C6.1 Chemicals and why we need them
Chemicals are all around us and we depend on them daily – chemical synthesis is the process by which raw materials are made into useful products such as:
- Food additives
- Fertilisers
- Dyestuffs
- Pigments
- Pharmaceuticals
- Cosmetics
- Paints
Bulk chemicals – Making chemicals in much larger quantities
Raw materials – E.g. crude oil and natural gas – these are converted into more useful products
Fine chemicals – chemicals made in much smaller quantities – thousands/ million tonnes made per year e.g. drugs
*need to know* Many chemicals are involved in the synthesis of other useful chemicals including:
There are dangerous acids such as concentrated sulfuric acid which is very corrosive and must be handled with care – however there are pure acidic compounds that are solids (citric and tartaric acids), liquids (sulfuric, nitric and ethanoic acids) or gases (hydrogen chloride)
Common alkalis include the hydroxides of sodium, potassium and calcium.
The pH scale is a measure of the acidity or alkalinity of an aqueous solution, across a 14-point scale
Acids are substances that have a pH scale less than 7
Bases are the oxides and hydroxides of metals – those which are soluble are called alkalis and they have a pH greater than 7
Indicators such as litmus paper, universal indicator and pH meters are used to detect whether a substance is acidic or alkaline
Acids react with metals, metal oxides, metal hydroxides and metal carbonates – during each of these reactions a salt is made.
Acids will react with reactive metals, such as magnesium and zinc to make a salt and hydrogen.
Acid + Metal → Salt + Hydrogen
Acids react with metal oxides and hydroxides to produce salt and water.
Acid + Metal Oxide → Salt + Water
Acid + Metal Hydroxide → Salt + Water
When acids react with carbonates, a salt, water and carbon dioxide are made.
Acid + Carbonate → Salt + Water + Carbon Dioxide
When an acid reacts with a metal, metal oxide, metal hydroxide or metal carbonate a salt is made – the name of the salt depends on the metal name and the acid.
The first part of the name comes from the metal or the metal in the oxide/hydroxide/carbonate.
The second part of the name comes from the acid used:
- H2SO4 (sulfuric acid) becomes a sulfate
- HCl (hydrochloric acid) becomes a chloride
- HNO3 (nitric acid) becomes a nitrate
When an acidic compound dissolves in water it produces aqueous hydrogen ions H+(aq) – these ions are responsible for the acidity of the solution.
When an alkali compound dissolves in water it produces aqueous hydroxide ions OH–(aq) – these ions are responsible for the alkalinity of the solution
When an acid and a base are mixed together in the correct amounts, they cancel each other out – this reaction is called neutralisation because the solution that remains has a neutral pH of 7.
H+(aq) + OH−(aq) → H2O(l)
The reaction between the hydrogen ions and hydroxide ions produces water molecules
When a chemical reaction occurs energy is transferred to or from the surroundings – and there is often a temperature change.
Exothermic reactions are accompanied by a temperature rise – this is because they transfer 
heat energy to the surroundings (they give out heat). Examples of exothermic reactions include:
- Burning
- Neutralisation reactions between acids and alkalis
- Combustion of carbon
During exothermic reactions energy is given out – this means that the energy of the products will be lower than the energy of the reactants:
This energy-level diagram shows energy has left the system.
Endothermic reactions are accompanied by fall in temperature – this is because these reactions take in energy from the surroundings (they take in heat). Examples of endothermic 
reactions include:
- Electrolysis
- The reaction between ethanoic acid and sodium carbonate
- The reaction between citric acid and sodium hydrogencarbonate.
During endothermic reactions energy is taken in – this means the energy of the products will be higher than the energy of the reactants.
This energy-level diagram shows energy has been absorbed in the reaction.
Since energy changes play a major role in chemical reactions – it is very important that these changes are managed during chemical synthesis – e.g. if too much energy was given out = accident