Drawing cells/ conventional cell notation Flashcards
A half cell with metal in solution with ions
The metal acts as electrode connected by wire
Electrode is in a beaker of 1 moldm-3 aqueous metal ions
Temperature 298K
A half cell with gas and solution
Uses a system to feed 100kPa gas into 1 moldm-3 solution of ions
Pt (platinum) electrode
A half cell with aqueous ions only
Contains Pt electrode in a beaker with 1 moldm-3 of 2 solutions of both ions
Why do we need to use a platinum electrode (in terms of the cell working)?
Because it provides a surface for atoms/ ions in a solution or in a gas to lose electrons or gain them
Aka for reaction to happen
Good qualities of using platinum as a material for an electrode
Inert so does not react with any atoms or ions to affect the position of equilibrium
Conducts flow of electrons between half cells
Covered in a porous coating to provide a large surface area for reaction
Drawing an electrochemical cell 5 aspects to remember
Salt bridge
Standard conditions
High resistance volt meter
Platinum electrode
Flow of electrons
What is a Salt bridge ?
an ionic connection between 2 half cells used to balance the charges
What are standard conditions?
Temperature is 298K
All solutions are 1 moldm-3
All gases are 100KPa
Why do we need to use standard conditions?
To standardise for all half cells because concentration, temperature and pressure can affect the position of equilibrium of the half cell so affects voltage measured
High resistance voltmeter
Connects the 2 half cells together to read potential difference
To allow conduction of electrons
What way round are half cells written?
One that favours reduction reaction (more positive, position of equilibrium to the right) is on the RIGHT
One that favours oxidation reaction (more negative, position of equilibrium to the left) is on the LEFT
But if a SHE is included, it will always be on the LEFT no matter what
Why do we use short hand cell notation?
Because it avoids drawing out the full half cell diagram but communicates what is present
Short hand cell notation: separating half cells
With a dashed vertical double line to represent the salt bridge
Short hand cell notation: per half cell, what are the symbols used?
The formula for the substances in each half cell
The oxidised form of each substance (that lost electrons) for each half cell is written IN THE CENTRE
And the reduced form of each substance (that gained electrons) for each half cell is written ON THE OUTSIDE
Using short hand cell notation, what half cell is left and which is right?
The reduced one is on the right
The oxidised one is on the left
Same as when drawing it normally
When using short hand cell notation, how do we represent a phase boundary?
With a single solid line
What is a ‘phase boundary’?
Shows a difference in state, eg the Pt electrode or metal electrode separated from solution
Which in short hand cell notation, is separated by a single solid line
When is the Pt electrode written in short hand notation?
If it’s present in a solution + solution or a gas + solution half cell
ALWAYS WRITTEN ON THE OUTSIDE WITH A VERTICAL DASHED LINE TO SHOW THE PHASE BOUNDARY
In short hand notation, how do we show 2 solutions?
Separate each component of solution with a comma (no line needed as this is not a phase boundary)
But the oxidised form of the solution is in the CENTRE and the reduced form is in the OUTSIDE
Anode
Electrode in the half Cell that is oxidised
Cathode
Electrode in the half cell that is reduced
Why use a high resistance volt meter?
To stop current flowing in a circuit so it is actually possible to measure maximum potential difference
Will the overall reaction in an electrochemical cell occur if a high resistance voltmeter is connected?
No because voltmeter stops flow of electrons so reactants in each half cell cannot be reduced/ oxidised to form the product
Just measures potential difference between half cells due to amount of electrons build up on each electrode in either electrode
Example of what to use in a salt bridge
Rope saturated in ionic solution eg potassium nitrate
