Cu2+(aq) | Cu(s)
Blue –> Red-brown
Zn2+(aq) | Zn(s)
Colourless –> Silver
H+(aq) | H2(g) | Pt(s)
Colourless
Fe3+(aq), Fe2+ (aq) | Pt (s)
Yellow –> Pale green
MnO4-(aq), Mn2+(aq) | Pt(s)
Purple –> Colourless / Pale pink
Cl2 (g) | Cl- (aq) | Pt(s)
Green/ Yellow –> Colourless
Br2(aq), Br-(aq) | Pt(s)
Orange/Brown –> Colourless
I2(g) | I-(aq) | Pt(s)
Purple –> Colourless
Steps for working out half equations
- Balance formulae of species before and after the change
- Calculate oxidations states before and after
- Add electrons to one side to balance oxidation states
- Add H+ to balance charges
- add H2O tto balance equation
What is the purpose of electrochemical cells?
To produce electricity through chemical reactions
In what direction do electrons flow?
Towards the strongest oxidising agent (more positive E°)
EMF =
E° for reduction - E° for oxidation
What is the purpose of the salt bridge?
This completes the circuit by allowing ions to move, without the two solutions mixing
Define standard electrode potential
The potential difference when any half-cell is connected to the standard hydrogen electrode under standard conditions
What are the standard conditions?
Concentration ~ 1mol dm-3
Temperature ~ 298K
Pressure ~ 1atn
What is the purpose of an inert platinum electrode in an electrochemical cell?
To allow for the transfer of electrons
Cr2O7^2-(aq) in acidified solution
colour change, product, redox?
= Cr3+
Orange –> green
Reduction
Cr2O7^2- in alkaline solution
colour change, product, redox?
= 2CrO4^2-
Orange –> Yellow
Neither, the oxidation state of chromium does not change
Observations for the reaction between copper (II) ions and iodide ions
~ White solid of CuI
~ Brown solution of iodine
What indicator is used in the iodine clock reaction? What is the colour change?
Starch, goes blue-black in the presence of iodine, which then goes colourless when all iodine has been reduced to iodide ions
Equations at the anode, the cathode, and overall for hydrogen fuel cells
Anode : H2 –> 2H+ + 2e-
Cathode : O2 + 4H+ + 4e- –> 2H2O
Overall : 2H2 + O2 –> 2H2O
3 advantages of hydrogen fuel cells
- Water is the only product, so no CO2
- Highly efficient, as less energy is wasted as heat
- Hydrogen gas can be produced using renewable resources by the electrolysis of water
- Hydrogen can be produced from water, which is a sustainable resource
3 disadvantages of hydrogen fuel cells
- Hydrogen gas is highly flammable
- Hydrogen gas is difficult to store due to low density and therefore large volume
- Hydrogen gas is usually produced from fossil fuels; net energy loss
