Chemistry - Electrochemistry

Define standard electrode (redox) potential

The standard electrode potential of a half-cell is defined as the potential difference between a standard hydrogen electrode and the half-cell in which the reacting species in the solution are at molar concentrations of 1 mol dm⁻³ ,298K and gaseous species at pressure of 1 bar

Define standard cell potential

The standard cell potential is the maximum potential difference between the two half-cells which are operated under standard conditions

What is the purpose for a salt bridge?

The salt bridge completes the entire circuit. It prevents the build-up of charges in the 2 half-cells and allows ions to move between the 2 solutions of the 2 half-cells.

How do you write a cell diagram?

example : Cu(s) | Cu²⁺(aq) || Fe³⁺(aq), Fe²⁺(aq) | Pt(s)

|| → salt bridge

| → phase boundary (when species of different states are present in a half-cell

, → species boundary in the same phase

Note: state symbols must be included and it must be written as oxidation first followed by reduction (write the species in the same direction as how the reaction would occur)

Whats the formula to calculate E°꜀ₑₗₗ?

E꜀ₑₗₗ = Eᵣ - Eₒ

Why does the voltmeter reading gives the E° of the half-cell against the standard hydrogen electrode (S.H.E)?

Since the standard hydrogen electrode is assigned 0.00V, the potential difference shown on the high-resistance voltmeter gives the standard electrode potential of the ____ half-cell.

How is E°cell used to calculate ΔG°?

ΔG°=-nFE°cell

What are the factors that affect E°?

temperature, concentration, pressure

Why is it not possible to use values reliably to decide whether a chemical reaction will occur?

Two reasons to explain this limitation of using the standard electrode potential to determine spontaneity: → non-standard conditions and high activation energy(not kinetically favourable)

E° values only tell you whether the reaction is thermodynamically favoured, not kinetically. (meaning, E°>0 tells you the reaction is spontaneous at standard conditions, but the reaction may not take place if it’s very slow) → it does not say anything about the rate of reaction.

NOTE:

ΔG° determines the spontaneity of a reaction (thermodynamically)

Eₐ determines the rate of reaction (kinetically); the smaller Eₐ, the more kinetically favoured

What effect would an increase in [I⁻] have on the E°cell of the I₂(aq)/I⁻(aq) system?

Write down the reversible reaction, including the two species above.

I₂(aq) + 2e (→ ←) 2I⁻(aq)

When [I⁻] increased, the position of equilibrium shifts left to decrease [I⁻]. Oxidation is favoured. E(I₂/I⁻) becomes less positive.

E°cell becomes less positive.

Describe and explain the effect of the addition of a few drops of AgNO₃ (aq) on the E°cell of the I₂(aq)/I⁻(aq) system against the standard hydrogen electrode?

I₂(aq) + 2e (→ ←) 2I⁻(aq)

E⁰_cell = E⁰(red) - E⁰(oxd) 

        = E⁰(I₂/I^‒) - E⁰(H⁺/H₂) 

Ag⁺ will react with I⁻ to form a yellow precipitate (AgI), decreasing [I⁻]. The position of equilibrium shifts right to increase [I⁻]. Reduction is favoured. E(I₂/I⁻) becomes more positive.

E°cell becomes more positive.

What happens to the E°cell when the a Pt electrode is doubled in mass?

No effect on the E°cell when doubling its mass as [Cu(s)] is constant.

What are the advantages and disadvantages of a primary cell?

characteristics: limited capacity, discard when exhausted

Advantages:

light and portable

sealed and not easily leaked

Disadvantages:

not rechargeable (to be discarded when supply of electricity is exhausted)

examples of primary cells : flashlight batteries, mercury cells - for digital calculators

What are the advantages and disadvantages of a secondary cell?

characteristics: can be recharged, has storage capacity

Advantages:

relatively cheap, reliable, has an adequate life (can be recharged 500-800 times), and produces very large currents (like 30A) required to start a car engine

Disadvantage:

has a LOW energy/mass ratio (voltage)

Example of a secondary cell: lead-acid accumulator

State two differences between a primary cell and a secondary cell

A primary cell is not rechargeable, while a secondary cell is rechargeable

A primary cell produces very low currents, while a secondary cell produces very large currents

What are the advantages of a fuel cell?

it is highly efficient as the electric current is produced directly from the reaction of hydrogen and oxygen without the inherent wasteful immediate conversion of chemical energy to heat.

It has no greenhouse effect as its chemical product is water

It has high energy/mass ratio