Electrode potential and cells Flashcards
What happens when a metal is dipped into a solution of its own ions
An equilibrium is set up between the solid metal and the aqueous metal ions
Write half equations for zinc (s) and zinc (II)
Zn (s) (equilibrium sign) Zn2+ (aq)+ 2e-
Write a half equation for copper (II) to copper (III)
Cu 2+(s) (equilibrium sign) Cu3+(aq)+ e-
Whats the simplest salt bridge made of ?
Filter paper soaked in saturated solution of KNO3 (potassium nitrate)
Why r salt bridges necessary
Its purpose is to not move electrons from the electrolyte
But To maintain charge balance
Cos electrons r moving from one half cell to the other
Salt bridge prevents diffusion of solution from one half cell to another
What symbol is used to represent a salt bridge in standard notation
||
What type of species goes on the outside (furthest from the salt bridge) in standard notation
The most reduced species
What does | indicate?
Phase boundary (s/l/g)
How would an aluminium/copper cell b represented
Al (s) | Al3+ (aq) || Cu2+ (aq) | Cu (s)
What happens at the left hand electrode ?
Oxidation occurs
Left hand electrode is the half cell with the most neg E° value
What happens at the right hand electrode
Reduction
Half cell w the most positive E° value
Which side of the cell has the most negative E° value?
What happens to the metal w the most neg E° value
Oxidation - left hand electrode
Whats E°
Standard electrode potential of a cell
Draw the standard hydrogen electrode
What conditions is the standard hydrogen electrode used in ?
What conditions is the standard hydrogen electrode used in ?
Temperature 298k
Pressure 100 kPa
[H+] = 1.00 mol dm-3
Whats the standard hydrogen electrode used for?
Comparing other cells against
E° of standard hydrogen electrode is defined as 0
So All other E° values are compared against it
Y might u use other standard electrodes occasionally?
They r cheaper and quicker to use and can provide just as a good reference
Pt is expensive
If an E° value is more positive
What does it mean in terms of oxidising/reducing power
Better oxidising agent (easier to reduce
What factors will change e° values
Concentration of ions
Temperature
Why happens if u reduce the concentration of the ions in the left hand half cell
Equilibrium moves to the left to oppose the change of removing ions
This releases more electrons
The e° of the left hand cell becomes more negative so the e.m.f of the cell increases
How do u calc the emf of a cell from e° values
e° (right) -e° (left) = e° cell
When would you use a pt electrode
When both the oxidised and reduced forms of the metal are in aqueous solutions
Whys pt used
Inert
Good conducter
How would u predict if a reaction would occur
Take the two half equations
Find the species thats being reduced ( this is effectively the right hand electrode)
Calculate its e° value minus the e° value of the species thats being oxidised ( effectively the left hand side)
If E° overall > 0, reaction will occur
What was the first commercial cell made from (daniell cell)
Zinc / copper (||)
What r zinc carbon cells more commonly known as?
Disposable batteries
What r the two reactions that take place in zinc carbon cells
Zn is oxidised to Zn2+
NH4+ reduced to NH3 at carbon electrode
What r the reactions that occur in a lead/acid battery (car batteries)
Pb + SO42- —-> PbSO4 (s) + 2e-
PbO2 + 4H+ + SO4 2- + 2e- —> PbSO4 + 2H2O
How r cells recharged (if they r rechargeable)
Reactions r reversible
Reversed by running a higher voltage thru the cell than the cell’s E°
Nickel / cadmium cells r rechargeable AA batteries etc.
What reactions occur at the electrodes
Cd(OH)2 (s) + 2e- —> Cd (s) + 2OH-
NiO(OH) (s) + H2O + e- —> Ni(OH)2 (s) + OH-
Where r lithium ion cells used?
Mobile phones
Laptops
What reactions occur on discharge in lithium ion cells
Li+ + CoO2 + e- —-> Li+[CoO2]-
Li —-> Li+ + e-
What is a fuel cell?
A cell thas used to generate electric current
Does not require electrical recharging
What r the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?
2H2 + 4OH- —-> 4H20 + 4e-
O2 + 2H2O + 4e- —-> 4OH-
Draw a diagram of a hydrogen fuel cell
Ugh
Y is it better to use a fuel cell than to burn H2 in air
Even tho the same overall reaction occurs
In combustions sulphur containing compounds and nitrogen containing compounds r produces die to high temperatures and the s and n in air
These r bad for the environment
This doesn’t occur in a fuel cell, the only product is water
More efficient
Disadvantages of fuel cells
h is a flammable gas w a low b.p.—> hard and dangerous to store and transport
Expensive to buy
Fuel cells have a Limited lifetime and use toxic chemicals in their manufacture
How do u find the weakest reducing agent from a table of electrode potential data?
Most pos e° value
Then it is the product of the reduction equation i.e. imagine equation going from left to right
Whats the reason that some cells cannot b recharged
Reaction of the cell is not reversible - a product is produced that either dissipated or cannot b converted back into the reactants
Why might the emf of a cell change after a period of time?
Concentrations of the ions change - the reagents are used up
How can the emf of a value b kept constant
Reagents r supplied constantlt so tye concentrations of the ions are constant
E° remains constant
Give two important conventions that you have to follow when your drawing electrochemical cells using the conventional representation
The half cell with the more negative potential goes on the left the oxidised forms go in the centre of the cell diagram the reduced forms go at the edge of the cell diagram
If three factors that can influence electrode potentials
Temperature
Concentration of reactants
Pressure
What is the electrode potential of the standard hydrogen electrode?
why is it this value
0.00 v
By definition
Describe how a standard hydrogen electrode is set up and give the standard conditions used when measuring electrode potentials
Hydrogen gas is bubbles into a solution of aqueous H+ ions
electrode is made of Pt
standard conditions
temperature 298k
pressure 100 kPa
all solutions of ions have a concentration of 1.00 mol dm-3
Describe the term standard electrode potential
The voltage measured under standard conditions when a half cell is connected to a standard hydrogen electrode
Aluminium has a standard electrode potential of -1.66 whilst Nickel has a standard electrode potential of -0.25 states which species is more reactive and explain how you know in terms of electrode potential
Al - they r both metals and Al has a more negative electrode potential
Electrode potential of bromine is +1.07 whilst chlorine has an electrode potential of +1.36
state which species is more reactive and explain how you know in terms of electrode potential
Chlorine - they are both nonmetals and chlorine has the more positive standard electrode potential
Electrode potential of silver is +0.80 whilst copper has an electrode potential of +0.34
state which species is more reactive and explain how you know in terms of electrode potential
Copper - They r both metals and copper has the more negative electrode potential
Calculate the EMF for the following reactions
Al(s) + 3Ag+(aq) —> Al3+(aq) + 3Ag(s)
Cu (s) + Cl2(aq) —> Cu2+(aq) + 2Cl-(aq)
Al3+ (aq) E° = -1.66
Ag+ (aq) E° = +0.80
Cu2+ (aq) E° = +0.34
Cl2(aq) E° = +1.36
- 80-(-1.66)=2.46v
1. 36-0.34=+1.02v
The direction of redox reactions of the following half-cells Mg2+(aq)/Mg(s) and Ni2+(aq)/Ni(s)
Mg2+(aq) +2e—-> Mg(s)
E° = -2.38v
Ni2+(aq) + 2e- —> Ni (s)
E°
