium Manganate

• In the redox titration between iron ions and manganate ions, the iron ions are oxidised while the manganate ions are reduced.
• The method described above can be used to find their half equations.
• The overall equation is: MnO4 + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O
• The endpoint of the titration is indicated when the solution in the conical flask has a permanent pale pink colour.

Reaction between Iodine and Sodium Thiosulphate

• In the titration between iodine and thiosulphate ions, the thiosulphate ions are oxidised while the iodine is reduced.
• The half equations can be found using the method described above.
• The overall equation is: 2S2O32- + I2 → S₄O₆²¯ + 21

Errors and Uncertainty

• All apparatus has uncertainties
• Percentage uncertainty is calculated as the uncertainty of the instrument / measurement * 100
• Errors in apparatus measurements have implications for validity
• The best way of reducing uncertainties in a titration is to increase the titre volume needed for the reaction.
• This can be done by increasing the volume and concentration of the substance in the conical flask and decreasing the concentration of the substance in the burette.