The pH scale

The pH scale is a continuous range of numbers from below 0 to above 14 which indicate the acidity or alkalinity of solutions.

• Acids have a pH of less than 7. (pH<7)

• Alkalis have a pH of more than 7. (pH>7)

• Pure water and neutral solutions have a pH equal to 7. (pH=7)

• Solids and gases do not have a pH value, they need to be dissolved in water before pH can be measured.

 
Non-metal oxides

• Non metal elements can react with air or oxygen to form compounds called oxides.

  C + O2	---------->	CO2
  S + O2	---------->	SO2
  N2 + 2O2	---------->	2NO2

• Non-metal oxides which dissolve in water produce acid solutions, e.g. CO₂, NO₂, SO₂, SO₃.

• Oxides of non-metals which do not dissolve do not affect the pH.

 
Metal oxides

Oxides of metals or hydroxides of metals which dissolve in water produce alkaline solutions. The data book (page 5) gives information about which ones dissolve or react.

• The oxides of Group 1 metals and some of some Group 2 metals produce alkaline solutions with water.

• Ammonia, which is not an oxide, also dissolves in water to produce an alkali.

 
Acids in the Laboratory and in the home

 
Alkalis in the Laboratory and in the home

You can quickly test the above information in the Virtual Lab.

 
Hydrogen and hydroxide ions

Water molecules can break up to form hydrogen ions and hydroxide ions as follows:

The double headed arrow between the reactants and the products shows a reversible reaction. A small proportion of water molecules break up (forward reaction), but a large proportion of these ions formed join back up together to form the water molecules (reverse reaction). At any one time, there are far more molecules than ions in pure water. For every mole of hydrogen ions and hydroxide ions there are approximately 555,000,000 moles of water molecules.

Water molecules are constantly breaking up and reforming. The speed of the forward reaction equals the speed of the reverse reaction, with both reactions taking place all of the time and the overall concentration of the reactants (water molecules) remains constant as well as the overall concentration of the products (hyrogen and hydroxide ions) remaining constant. This means the reversible reaction is in equilibrium. Note that the concentration of the reactants and the products do not need to be the same for it to be in equilibrium.

• In water and neutral solutions the concentration of hydrogen ions, H⁺(aq), is equal to the concentration of hydroxide ions OH^-(aq).

• Acid solutions contain more hydrogen ions than hydroxide ions.

• Alkaline solutions contains more hydroxide ions than hydrogen ions.

• The ions formed when water molecules dissociate (break up), helps to explain why water, although made of molecules, can conduct electricity (due to the ions made).

 
Dilution of acids and alkalis

When we dilute acids and alkalis, we are effectively adding more water molecules (only a tiny proportion of which dissociate into ions). This means that the concentration of these ions in the acids and alkalis decrease.

• Diluting acids decreases the concentration of H⁺(aq) ions. This means that the pH increases towards 7

• Diluting alkalis decreases the concentration of OH^-(aq) ions. This means that the pH decreases towards 7

 
Concentration

Solutions are formed when solutes dissolve in solvents. If the moles of solute and the volume of solvent used is know, the concentration can be calculated.

The concentration of a solution is expressed in mol l^-1 and can be calculated as follows:

The number of moles of solute, volume and concentration of a solution can be calculated from the other two variables.

Some example questions

1. If 30g of sodium hydroxide is dissolved in 500 ml of water, what will be the concentration of the solution?

If 1 mole of NaOH weighs (23 + 16 +1) = 40g
Then number of moles in 30g = 30g / 40g = 0.75 moles
From the moles / volume / concentration triangle -
Concentration (in mol l^-1) = Number of moles / Volume of solution in litres
500 ml = 500 / 1000 = 0.5 litres
Concentration = 0.75 / 0.5 = 1.50 mol l^-1

2. If the concentration of a solution of hydrochloric acid is 0.5 mol l^-1, what mass of acid will be present in 100 ml (0.1 litres) of this acid?

Number of moles	= Concentration x Volume
	= 0.5 x 0.1
	= 0.05 moles of Hydrochloric acid

1 mole of HCl weighs 1 + 35.5 = 36.5g
So 0.05 moles weighs 36.5 x 0.05 = 1.825 g

 
Strong and weak acids and bases

When acids and alkalis dissolve in water, the compounds are split up (dissociated) by the water molecules to form ions.

• Strong acids, such as hydrochloric acid, are completely dissociated in water to form ions:
  

  HCl(g) + (aq)

  H+(aq) + Cl-(aq)

• Strong acids include hydrochloric, nitric and sulphuric acid

• Weak acids, such as ethanoic acid, only partially dissociate in water. Only some ions are formed, and these ions are in equilibrium with the starting compound:
  

  CH3COOH(l) + (aq)

  H+(aq) + CH3COO-(aq)

• Strong bases, such as sodium hydroxide, are completely ionised in water:
  

  NaOH(s) + (aq)

  Na+(aq) + OH-(aq)

• Solutions of metal hydroxides are strong bases (the metal hydroxide has to be soluble to form a solution)

• Weak bases, such as ammonia, are only partially ionised in water:
  

  NH3(g) + H2O(l)

  NH4+(aq) + OH-(aq)

 
Properties of weak and strong acids and bases

Since strong acids and bases form ions more easily than weak acids and bases, their properties are slightly different

pH comparisons

• Equimolar solutions of strong acids have lower pH numbers (more 'acid') than weak acids

• Equimolar solutions of strong bases have higher pH numbers (more 'alkali') than weak alkalis

Conductivity comparisons

Because strong acids and bases form more ions, they are better at conducting electricity than weak acids and bases.

Rate of reaction

Due to the increase in acidity/alkalinity, equimolar solutions of strong acids/bases react quicker than weak acids/bases.

New words and their meanings

pH scale - A scale that ranges from below 0 to above 14 that is a measure of acidity/alkalinity of a solution.

reversible reaction - A chemical reaction in which reactants form products and products can also reform the reactants.

equilibrium - A reversible reaction when the concentration of reactants and products remains constant (though not necessarily the same as each other).

dissociate - The break up of compounds by water to form ions.