What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

Define oxidation and reduction in electronic terms.

Oxidation is the loss of electrons and reduction is the gain of electrons; both occur together in a redox reaction.

What is the oxidation number of an atom in a free element?

Zero, for example in \(H_2\), \(O_2\), \(P_4\) or metallic sodium.

What is the oxidation number of oxygen in peroxides?

It is \(-1\), as in hydrogen peroxide and sodium peroxide.

Give an example of a disproportionation reaction.

The decomposition of hydrogen peroxide, \(2H_2O_2\rightarrow2H_2O+O_2\), in which oxygen is both oxidised and reduced.

Why must fluorine be obtained by electrolysis of fluorides?

Because \(F_2\) is the strongest oxidising agent, no chemical reagent can oxidise \(F^-\); only electrolytic oxidation works.

What completes the internal circuit of a Daniell cell?

A salt bridge that allows ions to migrate between the two half-cells.

What does the standard hydrogen electrode potential equal?

It is assigned a value of exactly 0.00 volts by convention.

What does a negative standard electrode potential indicate?

That the redox couple is a stronger reducing agent than the hydrogen couple.

Redox Reactions

Q: Name the four types of redox reactions.

Combination, decomposition, displacement and disproportionation reactions.

Q: Name the two methods used to balance redox equations.

The oxidation-number method and the half-reaction (ion-electron) method.

Class 11 Chemistry · Chemistry Part II · Chapter 1

163 members in the CBSE community

View all

CBSE Class 11 · Chemistry

Organic basics: 11 marks, the biggest unit

Marks booster + cheatsheet: key concepts in 2 lines each, common traps
10 marks: Hydrocarbons
Every reaction with reagent and condition
Common traps that cost marks, listed
IUPAC naming rules
Oxidation-number rules and balancing

Page 1 free about 8 min read, Full kit about 55 min read

₹159See the full kit

CBSE Class 11 · Chemistry

Chemistry Part I: 45 of 70 theory marks

Marks booster + cheatsheet: key concepts in 2 lines each, common traps
9 marks each: Atom and Thermodynamics
Constants and formulas, with page numbers
Common traps, corrected
Periodic trends from the real data
pH, buffers and solubility product

Page 1 free about 8 min read, Full kit about 55 min read

₹159See the full kit

View all 44 amazing Marks Booster kits →

Summary

Redox reactions, in which oxidation and reduction occur simultaneously, form a major class of chemical change underlying combustion, metallurgy, batteries and corrosion. The unit develops three parallel ways of viewing them. The classical idea defines oxidation as the addition of oxygen or an electronegative element, or the removal of hydrogen or an electropositive element, with reduction as the reverse. The electronic idea defines oxidation as loss of electrons and reduction as gain of electrons, so the oxidising agent accepts electrons (and is itself reduced) while the reducing agent donates them. Competitive electron-transfer experiments, such as zinc displacing copper from copper sulphate, establish a relative order of reactivity. The oxidation-number concept assigns oxidation states by a fixed set of rules and redefines oxidation as an increase, and reduction as a decrease, in oxidation number; this allows redox reactions to be classified into combination, decomposition, displacement and disproportionation types. Two systematic methods balance redox equations — the oxidation-number method and the half-reaction (ion-electron) method — applicable in both acidic and basic media, and redox titrations use self-indicators such as permanganate. Finally redox is linked to electrode processes through the redox couple, the Daniell cell and the standard electrode potential, the standard hydrogen electrode being assigned a value of exactly zero volts.

Classical idea of oxidation and reductionRedox in terms of electron transferOxidation number and its rulesTypes of redox reactionsBalancing by oxidation-number methodBalancing by half-reaction methodRedox titrationsRedox couples and electrode potential

Key terms

Oxidation

Loss of electrons by a species, equivalently an increase in its oxidation number.

Reduction

Gain of electrons by a species, equivalently a decrease in its oxidation number.

Oxidation number

The charge an atom would have if the bonding electrons were assigned to the more electronegative atom, fixed by a set of rules.

Disproportionation

A redox reaction in which an element in an intermediate oxidation state is simultaneously oxidised and reduced.

Redox couple

The oxidised and reduced forms of a substance taking part together in a half reaction, e.g. \(Zn^{2+}/Zn\).

Standard electrode potential

The potential of an electrode at unit concentration and 298 K, measured relative to the standard hydrogen electrode taken as zero.

Extra questions & answers

Try one from this chapter

In electronic terms, oxidation is defined as the:

Flashcards & interactive practice