Chapter 4 - Reactions and Stoichiometry

Question 1

What is the law of conservation of matter? (2)

Answer 1

1) Every atom of an element is not destroyed or created, but rearranged.
2) sum of masses of all products is equal to sum of masses of all reactants.

Question 2

What are the 4 types of reactions?

Answer 2

1) Precipitation reaction
2) Thermal decomposition
3) Acid-base reaction
4) Redox reaction

Question 3

What is the temperature, pressure and molar volume at standard temperature and pressure (stp)?

Answer 3

Temperature: 273K (0C)
Pressure: 1 bar (100 kPa)
Molar volume: 22.7dm3/mol

Question 4

What is the temperature, pressure and molar volume at room temperature and pressure (rtp)?

Answer 4

Temperature: 293K (20C)
Pressure: 1 atm (101 kPa)
Molar volume: 24dm3/mol

Question 5

What is a standard solution?

Answer 5

It is a solution whose concentration is accurately known.

Question 6

What is the Arrhenius Theory of acids and bases? (3)

Answer 6

1) An acid is a substance that dissociates in water to produce aq H3O+ (interchangeably used with H+)
2) A base is a substance that dissociates in water to produce aq OH-.
3) When an acid is added to a base, the H+ ion reacts with OH- ion to produce a molecule of water, and undergoes neutralisation.

Question 7

What is a limitation of the Arrhenius Theory of acids and bases?

Answer 7

it is strictly restricted to aqueous solutions only.

Question 8

What is the Bronsted-Lowry Theory of acids and bases? (3)

Answer 8

1) An acid is defined as any species which donates a proton, H+. An acid must thus contain H.
2) 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.
3) The acid base reaction involves the transfer of a proton from an acid to a base, in gaseous, aqueous and non-aqueous systems.

Question 9

What is a limitation of the Bronsted-Lowry Theory of acids and bases?

Answer 9

It cannot explain why substances that do not contain any hydrogen atoms are still known to behave as acids.

Question 10

What is the Lewis Theory of acids and bases? (3)

Answer 10

1) The acid-base reaction is a transfer of a pair of electrons from the base to the acid.
2) An acid is a species that accepts an electron pair.
3) A base is a species that donates an electron pair.

Question 11

What is a limitation of the Lewis Theory of acids and bases?

Answer 11

It is too general. It is better to use Bronsted-Lowry whenever possible for acid-base reactions, and apply Lewis Theory only if the reaction does not involve proton transfer.

Question 12

What is back titration?

Answer 12

In a back titration, a known excess of one reagent A is allowed to react with an unknown amount of B. The amount of unreacted A is then determined by titration with a reagent C of known concentration. From the titration result, the amount of unreacted A and the amount of B can be found by simple stoichiometric calculations.

Question 13

Define disproportion.

Answer 13

It is a redox reaction where the same substance is both oxidised and reduced.

Question 14

What are 2 types of reaction that are definitely not redox reactions?

Answer 14

1) precipitation

2) neutralisation

Question 15

What are the 2 types of redox titrations?

Answer 15

1) Potassium Manganate (VII) titration

2) Iodometric titration - iodine/thiosulfate titration

Question 16

What is the relevant half equation for potassium manganate (vii) titration?

Answer 16

MnO4^- + 8H+ + 5e = Mn^2+ + 4H2O

Question 17

What are iodometric titrations?

Answer 17

It is where iodine is used to oxidise thiosulfate ions to tetrathionate ions, itself reduced to iodide.

Question 18

What is the relevant half equation for iodometric titrations?

Answer 18

2S2O3^2- + I2 = 2I- + S4O6^2-

Question 19

During iodometric titration, why should the iodine solution be titrated as soon as possible?

Answer 19

Iodine is volatile and will vaporise easily at room temperature, hence titration must be carried out as soon as possible. If this is not possible, cover all flasks containing iodine.

Question 20

During iodometric titration, why is the starch indicator added only towards the end of titration when the iodine solution is pale yellow, instead of right at the start?

Answer 20

Starch forms a blue-black water-soluble complex with iodine in which the the iodine is trapped within the starch molecules. Hence, starch should not be added at the start where there is a high concentration of iodine since some iodine may remain trapped in the starch even at equivalence point.

Question 21

After iodometric titration is complete, a slow return of blue colour is observed. Why is this so?

Answer 21

This is due to atmospheric oxidation of I- back to I2. Ignore any slow return of blue colour.