Chemistry Unit 1

WJEC A-Level Chemistry

Formulae of Sulfur Dioxide

SO₂

Formulae of Methane

CH₄

Formulae of Hydrochloric Acid

HCl

Formulae of Sulfuric Acid

H₂SO₄

Formulae of Nitric Acid

HNO₃

Formulae of Ammonia

NH₃

Formulae of Ammonium Chloride

NH₄Cl

Formulae of Sodium Hydroxide

NaOH

Formulae of Sodium Chloride

NaCl

Formulae of Sodium Carbonate

Na₂CO₃

Formulae of Sodium Hydrogencarbonate

NaHCO₃

Formulae of Sodium Sulfate

Na₂SO₄

Formulae of Calcium Hydroxide

Ca(OH)₂

Formulae of Calcium Carbonate

CaCO₃

Formulae of Calcium Chloride

CaCl₂

Formulae of Copper (II) Oxide

CuO

Formulae of Copper (II) Sulfate

CuSO₄

How to find charge of ion?

Use group number

e.g. group 7 has charge of ^-

Charge for Hydrogencarbonate

HCO₃^-

What is a redox reaction?

A reaction where one substance is reduced and another is oxidised.

What is a reducing agent?

• reduce other species

• give electrons

• are themselves oxidised

What is a oxidising agent?

• oxidise other species

• accept electrons

• are themselves reduced

What is the oxidation of an uncombined element?

0

e.g. O₂ = 0

What is the oxidation of a simple ion?

Charge of the ion

e.g. O^2- = -2

What is the oxidation of a compound?

The sum must equal 0

What is the oxidation of group 1 and 2 metals in a compound?

Same as their group number

e.g. BaSO₄ = Ba=+2

What is the oxidation of a complex ion?

The sum must equal the charge

e.g. CO₃^2- = C=+4, O=-2

What is the oxidation of Hydride?

H = -1

What is the oxidation of Fluorine?

F = -1

What is the oxidation of Peroxide?

O = -1

Steps to work out ionic equations

1. word equation

2. balanced symbol equation

3. balanced ionic equation (usually for precipitate (s) )

4. spectator ions

Charge for Hydroxide

OH^-

Charge for Nitrate

NO₃^-

Charge for Oxide

O₂^-

Charge for Sulfide

S₂^-

Charge for Carbonate

CO₃^2-

Charge for Sulfate

SO₄^2-

Charge for Phosphate

PO₄^3-

Charge for Ethanoate

CH₃COO^-

What is the mass/charge of proton/neutron/electrons?

What charge does the nucleus have?

Positive - contains protons and neutrons

What charge does an atom have in general?

Neutral - same amount of protons and electrons

What is an ion?

Charged atoms caused by the gain or loss of electrons

What is an isotope?

Atoms of the same element with a different number of neutrons.

Alpha particle

• Helium nucleus

• Mass - 4 units

• Absorbed by - paper or few cm of air

• High ionising power

Beta particle

• Fast moving electron

• Mass - negligible

• Absorbed by - Aluminium

• Medium ionising power

Gamma ray

• Electromagnetic wave

• Mass - 0

• Absorbed by - thick lead

• Low ionising power

Electric field effect on radioactive particles

• Beta - light so fully attracted

• Alpha - heavy so slightly attracted

Alpha decay

• mass number decreases by 4

• atomic number decreases by 2

Beta decay

• mass number remains the same

• atomic number increases by 1

Positron

• mass number remains the same

• atomic number decreases by 1

Electron capture

• mass number remains the same

• atomic number decreases by 1

Why is ionising radiation unsafe for humans?

Damages cells and DNA causing mutations to occur

What are the safe uses of radioactivity?

• tracers

• carbon dating

• smoke alarms

What is tracers?

• radioactive isotopes are inserted into a sample and tracked using x-rays

• medical tracers/water pipe blockages/biochemistry of plants

• must be low ionising power and short half life if used in the body

What is carbon dating?

1. Carbon-14 is a beta emitter

2. When an organism is living the amount of Carbon-14 is consistent

3. When an organism dies the amount of Carbon-14 decreases

4. This allows scientists to determine the age of organisms that have been dead for many years

What is half life?

Time taken for the mass/activity of a radioactive isotope to fall to half of its original value

What is an orbital?

Volume where there is a high probability of finding an electron

How many electrons can each orbital have?

2 electrons each

S orbital

• spherical

• all energy levels

• groups of 1

• size increases with energy level

P orbital

• dumbbell shape

• all energy levels except n=1

• groups of 3

• lobes become longer and larger with energy level

How many D orbitals are in a group?

groups of 5

How many F orbitals are in a group?

groups of 7

How many G orbitals are in a group?

groups of 9

What is the order of orbitals?

