Electrochemistry

Definition and Applications

Electrochemistry is the branch of chemistry that studies the relationship between electricity and chemical reactions, specifically those involving the movement of electrons. It is foundational to many technologies and natural processes.

• Electricity and Chemistry: Electrochemistry focuses on processes where electrons are transferred between substances, resulting in the generation or consumption of electrical energy.

• Spontaneous Reactions: Some chemical reactions can generate an electric current (e.g., batteries).

• Nonspontaneous Reactions: Some reactions require an external electric current to proceed (e.g., electrolysis).

• Applications:

  • Energy generation (batteries, fuel cells, solar panels)

  • Corrosion (e.g., rusting of iron)

  • Chemical analysis (e.g., sensors, medical tests like glucose monitors)

  • Industrial processes (e.g., metal refining, chlorine production, chrome plating)

Redox (Reduction-Oxidation) Reactions

Definition and Examples

Redox reactions are chemical processes involving the transfer of electrons between two species. These reactions are central to electrochemistry.

• Oxidation: The process in which a substance loses electrons.

• Reduction: The process in which a substance gains electrons.

• Examples:

  • Hydrogen fuel cell:

  • Combustion of methane:

  • Rusting of iron:

  • Rechargeable batteries:

In all these examples, the addition of oxygen (oxidation) or removal of oxygen (reduction) involves electron transfer.

Electron Transfer and Mnemonics

• Oxidation: Loss of electrons (e.g., )

• Reduction: Gain of electrons (e.g., )

• Mnemonic: OIL RIG — Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons).

• Redox Pairing: Oxidation and reduction always occur together; the electrons lost by one species are gained by another.

Oxidising and Reducing Agents

• Oxidising Agent (Oxidant): A substance that removes electrons from another (itself is reduced).

• Reducing Agent (Reductant): A substance that donates electrons to another (itself is oxidised).

• Example: In the reaction , Na is the reducing agent (loses electrons), and Cl2 is the oxidising agent (gains electrons).

Oxidation Numbers

Definition and Purpose

Oxidation number (or oxidation state) is a value assigned to an atom in a compound, representing the hypothetical charge it would have if all bonds to atoms of different elements were 100% ionic. This concept helps identify redox reactions, especially in covalent compounds where electron transfer is not obvious.

Rules for Assigning Oxidation Numbers

• The oxidation number of an atom in its elemental form is 0 (e.g., O2, H2).

• The oxidation number of a monatomic ion equals its charge (e.g., Na+ is +1, Cl- is -1).

• Group 1 metals: +1; Group 2 metals: +2 in compounds.

• Fluorine: always -1 in compounds.

• Oxygen: usually -2 (except in peroxides, where it is -1, and in OF2, where it is +2).

• Hydrogen: +1 when bonded to nonmetals, -1 when bonded to metals.

• The sum of oxidation numbers in a neutral molecule is 0; in a polyatomic ion, it equals the ion's charge.

• Other elements: assign based on electronegativity and known oxidation states.

Additional info: Fractional oxidation numbers can occur in some compounds (e.g., N in NaN3 is -1/3).

Examples of Assigning Oxidation Numbers

• Water (H2O):

  • H: +1 each (total +2)

  • O: -2

  • Sum: 2(+1) + (-2) = 0

• Potassium Fluoride (KF):

  • K: +1

  • F: -1

• Magnesium Phosphide (Mg3P2):

  • Mg: +2 each

  • P: -3 each

• Potassium Sulfate (K2SO4):

  • K: +1 each (total +2)

  • O: -2 each (total -8)

  • S: +6 (to balance the total to 0)

Notation and Interpretation

• Oxidation numbers are written with the sign before the number (e.g., +1, -2).

• Ionic charges are written with the sign after the number (e.g., 1+, 2-).

• Oxidation numbers may be shown above the element symbol or in Roman numerals in names (e.g., Fe(II) for +2, Fe(III) for +3).

• Many elements have variable oxidation numbers depending on the compound.

Oxidation Numbers and Redox Reactions

• Oxidation: Increase in oxidation number.

• Reduction: Decrease in oxidation number.

• Application: By tracking changes in oxidation numbers, chemists can identify which atoms are oxidised and which are reduced in a reaction.