Recognising Reactions Using Oxidation Numbers

Redox Chemistry

How can oxidation numbers be used to identify oxidation?

Oxidation occurs when the oxidation number of an element increases.

How can oxidation numbers be used to identify reduction?

Reduction occurs when the oxidation number of an element decreases.

What happens to electrons during oxidation?

Electrons are lost.

What happens to electrons during reduction?

Electrons are gained.

How are oxidation number and electron loss related?

Loss of electrons causes the oxidation number to increase.

How are oxidation number and electron gain related?

Gain of electrons causes the oxidation number to decrease.

What is a redox reaction?

A reaction in which oxidation and reduction occur simultaneously.

How can you recognise a redox reaction using oxidation numbers?

At least one element increases in oxidation number and another decreases in oxidation number.

How can you recognise that a reaction is not redox?

None of the elements change oxidation number.

What is a disproportionation reaction?

A reaction in which the same element in a single species is simultaneously oxidised and reduced.

How can you recognise disproportionation using oxidation numbers?

The oxidation number of the same element both increases and decreases during the reaction.

What happens to Zn in Zn → Zn²⁺ + 2e⁻?

Zn is oxidised because its oxidation number increases from 0 to +2.

What happens to Fe²⁺ in Fe²⁺ → Fe³⁺ + e⁻?

Fe²⁺ is oxidised because iron's oxidation number increases from +2 to +3.

What happens to iodide in 2I⁻ → I₂ + 2e⁻?

Iodide is oxidised because iodine's oxidation number increases from −1 to 0.

What happens to chlorine in Cl₂ + 2e⁻ → 2Cl⁻?

Chlorine is reduced because its oxidation number decreases from 0 to −1.

What happens to Mn in MnO₄⁻ → Mn²⁺?

Manganese is reduced because its oxidation number decreases from +7 to +2.

What happens to hydrogen in 2H₂O + 2e⁻ → 2OH⁻ + H₂?

Hydrogen is reduced because its oxidation number decreases from +1 to 0.

Is H₂S + Cl₂ → 2HCl + S a redox reaction?

Yes. Sulfur is oxidised from −2 to 0 and chlorine is reduced from 0 to −1.

What happens to sulfur in H₂S + Cl₂ → 2HCl + S?

Sulfur is oxidised because its oxidation number increases from −2 to 0.

What happens to chlorine in H₂S + Cl₂ → 2HCl + S?

Chlorine is reduced because its oxidation number decreases from 0 to −1.

Is NaOH + HCl → NaCl + H₂O a redox reaction?

No. None of the elements change oxidation number.

Why is an acid-base neutralisation such as NaOH + HCl → NaCl + H₂O not redox?

There is no change in oxidation numbers.

What happens to chlorine in 2NaOH + Cl₂ → NaCl + NaClO + H₂O?

Chlorine is both oxidised and reduced.

How is chlorine oxidised in 2NaOH + Cl₂ → NaCl + NaClO + H₂O?

Its oxidation number increases from 0 in Cl₂ to +1 in NaClO.

How is chlorine reduced in 2NaOH + Cl₂ → NaCl + NaClO + H₂O?

Its oxidation number decreases from 0 in Cl₂ to −1 in NaCl.

Why is 2NaOH + Cl₂ → NaCl + NaClO + H₂O a disproportionation reaction?

The same element, chlorine, is simultaneously oxidised from 0 to +1 and reduced from 0 to −1.

What usually happens to Group 1 metals when they react?

They lose one electron and are oxidised from oxidation number 0 to +1.

What is the general oxidation half-equation for a Group 1 metal M?

M → M⁺ + e⁻.

What happens to sodium when it reacts?

Na → Na⁺ + e⁻; sodium is oxidised from 0 to +1.

What usually happens to Group 2 metals when they react?

They lose two electrons and are oxidised from oxidation number 0 to +2.

What is the general oxidation half-equation for a Group 2 metal M?

M → M²⁺ + 2e⁻.

What happens to magnesium when it reacts?

Mg → Mg²⁺ + 2e⁻; magnesium is oxidised from 0 to +2.

What happens to aluminium when it reacts to form Al³⁺?

Aluminium loses three electrons and is oxidised from 0 to +3.

What is the half-equation for oxidation of aluminium?

Al → Al³⁺ + 3e⁻.

What happens to fluorine when it forms fluoride ions?

Fluorine gains electrons and is reduced from oxidation number 0 to −1.

What is the reduction half-equation for fluorine?

F₂ + 2e⁻ → 2F⁻.

Why are reactions of p-block elements less predictable than those of s-block elements?

p-block elements have a wider range of electronegativities and can therefore be oxidised or reduced depending on what they react with.

What happens to nitrogen when N₂ reacts with sodium to form Na₃N?

Nitrogen is reduced because its oxidation number decreases from 0 to −3.

What happens to sodium when it reacts with nitrogen?

Sodium is oxidised because its oxidation number increases from 0 to +1.

What happens to nitrogen when N₂ reacts with F₂ to form NF₃?

Nitrogen is oxidised because its oxidation number increases from 0 to +3.

What happens to fluorine when it reacts with nitrogen to form NF₃?

Fluorine is reduced because its oxidation number decreases from 0 to −1.

Why can nitrogen be either oxidised or reduced in reactions?

Whether nitrogen gains or loses electron density depends on the electronegativity of the element it reacts with.

Is Mg + 2HCl → MgCl₂ + H₂ a redox reaction?

Yes.

Which element is oxidised in Mg + 2HCl → MgCl₂ + H₂?

Magnesium; its oxidation number increases from 0 to +2.

Which element is reduced in Mg + 2HCl → MgCl₂ + H₂?

Hydrogen; its oxidation number decreases from +1 to 0.

Is CaO + H₂O → Ca(OH)₂ a redox reaction?

No, because no oxidation numbers change.

Is 2H₂O₂ → 2H₂O + O₂ a redox reaction?

Yes; it is also a disproportionation reaction.

Why is 2H₂O₂ → 2H₂O + O₂ a disproportionation reaction?

Oxygen in H₂O₂ has oxidation number −1; some oxygen is reduced to −2 in H₂O while some is oxidised to 0 in O₂.

Is KOH + HNO₃ → KNO₃ + H₂O a redox reaction?

No, because no oxidation numbers change.

Is Cl₂ + H₂O → HCl + HClO a redox reaction?

Yes; it is also a disproportionation reaction.

Why is Cl₂ + H₂O → HCl + HClO a disproportionation reaction?

Chlorine changes from 0 to −1 in HCl and from 0 to +1 in HClO.

What does an increase in oxidation number always indicate?

Oxidation.

What does a decrease in oxidation number always indicate?

Reduction.

What should you compare when deciding whether a reaction is redox?

The oxidation numbers of each element in the reactants and products.

What should you look for to identify disproportionation in an exam question?

One element starting in a single oxidation state and ending in both a higher and a lower oxidation state.