P2: Electrochemistry Flashcards
What is a displacement reaction?
When one element takes the place of another
The one that takes over displaces, is the stronger reducing agent
What is the order of reducing agent power from strongest to weakest?
Li
K
Ca
Mg
Zn
Ni
Pb
Cu
Ag
What is the electrochemical series?
Ranks the reducing power of half equations
Forwards is…
oxidising agent
Backwards is…
metal reducing agent
What is a half-cell?
Metal dipped in a solution of its own ions (an electrode)
What is a cell?
Two half cells connected together
What is a battery?
Two or more cells connected together
Which way do electrons flow?
Left to right
Negative to Positive
What is the electromotive force ‘E’?
The maximum voltage possible between the 2 electrodes
What do we use as a comparison fro all oxidising and reducing agents?
The hydrogen electrode
What does the hydrogen electrode consist of?
-Strip of platinum dipped in a solution containing H+ ions
-Hydrogen gas bubbled through
What are the standard conditions?
298K
100kPa
1 mol dm-3
What does E (and standard symbol) stand for?
Standard electrode potential
What is the EMF?
Electromotive force
How do you calculate EMF?
‘E’ of cell= ‘E’ right - ‘E’ left
What does a negative EMF value tell us?
Negative electrode on right
What side is the positive electrode normally on?
Right
EMF is +
What happens is + charge builds up in the left hand solution, and - charge builds up in the right solution?
Reactions come to a halt, voltage drops
What is the purpose of a salt bridge?
Prevents build up of charge by allowing the movement of ions
What is required for a salt bridge?
-Salt should not react with the ions in solution in either electrodes
How do you represent cells?
- Salt bridge is ll
- electrode on right
- Ignore spectator ions
- Most oxidised substances next to the salt bridge
- Separate substances in different states with a l
- Separate multiple substances in the same state with a comma
What are some examples of non-rechargeable cells?
Zinc chloride
Zinc carbon
Alkaline
Button
What is in a Daniell Cell?
Zinc and Copper
What is a separator?
Used when the the + and - electrode are very close together, prevents them from reacting directly.
Porous, allows transfer of ions.
What is a dry cell?
Have no free liquid, only paste
What is an electrolyte?
Substance that contains ions that are free to move, completing an electrical circuit
What is in an alkaline battery?
-Zinc is powdered, increase SA
-Next to a brass pin
-Electrolyte is KOH paste
How does recharging work?
Works in an equilibrium, so applying a strong electrical force pushes the electrons back towards the negative terminal
Why do cars have rechargeable batteries?
To start their internal combustion engines
Describe the positive electrode of a Lead-acid battery
-Lead IV oxide
-Dipped in sulphuric acid
-Becomes coated in lead sulphate as battery discharges
Describe the negative electrode of a Lead-acid battery
-Lead
-Dipped in sulphuric acid
-Becomes coated in lead sulphate as battery discharges
Describe how lithium ion cells work
- Lithium starts at the negative electrode, with its support medium of powdered graphite
- As lithium gives up electrons at the negative electrode, it migrates through the electrolyte to the positive electrode (cobalt IV oxide)
Give the overall reaction that occurs in a lithium ion cell
Li (s) + CoO2 (s) —-> LiCoO2 (s)
What are the advantages of lithium ion cells?
-Li is the lightest metal
-Strongest reducing agent (so a high voltage)
-Constructed without any moisture
What do fuel cells require?
Continuous supply of reactants
How do alkaline fuel cells work?
- H2 is oxidised to H+ at the negative electrode
- O2 is reduced to OH- at the positive electrode
- H2O is produced at the negative electrode as the OH- migrates to the - electrode and combines with H+
What is the overall equation for an alkaline fuel cell?
2 H2 + O2 —–> 2 H2O
Do hydrogen fuel cells produce fossil fuels?
Not directly, but producing H2 does
Why are hydrogen fuel cells dangerous?
Hydrogen is a hazard as it is explosive
