Redox Reactions and Oxidation Numbers

Flashcards covering the definitions, rules, and examples of redox reactions, oxidation numbers, and chemical agents based on the Cambridge IGCSE Chemistry coursebook.

Oxidation (general)

A reaction in which oxygen is added, electrons are lost from an atom or ion, or the oxidation state of an element increases.

Reduction (general)

A reaction in which oxygen is removed, electrons are gained by an atom or ion, or the oxidation state of an element decreases.

Redox reaction

A chemical reaction involving both reduction and oxidation taking place together.

Oxidation number

A number given to each atom in a compound to show how oxidised or reduced an element is; for a simple monoatomic ion, it is the charge on the ion (e.g., $+2$ for $Zn^{2+}$).

Oxidising agent

A substance that oxidises another substance during a redox reaction and is itself reduced. Examples include oxygen ($O_2$), hydrogen peroxide ($H_2O_2$), and potassium manganate(VII).

Reducing agent

A substance that reduces another substance during a redox reaction and is itself oxidised. Common examples include hydrogen ($H_2$), carbon ($C$), and carbon monoxide ($CO$).

OIL RIG

A memory aid for redox reactions: Oxidation Is the Loss of electrons, Reduction Is the Gain of electrons.

Half-equation

An equation representing either the gain or loss of electrons separately, such as $Cl_2(g) + 2e^- \rightarrow 2Cl^-(aq)$.

Displacement reaction

A reaction in which a more reactive element displaces a less reactive element from a solution of its salt.

Oxidation number: Uncombined element

The oxidation number of any uncombined element, such as $H_2$ or $Cl_2$, is zero ($0$).

Oxidation number: Hydrogen

The oxidation number of hydrogen in compounds is usually $+1$, as seen in $HCl$ or $H_2O$.

Oxidation number: Oxygen

The oxidation number of oxygen in compounds is usually $-2$, as seen in $H_2O$ or $MnO_2$.

Oxidation number sum (Compound)

The sum of the oxidation numbers of all atoms in a neutral compound is zero ($0$).

Oxidation number sum (Ion)

The sum of the oxidation numbers in a polyatomic ion is equal to the charge on the ion, such as the manganate(VII) ion ($MnO_4^-$) where the sum is $-1$.

Transition metals

Hard, strong, dense elements from the central region of the Periodic Table that often form coloured compounds and show variable oxidation numbers (e.g., Iron can be $+2$ or $+3$).

Corrosion

The process where metals and alloys are chemically attacked by oxygen, water, or other environmental substances, such as iron forming rust (iron(III) oxide).

Rancid

A term used to describe food where fats and oils have been oxidised, resulting in an unpleasant change in taste and smell.

Electrolysis: Anode

The positive electrode where negative ions travel to lose electrons, meaning oxidation always takes place here.

Electrolysis: Cathode

The negative electrode where positive or hydrogen ions travel to gain electrons, meaning reduction always takes place here.

AN OIL RIG CAT

An extended memory aid for electrolysis: Anode Oxidation Is Loss (of electrons), Reduction Is Gain (at the) Cathode.

Manganese variable oxidation numbers

Manganese can exist in several oxidation states including $+2$ ($MnO$), $+4$ ($MnO_2$), and $+7$ ($Mn_2O_7$).