A2 Chemistry - Topic 1: Electrochemistry

reactions that involve the transfer of electrons

what is a redox reaction

how easily an ion is discharged during electrolysis

what does the relative (E^ꝋ ) electrode potential of ions tell you

gets reduced at the cathode

where does the positively charged cation/the more positive electrode potential half cell value get discharged

gets oxidised at the anode

where does the negatively charged anion/the more negative electrode potential half cell value get discharged

• cathode - hydrogen forms unless metal is less reactive than hydrogen in the reactivity series, then that metal forms instead

• anode - oxygen forms

what forms at the cathode and anode during the electrolysis of an aqueous solution

• cathode - cation/metal

• anode - anion/non-metal

what forms at the cathode and anode during the electrolysis of a molten solution

• cathode - hydrogen will form unless metal is less reactive than hydrogen, then that metal will form

• anode - is negative ion is a halide, then halogen gas will form, otherwise oxygen will form

what forms at the cathode and anode during the electrolysis of a concentrated aqueous solution

the amount of substance that is formed at an electrode during electrolysis is proportional to

• the amount of time where a constant current to pass

• the amount of charge, in coulombs, that passes through the electrolyte (strength of electric current)

what does Faraday’s Law state

Q = Ixt

what is the formula for charge

96500 Cmol^-1

what is the value of Faraday’s constant

the amount of electric charge carried by 1 mole of electrons or 1 mole of singly charged ions

what does Faraday’s unit tell us

• F = L x e

• F → faraday’s constant (96500 Cmol^-1)

• L → Avogadro’s constant (6.02 × 10²³ mol^-1)

• e → charge on an electron

what is the formula for the relationship between Faraday’s constant and the Avogradro constant

the voltage when a standard half-cell is connected to a standard hydrogen cell under standard conditions

define the standard electrode potential

the potential difference between two half cells/two electrodes in a cell under standard conditions of 1 atm, 298K, and all solutions being 1 moldm^-3

define standard cell potential

• 1 atm

• 298K

• 1 moldm^-3 solution

what are standard conditions

• typically a strip of filter paper soaked in a saturated solution of potassium nitrate of potassium chloride (as nitrates and chlorides are usually soluble)

• a salt bridge has mobile ions that complete the circuit

• salt bridge ensures that no precipitates form which can affect the equilibrium position of the half cells

what is a salt bridge and what does it do

how does a metal/metal ion half cell look like

how does a non-metal/non-metal ion half cell look like

how does a ion/ion half cell look like

• the positive pole can more readily accept electrons from the other half cell so gets more readily reduced

• the negative pole more readily loses electrons to the other half cell so gets more readily oxidised

• the flow of electrons is from the negative pole to the positive pole

how can the direction of electron flow be determined by comparing Ecell values of two half cells in an electrochemical cell

• The Ecell values of a species indicate how easily they can get oxidised or reduced

• the more positive the value, the easier it is to reduce the species on the left of the half-equation - the reaction will tend to proceed in the forward direction

• the less positive the value, the easier it is to oxidise the species on the right of the half-equation, the reaction will tend to proceed in the backward direction

• a reaction is feasible (likely to occur) when the E_cell^ꝋ is positive

predict the feasibility of a reaction using standard cell potentials

• if the concentration of the species on the left is increased, the position of equilibrium will shift to the right

• this means that the species on the left gets more easily reduced

• the Evalue becomes more positive (or less negative)

what is the effect of increasing the concentration of the species on the left on the electrode potential

• equilibrium shifts to the left

• species on the left gets less easily reduced

• Evalue becomes less positive (or more negative)

what is the effect of increasing the concentration of the species on the right on the electrode potential

• E = E^θ+ 0.059/z log [oxidised species]/[reduced species]

• E - electrode potential under nonstandard conditions

• E^{θ - standard electrode potential}

• z - number of electrons transferred in the reaction

• The Nernst equation only depends on aqueous ions and not solids or gases

• the concentrations of solids and gases are therefore set to 1 moldm^-3

what is the Nernst equation

• ΔG^ꝋ = - n x E_cell^ꝋ x F

• ΔG^ꝋ - standard Gibbs free energy

• n - number of electrons transferred in the reaction

• E_cell^ꝋ - standard cell potential (V)

• F - Faraday constant

what is the formula to calculate free energy change using standard electrode potentials