Module 2.1 | Atoms and Reactions

Question 1

2.1.1 a) Define ‘isotopes’.

Answer 1

Atoms of the same element with different numbers of neutrons and different masses.

Question 2

2.1.1 b) Determine atomic structure in terms of protons, neutrons and electrons for atoms and ions, given the atomic number, mass number and any ionic charge.

Answer 2

Number of protons = atomic number
Number of electrons = atomic number
Number of neutrons = mass number - atomic number

To find the number of electrons in an ion, identify whether it is a cation or an anion, and add or subtract from the atomic number respectively.

Question 3

2.1.1 c) Define the term relative isotopic mass.

Answer 3

Mass of an atom of an isotope relative to 1/12th mass of carbon-12.

Question 4

2.1.1 c) Define the term relative atomic mass.

Answer 4

Weighted mean mass of an atom of an element relative to 1/12th mass of carbon-12.

Question 5

2.1.1 d) Understand how to calculate the relative atomic mass of an element from the relative abundances of its isotopes.

Answer 5

Ar = Sum of (m/z x abundance) / Total abundance

Question 6

2.1.1 e) Understand when to use relative molecular mass (Mr) and relative formula mass.

Answer 6

For simple molecules, the term relative molecular mass will be used. For compounds with giant structures, the term relative formula mass will be used.

Question 7

2.1.1 e) Understand how to calculate relative molecular mass or relative formula mass from relative atomic masses.

Answer 7

Mr = Sum of all of the Ars of the component elements.

Question 8

2.1.2 a) Understand how to write formulae of ionic compounds from ionic charges by predicting ionic charge from the position of an element in the periodic table.

Answer 8

Group 1 metals lose an electron to form 1+ ions.
Group 2 metals lose two electrons to form 2+ ions.
Group 7 non-metals gain an electron to form 1- ions.

Question 9

2.1.2 a) Recall the formulae for the following ions - nitrate, carbonate, sulfate, hydroxide, ammonium, zinc (II), silver.

Answer 9

Nitrate - (NO3) -
Carbonate - (CO3) 2-
Sulfate - (SO4) 2-
Hydroxide - (OH) -
Ammonium - (NH4) +
Zinc - (Zn) 2+
Silver - (Ag) +

Question 10

2.1.3 a) Define ‘amount of substance’.

Answer 10

Amount of substance defines the number of particles in a substance.

Question 11

2.1.3 a) Define ‘mole’.

Answer 11

Symbol ‘mol’ - the unit for amount of substance.

Question 12

2.1.3 a) Define ‘the Avogadro constant’.

Answer 12

The number of particles per mole, 6.02 x 10^23 mol^-1

Question 13

2.1.3 a) Define ‘molar mass’.

Answer 13

Mass per mole, units g mol^-1

Question 14

2.1.3 a) Define ‘molar gas volume’

Answer 14

Gas volume per mole, units dm^3 mol^-1. Molar gas volume at RTP = 24.0 dm^3

Question 15

2.1.3 c) Understand how to calculate empirical formula.

Answer 15

For each individual element:

Mass/Ar = Number of moles
Divide each number of moles by the smallest number of moles to obtain ratio
Write empirical formula

Question 16

2.1.3 c) Understand how to calculate molecular formula.

Answer 16

Mr of molecule / empirical Mr = multiplier
Use this multiplier to scale up the empirical formula

Question 17

2.1.3 d) Define ‘anhydrous’.

Answer 17

An anhydrous substance contains no water of crystallisation.

Question 18

2.1.3 d) Define ‘hydrated’.

Answer 18

A hydrated substance contains water of crystallisation.

Question 19

2.1.3 d) Define ‘water of crystallisation’

Answer 19

Water that is part of the crystalline structure. The molecules are stoichiometrically chemically bonded into the crystal structure.

Question 20

2.1.3 d) Understand how to calculate the formula of a hydrated salt.

Answer 20

Work out the mass of the water by subtracting the mass of the anhydrous salt from the mass of the hydrated salt.
Use mass / Mr to work out the number of moles of the salt and the water separately.
Divide each number of moles by the smallest number of moles to obtain ratio.
Write the formula of the hydrated salt.

Question 21

2.1.3 e) Understand how to calculate amount of substance involving mass.

Answer 21

Number of moles = mass / Mr

Question 22

2.1.3 e) Understand how to calculate amount of substance involving gas volume.

Answer 22

Number of moles = volume (in dm^3) / 24

Question 23

2.1.3 e) Understand how to calculate amount of substance involving volume of solution and concentration.

Answer 23

Number of moles = concentration x volume

Question 24

2.1.3 f) Recall and use the ideal gas equation.

Answer 24

pV = nRT

p - pressure (Pa)
V - volume (m^3)
n - number of moles (mol)
R - Ideal gas constant, 8.31JK^-1mol^-1
T - temperature (Kelvin, °C + 273)

Question 25

2.1.3 h) Understand how to calculate percentage yield of a reaction.

Answer 25

Percentage yield = (actual yield / theoretical yield) x 100

Question 26

2.1.3 h) Understand how to calculate atom economy of a reaction.

Answer 26

Atom economy = (Mr of desired product / Mr of reactants) x 100

Question 27

2.1.3 i) Understand the techniques and procedures required during experiments.

Answer 27

-Use a digital mass balance to measure mass. It is important to use a balance with a suitable resolution for the experiment. -Use a measuring cylinder to measure the volume of a solution. If making up a standard solution, a volumetric flask of the required volume should be used. -Use a gas syringe to measure the volume of gas produced during an experiment. Alternatively, measure the mass lost on a balance and use this value to calculate the moles of gas produced during the experiment.

Question 28

2.1.3 j) Understand the benefits for sustainability of developing chemical processes with a high atom economy.

Answer 28

A high atom economy means: -There is little or no waste product, only the desired product. -The process is more economically viable for industrial scale manufacture. -Helps to preserve raw materials.