When an Arrhenius acid is added to an Arrhenius base, we say that the acid will neutralise the base.
When an Arrhenius base is added to an Arrhenius acid, we say that the base will neutralise the acid.
A salt (MX) is a compound whose ions are left after an acid has neutralised a base:
HX(aq) + MOH(aq) → H2O + MX(aq)
The salt formed is an ionic compound and is named by placing the name of the metal first, followed by the the name of the non-metal with an appropriate ending:
hydrochloric acid (HCl)
sodium chloride (NaCl)
sulfuric acid (H2SO4)
sodium sulfate (Na2SO4)
nitric acid (HNO3)
sodium nitrate (NaNO3)
phosphoric acid (H3PO4)
sodium phosphate (Na3PO4)
acetic acid (ethanoic acid) (CH3COO-)
acetate (ethanoate) (CH3COO-)
sodium acetate (sodium ethanoate) (CH3COONa)
oxalic acid (ethanedioc acid) (HOOCCOOH)
oxalate (ethanedioate) (-OOCCOO-)
sodium oxalate (sodium ethanedioate) (NaOOCCOONa)
A neutralisation reaction can be summarized as:
general word equation:
general molecular chemical equation: (all species shown as molecules)
general ionic chemical equation: (showing which species are ions and which are molecules)
H+(aq) + X-(aq)
M+(aq) + OH-(aq)
M+(aq) + X-(aq)
general net ionic chemical equation2: (showing only the species that react)
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Example: Writing a Molecular Equation for a Neutralisation Reaction
In a molecular equation, all the species are represented as molecules, even the compounds that exist only as ions in the solution.
Example: Write a balanced molecular equation for the reaction in which sodium hydroxide is neutralised by hydrochloric acid.
Determine which species is the Arrhenius acid and which is the Arrhenius base:
An Arrhenius acid contains hydrogen: hydrochloric acid
An Arrhenius base contains hydroxide ions: sodium hydroxide
Write the word equation for the neutralisation reaction:
general word equation:
word equation for this reaction:
Write the chemical formula for each species and include its state:
Write the skeletal chemical molecular equation by substituting the names of each species with its chemical formula:
skeletal molecular equation:
Determine which of these "molecular" species exists as ions in the aqueous solution:
Sulfuric acid (H2SO4(aq)) dissociates in water 3 producing 2H+(aq) + SO42- Potassium hydroxide (KOH(aq)) dissociates in water producing K+(aq) + OH-(aq) Water: H2O(l) (water dissociates only very, very slightly, so we can treat it as a molecule)
Potassium sulfate (K2SO4(aq)) is soluble in water because the salts of Group 1 metals are soluble.
Potassium sulfate (K2SO4(aq)) dissociates in water producing 2K+(aq) + SO42-(aq)
Write the skeletal ionic equation by substituting the molecular formula of each soluble ionic species with the formula of its ions:
1. Since we are using the Arrhenius definition of an acid it is quite acceptable to use H+ (or H+(aq)) to represent the hydrogen ion.
2. These equations assume that the salt formed is soluble in water.
It is possible that the salt formed is not soluble in water, in which case the equation would be written as for a precipitation reaction.
3. The assumption here is that the neutralisation reaction will go to completion, driving the sulfuric acid to complete dissociation.