Redox and Standard Electrode Potetentials

Meaning of oxidation

• Loss of electrons

• Gain of oxygen

• Loss of hydrogen

• Increase in oxidation number

Meaning of Reduction

• Gain of electrons

• Loss of oxygen

• Gain of hydrogen

• Decrease in oxidation number

Reduction half equation of iron(II)-dichromate(VI) reaction

Oxidation half equation for iron(II)-dichromate(VI) reaction

Overall equation for iron(II) - dichromate (VI) reaction

Colour change of the reduction process in the iron dichromate reaction

orange to green

Colour change of oxidation process in the iron dichromate reaction

Green to orange-rust

What happens and what are the observations for when OH- is added to dichromate ions

Orange to yellow

Reduction half equation for iron(II) - manganate (VII) reaction

Oxidation half equation for iron (II) manganate (VII) reaction

Overall equation for iron (II) manganate (VII) reaction and colour change

pink to colourless

Equation for the reaction of Cu²⁺ and I^-

Equation of the reaction of S₂O₃^2- and I₂

What indicator should be used for a titration of iodine against thiosulphate solution

starch blue/black to colourless

What is the salt bridge

a piece of filter paper soaked in KNO₃ or NH₄NO₃

Function of salt bridge

completes the circuit and allows ions to transfer between the 2 half cells without any mixing of solutions

State structures present in a battery

Salt bridge

Potentiometer

Wire

2 beakers

What happens on the left side of the cell

• Oxidation

• Electrons are being lost

• Cell is negatively charged

What happens on the right side of the cell

• Reduction

• Electrons are being gained

• Cell is positively charged

Draw a cell diagram

Define a standard electrode potential

The EMF of a half cell when connected to the standard hydrogen half cell.

Diagram of standard hydrogen electrode

Conditions of standard hydrogen electrode

298K

1 atm hydrogen

1 mol dm^-3

inert platinum electrode

Function of platinum electrode

Such low reactivity that it doesn’t interfere with the reaction

Completes the circuit because there is no metal in the half cell

Predict whether the zinc half cell or the copper half cell will be reduced and which will be oxidised

• Copper is on the RHS and is reduced

• Copper has a more +ive E feta value

• Copper is a stronger oxidising agent

• Zinc is on the LHS and is oxidised

• Zinc has a less +ive E feta value

• Zinc is a stronger reducing agent

EMF = 0.34 - - 0.76 = 1.10V

Function of hydrogen fuel cell

uses electrochemical methods to get energy from hydrogen

How does a hydrogen fuel cell work

Hydrogen gas is fed into the anode

oxygen from the air is fed into the cathode

Catalyst in the hydrogen cell

platinum catalyst

Write the half-equations for the processes occurring at the electrodes in an hydrogen fuel cell and an equation for the overall reaction.

At the anode: H2 → 2 H+ +2 e-

At the cathode: O2 + 4 H+ + 4 e- → 2 H2O

Overall reaction: 2 H2 + O2 → 2 H2O

State the disadvatages of a hydrogen fuel cell

• expensive

• Storage of hydrogen is problematic as it is expensive

• very flammable and a gas

• Lots of hydrogen is needed to produce enough energy

• Cell does not last for long

• A net energy loss as the energy produced by the cell is less than the energy required to produce the hydrogen for it

State the advantages of a hydrogen fuel cell

• Clean technology

• High efficiency

• No combustion process required

• simple to construct

• No CO2 is produced