Separation and Purification

States of matter

State changes are physical changes - they cannot be reversed and the chemical properties of what you’re changing stay the same

Particles

Filtration and Crystallisation

Filters separate mixtures by letting smaller things through but trapping bigger things (aka insoluble substances)

Solute: a dissolved substance

Solution: a mix of solutes

Solvent: what the solutes dissolve in

Saturated solution: a solution containing the most amount of solute that can dissolve in that amount of solvent

Crystallisation

Crystallisation: a method of separation involving evaporating the solvent to leave the solutes behind. This forms solid crystals.

If crystals form slowly, the particles have longer to form an ordered pattern and will make larger crystals. If crystals form quickly, they will be smaller.

Crystallisation risk assessment:

Wear eye protection

Use steam to heat the evaporating basin gently

Remove the bunsen burner before the solution is completely dry

Care must be taken not to overheat the solution as hot crystals may spit out

Crystallisation in the lab:

1. The filtrate is placed above a bunsen burner
2. The bunsen burner evaporates the filtrate carefully
3. Crystals are left to cool
4. Crystals are scraped out
5. Crystals are patted dry with cloth / filter paper

Further heating may cause the crystals to change chemically

Filtration

1. A filter tunnel is lined with filter paper
2. The solvent and the solute pass through
3. This forms the filtrate
4. Bits of insoluble substances that can’t fit through form the residue

Residue: bits of insoluble substances that don’t get through filter paper and are left in it

Distillation

Distillation is a way of separating multiple liquids by boiling them one by one. The one with the lowest boiling point evaporates first, and so on

1. Liquid is evaporated and turns to gas (vapour)
2. Other materials with higher boiling points are left behind
3. The vapour of the first liquid is now pure. It is condensed back into a liquid

Still: the apparatus used for distillation

Ways to make a still more efficient:

Plugging gaps in apparatus with bungs to stop any gas escaping

Condenser: a tube surrounded with a jacket of cold water (used instead of a delivery tube). Helps liquid condense quicker with lowered temperatures

Anti-bumping granules: makes the liquid boil more smoothly, reduces risk of liquid boiling over (put in the solution)

Fractional distillation

1. Solution is put in a distillation flask and heated
2. The first substance’s vapour (the one with the lowest boiling point) rises up a fractioning column
3. A temperature gradient is created, with the hottest temperatures at the bottom
4. The substance with the lowest boiling point reaches the top first and passes into the condenser and is collected
5. The same happens with the rest of the substances, one by one

Apparatus for fractional distillation

Examples of uses of fractional distillation:

Separate different products of crude oil

Make alcoholic drinks like whisky and vodka

Separate gasses in the air

Mixtures

The composition of a pure substance can’t be changed, and is the same everywhere in the substance. For example, pure gold contains only gold atoms.

Compounds can be pure if it is the only thing in the substance e.g. pure water is only H2O

A pure substance can’t be separated into other substances

Mixture: contains elements and/or compounds not chemically joined together. They can be separated and don’t have a fixed composition

Melting points

When something melts its particles gain enough energy to overcome the forces of attraction between them. The temperature at which this happens is the melting point

Pure substances have fixed melting points.

Mixtures don’t have fixed melting points and instead melt over a range of temperatures. This is because there are different things with different melting points inside it.

Chromatography

When a solvent moves along paper, different substances in it move at different speeds, so they separate.

Mobile phase = the solvent (e.g. water)

Stationary phase = the paper

Chromatogram: the paper with the separated components

Rf value = distance moved by spot distance moved by solvent

Measurements are made from the starting points of samples

Atomic Structure

Periodic Table

Mendeleev was a Russian chemist credited with the first periodic table

He arranged the elements in increasing order of atomic masses, and elements with similar properties were near each other.

He left gaps in his table for unknown elements, and so was able to predict the properties of them by looking at where they fitted and the properties of the elements around them

His law couldn’t explain the existence of isotopes or where they would go on his table. As they have different atomic masses, they should have been in a separate place, but Mendeleev didn’t give them their column, which didn’t make sense with his law.

Structure of atoms

Atoms have a central nucleus containing protons and neutrons, and electrons arranged in shells surrounding it.

Protons have a positive charge (+1)

Neutrons have a neutral charge (0)

Electrons have a negative charge (-1)

Atomic number: number of protons/electrons

Mass number: number of protons + number of neutrons

aka atomic number + neutrons

The number of protons and electrons is always the same

To work out:

number of protons: atomic number

number of electrons: atomic number

number of neutrons: mass number - atomic number

The mass number is always the largest of the two numbers

The atomic number determines what element and atom is

Isotopes

Isotope: an atom with the same atomic number but a different mass number

They are the same element but are chemically slightly different as they have a different number of neutrons

Example: Chlorine

Chlorine has two isotopes: chlorine-35 and chlorine-37

Both of their atomic numbers are 17 - they are still chlorine, but their mass numbers are 35 and 37

Isotopes have the same chemical reactions as they have the same electron arrangement (the only thing that is different is the number of neutrons)

