Define oxidation number .

Oxidation number is the charge that would exist on an individual atom if the bonding were completely ionic. It represents the electronic 'status' of an element and is used to determine whether oxidation or reduction has occurred.

True or False? The oxidation number of an uncombined element is always zero.

True. Uncombined elements — whether monatomic (e.g. Na) or diatomic (e.g. Cl 2 ) — always have an oxidation number of 0.

Oxygen has an oxidation number of .......... in most compounds, but .......... in peroxides.

Oxygen has an oxidation number of -2 in most compounds, but -1 in peroxides.

What is the oxidation number of Mn in MnO 4 - ?

The oxidation number of Mn in MnO 4 - is +7 . Oxygen contributes 4 × (−2) = −8; the ion has an overall charge of −1, so Mn must be +7.

OIL RIG is a mnemonic for remembering the definitions of oxidation and reduction: O xidation I s L oss (of electrons); R eduction I s G ain (of electrons).

What are the oxidation number rules for hydrogen?

Oxidation number rules for hydrogen: hydrogen has an oxidation number of +1 in most compounds, but −1 when bonded to a metal (metal hydrides, e.g. NaH).

True or False? In a neutral molecule, the sum of all oxidation numbers equals zero.

True. The oxidation numbers of all atoms in a neutral molecule must sum to zero. In an ion , the sum equals the overall charge on the ion.

Iron(III) sulfate contains iron in which oxidation state? What is the formula of the iron ion?

Iron(III) sulfate contains iron in the +3 oxidation state . The iron ion is Fe 3+ . Roman numerals in compound names indicate the oxidation state of the element with variable oxidation number.

The oxidation number of Cr in Cr 2 O 7 2- is .......... , because oxygen contributes a total of .......... and the ion has an overall charge of .......... .

The oxidation number of Cr in Cr 2 O 7 2- is +6 , because oxygen contributes a total of −14 and the ion has an overall charge of −2 .

Give three uses of oxidation numbers in chemistry.

Oxidation numbers can be used to: Determine whether oxidation or reduction has taken place. Identify what has been oxidised and/or reduced in a reaction. Construct half-equations and balance redox equations .

Define oxidising agent .

An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons. The oxidising agent itself is reduced (gains electrons), so its oxidation number decreases.

An oxidising agent is itself .......... and its oxidation number .......... , whereas a reducing agent is itself .......... and its oxidation number .......... .

An oxidising agent is itself reduced and its oxidation number decreases , whereas a reducing agent is itself oxidised and its oxidation number increases .

True or False? An oxidising agent gains electrons and is itself reduced.

True. An oxidising agent causes another species to lose electrons; in doing so, it gains those electrons and is itself reduced , with a decrease in oxidation number.

In the reaction 2NH 3 + 3Br 2 → N 2 + 6HBr, which element is oxidised and which is reduced? How do oxidation numbers change?

Nitrogen is oxidised: its oxidation number increases from −3 (in NH 3 ) to 0 (in N 2 ). Bromine is reduced: its oxidation number decreases from 0 (in Br 2 ) to −1 (in HBr).

What is electron transfer in the context of redox reactions?

Electron transfer is the movement of electrons from the species being oxidised to the species being reduced during a redox reaction. The species that loses electrons is oxidised; the species that gains electrons is reduced.

True or False? A substance cannot act as both an oxidising and a reducing agent.

False. Some substances can act as both an oxidising agent and a reducing agent , depending on what they are reacting with and the reaction conditions.

In the reaction Mg + Fe 2+ → Mg 2+ + Fe, which species is the oxidising agent and why?

Fe 2+ is the oxidising agent : it oxidises Mg (from 0 to +2) and is itself reduced from +2 to 0. The species that gains electrons and decreases in oxidation number acts as the oxidising agent.

For a reaction to be classified as a redox reaction, there must be both an .......... and a .......... present.

For a reaction to be classified as a redox reaction, there must be both an oxidising agent and a reducing agent present.

What happens to the oxidation number of a reducing agent during a redox reaction?

The oxidation number of a reducing agent increases during a redox reaction, because the reducing agent is itself oxidised (loses electrons).

Define reducing agent .

A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons. The reducing agent itself is oxidised (loses electrons), so its oxidation number increases.

Define disproportionation .

Disproportionation is a reaction in which the same species is simultaneously oxidised and reduced . The oxidation number of that species both increases and decreases in the same reaction.

