Topic 9.1: Redox processes – Oxidation and reduction
Redox (reduction–oxidation) reactions play a key role in many chemical and biochemical processes.
• Understanding: Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in
oxidation number.
Oxidation Reduction
In terms of oxygen Addition of oxygen Removal of oxygen
In terms of hydrogen Removal of hydrogen Removal of oxygen
Electron transfer Loss of electrons Gain of electrons
Oxidation state Increases Decreases
• Understanding: An oxidizing agent is reduced and a reducing agent is oxidized.
▪ Oxidizing agent: causes another species to be oxidized and is itself reduced in the process
▪ Reducing agent: causes another species to be reduces and is itself oxidized in the process
• Understanding: Variable oxidation numbers exist for transition metals and for most main-group non-metals.
▪ Transition metals: d-block valence elements have variable oxidation numbers
• +2 oxidation state: is stable for most transition metals as it removes a pair of
electrons from the valance shell
• Other oxidation state: other oxidation number arises from removal of electrons
from the d-orbital that ensures stability
▪ Main group non-metals: have variable oxidation number that depends on the type of
molecule the atom is bonded to in a molecule
• Understanding: The activity series ranks metals according to the ease with which they undergo oxidation.
▪ Activity series: ranking of metals according to the ease with which they undergo oxidation
• As the ranking increase: the more reactive the metal; becomes more likely to be oxidized (loss electrons)
• Metals that are higher in the activity series displace metals below them to become ions
• Understanding: The Winkler Method can be used to measure biochemical oxygen demand (BOD), used as a measure of the degree
of pollution in a water sample.
▪ Biochemical oxygen demand (BOD): amount of oxygen required to oxidize organic matter in water at specific temperature
▪ Winkler Method: technique that can be used to measure oxygen content and BOD in water to measure the degree of pollution
• Applications and skills: Deduction of the oxidation states of an atom in an ion or a compound.
▪ Oxidation state: charge of an atom in an element, molecule or an ion
• Neutral molecule: sum of all oxidation state of all atoms must equal to zero
• Polyatomic ion: sum of all oxidation state of all atoms must equal the overall charge
Condition Oxidation state Example
Free element Always 0 S8, O2, P4
Group 1 metal Always +1 Na, K
Group 2 metal Always +2 Ca, Mg
Group 3 metal Usually +3; can vary Al
Group 17 halogen Usually -1; can be different in oxoanions or oxoacids F
Oxygen Usually -2; -1 in hydrogen peroxide
Hydrogen Usually +1; -1 in metal hydrides
• Applications and skills: Deduction of the name of a transition metal compound from a given formula, applying oxidation numbers
represented by Roman numerals.
▪ Process of deducting the name of a transition metal compound from given formula
• Identify atoms with known oxidation states in the compound
• Add up the known oxidation states
• Subtract the sum of known oxidation states from the overall charge (if any)
• Divide the number by the number of transition metal in the compound
• Apply oxidation number of the metal represented by Roman numeral
• Example: CoF3; Cobalt (III) fluoride, Cu(OH)2; copper (II) hydroxide, MnO2 (manganese (IV) oxide)
• Applications and skills: Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox
reactions.
▪ Process of identifying species oxidized and reduced in a redox reaction
• Identify the two half equations
• Identify species being oxidized and reduced, and respective agents
Redox (reduction–oxidation) reactions play a key role in many chemical and biochemical processes.
• Understanding: Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in
oxidation number.
Oxidation Reduction
In terms of oxygen Addition of oxygen Removal of oxygen
In terms of hydrogen Removal of hydrogen Removal of oxygen
Electron transfer Loss of electrons Gain of electrons
Oxidation state Increases Decreases
• Understanding: An oxidizing agent is reduced and a reducing agent is oxidized.
▪ Oxidizing agent: causes another species to be oxidized and is itself reduced in the process
▪ Reducing agent: causes another species to be reduces and is itself oxidized in the process
• Understanding: Variable oxidation numbers exist for transition metals and for most main-group non-metals.
▪ Transition metals: d-block valence elements have variable oxidation numbers
• +2 oxidation state: is stable for most transition metals as it removes a pair of
electrons from the valance shell
• Other oxidation state: other oxidation number arises from removal of electrons
from the d-orbital that ensures stability
▪ Main group non-metals: have variable oxidation number that depends on the type of
molecule the atom is bonded to in a molecule
• Understanding: The activity series ranks metals according to the ease with which they undergo oxidation.
▪ Activity series: ranking of metals according to the ease with which they undergo oxidation
• As the ranking increase: the more reactive the metal; becomes more likely to be oxidized (loss electrons)
• Metals that are higher in the activity series displace metals below them to become ions
• Understanding: The Winkler Method can be used to measure biochemical oxygen demand (BOD), used as a measure of the degree
of pollution in a water sample.
▪ Biochemical oxygen demand (BOD): amount of oxygen required to oxidize organic matter in water at specific temperature
▪ Winkler Method: technique that can be used to measure oxygen content and BOD in water to measure the degree of pollution
• Applications and skills: Deduction of the oxidation states of an atom in an ion or a compound.
▪ Oxidation state: charge of an atom in an element, molecule or an ion
• Neutral molecule: sum of all oxidation state of all atoms must equal to zero
• Polyatomic ion: sum of all oxidation state of all atoms must equal the overall charge
Condition Oxidation state Example
Free element Always 0 S8, O2, P4
Group 1 metal Always +1 Na, K
Group 2 metal Always +2 Ca, Mg
Group 3 metal Usually +3; can vary Al
Group 17 halogen Usually -1; can be different in oxoanions or oxoacids F
Oxygen Usually -2; -1 in hydrogen peroxide
Hydrogen Usually +1; -1 in metal hydrides
• Applications and skills: Deduction of the name of a transition metal compound from a given formula, applying oxidation numbers
represented by Roman numerals.
▪ Process of deducting the name of a transition metal compound from given formula
• Identify atoms with known oxidation states in the compound
• Add up the known oxidation states
• Subtract the sum of known oxidation states from the overall charge (if any)
• Divide the number by the number of transition metal in the compound
• Apply oxidation number of the metal represented by Roman numeral
• Example: CoF3; Cobalt (III) fluoride, Cu(OH)2; copper (II) hydroxide, MnO2 (manganese (IV) oxide)
• Applications and skills: Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox
reactions.
▪ Process of identifying species oxidized and reduced in a redox reaction
• Identify the two half equations
• Identify species being oxidized and reduced, and respective agents