Isotopes can be written in two ways:

1. element-mass number e.g. chlorine-35
2. with the mass number in small above and the atomic number in small below e.g:

Relative atomic mass

Relative atomic mass: the weighted average mass of an atom’s isotopes

How to work it out:

(percentage x mass number of isotope 1) + (percentage x mass number of isotope 2) 100

If there are more than two isotopes you just add more brackets on the end

Example:

chlorine-35 is 75% and chlorine-37 is 25%

(75 x 35) + (25 x 37)

100

= 35.5

So chlorine’s relative atomic mass is 35.5

Electron Configuration

The first (most inner) shell can hold max 2 electrons

All the other shells can hold max 8 electrons

When a shell is full you have to go onto the next one out

Writing electron configuration:

Numbers of electrons in each shell starting in the centre and going outwards e.g. 2, 8, 6

Atoms in the same group have the same number of electrons in their outer shell

Atoms in the same period have the same number of shells

Metals have 1, 2, 3 electrons in their outer shell

Non-metals have 4, 5, 6, 7 electrons in their outer shell

Periodic Table

Staircase line: (remember down from B) metals to the left, non-metals to the right

Transition metals: middle section of un-grouped metals, the most common metals

Vertical columns are groups (0-7, missing out transition metals)

Horizontal rows are periods (1-7)

Ionic bonding

Bonds are the forces of attraction that hold atoms together. When bonds form between atoms, energy is released, making them more stable & less reactive. The most stable atoms are the ones with a full outer shell - they contain as many electrons as possible.

Atoms can achieve a full outer shell by transfer of electrons between atoms, which form charged particles called ions.

Metal atoms loose electrons and form positive ions aka cations, which have more protons than electrons.

Non-metal atoms gain electrons and form negative ions aka anions, which have more electrons than protons

When non-metals form negative ions the end of the name changes to -ide

Electrostatic forces: forces of attraction between all positively and negatively charged objects. They hold the oppositely charged ions together and form an ionic bond between them.

Atoms that easily form ions will either have a nearly full or nearly empty outer shell.

Most ionic bonds will be formed between a metal and a non-metal.

The ion formed depends on the element’s position in the periodic table and its number of outer electrons

Groups of the periodic table

The periodic table is arranged so that elements in the same vertical column or group have similar chemical and physical properties and show trends in those properties

Group 1 - Alkali metals

They are called alkali metals because they react with water to form alkalis

The alkali metals in group 1 have similar physical properties that are specific to the group - they have low melting points, are soft and easily cut, are very reactive and readily form compounds with non-metals.

Reactivity of the metals increases down the group.

Alkali metals are so reactive because they only have one electron they have to loose for a complete outer shell, making it easy to happen. They all create positive (1+) ions.

As we go down the group, an extra electron shell is added, making the force of attraction between the nucleus and the outer electron weaker, as there is more space between them. This means that it is easier to remove the outer electron, explaining the trend in reactivity

Group 7 - Halogens

Similarities in group 7:

• All halogens exist as diatomic molecules (two atoms held together by a covalent bond).
• They are all non-metallic elements
• They are all poor conductors of eat and electricity
• They are all toxic and corrosive

As you go down the group, the melting points, boiling points and densities all increase:

Halogens react with metals forming salts. They can also all be used as disinfectants and bleaches, as they can kill microorganisms and remove the colour from materials.

If you put damp blue litmus paper in chlorine, it will turn red and then bleach white. This is the test for chlorine

Halogens react with hydrogen to form hydrogen halides, which dissolve in water to form acids including hydrochloric acid

Reactivity increases up the group in halogens.

Displacement reaction: a reaction where a more reactive element takes the place of a less reactive element in a compound.

Group 0 - Noble gasses

This group wasn’t known until the 19th centry, and were so difficult to detect because they don’t react with anything. There are only very small amount of each noble gas in our atmosphere.

Noble gasses are:

• colourless
• have very low melting and boiling points
• are poor conductors of heat and electricity

They are also all inert, meaning they don’t react easily with anything. This is because they all already have full outer shells, meaning they don’t need to loose/gain any more electrons.

They all exist as single atoms, because they don’t form bonds easily with other atoms

Trends:

Uses:

Krypton is used in photography lighting because it produces a brilliant white light when electricity is passed through it.

Argon is denser than air, so it is added to the space above the wine in wine barrels to stop oxygen reacting with the wine.

Helium has a very low density and is non-flammable, so it is used in balloons and airships.

Neon produces a red-orange light when electricity is passes through it, and so it is used to make neon lights.

Redox

When a metal reacts with oxygen in looses electrons, and the opposite of it is reduction, gain of electrons.

OIL RIG

Oxidation Is Loss Reduction Is Gain

These two processes occur at the same time in displacement reactions and they are called reduction-oxidation or redox reactions.

For example, when one element looses electrons to become a positively charged ion it is oxidised, and when another gains electrons to become a negatively charged ion it is reduced.