3.1 Redox and Standard Electrode Potential Flashcards
Define oxidation in terms of electron transfer
Loss of electrons
Define reduction in terms of electron transfer
Gain of electrons
What is a redox reaction?
A reaction in which oxidation and reduction occur on different species simultaneously
What is standard electrode potential?
Standard electrode potential is the potential across the electrodes when a redox system is connected to a hydrogen half-cell under standard conditions.
What conditions are required for measuring the standard electrode potential?
- 298 K temperature
- 100 kPa pressure
- 1.00 mol dm^-3 concentration of ions
Why is a hydrogen half-cell needed as a reference?
The hydrogen half cell is used to allow for easy comparison between the electrode potential of different elements. The standard electrode potential for hydrogen is assumed to be zero volts at any temperature.
What is the experimental setup to calculate the standard electrode potential for zinc?
diagram with hydrogen half-cell on the left (H2 gas, platinum cathode, acid(HNO3)) and zinc on the right (zinc anode, Zn(NO3)2 solution), NaCl salt bridge and voltmeter between electrodes
Why must metal electrodes be cleaned with sandpaper before creating an electrochemical cell?
To remove any metal oxide that has formed on the surface and improve electrical conductivity.
Describe the movement of electrons in an electrochemical cell
Electrons flow through the wire from the positive electrode to the negative electrode.
Why is a salt bridge used in an electrochemical cell?
To maintain the charge balance and complete the circuit.
Negative electrons are moving from one half cell to another. Without the salt-bridge, positive charge would build up in the half cell containing the anode and negative charge would build up in the half cell containing the cathode. This would cause the reaction to stop.
Why must an inert salt be used in the salt bridge?
The salt must be inert so that it doesn’t react with the solutions and alter the ion concentrations. If a reactive salt was used, the cell potential would change.
What moves across the salt bridge?
Ions
For what range of cell potential values is a process feasible?
Cell potential must be greater than 0
How can cell potential be calculated?
E cell = E (positive terminal) - E (negative terminal)
Why might theoretical cell potential values be different to values obtained experimentally?
Conditions may be non-standard
Draw a diagram showing a standard Fe^2+/Fe^3+ cell
platinum electrodes in Fe^2+/Fe^3+ solution and H+ ions, Hydrogen gas, salt bridge and voltmeter
A cell is made up of the following half cells:
Ag+(aq) + e- <—> Ag (s) E= +0.80V
Cu2+(aq) + 2e- <—> Cu(s) E= +0.34V
Write the overall cell equation and calculate the standard cell potential
2Ag+(aq) + Cu(s) —> Ag(s) + Cu2+(aq)
E cell = +0.80 - (+0.34)= 0.46V
In an electrochemical cell, is the more negative cell oxidised or reduced?
Oxidised
What is a fuel cell?
A cell that continually produces a voltage as long as it is supplied with oxygen and a fuel (like hydrogen).
What is the only product of a hydrogen-oxygen fuel cell?
Water
How does a hydrogen fuel cell work?
Hydrogen and oxygen are pumped through porous electrodes. The electrolyte is often an acid such as phosphoric acid.
Hydrogen and oxygen react, producing energy and water.
What are the two half equations taking place in a hydrogen-oxygen fuel cell?
2H2 + 4OH- <—> 4H2O + 4e-
O2 + 2H2O + 4e- <—> 4OH-
Write an equation for the overall reaction that takes place in a hydrogen-oxygen fuel cell
2H2 + O2 —> 2H2O
What are the advantages of using fuel cells?
- No pollution.
- They produce more energy than an alternative fuel like petrol.
- The process is continuous as long as fuel is supplied.
What are the disadvantages of using fuel cells?
- Materials used to make them are expensive.
- High pressure tanks are required to store oxygen and fuels like hydrogen.
- Hydrogen is expensive and hard to store.
- Efficiency is affected by temperature.
