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2.1.4 - Acids and Bases

Acids

An acid is a chemical that releases protons, or hydrogen ions (H+) into solution when dissolved in water. They have a pH less than 7.0 at 298K and can be neutralised using a base or alkali.

Commons compounds that become acids when mixed with water:

  • Hydrogen chloride:
    • Dissolves to become hydrochloric acid, HCl
    • Monoprotic, meaning it releases only 1 proton
    • Strong acid
  • Hydrogen sulphate:
    • Dissolves to become sulfuric acid, H2SO4
    • Diprotic (2 protons)
    • Strong acid
  • Hydrogen nitrate:
    • Dissolves to become nitric acid, HNO3
    • Monoprotic
    • Weak acid
  • Hydrogen phosphate:
    • Dissolves to become phosphoric acid, H3PO4
    • Triprotic (3 protons)
    • Weak acid
  • Ethanoic anhydride:
    • Reacts with water to produce ethanoic acid, CH3COOH
    • Monoprotic
    • Weak acid

Bases

A base is a chemical that accepts protons. A base that is dissolved in water is known as an alkali, which release hydroxide ions (OH−) when dissolved in water. They have a pH of greater than 7.0 at 298K and can be neutralised using an acid.

Common bases include:

  • Ammonia, NH3:
    • Has a lone pair on the nitrogen which can be used to form a covalent bond with a proton, thus ‘accepting’ it
    • Can dissolve in water to from ammonium ions and hydroxide ions (ammonium hydroxide), NH4OH
    • Weak base
  • Sodium hydroxide, NaOH, a strong base
  • Potassium hydroxide, KOH, a strong base

Strong and Weak Acids or Bases

A strong acid or base is defined as one that fully dissociates into its ions when dissolved in water, whereas a weak acid or base is defined as one that only partially dissociates into its ions when dissolved in water.

The ionic equations of a strong acid and strong base are shown below as complete equations with only the right hand side produced:

H2SO4(aq)→2H+(aq)+SO2−4(aq)

NaOH(aq)→Na+(aq)+OH−(aq)

On the other hand, the ionic equations for weak acids or bases are not complete, and instead are shown as equilibria, as both the left and right hand sides are produced:

CH3COOH(aq)⇋H+(aq)+CH3COO−(aq)

NH4OH(aq)⇋NH+4(aq)+OH−(aq)

Neutralisation

When an acid and a base or alkali are mixed together, they react to form a salt and water, with carbonates as bases also producing carbon dioxide. This is neutralisation, and produces a solution with pH equal to 7.0 at 298K. For every neutralisation reaction, the ionic equation is:

H+(aq)+OH−(aq)→H2O(l)

while the spectator ions are the only things that change.

As well as water, a salt is produced, which is a pH neutral ionic compound made up of a metal and a non-metal (except an ammonium salt, where both are non-metals). The metal comes from the base and the non-metal comes from the acid. For example:

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)

or:

2HCl(aq)+Na2O(aq)→2NaCl(aq)+H2O(l)

the Cl− anion from the HCl and the Na+ cation from the NaOH or Na2O combine to form NaCl.

Another type of base is a carbonate, which contains the carbonate ion (CO2−3). When it is involved in a neutralisation reaction, carbon dioxide (CO2) is formed.

2HCl(aq)+Na2CO3(aq)→2NaCl(aq)+H2O(l)+CO2(g)

Acid-Base Titrations

As seen in the ionic equation above, one mole of H+ ions and one mole of OH− ions together form one mole of water. If neither ion is in excess, i.e., the number of hydrogen and hydroxide ions are exactly equal, a solution is perfectly neutralised.

An acid-base titration is an experiment to determine the exact volume of acid/base that is needed to neutralise a known volume of the other. This is usually done to find the concentration of one of the solutions. An example of a calculation was shown in notes of 2.1.3 - Amount of Substance.

Standard Solutions

A solution with a known concentration is known as a standard solution. One is usually made by measuring a specific mass of pure, dry solid substance and dissolving it carefully in a specific volume of distilled water such that the substance is dissolved equally throughout the solution. An example of a calculation was shown in notes of 2.1.3 - Amount of Substance.

Below is a video with further explanation of titrations, as well as how to set up and conduct an acid-base titration, and making a standard solution: