IB Chemistry home > Syllabus 2016 > Acids and Bases > The properties of bases

Syllabus ref: 8.1

Bases constitute a class of compounds that are considered to be the chemical opposite of acids.

Bases

Bases are the chemical opposites of acids. Strong bases are corrosive substances with a greasy slippery feel.

Bases contain ions that can neutralise (react with and cancel out) the H+ ions of the acids, such as the hydroxide ion, the oxide ion and the carbonate ion.

basic ion formula
hydroxide OH-
oxide O2-
carbonate CO32-
hydrogen carbonate HCO3-
sulfate(IV) SO32-

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Neutralisation

Just as acids neutralise bases, so bases neutralise acids by reacting with the H+ ions of the acidic solution. In all neutralisation reactions water is formed:

Base neutralisation
hydroxide OH- + H+ H2O
oxide
O2- + 2H+ H2O
carbonate
CO32- + 2H+ H2O + CO2
hydrogen carbonate
HCO3- + H+ H2O + CO2
sulfate(IV)
SO32- + 2H+ H2O + SO2

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Alkalis

Alkali is simply the term given to a soluble base, such as sodium hydroxide, potassium hydroxide or barium hydroxide.


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Ammonia

Ammonia has the molecular formula NH3. It is a pungent (sharp) smelling gas that is used in household cleaners. It is very soluble in water giving a solution of about pH 10.

Ammonia gas dissolves in water to give a solution that behaves as a base in that it neutralises acids producing a salt and water. The reason for this is that the ammonia molecule partially removes some of the hydrogen ions from water leaving the solution with an excess of OH´(hydroxide ) ions. These ions can then react with acids.

ammonia
+
water
ammonium ions
+
hydroxide ions
NH3
 
H2O
 
NH4+
 
OH-

This equilibrium reaction lies very much to the left hand side, meaning that there is much more of the molecular form of ammonia in the solution than there is of the ions.

However, any reaction of the hydroxide ions removes them from the equilibrium which responds (Le Chatelier, take from the right hand side, make more of the right hand side) by making more. Thus, any reaction which neutralises the OH- ions goes all the way to completion.


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Ammonia reactions

sulfuric acid
+
ammonia
ammonium sulfate
H2SO4
 
2NH3
 
(NH4)2SO4
         
hydrochloric acid
+
ammonia
ammonium chloride
HCl
 
NH3
 
NH4Cl
         
nitric acid
+
ammonia
ammonium nitrate
HNO3
 
NH3
 
NH4NO3
         
ethanoic acid
+
ammonia
ammonium ethanoate
CH3COOH
 
NH3
 
CH3COONH4

Note: There is no water formed in the reaction. Some textbooks refer to a solution of ammonia in water as 'ammonium hydroxide' solution, NH4OH, in which case there will be water formed when it reacts with acids.


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Indicators

Indicators may be used to test for bases in the same way as they are used to test for acids (only with different colours of course)

Common indicator colour changes are:

medium
Universal indicator
Litmus
Phenolphthalein
Neutral
green
purple
pink
Basic
purple-blue
blue
red

Bases register values from 7 (neutral) to 14 on the pH scale.


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