Chemical Reactions, Acids, Bases & Redox

11 methods · 28 questions

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A reaction is redox only when at least one element changes oxidation state (electrons actually move); whole families of reaction never do, and naming the family answers the question faster than assigning every number.

Trigger: 'which of these equations represent(s) a redox reaction' or 'which involve(s) oxidation'.

Instances:
(i) Never redox: precipitation (ions pair up into a solid), neutralisation (acid with a base, alkali or carbonate), and combining a basic oxide with an acidic oxide into a salt; every element keeps its state.
(ii) Always redox: displacement (a more reactive element pushes out a less reactive one), disproportionation (one element goes both up and down at once), and decomposition of a compound into its free elements (state $0$ on the right).
(iii) Gain of oxygen is oxidation and loss of oxygen is reduction, so a species that visibly picks up or sheds oxygen (CO to $C O_{2}$ , $C O_{2}$ to CO) flags a redox change without any number crunching.

Linked questions (3)

NSAA 2016 Section 1, Part C (Chemistry questions) · Q45

Two of the following equations represent redox reactions:

1

C l_{2} + 2 KI \to I_{2} + 2 KCl

C l_{2} + 2 e^{-} \to 2 Cl^{-}

HCl + AgN O_{3} \to AgCl + HN O_{3}

C l_{2} + H_{2} O \to HCl + HClO

Which two equations represent redox reactions?

A1 and 2
B1 and 3
C1 and 4
D2 and 3
E2 and 4
F3 and 4

Worked solution

Answer:C

A redox reaction involves a change in oxidation state, with one species oxidised and another reduced.

Equation 1: chlorine goes from 0 in

C l_{2}

- 1

in KCl (reduced) while iodine goes from

- 1

in KI to 0 in

I_{2}

(oxidised). Redox.

Equation 2: this is a half-equation showing reduction only (chlorine gains electrons); nothing is oxidised, so it does not represent a complete redox reaction.

Equation 3: this is a precipitation reaction. Every element keeps its oxidation state (H

+ 1

, Cl

- 1

, Ag

+ 1

, N

+ 5

, O

- 2

). Not redox.

Equation 4: chlorine goes from 0 in

C l_{2}

- 1

in HCl and to

+ 1

in HClO. The same element is both oxidised and reduced (disproportionation), which is a redox reaction.

The two redox reactions are 1 and 4.

The answer is C.

NSAA 2017 Section 1, Part C (Chemistry questions) · Q38

Which two of the following reactions involve oxidation?

1

Ca \to Ca^{2 +} + 2 e^{-}

C l_{2} + 2 e^{-} \to 2 C l^{-}

F e_{2} O_{3} + 3 CO \to 2 Fe + 3 C O_{2}

MgC O_{3} + 2 HCl \to MgC l_{2} + C O_{2} + H_{2} O

A1 and 2 only
B1 and 3 only
C1 and 4 only
D2 and 3 only
E2 and 4 only
F3 and 4 only

Worked solution

Answer:B

Oxidation is loss of electrons, or equivalently gain of oxygen or an increase in oxidation state.

Reaction 1: calcium loses two electrons to form

Ca^{2 +}

. This is oxidation.

Reaction 2: chlorine molecules gain electrons to form chloride ions. This is reduction only.

Reaction 3: carbon monoxide gains oxygen to become carbon dioxide (carbon goes from

+ 2

+ 4

), while iron(III) oxide is reduced to iron. The reaction therefore involves oxidation (it is a redox reaction).

Reaction 4: an acid reacts with a carbonate. Every element keeps its oxidation state (

Mg

stays

+ 2

C

stays

+ 4

Cl

stays

- 1

), so there is no oxidation.

The two reactions involving oxidation are 1 and 3.

The answer is B.

NSAA 2018 Section 1, Part C (Chemistry questions) · Q43

The following equations show the main reactions that take place in a blast furnace during the extraction of iron and the removal of the impurities:

C + O_{2} \to C O_{2}

[[U]] C O_{2} [[/ U]] + C \to 2 CO

F e_{2} O_{3} + 3 CO \to 2 Fe + 3 C O_{2}

[[U]] CaC O_{3} [[/ U]] \to CaO + C O_{2}

[[U]] CaO [[/ U]] + Si O_{2} \to CaSi O_{3}

Which row in the following table correctly identifies whether the underlined substance is oxidised, or reduced, or neither?

The options list, in order:

C O_{2}

CaC O_{3}

and

CaO

Aoxidised; reduced; neither
Boxidised; neither; neither
Coxidised; reduced; oxidised
Doxidised; neither; oxidised
Ereduced; reduced; neither
Freduced; neither; neither
Greduced; reduced; oxidised
Hreduced; neither; oxidised

Worked solution

Answer:F

Oxidation is gain of oxygen (or loss of electrons, a rise in oxidation number); reduction is loss of oxygen (a fall in oxidation number).

C O_{2}

C O_{2} + C \to 2 CO

: each carbon dioxide molecule loses an oxygen atom to become carbon monoxide, and the oxidation number of carbon falls from

+ 4

+ 2

. Carbon dioxide is reduced.

CaC O_{3}

CaC O_{3} \to CaO + C O_{2}

: this is thermal decomposition. Calcium stays at

+ 2

, carbon stays at

+ 4

and oxygen stays at

- 2

, so no oxidation number changes. Neither.

CaO

CaO + Si O_{2} \to CaSi O_{3}

: a basic oxide combines with an acidic oxide to form a salt. Again every oxidation number is unchanged. Neither.

Reduced, neither, neither is row F.

The answer is F.