Neutralisation Investigation

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Peter Man


Chemistry GCSE:



Background information

Substances that neutralise acids are called bases.  Bases are alkali – they have pH’s above 7; acids are acidic, and have pH’s below 7.  Bases that can dissolve in water are also known as alkalis.


Acids are compounds of non-metals with simple molecular structures.  They all contain hydrogen (H) covalently bonded to other elements, like HCl, or H2SO4 (hydrochloric and sulphuric acid respectively).  When an acid is dissolved in water, its molecules ionise.  The hydrogen present in every acid has the potential to ionise; if it does, it becomes an H+ ion.  It is the H+ ions that gives acids their acidic properties.  The amount of an acid’s molecules that ionise in water determines the strength of its acidity.  Vinegar, for instance, is a weak acid, an only 1 in about 100,000 of its molecules ionise; on the other hand, almost all molecules of HCl ionise.  This means that in an acid-base reaction involving vinegar, there are less H+ ions available to react, and so less bonds are made compared to an acid-base reaction involving HCl.  Bond making is exothermic, and so less energy is given out.  Stronger acids (those with a higher number of H+ particles that can dissolve in water) have a lower pH.


Just as all acids contain H+ ions, all alkalis contain OH- ions, and it is this OH- (hydroxide) ion that gives alkalis their alkali properties.

Bases are usually oxides, hydroxides, or carbonates of metal.  Ammonia is unusual in this respect, as it contains no metal element.  Alkali substances are also very corrosive, and can do even more damage to living cells than acids.

The neutralisation reaction

NaOH(aq) + HCl (aq)                        NaCl(aq) + H2O(l)

The above is an example of a neutralisation reaction, involving an alkali and an acid.  The result is a salt and water.  In every neutralisation reaction, the metal in the base (Na+ here) takes the place of the Hydrogen in the acid, forming a metal compound called a salt.  The term salt is used to describe any compound formed by the reaction between a base and an acid.

From the above equation, we can break up these molecules into this:

Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq)                        Na+(aq) + Cl-(aq) + HHO(l)

The Na+ and Cl- on the left side of the equation are present at the end of the equation (on the right side), and are known as spectator ions, as they have not chemically reacted.  Therefore, the equation can be rewritten, with just the reactants, as:

OH-(aq) + H+(aq)                                H2O(l)

This equation occurs in all acid/alkali neutralisation reactions where the salt is soluble – hydrogen ions from the acid react with hydroxide ions from the alkali to form water.  The neutralisation reaction is exothermic because of the bonds being made (OH- + H+ bonding together to make H2O).  This reaction can be generalised to apply to any reaction between a base and an acid:

acid + base                                 salt + water

If the base happens to be a carbonate, then carbon dioxide is formed as well (e.g. calcium carbonate, CaCO3):

CaCO3 + 2HCl                         CaCl2 + CO2 + H2O

       Calcium  +  Hydrochloric                 Calcium + carbon + water

       carbonate    acid                               chloride   dioxide

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A single mole of an element contains 6 × 1023 atoms of that element.  It is a convenient unit of measurement as this amount of atoms has a mass equal to that of the relative atomic mass of the element in grams.  This number is called the Avogadro Constant.  Therefore one mole of Hydrogen (6 × 1023 atoms of hydrogen) weighs 1g, and one mole of Magnesium (6 × 1023 atoms of Magnesium) weighs 24g.  When we refer to solutions as having 1M concentration, we mean that it has 1M of hydrogen in every litre of the solution.


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