electrochemical cells Flashcards
What is a half cell?
half an electrochemical cell, made up of an electrode (platinum or using the metal itself) in a solution containing aqueous ions
What makes up an electrochemical cell?
2 half cells
salt bridge
voltmeter
wire
What does the salt bridge contain?
KNO3/NaNO3
What is the role of the salt bridge?
allows the transfer of ions/charge
What is the electrode potential of a cell?
a value that tells us how easily the half cell is to be oxidised
What will the most negative half cell undergo?
oxidation
What will the most positive half cell undergo?
reduction
whats SHE
Standard Hydrogen Electrode
used as a reference for electrode potential values
What is the set up of the SHE?
Pt electrode
1mol dm3 H+ ions in solution
298K
100kPa
What are the standard conditions for electrochemical cells?
100kPa
298K
1 mol dm-3 ions
What is the function of the porous separator in a chemical cell (battery)?
salt bridge - allows transfer of ions
What happens to the chemical reaction when a cell is recharged?
it is reversed
Why are some chemical cells non-rechargeable?
reaction cannot be reversed
reaction produces a gas
Why does the metal case of some chemical cells break down over time?
the metallic case is a reactant and gets used up
What happens to cell emf over time?
decreases as reactants get used up
Which half cell is on the left?
half cell with most - electrode potential
What is the equation showing the positive electrode in a lithium ion cell?
Li+(aq) + CoO2(s) + e- –> Li+CoO2- (aq)
What is the equation showing the negative electrode in a lithium ion cell?
Li(s) –> Li+(aq) + e-
What is the standard cell representation for a lithium ion cell?
Fe / Li+ / Li // Li+ / CoO2 / Li+CoO2- / Fe
What is the discharge equation for a lithium ion cell?
Li (s) + CoO2 (s) –> Li+CoO2- (aq)
What metals make up a Daniell cell?
Zn and Cu
How is cell emf calculated from electrode potentials?
right - left
What is an electrochemical series?
list of electrode potentials in numerical order
What is one property a salt bridge MUST have?
must not react with the electrolyte/ions in solution
Why would a salt bridge containing a carbonate ion not be suitable for use (when H+ is present)?
carbonate ions will react with acid
What are 2 advantages of using Hydrogen Fuel Cells compared to using hydrogen in combustion?
HFCs:
- less energy wasted as thermal energy
- reaction happens at low temperatures
What are the half equations for a hydrogen-oxygen fuel cell operating in alkaline conditions? Use these half equations to explain how an electric current can be generated
O2 + 2H2O + 4e- ⎯→ 4OH-
H2 + 2OH- ⎯→ 2H2O + 2e-
Oxygen electrode gains electrons (is reduced), hydrogen electrode loses electrons (is oxidised)
Why does a hydrogen fuel cell does not need to be recharged?
emf is constant (given that supply of reactants is constant)
What is one major hazard associated with the use of a hydrogen–oxygen fuel cell in a vehicle?
hydrogen is flammable/ explosive
What would happen to the cell emf if the surface area of the electrodes was increased?
no change / nothing
What are the advantages of using hydrogen fuel cells?
-constant emf as long as reactant supply is constant
-works at room temperature so low thermal energy loss (compared to burning hydrogen)
-produces H2O as by-product, no greenhouse emissions or other pollutants such as NOx, SO2 (be specific)
What are the disadvantages of using hydrogen fuel cells?
-little hydrogen infrastructure in place (petrol stations)
-production of H2 currently energy intensive (produces greenhouse gases in power stations)
-storage of hydrogen is difficult and dangerous (explosive gas)
what do you do to most negative half cell
flip equation
wheres oxidising agents in table
left hand side
wheres reducing agent in table
right hand side
strongest oxidising agent and weakest reducing agent
most + sep
weakest oxidising agent and strongest reducing agent
most - sep
e cell equation
reduced - oxidised
positive - negative
right-left
when is pt electrode used
2 non metal half cells used
