Alkalis and bases

• Bases are substances that can neutralise acids.
• Alkalis are formed when bases (normally metal oxides, which are called basic oxides) dissolve in water forming hydroxide ions (OH^-).
• Alkalis have a higher concentration of OH^- than H⁺ ions.
• Soluble bases form solutions (alkalis) which have pH numbers greater than 7.
• Insoluble bases can still neutralise acids, but do not have any effect on pH indicator.
• Metal oxides, metal hydroxides and metal carbonates are examples of bases.

Some examples of bases, and alkalis formed are shown in the table below:

All of the above substances can neutralise acids (acids are substances that dissolve in water to produce the hydrogen ion, H⁺).

 
Neutralisation

• Neutralisation moves the pH of an acid up towards 7.
• Neutralisation moves the pH of an alkali down towards 7.

Reactions of Acids with Neutralisers (examples of neutralisation)

1. When acids react with alkalis, the hydrogen ions from the acid (H⁺) reacts with the hydroxide ion from the alkali (OH^-) to form water as shown below:
   

   H+(aq) + OH-(aq)

   H2O(l)

2. When acids react with metal oxides, the hydrogen ions from the acid (H⁺) reacts with the oxide ion from the metal oxide (O^2-) to form water as shown below:
   

   2H+(aq) + O2-(aq)

   H2O(l)

3. When acids react with metal carbonates, the hydrogen ions from the acid (H⁺) reacts with the carbonate ion from the metal carbonate (CO₃^2-) to form water and carbon dioxide as shown below:
   

   2H+(aq) + CO32-(aq)

   H2O(l) + CO2(g)

 
Everyday neutralisation reactions

Acids are often neutralised in everyday chemical reactions:

• The use of indigestion (or ant-acid) tablets to neutralise excess stomach acid.
• The use of lime to reduce soil acidity.
• The use of lime to reduce acidity in lochs.

 
Acids and metals

Some metals react with acids to give off hydrogen gas. The hydrogen gas can be collected and tested with a lit splint. Hydrogen burns with a 'pop'.

• When acids react with metals, the hydrogen ions from the acid (H⁺) form hydrogen molecules as shown below:
  

  2H+(aq)

  H2(g)

Metals such as copper, silver and gold do not react with dilute acids.

 
Acid rain

Sulphur dioxide, produced by the burning of fossil fuels, and nitrogen dioxide, produced by the sparking of air in car engines, dissolve in water in the atmosphere to produce acid rain (this was covered in the Fuels topic).

Acid rain has damaging effects on buildings made from carbonate rock, structures made of iron and steel, soils and plant and animal life.

 
Volumetric titrations

The concentration of acids/alkalis can be calculated from the results of a volumetric titration.

You can quickly try a simulated titration in the Virtual lab.

After carrying out a titration it is important to be able to write a balanced chemical equation for the reaction and to be able to use the concentration formula triangle. From this, you can work out the following formulae:

• Concentration = number of moles / volume
• Number of moles = concentration * volume
• Volume = number of moles / concentration

Concentration is measured in moles per litre, which is shown as mol l^-1, or mol/l
Volume in these formulae is always written in litres, which is shown as l

A titration was carried out between sulphuric acid solution and sodium hydroxide solution. The concentration of the acid was 0.1150 mol l^-1 and 28.60 ml of it was used to neutralise 20.00 ml of the sodium hydroxide solution.
The reaction formed sodium sulphate (a salt) and water

What was the concentration of the sodium hydroxide solution?

1)	Write a balanced chemical equation		H2SO4 + 2NaOH ---> Na2SO4 + 2H2O
2)	Identify the moles of reactants involved		1 mole of acid reacts with 2 moles of alkali
3)	Calculate the moles of acid actually used		Number of moles = concentration * volume (in litres) Number of moles = 0.1150 mol l-1 * 0.02860 l Number of moles = 0.003289 moles
4)	Calculate the number of moles of sodium hydroxide used from the balanced chemical equation		1 mole of acid reacts with 2 moles of alkali 0.003289 moles of acid reacts with (2*0.003289) moles of alkali 0.003289 moles of acid reacts with 0.006578 moles of alkali
5)	Calculate the concentration of the sodium hydroxide		Concentration = number of moles / volume (in litres) Concentration = 0.006578 / 0.02000 l Concentration = 0.3289 mol l-1
The concentration of the sodium hydroxide solution is 0.3289 mol l-1

Note : When carrying out these calculations, never round off values halfway through the calculation - you should only round your answer at the final stage. Also, never give too many decimal places in your answer. In the above example, the concentration of the acid was given to 4 decimal places, so it is not possible to calculate the concentration of the alkali to any more precision than 4 decimal places.

