(I) Topic 20 - Electrochemical Cells
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What is a SHE cell?
A Standard Hydrogen Electrode. It is the primary standard used as a point of reference (0E) when measuring other electrode potentials
What are the standard electrode potential conditions
100kPa (only affects gases), 298K, 1moldm-3
What is an electrochemical cell
Two connected half cells
What is the purpose of a salt bridge?
To allow the flow of ions between half cells
What salt is usually used in the salt bridge and why?
KNO3 as K+ and NO3- ions are quite unreactive
How would decreasing concentration of the metal ion effect the electrode potential value of the half cell?
M+ +e- -> M The equilibrium would shift to the left to oppose the change in conditions, the decrease in reactant. This would make the half cell more negative.
Equation for Electrode Potential Value of a cell (emf)
emf = +E - -E or emf = (right)E - left(E) The more negative electrode should always be on the left, but check
What is the anode? Which part of the redox reaction occurs here?
The negative electrode. Oxidation occurs.
What is the Cathode? Which part of the redox reaction occurs here?
The positive electrode. Reduction occurs here.
what is an electrochemical series
A list of electrode potentials in order of decreasing (or increasing) potential
Which electrode potentials are the best oxidising agents?
The most positive
Which electrode potentials are the best reducing agents?
The most negative
What are 3 types of commericial cell?
• Non-rechargable • Regchargable • Fuel Cells
What are the simplified electrode reactions in a lithium cell?
Positive Electrode: Li(+) +CoO2 + e- -> Li+[CoO2]- Negative Electrode: Li -> Li(+) + e-
What occurs during recharging of a cell?
The expected (discharge) reaction in reverse. (occurs by applying external current to regenerate the chemicals)
What are Fuel Cells?
Fuel Cells are cells that have a continuous supply of chemicals into the cell and so neither run out of chemicals or need recharging. Most common example of this is a Hydrogen-Oxygen Fuel Cell
Outline the diagram and reactions in a Alkaline Hydrogen-Oxygen Fuel cell
Refer to notes for Diagram: * Hydrogen flows into anode, excess water flows out Anode: { H2 +2OH(-) -> 2H2O + 2e(-) } • Electrolyte in between - H2O and OH- move between electrodes through the electrolyte Cathode: { O2 + 2H2O + 4e(-) -> 4OH(-) } • O2 moves into the cathode • Electrons flow from anode to cathode through a wire forming current Net reaction: 2H2 + O2 -> 2H2O
Pros and cons of using non-rechargable cells
Pros: Cheap Cons: waste issues, e.g. disposing of precious metals
Pros and cons of using re-chargable cells
Pros: Less waste, cheaper in the long run, lower environmental imapct Cons: Still some waste issues at end of useful life
Pros and cons of using fuel cells
Pros: Only waste product is water, does not need recharging, very efficient Cons: Need constant supply of fuels, hydrogen is flammable and explosive, hydrogen usually made using fossil fuels, high cost of fuel cells
How do cells generate an electric current?
Electrons flow from the andode to the cathode through a wire, forming an electrical current.
Outline the diagram and reactions in an acidic Hydrogen-Oxygen Fuel cell
Refer to notes for Diagram: * Hydrogen flows into anode Anode: { H2 -> 2H+ + 2e- } • Electrolyte in between - H+ ions move through between electrodes from anode to cathode Cathode: { O2 + 4H+ + 4e- -> 2H2O } • O2 moves into the cathode, and excess water flows out • Electrons flow from anode to cathode through a wire forming current Net reaction: 2H2 + O2 -> 2H2O