Electrochemistry ⚡️

definition of electrolysis

the breakdown of a molten or aqueous ionic compound by the passage of electricity through it

charge formula

Q = I t

Q: charge / coulombs

I: current / amperes

t: time / seconds

Faraday's Constant formula

F = le

F: charge carried by one mole of electrons

l: Avogadro's constant

e: electronic charge

*All values given in the data booklet

electrode potential

numerical value that shows the ease of reduction for metals as well as non-metals

higher positive/less negative E° value

more reduction

higher negative/more negative E° value

more oxidation

salt bridge

maintains the ionic balance at 1 moldm⁻³, any excess of ions is absorbed by the salt bridge

standard electrode potential (E°)

emf of an electrode which is connected to a standard hydrogen half cell at 298K, 1 atm and concentration 1 moldm-3

standard cell potential (E°cell)

voltage produced under standard conditions when 2 half cells are connected and none of them is a reference half cell

E°cell =

E°reduced − E°oxidised

prove Cl oxidises Fe²⁺ to Fe³⁺ using E° values

½Cl₂ + e ⇌ Cl⁻ (E°= +1.36V)

Fe³⁺ + e ⇌ Fe²⁺ (E°= +0.77V)

Cl⁻ has a more positive E° value meaning it's reduced so the reaction is feasible

Predict if Fe³⁺ can oxidise Cl⁻ to Cl₂ by calculation

1) Change the sign of E° for the substance that's being oxidised

2) Add the 2 E° values

3) If the final answer is positive, it's a feasible reaction. If it's negative, the reaction does not occur.

½Cl₂ + e ⇌ Cl⁻ (E°= +1.36V)

Fe³⁺ + e ⇌ Fe²⁺ (E°= +0.77V)

-1.36 + 0.77 = -0.59 (negative so reaction does not occur)

a substance with a high positive E°

is a strong oxidising agent

a substance with a high negative E°

is a strong reducing agent

Fe³⁺ + e ⇌ Fe²⁺ (E°= +0.77V)

If Fe³⁺ > 1 moldm⁻³, what happens to the E° value?

- equilibrium shifts to the RHS

- more reduction occurs

- E° becomes more positive

Fe³⁺ + e ⇌ Fe²⁺ (E°= +0.77V)

If Fe²⁺ > 1 moldm⁻³, what happens to the E° value?

- equilibrium shifts to the LHS

- more oxidation occurs

- E° becomes more negative

disadvantages of hydrogen fuel cell

1. water freezes at low temp

2. expensive

3. hydrogen storage is difficult

4. producing the hydrogen fuel cell itself is bad for the environment

half equations of hydrogen fuel cell

cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻

anode: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻

overall: 2H₂ + O₂ → 2H₂O

half equations of hydrogen fuel cell under acidic conditions

reduction: O₂ + 4H⁺ + 4e⁻ ⇌ 2H₂O

draw a fully labelled diagram to show how you could use a standard hydrogen electrode to measure the standard electrode potential of zinc

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In the alkaline hydrogen-oxygen fuel cell, H₂(g) and O₂(g) are passed over two inert electrodes immersed in an alkaline solution.

Write the half-equations for the reactions taking place at each of these electrodes.

hydrogen electrode: H₂ + 2OH⁻ → 2H₂O + 2e⁻

oxygen electrode: O₂ + 2H₂O + 4e⁻ → 4OH⁻

overall: 2H₂ + O₂ → 2H₂O