C4 Stoichiometry

Question 1

Arrhenius Theory of Acids & Bases

Answer 1

An acid is a substance that dissociates in water to produce H3O+(aq).
E.g. HCl(g) + H2O(l) → H3O+(aq) + Cl−(aq)

A base is a substance that dissociates in water to produce OH−(aq).
E.g. NaOH(s) → Na+(aq) + OH−(aq)

The H+ ion reacts with the OH− ion to produce a molecule of water, and undergoes neutralisation.
E.g. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Ionic equation: H+(aq) + OH−(aq) → H2O(l)

Restricted to aqueous solutions only

Question 2

Bronsted-Lowry Theory of Acids & Bases

Answer 2

An acid is defined as any species which donates a proton, H+
An acid must thus contain H in its formula. E.g. HNO3 and H2PO4–

A base is defined as any species which accepts a proton, H+
A base must contain a lone pair of electrons to bind the H+ ion. E.g. NH3, CO3^(2–), F– and OH–

Transfer of a proton from an acid to a base

Question 3

empirical formula

Answer 3

The empirical formula of a compound is the simplest formula which shows the ratio of the atoms of the different elements in the compound.

Question 4

molecular formula

Answer 4

The molecular formula of a compound is the formula which shows the actual number of atoms of each element in one molecule of the compound.

Question 5

Redox reaction:

Answer 5

Redox reaction: A reaction that involves reduction and oxidation simultaneously.

Question 6

Reduction

Answer 6

A process whereby a substance gains electrons, resulting in a decrease in oxidation number.

Question 7

Oxidation

Answer 7

A process whereby a substance loses electrons, resulting in an increase in oxidation number.

Question 8

Reducing agent

Answer 8

A substance that gives electrons to another, itself being oxidised in the process.

Question 9

Oxidising agent

Answer 9

A substance that takes in electrons from another, itself being reduced in the process.

Question 10

Disproportionation

Answer 10

A redox reaction in which the same substance is both oxidised and reduced.

Question 11

Oxidation number (oxidation state):

Answer 11

The total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom.

Question 12

Is CrO4^2- an oxidising or reducing agent? Suggest why it was necessary to remove CrO4^2- in step IV before carrying out Step V.

Answer 12

CrO4^2– is an oxidising agent.
In Step V, the intended reaction is for the BrO3– to oxidise the KI to I2.
KI is added in excess.
If CrO4^2– remains present, it also oxidises the KI to I2.

Remarks: Cr2O7^2– is an oxidising agent.
So CrO42– which also contains Cr in +6 state, is most likely an
oxidising agent too.

Question 13

Suggest why NaNO2 was placed in the burette instead of the conical flask for titration in this experiment. [1]

Answer 13

An acidic medium is required for the reduction of MnO4
− to Mn2+.
If NaNO2 solution is placed in conical flask, we would acidify the NaNO2
with H2SO4(aq), then titrate with KMnO4. However, adding of H2SO4(aq) to
NaNO2 would convert it into products like NO2 and HNO3. But we want a
reaction to occur between the NaNO2 and KMnO4.

Question 14

Suggest why sulfuric acid was used in Step III instead of hydrochloric acid? [1]

Answer 14

The hydrochloric acid or Cl−(aq) ion will be oxidised by KMnO4 (to Cl2).
(sulfuric acid or the SO42−(aq) ion does not react with KMnO4)

Question 15

Lewis Theory of Acids & Bases

Answer 15

An acid is a species that accepts an electron pair, e.g. BF3
A base is a species that donates an electron pair, e.g. NH3
Transfer of a pair of electrons from the base to the acid.