Chemical Reactions

Acids, alkalis and bases

An acid is a substance that forms hydrogen ions, H⁺(aq), when added to water.

Acids have pH values below 7.
 

Some common lab acids are:

• Hydrochloric acid, HCl
• Sulphuric acid, H₂SO₄
• Nitric acid, HNO₃

A base is any substance that neutralises an acid.

A neutral solution has a pH of 7.
 

An alkali is a soluble base.

All alkalis form hydroxide ions, OH_-(aq), when added to water.
Alkalis have pH values above 7.

Some common lab alkalis are:

• sodium hydroxide, NaOH
• potassium hydroxide, KOH

Neutralisation reactions 

When an acid is neutralised a salt is formed.

   acid   +   alkali       salt   +   water

The water is formed when the hydrogen and hydroxide ions combine.

    H⁺_(aq)     +     OH^-_(aq)      H₂O_(l)

The salt formed depends on the acid used and the metal in the alkali.

Hydrochloric acid forms salts called chlorides.

Sulphuric acid forms sulphates.

Nitric acid forms nitrates.
 

Ammonia gas dissolves in water to form an alkaline solution, ammonium hydroxide.

This can be neutralised with acid to produce ammonium salts.

    sulphuric acid  +  ammonium hydroxide    ammonium sulphate   +   water

When preparing a salt using an alkali a titration is usually carried out, using a burette. This is a piece of equipment that can accurately deliver a small volume of liquid.

To prepare the salt sodium chloride hydrochloric acid is reacted with sodium hydroxide solution.
   hydrochloric acid   +   sodium hydroxide      sodium chloride   +   water
           HCl(aq)         +          NaOH(aq)               NaCl(aq)        +    H₂O(l)

A primary titration is carried out to find out how much acid is needed to exactly neutralise a known volume of alkali.

The alkali is placed in a flask with a few drops of indicator, e.g. phenolphthalein. This indicator is pink in alkali and colourless in acid.

Acid is then slowly added whilst swirling the flask.

When the indicator just changes colour, the alkali has been neutralised and the volume of acid is read off the burette.
 

The titration is then repeated without indicator, to produce a neutral solution of salt and water.

The water can be evaporated off to leave white crystals of pure sodium chloride.

Preparing salts using an insoluble base

Acids can also be neutralised with insoluble bases, such as transition metal oxides and hydroxides.

   acid   +   metal oxide      salt   +   water

e.g.   nitric acid   +   copper oxide      copper nitrate   +   water

   acid   +   metal hydroxide       salt   +   water

e.g.   sulphuric acid   +   iron hydroxide      iron sulphate   +   water

Metals and metal carbonates also neutralise acids.

   acid   +   metal      salt   +   hydrogen

e.g.   hydrochloric acid   +   zinc      zinc chloride   +   hydrogen

   acid   +   metal carbonate     salt   +   carbon dioxide   +   water

e.g. hydrochloric acid +calcium carbonate calcium chloride + carbon dioxide + water

To prepare a salt using an insoluble base an excess amount of base is added to the acid, to make sure all the acid has reacted.

The excess is then filtered off leaving the salt solution, which can be evaporated to form the salt.

The chlor-alkali industry

Sodium chloride (common salt) is a compound of an alkali metal and a halogen. It is found in large quantities in the sea and in underground deposits.

Sodium chloride solution is also called brine.

Electrolysis of brine is an important industrial process.

Chlorine gas is formed at the positive electrode.

Chlorine is a poisonous gas. It is used to kill bacteria in drinking water and swimming pools.

It is also used to make hydrochloric acid, disinfectants, bleach and the plastic known as PVC.
 

The test for chlorine is that it bleaches the colour out of damp litmus paper.
 

Hydrogen gas is formed at the negative electrode.

Hydrogen is used in the manufacture of ammonia and margarine.
 

The test for hydrogen is to put a lit splint in the gas. It explodes with a squeaky pop.
 

Sodium ions, Na⁺, and hydroxide ions, OH^-, are left behind in the solution. These form sodium hydroxide solution.
 
Sodium hydroxide is used in the manufacture of soap, paper and ceramics.

Halides

Halides are compounds which contain a halogen.

Hydrogen halides, e.g. hydrogen chloride, are gases that dissolve in water to produce acidic solutions, due to the formation of H⁺ ions.

Silver halides, e.g silver chloride, silver bromide and silver iodide, are reduced to silver by light, X-rays and radiation from radioactive sources.

They are used to make photographic film and photographic paper.

Silver forms a dark image where light hits it.

                        light
silver chloride     silver  +  chlorine