You can quickly try more of these calculations.

 
Naming salts

• A salt is a compound in which the hydrogen ions of an acid have been replaced by metal ions (or ammonium ions).

• Salts are formed in the reaction of acids with bases or metals.

1. Hydrochloric acid (HCl) always makes CHLORIDE salts
2. Nitric acid (HNO₃) always makes NITRATE salts
3. Sulphuric acid (H₂SO₄) always makes SULPHATE salts

NB. Learn the formulae of these acids.
NB. The acid forms the 'Surname' of the salt.
NB. The neutraliser forms the 'Forename' of the salt

e.g.

1. If hydrochloric acid is neutralised by potassium hydroxide, the salt potassium chloride will be formed (+ water)

2. If sulphuric acid is neutralised by sodium carbonate, the salt sodium sulphate will be formed (+ water + carbon dioxide)

3. If nitric acid is neutralised by sodium carbonate, the salt sodium nitrate will be formed (+ water + carbon dioxide)

4. If hydrochloric acid is neutralised by zinc metal, the salt zinc chloride will be formed (+ hydrogen)

Some nitrogen salts, including ammonium nitrate, ammonium sulphate and potassium nitrate are made by neutralisation reactions for use as fertilisers; these salts are soluble in water.

In the preparation of a soluble salt, it is often easier to use an insoluble metal carbonate or metal oxide as the base, so that the excess base can be filtered off before evaporating the reaction mixture to obtain the salt.

 
Precipitation

• Precipitation is the reaction of two solutions to form an insoluble product called a precipitate.
• Insoluble salts can be formed by precipitation.

To make insoluble lead (II) iodide, mix together two separate solutions of soluble salts one containing lead (II) ions and the other containing iodide ions e.g. lead (II) nitrate solution and sodium iodide solution.

Lead iodide is formed as a yellow precipitate which can be removed easily by filtering, followed by washing the precipitate with distilled water and then drying it. The method is quick and doesn't need careful measurement as excess reactants are soluble and are left in solution (and are not removed by the filtration process).

The data book indicates the solubility of many substances.

e.g. Will a precipitate be formed if sodium carbonate solution is added to copper (II) sulphate solution?

As these two chemicals are both soluble salts, there will only be a precipitate formed if they can react to give an insoluble substance.

Possible products are sodium sulphate and copper (II) carbonate. The data book indicates that copper (II) carbonate is insoluble and so copper (II) carbonate will be formed as a precipitate and sodium sulphate (which is soluble) will be left in the solution.

 
Ionic equations

Spectator ions are ions which are free to move (aqueous or molten) at the beginning and end of a chemical reaction, and can therefore be omitted.

Some examples follow:

Acid and alkali

Acid and metal carbonate

Acid and metal

You can quickly test your knowledge of the above information.

New words and their meanings

base - A substance that can neutralise an acid, such as a metal oxide, metal hydroxide, metal carbonate, or a metal.

basic oxide - A metal oxide.

alkali - Formed when a base dissolves in water.

neutralisation - The reaction between an acid and a base.

volumetric titration - A chemical technique carried out using a burette that is used to meause a volume of chemical used in a chemical reaction (volumetric titration is often simplified to titration).

precipitation - A chemical reaction that produces a solid in a liquid.

concentration - The number of moles of a solute dissolved in 1 litre of solvent.

salt - Formed in a neutralisation reaction. This is an ionic compound containing the metal part (or ammonium ion) of the base and the non-hydrogen part of the acid, e.g. sodium chloride is a salt formed from sodium oxide and hydrochloric acid.