1.11 Electrode Potentials and Electrochemical Cells
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Composition of a half-cell
• Two species of the same element in different oxidation states • Contain a solution and a solid electrode
In half equations for electrode reactions the forwards reaction is __________
Reduction
How are half cells connected?
• Wire through an external circuit • Salt bridge between the two half cells
Salt bridge
• Filter paper soaked with saturated KCl or KNO₃ solution • Allows flow of ions
Condition for ions in salt bridge
Must not react with ions in the half-cells
Standard conditions for cells
• 100kPa • 298K • 1.00 mol dm⁻³
Which side should positive electrode be drawn on?
The RHS (apart from when there is a hydrogen half cell, which is always drawn on the left)
Standard electrode potentential
The e.m.f. of a half-cell compared with a standard hydrogen half-cell. Measured at 298K, all solutions having concentrations of 1 mol dm⁻³ and all gasses having a pressure of 100kPa.
What electrode is used if there is no solid?
Platinum (Pt) Used as it in inert and conducts electricity
Conventional representation of a cell
• Vertical line for phase boundary • Double vertical line for salt bridge • Positive electrode on the right (unless H used) • Species written in order of oxidation state (highest oxidation state next to salt bridge)
Limitations of using electrode potential to predict redox reactions
• Activation energy mat be too high for the reaction to occur • Many reactions do not occur at standard conditions
v-t graph for a cell
Straight line with a negative gradient
Primary cell
Non-rechargeable
Secondary cells
Rechargeable
Why are lithium ion cells used in phones and laptops?
Lithium is the least dense metal
v-t graph for a fuel cell
Horizontal line (constant v)
Fuel cell
• Uses energy from the reaction of fuel with oxygen • Can operate continuously
Half equations for alkaline fuel cells
H + 2OH⁻ –> 2H₂O + 2e⁻ O₂ + 2H₂O + 4e⁻ –> 4OH⁻
Half equations for acidic fuel cells
H₂ –> 2H⁺ +2e⁻ O₂ + 4H⁺ +4e⁻ –> 2H₂O
Advantages of fuel cells (compared to combustion engines)
• Efficient - Convert more of their available energy to KE • No CO₂ produced - only waste product is water • No recharging - can operate continuously if supplied with fuel
Disadvantages of fuel cells (compared to combustion engines)
• CO₂ released in H₂ production • H₂ is highly flammable • Lack of infrastructure of H₂ cars