Note
Studied by 13 peopleElectrochemical cells
Q: What is a standard hydrogen electrode (SHE)?
A: A reference electrode with a standard electrode potential of 0.00 V, consisting of hydrogen gas at 1 atm pressure in contact with a 1 mol dm⁻³ solution of H⁺ ions, using a platinum electrode.
Q: Why is platinum used in the SHE?
A: Platinum is inert and conducts electricity, providing a surface for electron transfer.
Q: Conditions of the SHE?
A: 1. 1 mol dm⁻³ H⁺ solution
2. Hydrogen gas at 100 kPa
3. 298 K (25°C)
4. Inert platinum electrode
Q: What is meant by standard electrode potential (E⁰)?
A: The voltage of a half-cell compared with the standard hydrogen electrode under standard conditions (298 K, 100 kPa, 1 mol dm⁻³ solutions).
Q: What does a more positive E⁰ value mean?
A: The half-cell has a greater tendency to gain electrons (reduction); the species is a stronger oxidising agent.
Q: What does a more negative E⁰ value mean?
A: The half-cell has a greater tendency to lose electrons (oxidation); the species is a stronger reducing agent.
Q: How do you calculate EMF of a cell from E⁰ values?
A: EMF = E⁰ (more positive) – E⁰ (more negative)
Q: Which electrode is the positive terminal in an electrochemical cell?
A: The one with the more positive E⁰ value.
Q: What flows in the external circuit of an electrochemical cell?
A: Electrons from the more negative electrode to the more positive electrode.
Q: What flows in the salt bridge and why?
A: Ions flow to maintain charge balance in each half-cell; the salt bridge allows ion movement but not electron flow.
Q: Why must a salt bridge contain inert ions (e.g. KNO₃)?
A: So it does not react with the ions in the half-cells.
Q: Why is a salt bridge necessary?
A: To complete the circuit and prevent charge build-up.
Q: What is the purpose of a high-resistance voltmeter in an electrochemical cell setup?
A: To measure EMF without drawing current, ensuring standard conditions.
Q: How does concentration affect EMF?
A: Increasing ion concentration in the reduction half-cell increases EMF; in the oxidation half-cell, it decreases EMF.
Q: How does temperature affect EMF?
A: EMF can increase or decrease depending on the enthalpy change and entropy effects of the cell reaction.
Q: What is a fuel cell?
A: A device that converts the chemical energy of a fuel directly into electrical energy by a redox reaction.
Q: What are the products of a hydrogen fuel cell (acidic conditions)?
A: Water.
Q: Half-equations in acidic hydrogen fuel cell?
A:
Anode: H₂ → 2H⁺ + 2e⁻
Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
Overall: 2H₂ + O₂ → 2H₂O
Q: Half-equations in alkaline hydrogen fuel cell?
A:
Anode: H₂ + 2OH⁻ → 2H₂O + 2e⁻
Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻
Overall: 2H₂ + O₂ → 2H₂O
Q: Advantages of fuel cells over combustion engines
A:
Higher efficiency
Less CO₂ produced
Continuous operation with fuel supply
Q: Disadvantages of hydrogen fuel cells
A:
Hydrogen is flammable
Storage and transport are difficult
Production may involve fossil fuels
Q: Why does increasing electrode surface area increase EMF slightly?
A: More surface area for redox reactions allows more efficient electron transfer.
Q: What is meant by a feasible reaction in electrochemistry?
A: One where the cell EMF is positive, indicating a thermodynamically favorable redox reaction.
Q: When is a redox reaction not feasible despite a positive EMF?
A: If kinetic factors like activation energy or lack of a catalyst prevent the reaction.
Q: How to predict feasibility of redox reactions using E⁰ values
A:
Identify the half-equations
Reverse the more negative one (oxidation)
Add the half-equations
Ensure EMF is positive
Q: Why must platinum or graphite be used in some half-cells?
A: When neither redox species is a solid conductor, a solid electrode is needed to transfer electrons.
Q: Example: Why can Fe³⁺/Fe²⁺ not use a metal electrode?
A: Both Fe³⁺ and Fe²⁺ are aqueous ions, so platinum is needed to provide a surface for redox reactions.
Q: Can non-metal species like Cl₂ be used in electrochemical cells?
A: Yes, but a solid inert electrode (e.g. Pt) must be used to allow electron transfer.
Q: How is a conventional cell diagram written?
A:
Most oxidised species on the left
Vertical line represents a phase boundary
Double vertical line for the salt bridge
Example: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
Q: When is an inert electrode needed in a cell diagram?
A: When redox species are in solution only (no metal present).
Example: Pt(s) | Fe²⁺(aq), Fe³⁺(aq)
Q: How to draw an electrochemical cell setup?
A:
Two beakers with half-cell solutions
Metal electrodes connected by wires and voltmeter
Salt bridge dipping into both solutions
Q: What makes a cell standard?
A: 1 mol dm⁻³ concentrations, 100 kPa pressure, 298 K temperature.