Cells Flashcards
Primary cells
Cells that are Not rechargeable and will eventually run out when reactants get used up
Secondary cells
Rechargeable cells that use a reversible reaction
What is the theory behind electrochemical cells?
There is a transfer of electrons in the redox reaction between half cells which acts as an electric current
Storage cells
A rechargeable cell
Example of a primary cell
Alkaline batteries: Zinc + Manganate (IV)
Why can Storage cells be recharged?
Because the overall reaction is reversible
Why can a current occur in an electrochemical cell?
Because the voltmeter was replaced with an ammeter to allow flow of electrons between half cells so overall, reactions occur
Why does the current in a non rechargeable cell eventually fall to 0?
Because the reactants get used up and form products of overall reaction
So no current will flow as no more electrons are transferred
What is the discharge reaction in an electrochemical cell?
The forward overall reaction because charge is given out when electrons from oxidised reactant is transferred to reduced reactant
What is the charging reaction in an electrochemical cell?
The backward reaction because reactants are reformed for the forward reaction to proceed again and give off more charge
Why do chargeable cells eventually run out of charge?
Because of leakage of the electrolyte solvent or side reactions occur that decrease concentration of reactants over time
Example of rechargeable secondary cell?
Lithium ion batteries
Why is graphite used in lithium ion batteries?
Because absorbs reagents so allows ions to react in absence of a solvent
Why can water not be used as a solvent in lithium ion batteries
Because it is not inert so water will react with the lithium
Fuel cell
Uses a constant supply of fuel and oxidising agent to produce a voltage via a chemical reaction (without burning the fuel)
General outline of how a fuel cell works
Fuel reacts with oxygen
Uses energy in this to create a voltage
Examples of fuel in a fuel cell
Methanol
Hydrogen
2 types of hydrogen fuel cells
In acidic electrolyte conditions
In alkaline electrolyte conditions
Summary of alkaline hydrogen fuel cell: anode
hydrogen fed into cell and oxidised to H+ (but combines with hydroxide ions from electrolyte) to produce water
Electrons are released from oxidation
Equation at anode of alkaline hydrogen fuel cells
2H₂ + 4OH⁻ —-> 2H₂O + 4e⁻
OXIDATION OF HYDROGEN GAS
Electrolyte in an alkaline hydrogen fuel cell
Potassium hydroxide
Summary of alkaline hydrogen fuel cell: cathode
Oxygen is fed into cell
Accepts electrons (reduced) from circuit and reacts with water to reform hydroxide ions
Equation at cathode of an alkaline hydrogen fuel cell
O₂ + 2H₂O +4e⁻ —-> 4OH⁻
REDUCTION OF OXYGEN
Why is the reaction at the anode written as reduction when it is oxidation?
Because the standard notation is to write electrode equations as reduction :)
Exchange of hydroxide ions in an alkaline hydrogen fuel cell
between a membrane from the positive electrode (produced from reduction of oxygen) to the negative electrode (gained after hydrogen was oxidised to
Overall hydrogen fuel cell equation
2H₂ + O₂ —–> 2H₂O
Summary of acid hydrogen fuel cell: anode
Hydrogen is fed into the fuel cell
Hydrogen is oxidised by losing electrons to form H+ and releases electron
Equation for the hydrogen fuel cell in acidic conditions
H₂ —–> 2H⁺ + 2e-
Summary of acid hydrogen fuel cell: cathode
Oxygen is fed into the fuel cell and combines with H+ (from the anode) to produce water
Accepts electrons from the anode
Electrolyte used in acid hydrogen fuel cell
Phosphoric acid
How many electrons are transported in each half cell in a fuel cell?
4
Why do fuel cells maintain a constant voltage over time?
Continuously feeding the fuel cell with H2 and O2 to maintain concentration
So concentrations of reactant remains constant over time
Can reactions in the hydrogen fuel occur under standard conditions?
No, rate is too slow because the activation energy is too high
Increasing temperature of a hydrogen fuel cell
Too high shifts position of equilibrium to prefer the backward endothermic reaction to take energy in so EMF falls
How can hydrogen be stored?
As a liquid under pressure
Adsorbed on the surface of a solid
Absorbed on solid surface material
Catalysts in a hydrogen fuel cell
Used to lower activation energy to provide an alternative reaction pathway so reaction can actually occur
