Electrochemical Cells
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How to make a cell?
A metal diped in it’s ions
How would a cell be made if there was 2 aqueous ions present?
A platinum electrode is used as it is electrically conductive and inert
How to make an electrochemical cell?
Join 2 half cells together joined by a wire, voltmeter and salt bridge
What reaction occurs in an electrochemical cell?
Redox reaction. Electrons flow from the more reactive metal to the less reactive.
In a zinc, copper cell - which would be undergoing reduction and oxidation? Reaction? Observation?
Zinc- Oxidation Zn(s) ⇌ Zn2+(aq) + 2e- Zinc electrode becomes thinner as Zn2+ is produced to make electrons Copper- Reduction Cu2+(aq) + 2e- ⇌ Cu(s) Copper electrode gets thicker as Cu2+ ions recieve the electrons and turn into copper.
Requirements for salt bridge
Must dip into each solution without touching the electrodes
What is an electrode potential?
How easily the half cell gives up electrons and is oxidised
What are the standard conditions for standard hydrogen electrodes?
Temperature at 298K Pressure at 100kPa Concentration of ions at 1 moldm-3
What does a higher standard electrode potential mean in terms of oxidising potential?
Higher = stonger oxidising agent Lower = weaker oxidising agent
Standard cell potential equation
Eθcell = Eθreduced - Eθoxidised
Standard cell notation
Reduced form | Oxidised form || Oxidised form | Reduced form Single line shows a state change Double line shows salt bridge
What can standard electrode potentials be used to predict?
If a stated procedure is likely to occur under standard conditions
Process to predict if a reaction will occur
Work out which species is oxidised and reduced. Combine two reactions. Calcute the Eθcell value. If it is feasible then the value is positive.
What is a common example of a rechargeable battery?
Lithium ion
Example of where rechargeable batteries are used
Tools, tablets, mobile phones, electric cars
How do rechargeable batteries work?
Supplying a current forces electrons to flow the opposite direction and reverse the overall discharge equarion
How are fuel cells fuelled?
Continous external supply of chemicals so does not require being recharged
Positive and negarive electrode reactions in a lithium ion cell. Discharge and recharge equations?
Positive electrode: Li+ + CoO2 + e– ⇌ Li+[CoO2]– Negative electrode: Li ⇌ Li+ + e– Discharge: Li + CoO2 ⇌ Li+[CoO2]- Recharge: Li+[CoO2]- ⇌ Li + CoO2
What happens within an alkaline hydrogen-oxygen fuel cell?
Hydrogen is fed into the negative electrode, reacting with OH- ions. H2(g) + 2OH-(aq) -> 2H20(l) + 2e- H+ ions moves through electrolyte to the positive electrode. Electrons move through a wire from negative to positive electrode. O2 fed in while e- and H+ enter. Recreating OH- ions. O2 + 2H2O + 4e- -> 4OH- Warer emitted into surroundings.
Overall equation of an alkaline hydrogen fuel cell
2H2 + O2 -> 2H2O
In an alkaline fuel cell, what is the overall EMF, EMF at cathode and EMF at anode. Respective equations with each.
Anode: -0.83V H2(g) + 2OH-(aq) -> 2H20(l) + 2e- Cathode: 0.4V O2 + 2H2O + 4e- -> 4OH- Overall: 1.23V 2H2 + O2 -> 2H2O
What are ion exchange membranes within alkaline hydrogen fuel cells?
Line platinum electrodes to allow OH- ions to pass through but not hydrogen and oxygen gas
Advantages of fuel cells
More efficient as a lot more energy into kinetic energy instead of thermal. Do not need to be recharged. Only waste product is water.
Disadvantage of fuel cells
Highly flammable. Needs pressured to store and transport so is expensive . Use of fossil fuels still required to pass the water through the electrolyte to make hydrogen and oxygen.