Topic 1 Formulae, Equations & Amount of Substance

Question 1

Define ‘element’.

Answer 1

Chemically the simplest substances, which cannot be broken down further using chemical reactions

Question 2

Define ‘compound’.

Answer 2

A substance containing atoms of different elements chemically bonded together

Question 3

Define ‘ion’.

Answer 3

A species consisting of one or more atoms joined together and having a positive or negative charge

Question 4

How do you write an ionic equation?

Answer 4

1. Replace the formulae of ionic compounds by their separate ions.
2. Delete any ions that appear identically on both sides (same formula and state symbol).

Question 5

Name the typical reactions of acids.

Answer 5

With metals
With metal oxides/insoluble metal hydroxides
With alkalis
With carbonates/hydrogencarbonates

Question 6

General equation of reaction between acids and metals

Answer 6

metal + acid → salt + hydrogen

Question 7

General equation of reaction between acids and metal oxides/insoluble metal hydroxides

Answer 7

metal oxide/hydroxide + acid → salt + water

Question 8

General equation of reaction between acids and alkalis

Answer 8

alkali + acid → salt + water

Question 9

General equation of reaction between acids and carbonates/hydrogencarbonates

Answer 9

carbonate/hydrogencarbonate + acid → salt + water + carbon dioxide

Question 10

Generally, how do you describe observations of metal displacement reactions in aqueous solutions?

Answer 10

• Any colour change of the metal due to formation of another metal
• Any colour change of the solution due to the formation of ions

Question 11

Describe the thermite method of joining rails together.

Answer 11

A mixture of aluminium and iron(III) oxide is positioned above the place where the rails are to be joined. The following reaction occurs:

2Al(s) + Fe₂O₃(s) → 2Fe(s) + Al₂O₃(s)

The reaction is so exothermic that the iron formed is molten, which flows into the gap between the rails. The molten iron cools, joining the rails together.

Question 12

Name the compound used for the test for sulphate and explain why it is used.

Answer 12

Barium nitrate
Barium: most sulphates are soluble but barium sulphate isn’t
Nitrate: all nitrates are soluble

Question 13

Describe the test for carbon dioxide. Include a chemical equation.

Answer 13

Bubble the gas into calcium hydroxide solution. A white precipitate of calcium carbonate will be formed.

CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

Question 14

Describe the test for halide ions. Explain why those specific compounds are used.

Answer 14

Add dilute nitric acid and then silver nitrate solution. A white precipitate of silver halide will be formed.
The dilute nitric acid will react with any ions that also form precipitates with Ag⁺ (e.g. carbonate ions). Hydrochloric acid and sulphuric acid cannot be used since AgCl and Ag₂SO₄ are both insoluble, while all nitrate compounds are soluble.

Question 15

Describe the test to distinguish between chloride, bromide and iodide ions.

Answer 15

Precipitate color:
Chloride – white
Bromide – pale yellow
Iodide – creamy

Add NH₃:
Chloride – dilute NH₃ → precipitate dissolves
Bromide – concentrated NH₃ (but not dilute NH₃) → precipitate dissolves
Iodide – any concentration of NH₃ → precipitate remains

Question 16

Avogadro’s constant

Answer 16

6.02 × 10²³

Question 17

Formula for atom economy

Answer 17

(molar mass of the desired product/sum of the molar masses of all products) × 100

Question 18

Molar volume of all gases at r.t.p.

Answer 18

24 dm mol^–1

Question 19

Formula for ppm

Answer 19

(mass of solute/mass of solution) × 10⁶