True or False? In a disproportionation reaction, two different species are simultaneously oxidised and reduced.

False. In a disproportionation reaction, the same species is simultaneously oxidised and reduced — its oxidation number both increases and decreases in the same reaction.

Chlorine undergoes disproportionation when added to hot concentrated aqueous sodium hydroxide. What are the two chlorine-containing products formed?

The two chlorine-containing products are Cl - and ClO 3 - ions. Chlorine is simultaneously reduced (to Cl - , oxidation number −1) and oxidised (to ClO 3 - , oxidation number +5), making this a disproportionation reaction.

In a disproportionation reaction, the oxidation number of the same species both .......... and .......... in the same reaction.

In a disproportionation reaction, the oxidation number of the same species both increases and decreases in the same reaction.

In the reaction CO + Ag 2 O → 2Ag + CO 2 , which reactant has been oxidised and which has been reduced?

Carbon (in CO) has been oxidised: it has gained oxygen, increasing its oxidation number. Silver (Ag in Ag 2 O) has been reduced: it has lost oxygen, decreasing its oxidation number from +1 to 0.

What are the steps for balancing a disproportionation reaction using oxidation numbers?

Steps for balancing a disproportionation reaction: Write the unbalanced equation and identify the atoms that change in oxidation number. Deduce the oxidation number changes. Balance the oxidation number changes. Balance the charges. Balance the atoms.

True or False? Oxidation always involves a gain of oxygen.

False. Oxidation can be defined as loss of electrons, increase in oxidation number, or gain of oxygen — but not all three definitions apply in every case. Loss of electrons is the most general definition.

In the reaction 2Na (s) + Cl 2 (g) → 2NaCl (s), which species has been oxidised and which has been reduced?

Na has been oxidised : its oxidation number increases from 0 (in Na) to +1 (in NaCl). Cl has been reduced : its oxidation number decreases from 0 (in Cl 2 ) to −1 (in NaCl).

When balancing a redox equation, after identifying oxidation number changes, the next step is to balance the .......... changes before balancing the .......... .

When balancing a redox equation, after identifying oxidation number changes, the next step is to balance the oxidation number changes before balancing the charges .

What is an ionic half-equation ?

An ionic half-equation is an equation that shows either the oxidation or the reduction process separately, including the electrons gained or lost. Half-equations are combined to give the overall redox equation .

What are the five steps for writing an overall redox equation from ionic half-equations?

Write the unbalanced equation and identify atoms that change in oxidation number . Deduce the oxidation number changes. Balance the oxidation number changes. Balance the charges . Balance the atoms.

True or False? Non-metals generally form positive ions by losing electrons.

False. Non-metals generally form negative ions by gaining electrons and are therefore reduced, with a decrease in oxidation number.

Metals generally form .......... ions by .......... electrons and are therefore .......... in reactions.

Metals generally form positive ions by losing electrons and are therefore oxidised in reactions.

How are Roman numerals used in the names of ionic compounds such as iron(II) sulfate and iron(III) sulfate?

Roman numerals indicate the oxidation state of the element with a variable oxidation number. iron(II) sulfate contains Fe 2+ ; iron(III) sulfate contains Fe 3+ . This naming system applies to compounds of transition metals and other elements with multiple oxidation states.

True or False? Sodium is oxidised when it reacts with water.

True. When sodium reacts with water, the oxidation number of sodium increases from 0 to +1, meaning sodium is oxidised and loses electrons.

What is the oxidation state of Cr in K 2 CrO 4 ?

The oxidation state of Cr in K 2 CrO 4 is +6 . Two K + ions contribute +2 overall; CrO 4 therefore carries a 2− charge; oxygen is −2 each (4 × −2 = −8), so Cr must be +6 to give the overall −2 charge.

In MnO 4 - reacting with Fe 2+ (aq) in acid, Mn is reduced from .......... to .......... and Fe is oxidised from .......... to .......... .

In MnO 4 - reacting with Fe 2+ (aq) in acid, Mn is reduced from +7 to +2 and Fe is oxidised from +2 to +3 .

Name the compound Cu 2 O, giving the oxidation state of copper.

Cu 2 O is copper(I) oxide . Oxygen has an oxidation state of −2; the overall compound is neutral, so both Cu atoms together contribute +2, giving each Cu an oxidation state of +1 .

A LevelChemistryEdexcelFlashcards2. Inorganic ChemistryRedox I

Redox I (Edexcel A Level Chemistry): Flashcards

Exam code: 9CHO

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FrontOxidation Number
Define oxidation number.

