Electrochemistry

What are the rules for assigning oxidation states?

Uncombined elements have an oxidation state of 0

In neutral compounds, the sum of the oxidation states is 0

The oxidation state of common ions is equal to their charge

Highly electronegative element is negative

What are some exceptions to the rule for oxidation states?

Hydrogen in metal hydrides has an oxidation state of -1 instead of +1

Oxygen in peroxides has an oxidation state of -1 instead of -2

What is oxidation?

The loss of electrons/ increase in oxidation number

What is reduction?

The gain of electrons/ decrease in oxidation number

What is the equation F=Le

• L is the Avogadro constant (6.02x10²³)

• e is the charge of an electron (1.6×10^-19)

• F is the Faraday constant, the charge of 1 mole of electrons

• F= 6.02x10²³ x 1.6x^-19

What is electrolysis?

The decomposition of a compound using an electric current

What is an electrolyte?

The molten ionic compound or aqueous solution of ions that is decomposed during electrolysis

Describe what happens at the electrodes during electrolysis

Negative ions are attracted to the anode where they are then oxidised to become neutral

Positive ions are attracted to the cathode where they are then reduced to become neutral

What forms at each electrode when a molten electrolyte containing two simple ions undergoes electrolysis?

A molten electrolyte containing two simple ions will contain metal and non-metal ions

A metal will form at the cathode and a non-metal will form at the anode

What forms are each electrolyte when an aqueoues electrolyte undergoes electrolysis?

• H+ and OH- ions are present from the water, metal and non-metal ions from the ionic compound

• If a halogen is present, it will form at the anode, if not oxygen is produced

• At the cathode, atoms with the more positive electrode potential will form either metal or hydrogen

What happens during discharge?

Ions are converted to atoms/ molecules at electrodes in electrolysis

What does discharge depend upon?

1. The concentration of the ions

2. Position in the redox series i.e. relative electrode potentials of the ion

How does concentration affect discharge at the cathode when aqueous solutions undergo electrolysis?

From around lead to zinc in the electrochemical series:

• If the solution is very concentrated then the metal will form

• If the solution is very dilute then hydrogen will form

• If the concentration of metal and hydrogen ions is similar both may form

How does concentration affect discharge at the anode when aqueous solutions undergo electrolysis?

Generally, if a halogen is present, it will form at the anode if not oxygen is produced

However when a more concentrated solution of NaCl is electrolysed, more chlorine is produced at the anode. If the solution is dilute, little chloride would be produced and the product would mostly be oxygen

How does discharge depend upon the relative electrode potentials of ions?

The cation is more easily reduced when the electrode potential is positive

The anion is more easily oxidised when the electrode potential is negative

If the cation has a greater electrode potential than hydrogen, the cation is discharged. If not hydrogen is discharged

How do you calculate the quantity of charge passed during electrolysis?

Using Q= It

Q is the charge in coulombs

I is the current, in amps

t is the time in seconds

Describe how to determine a value for the Avogadro constant

• Get the half equation at the cathode

• Measure the current in amps and time taken in seconds

• Calculate the charge transferred using Q=It

• Measure the mass of the cathode before and after electrolysis. The difference in mass is equal to mass of the metal produced at the cathode

• Use the moles equation to find the moles of the metal produced

• The ratio of no. of moles of electrons to the metal will give the charge transferred for every mole of the metal formed

• This gives the value of ‘F’, divide it by ‘e’ to find a value for Avogadro constant, L

What must a half-cell contain?

An element in two different oxidation states

Describe a metal/metal ion half-cell

A solid metal rod is dipped into a solution containing ions of the metal

An equilibrium will be set up on the surface of the metal

Describe an ion/ion half-cell

An ion/ion half-cell contain a solution of ions of the same element but of different oxidation states

The electrode is usually graphite or platinum

Define EMF

Electromotive force, the voltage when no current flows

Define Standard electrode potential

The voltage produced when a standard half-cell is connected to a standard hydrogen electrode under standard conditions

What is the Standard Hydrogen Electrode (SHE) used to for?

To calibrate the electrode potentials of half cells

What are the components of the Standard Hydrogen Electrode (SHE)?

Has a platinum wire that holds the platinum plate/electrode in place

H2 gas is pumped in at a pressure of 1 atm

HCl is the electrolyte and has a concentration of 1 moldm-3

What are the standard conditions when dealing with electrolysis?

• Pressure: 101 kPa/ 1 atm

• Temperature: 298K

• Solution concentration: 1 moldm-3

What is a hydrogen half-cell?

A half-cell contain hydrogen gas and a solution of hydrogen ions

An inert platinum electrode provides a surface for the equilibrium

2H+(aq) + 2e- ⇌ H2(g)

What is standard cell potential?

The difference in standard electrode potential between two half cells

How do you use standard electrode potentials to calculate standard cell potential?

Electrode potential of cell= electrode potential of positive electrode - electrode of negative electrode

What are the conditions kept when working out Standard Electrode Potentials?

Metals must be pure

Electrolyte concentration: 1 moldm-3

Pressure of gases: 101kPa/ 1 atm

Temperature: 298K

How do you predict which half-cell is being oxidised and which is being reduced?

• A system has a greater tendency to be oxidised when electrode potential is more negative

• A system has a greater tendency to be reduced when the electrode potential is more positive

Describe the flow of electrons in a simple electrochemical cell

Electrons flow from the negative electrode to the positive electrode due to the redox reactions that take place

How can you predict the feasibility of a redox reaction in an electrochemical cell?

The reaction is feasible if the oxidising agent (substance being reduced) has a lower standard cell potential than the reducing agent

The greater the difference in the electrode potential, the more likely the reaction is to occur

It has been predicted using standard electrode potentials that a redox reaction is feasible. Why might this reaction not occur spontaneously?

Non-standard conditions

Ambient energy of the system is lower than the activation energy

How can standard cell/electrode potentials be used to deduce the relative reactivity of the halogens?

As you go down group 17, standard electrode potentials decrease

So as they go down the group they are more likely to be oxidised and less likely to be reduced

Therefore oxidising ability decreases and reactivity decreases down the group

What must be balanced before combining half equations to write a redox reaction?

The number of electrons must be the same in both half equations

A half cell contains this equilibrium Cu2+(aq) + 2e- ⇌ Cu(s)

How does decreasing copper ion concentration affect electrode potential?

If the concentration of copper ions decreased, there are fewer copper ions in the solution so the position of equilibrium shifts to the left

This causes the electrode potential to become less positive because oxidation occurs more

What is Nernst equation?

E= Electrode potential + (0.059/z)log [oxidised species]/[reduced species]

The standard cell potential only applies if the concentration of the solutions is 1 moldm-3. If the concentrations are different, the Nernst equation is used to calculate electrode potential (E)

• Oxidised species = species with greater oxidation state

• Reduced species - species with lower oxidation state

• Z= the number of electrons transferred

List some advantages of H2/O2 fuel cells

• Only waste product is water

• High efficiency conversion

• No pollutants produced

• Potentially renewable if the H2 is obtained from water

• Don’t have to be recharged

• Run continuously if there is a constant supply of fuel and oxygen

• No need for fossil fuels

List some advantages of secondary cells

Examples: Nickel-metal hydride and lithium ion batteries

• They are rechargeable

• More cost efficient long term since fewer batteries need to be bought

• Saves wasting finite and expensive resources