Electrochemistry Thermodynamics and Equilibrium

ΔG = -nF E_{cell}

The reaction is spontaneous and capable of producing electrical energy.

A spontaneous reaction that can perform electrical work.

Because energy release (negative ΔG) corresponds to positive electrical potential (Ecell).

Maximum electrical work equals the decrease in free energy (ΔG).

wₘₐₓ = ΔG

It represents the charge of one mole of electrons (~96,500 C/mol).

The number of moles of electrons transferred in the reaction.

Charge = nF, where n is moles of electrons and F is Faraday constant.

ΔG° = -nF E°_{cell}

ΔG° = -RT ln K

E°_{cell} = (RT/nF) ln K

E°_{cell} = (0.0592/n) log K

log K = (n E°_{cell}) / 0.0592

The reaction strongly favors products and is highly spontaneous.

The system is at equilibrium with no net reaction tendency.

The reaction favors reactants and is non-spontaneous.

ΔG = 0, Ecell = 0, and Q = K.

Because there is no potential difference (Ecell = 0), so no current flows.

ΔG ∝ −Ecell (they are directly proportional with opposite signs).

Higher E°cell means greater driving force and more spontaneous reaction.

It changes K and thus affects Ecell via thermodynamic relationships.

Volt (V), equivalent to J/C.

It depends on the amount of charge transferred and the cell potential.

Work = Ecell × charge = nFEcell

To obtain maximum voltage without energy loss due to heat.

ΔG becomes more negative, indicating a stronger spontaneous reaction.

By evaluating Ecell or ΔG values to determine spontaneity.

Strong oxidizing agents have high positive E°, while strong reducing agents have negative E°.

It links chemical energy changes with electrical energy and thermodynamic principles.

A battery is an electrical device consisting of one or more galvanic cells that convert chemical energy into electrical energy through redox reactions.

ΔG is negative (ΔG < 0), indicating the reaction is spontaneous and capable of doing work.

The battery reaches equilibrium, no work can be done, and it is fully discharged.

Metals (e.g., Zn, Li, Pb) are used because they easily lose electrons (good reducing agents).

Metal oxides (e.g., MnO₂, PbO₂) are used because they readily accept electrons (good oxidizing agents).

The electrolyte allows ionic conduction between electrodes to maintain charge balance.

A primary battery is a non-rechargeable battery that cannot be restored to its original state after discharge.

The zinc container acts as the anode.

A graphite rod surrounded by MnO₂ acts as the cathode.

Potassium hydroxide (KOH) is used as the electrolyte.

It uses purer and denser MnO₂ and contains more active material in a smaller volume.

Capacity = current × discharge time (Cd = I × t).

Button batteries are small, compact primary batteries commonly made using mercury or silver oxide systems.

Both use zinc as the anode and operate in a basic electrolyte medium (usually KOH).

Mercury batteries use HgO, while silver batteries use Ag₂O as oxidizing agents.

Zn(s) + Ag₂O(s) → ZnO(s) + 2Ag(s) (Cell potential: ~1.6 V).

Zn(s) + HgO(s) → ZnO(s) + Hg(l) (Cell potential: ~1.3 V).

They have an extremely high energy-to-mass ratio (high energy density) because lithium has a very low atomic mass.

Lithium provides the same charge as zinc (1 F per mole) but at a much lower mass (7 g vs 65 g per mole).

A nonaqueous electrolyte is required.

Anode: Lithium metal foil; Cathode: Metal oxides such as silver vanadium oxide (AgV₂O₅.₅).

AgV₂O₅.₅ + 3.5Li → Li₃.₅AgV₂O₅.₅

A secondary battery is a rechargeable battery whose redox reactions can be reversed by applying an external electric current.

Approximately 2.1 V for a single cell; about 12.6 V for a typical car battery (6 cells in series).

PbO₂ + Pb + 2H₂SO₄ → 2PbSO₄ + 2H₂O (Spontaneous voltaic process).

They are the cheapest type of secondary battery, highly reliable, mature technology, and highly recyclable.

Nickel oxide hydroxide cathode, cadmium anode, and KOH electrolyte (~1.2 V typical voltage).

Ni-MH batteries use hydrogen-absorbing metal alloys as the anode and avoid toxic cadmium, making them more environmentally friendly.

Anode: Lithium intercalated graphite (LixC₆); Cathode: Lithium metal oxides (e.g., LiCoO₂).

High energy density, low self-discharge rate, and approximately 3.7 V cell voltage.

Lithium atoms at the anode release electrons to form Li⁺ ions, which then insert into the cathode's metal oxide structure.

They are expensive to manufacture and undergo ageing (gradual loss of capacity over time).

A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy via a continuous supply of fuel and oxidant, unlike batteries which are self-contained.

Typical efficiency is 60% or higher.

Anode (oxidation): 2H₂ → 4H⁺ + 4e⁻; Cathode (reduction): O₂ + 4H⁺ + 4e⁻ → 2H₂O; Overall: 2H₂ + O₂ → 2H₂O.

Approximately 1.2 V.

Energy density is the energy stored per unit mass/volume; Power density is the power delivered per unit mass.

Energy Density = (i × Ecell × t) / weight; Power Density = (i × Ecell) / weight.

A Ragone plot is a graph comparing energy density versus power density, used to analyze the performance of batteries, fuel cells, and capacitors.

High energy density (e.g., house batteries, mobile phones) vs. High power density (e.g., car starters, camera flashes).

Hydrogen has an extremely low molecular weight and high energy content.

Operating temperature is 800-1000°C; uses a solid oxide (ceramic) electrolyte.

1 volt (V) = 1 joule per coulomb (J/C), meaning 1 J of work is done per coulomb of charge.

Because cell potential is an intensive property and does not depend on size.

Hydrogen (H₂) is the electron source; Oxygen (O₂) is the electron sink (final acceptor).

Both moisture (water) and oxygen must be present.

Aluminum (Al), because it has the most negative reduction potential among metals like Sn, Pb, and Ni.

Anodic: Fe → Fe²⁺ + 2e⁻; Cathodic (oxygen-rich): O₂ + 2H₂O + 4e⁻ → 4OH⁻.

A galvanic cell with identical electrodes but different ion concentrations in each half-cell.

The standard cell potential is zero (E°cell = 0) because both electrodes are identical.

Ecell = 0.059 × pH (at 25°C).

The Nernst equation.