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Cards in this collection (38)

Define oxidation number.
Oxidation number is the charge that would exist on an individual atom if the bonding were completely ionic.
It represents the electronic 'status' of an element and is used to determine whether oxidation or reduction has occurred.
True or False?
The oxidation number of an uncombined element is always zero.
True.
Uncombined elements — whether monatomic (e.g. Na) or diatomic (e.g. Cl₂) — always have an oxidation number of 0.
Oxygen has an oxidation number of .......... in most compounds, but .......... in peroxides.
Oxygen has an oxidation number of -2 in most compounds, but -1 in peroxides.
What is the oxidation number of Mn in MnO₄^-?
The oxidation number of Mn in MnO₄^- is +7.
Oxygen contributes 4 × (−2) = −8; the ion has an overall charge of −1, so Mn must be +7.
What is OIL RIG?
OIL RIG is a mnemonic for remembering the definitions of oxidation and reduction:
Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons).
What are the oxidation number rules for hydrogen?
Oxidation number rules for hydrogen: hydrogen has an oxidation number of +1 in most compounds, but −1 when bonded to a metal (metal hydrides, e.g. NaH).
True or False?
In a neutral molecule, the sum of all oxidation numbers equals zero.
True.
The oxidation numbers of all atoms in a neutral molecule must sum to zero. In an ion, the sum equals the overall charge on the ion.
Iron(III) sulfate contains iron in which oxidation state? What is the formula of the iron ion?
Iron(III) sulfate contains iron in the +3 oxidation state. The iron ion is Fe³⁺.
Roman numerals in compound names indicate the oxidation state of the element with variable oxidation number.
The oxidation number of Cr in Cr₂O₇^2- is .........., because oxygen contributes a total of .......... and the ion has an overall charge of .......... .
The oxidation number of Cr in Cr₂O₇^2- is +6, because oxygen contributes a total of −14 and the ion has an overall charge of −2.
Give three uses of oxidation numbers in chemistry.
Oxidation numbers can be used to:
1. Determine whether oxidation or reduction has taken place.
2. Identify what has been oxidised and/or reduced in a reaction.
3. Construct half-equations and balance redox equations.
Define oxidising agent.
An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons.
The oxidising agent itself is reduced (gains electrons), so its oxidation number decreases.
An oxidising agent is itself .......... and its oxidation number .........., whereas a reducing agent is itself .......... and its oxidation number .......... .
An oxidising agent is itself reduced and its oxidation number decreases, whereas a reducing agent is itself oxidised and its oxidation number increases.
True or False?
An oxidising agent gains electrons and is itself reduced.
True.
An oxidising agent causes another species to lose electrons; in doing so, it gains those electrons and is itself reduced, with a decrease in oxidation number.
In the reaction 2NH₃ + 3Br₂ → N₂ + 6HBr, which element is oxidised and which is reduced? How do oxidation numbers change?
Nitrogen is oxidised: its oxidation number increases from −3 (in NH₃) to 0 (in N₂).
Bromine is reduced: its oxidation number decreases from 0 (in Br₂) to −1 (in HBr).
What is electron transfer in the context of redox reactions?
Electron transfer is the movement of electrons from the species being oxidised to the species being reduced during a redox reaction.
The species that loses electrons is oxidised; the species that gains electrons is reduced.
True or False?
A substance cannot act as both an oxidising and a reducing agent.
False.
Some substances can act as both an oxidising agent and a reducing agent, depending on what they are reacting with and the reaction conditions.
In the reaction Mg + Fe²⁺ → Mg²⁺ + Fe, which species is the oxidising agent and why?
Fe²⁺ is the oxidising agent: it oxidises Mg (from 0 to +2) and is itself reduced from +2 to 0.
The species that gains electrons and decreases in oxidation number acts as the oxidising agent.
For a reaction to be classified as a redox reaction, there must be both an .......... and a .......... present.
For a reaction to be classified as a redox reaction, there must be both an oxidising agent and a reducing agent present.
What happens to the oxidation number of a reducing agent during a redox reaction?
The oxidation number of a reducing agent increases during a redox reaction, because the reducing agent is itself oxidised (loses electrons).
Define reducing agent.
A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons.
The reducing agent itself is oxidised (loses electrons), so its oxidation number increases.
Define disproportionation.
Disproportionation is a reaction in which the same species is simultaneously oxidised and reduced.
