Additional Science

C2- Chemistry

Analysing substances, making salts, acids and bases

Instrumental methods of analysing substances such as elements and compounds are;accurate, rapid and sensitive

Paper chromatography can be used to detect additives in food.

Artificial colours can be identified.

Solvent; dissolves the colours and is absorbed through the paper.

Known samples.Unknown sample.

The colour dots from the unknown sample which match the known samples can be identified as to whether the additive is safe or not.

Pencil line; starting point, pencil does not dissolve in the solvent.

Lid to prevent evaporation of the solvent.

Gas chromatography linked to mass spectroscopy (GC-MS) is an example of an instrumental method:

Gas chromatography separates mixtures of compounds, it uses a column packed with solid.

The substance to be analysed is carried through the column using a gas

The time taken for a substance to travel through the column can be used to help identify the substance

The number of peaks corresponds to the number of compounds present

The position of the peak is the ‘retention time’, in other words how long they have spent in the column

The gas chromatography column can be linked to a mass spectrometer which can identify substances quickly and accurately and in very small quantities.

The mass spectrometer can give the relative molecular mass of the substances (you look at the molecular ion peak on a graph)

H+(aq) + OH-(aq) H2O (l)

State symbols are shown after substances in reactions

(s)= solid

(l)= liquid

(aq) = aqueous (dissolved in water)

(g) = gas

Soluble salts can be made by reacting acids with:1. metals – not all metals are suitable; some are too reactive and others are not reactive enough (MASH)2. insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off (copper sulphate)3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation)

Soluble salts can be made by reacting acids with:1. metals – not all metals are suitable; some are too reactive and others are not reactive enough (MASH)

Metal + Acid Salt + Hydrogen

• Hydrochloric acid produces metal chlorides e.g. Sodium chloride

• Sulphuric acid produces metal sulphates e.g. Sodium Sulphate

• Nitric acid produces metal nitrates e.g. sodium nitrate

Soluble salts can be made by reacting acids with:2. insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off (copper sulphate)

Soluble salts can be made by reacting acids with:3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation)

H+(aq) + OH-(aq) H2O (l)

Salt + WaterHydrochloric + Sodium acid hydroxide

Sodium +Chloride

Water

Acid + Alkali

Salts solutions (aq) can be crystallised to produce solid salts (s)

Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction.

Soluble except when with silver, mercury or lead.

Soluble except when with silver, lead, mercury, barium, strontium and calcium.

All soluble

Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction.

All insoluble except for NH4+,

barium and those of group 1 elements

All insoluble except when with NH4

+ and group 1 elements

All insoluble except for calcium, barium and group 1

An example precipitation reaction

Silver + Sodium nitrate chloride

Sodium + nitrate

Silverchloride

AgNO3(aq) + NaCl(aq) → NaNO3(aq) +AgCl(s)

Precipitation reactions are used to remove unwanted ions from solutions, for example to treat drinking water

or treating effluent

Metal oxides and metal hydroxides are bases e.g. sodium hydroxide, copper oxide.

If the hydroxides are soluble they are called alkalis

Hydrochloric + Sodium acid hydroxide

Sodium +Chloride

Water

The name of the salt produced when an acid and alkali react depends on the acid used and the metal in the base

Ammonia (NH3) dissolves in water to form an alkali solution, NH3 (aq).

It is used to produce ammonium salts

These salts are used as fertilisers

The presence of H+ ions makes solutions acidic.

The presence of OH- ions make solutions alkali.

1 2 14131211109876543

Strong acid Neutral Strong alkali

Weak acid Weak alkali

More alkaliMore acid

More OH- ionsMore H+ ions

H+(aq) + OH-(aq) H2O (l)

Salt + WaterAcid + Alkali

In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water.

nitric + potassium acid hydroxide

potassium + nitrate

Water