S, P, D, F, G

Subshells

What is the electronic configuration?

gives information on atom

What are the rules of electronic confurguration?

1. Put electron in lowest possible energy level as there’s more energy away from nucleus

2. Fill electrons into orbitals singly before adding 2 electrons to minimise electron repulsion

Example of electronic configuration

Exceptions to the electronic configuration rules

Copper and Chromium

• Orbitals are more stable if half-filled or fully-filled

• 4s and 3d are similar in energy one electron can move from 4s to 3d

Copper - [Ar] 4s¹ 3d¹⁰

Chromium - [Ar] 4s¹ 3d⁵

What is the Molar first ionisation energy?

The energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions

What is the equation for Molar first ionisation energy?

X_(g) → X⁺_(g) + e^-

X⁴⁺_(g) → X⁵⁺_(g) + e^-

What happens down a group with Molar first ionisation energy?

• distance from nucleus increases

• shielding from inner electrons increases

• weaker electrostatic forces of attraction

• outer electron easier to remove

What happens across a period with Molar first ionisation energy?

• increased nuclear charge

• outer electron harder to remove

What happens in successive ionisation energy?

A single atom loses it’s electrons one by one

• outer electron is easiest to remove as most shielding and furthest

• 3rd shell/2nd shell harder to remove as closer, slowly increases as protons are holding electrons more tightly because charge becomes imbalanced (effective nuclear charge)

• 1st shell hardest as no shielding and closest

Order of electromagnetic spectrum

What is quantisation?

The amount of energy that the molecule can absorb/emit and is a specific value, this allows us to identify a molecule

Energy =

energy (J) = Planck’s constant x frequency (Hz)

Speed of light =

speed of light = frequency (Hz) x wavelength (m)

Absorption spectra

1. Electrons are in ground state so lowest energy levels

2. Giving atoms energy can cause electrons to move to a higher energy level. They are then in an excited state

3. The difference between energy levels is a fixed value

4. The energy relates to a particular frequency (E=hf), the frequency relates to a specific wavelength/colour of light (C=fλ)

5. The missing wavelengths show dark bands on a coloured background

Emission spectra

1. Electrons have been absorbed so in an excited state

2. The electron falls to a lower energy level

3. The difference between energy levels is a fixed value

4. The energy relates to a particular frequency (E=hf), the frequency relates to a specific wavelength/colour of light (C=fλ)

5. The emitted wavelengths show coloured bands on a dark background

Atomic spectrum of Hydrogen

Why is the spectrum a series of lines?

Each line is equal to the difference in energy between particular energy levels which is a fixed value

Why do the lines converge?

The energy levels become closer and eventually converge

What is the significance of the convergence limit?

In the Lymans series, it is the ionisation energy

Lyman series

• UV part of spectrum

• electron moves to and from 1st energy level

• convergence limit tells us ionisation energy

• highest energy

Balmer series

• visible part of spectrum

• electron moves to and from 2nd energy level

Paschen series

• infrared part of spectrum

• electron moves to and from 3rd energy level

• lowest energy

Series on a graph

What is the relative isotopic mass?

The mass of an atom of an isotope relative to one-twelfth the mass of one atom of carbon-12.

e.g. ³⁵Cl has a relative isotopic mass of 35

What is the relative atomic mass (Ar)?

The average mass of one atom of the element relative to one-twelfth the mass of one atom of carbon-12.

• found on a periodic table

What is the relative formula mass (Mr)?

The average mass of a molecule relative to one-twelfth the mass of one atom of carbon-12.

• add up all the Ar

e.g. CaCO₃ 40.1 + 12 + (16×3) = 100.1

What is a mole?

One mole is the amount of substance that contains the same number of particles as there are atoms of carbon in exactly 12g of carbon-12.

Mass =

Mr x moles

What is the molar mass?

One mole of any substance has a mass equivalent to the Mr of that substance in grams (gmol^-1).

e.g. MgCl₂ - Mr=95.3, molar mass = 95.3 gmol^-1

What is the empirical formula?

Simplest formula of a substance

1. symbol of elements

2. % or mass

3. Ar

4. moles

5. divide by smallest mole

6. ratio

How to calculate reacting masses?

1. balanced equation

2. moles of one

3. ratio for moles of what we need

4. mass=mrxmoles

Percentage yield =

mass of product obtained / maximum theoretical mass x 100

Why will percentage yield always be less than 100%?

Due to loss of product during transfer,separation or an incomplete reaction

Atom economy =

mass of required product / total mass of reactants x 100