The oxidation number of that species both increases and decreases in the same reaction.
True or False?
In a disproportionation reaction, two different species are simultaneously oxidised and reduced.
False.
In a disproportionation reaction, the same species is simultaneously oxidised and reduced — its oxidation number both increases and decreases in the same reaction.
Chlorine undergoes disproportionation when added to hot concentrated aqueous sodium hydroxide. What are the two chlorine-containing products formed?
The two chlorine-containing products are Cl^- and ClO₃^- ions.
Chlorine is simultaneously reduced (to Cl^-, oxidation number −1) and oxidised (to ClO₃^-, oxidation number +5), making this a disproportionation reaction.
In a disproportionation reaction, the oxidation number of the same species both .......... and .......... in the same reaction.
In a disproportionation reaction, the oxidation number of the same species both increases and decreases in the same reaction.
In the reaction CO + Ag₂O → 2Ag + CO₂, which reactant has been oxidised and which has been reduced?
Carbon (in CO) has been oxidised: it has gained oxygen, increasing its oxidation number.
Silver (Ag in Ag₂O) has been reduced: it has lost oxygen, decreasing its oxidation number from +1 to 0.
What are the steps for balancing a disproportionation reaction using oxidation numbers?
Steps for balancing a disproportionation reaction:
1. Write the unbalanced equation and identify the atoms that change in oxidation number.
2. Deduce the oxidation number changes.
3. Balance the oxidation number changes.
4. Balance the charges.
5. Balance the atoms.
True or False?
Oxidation always involves a gain of oxygen.
False.
Oxidation can be defined as loss of electrons, increase in oxidation number, or gain of oxygen — but not all three definitions apply in every case. Loss of electrons is the most general definition.
In the reaction 2Na (s) + Cl₂ (g) → 2NaCl (s), which species has been oxidised and which has been reduced?
Na has been oxidised: its oxidation number increases from 0 (in Na) to +1 (in NaCl).
Cl has been reduced: its oxidation number decreases from 0 (in Cl₂) to −1 (in NaCl).
When balancing a redox equation, after identifying oxidation number changes, the next step is to balance the .......... changes before balancing the .......... .
When balancing a redox equation, after identifying oxidation number changes, the next step is to balance the oxidation number changes before balancing the charges.
What is an ionic half-equation?
An ionic half-equation is an equation that shows either the oxidation or the reduction process separately, including the electrons gained or lost.
Half-equations are combined to give the overall redox equation.
What are the five steps for writing an overall redox equation from ionic half-equations?
1. Write the unbalanced equation and identify atoms that change in oxidation number.
2. Deduce the oxidation number changes.
3. Balance the oxidation number changes.
4. Balance the charges.
5. Balance the atoms.
True or False?
Non-metals generally form positive ions by losing electrons.
False.
Non-metals generally form negative ions by gaining electrons and are therefore reduced, with a decrease in oxidation number.
Metals generally form .......... ions by .......... electrons and are therefore .......... in reactions.
Metals generally form positive ions by losing electrons and are therefore oxidised in reactions.
How are Roman numerals used in the names of ionic compounds such as iron(II) sulfate and iron(III) sulfate?
Roman numerals indicate the oxidation state of the element with a variable oxidation number.
iron(II) sulfate contains Fe²⁺; iron(III) sulfate contains Fe³⁺. This naming system applies to compounds of transition metals and other elements with multiple oxidation states.
True or False?
Sodium is oxidised when it reacts with water.
True.
When sodium reacts with water, the oxidation number of sodium increases from 0 to +1, meaning sodium is oxidised and loses electrons.
What is the oxidation state of Cr in K₂CrO₄?
The oxidation state of Cr in K₂CrO₄ is +6.
Two K⁺ ions contribute +2 overall; CrO₄ therefore carries a 2− charge; oxygen is −2 each (4 × −2 = −8), so Cr must be +6 to give the overall −2 charge.
In MnO₄^- reacting with Fe²⁺ (aq) in acid, Mn is reduced from .......... to .......... and Fe is oxidised from .......... to .......... .
In MnO₄^- reacting with Fe²⁺ (aq) in acid, Mn is reduced from +7 to +2 and Fe is oxidised from +2 to +3.
Name the compound Cu₂O, giving the oxidation state of copper.
Cu₂O is copper(I) oxide.
Oxygen has an oxidation state of −2; the overall compound is neutral, so both Cu atoms together contribute +2, giving each Cu an oxidation state of +1.

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