top of page ###### The Self-Ionisation of Water and pH

Chemistry (Year 12) - Acids and Bases

Michael Swift

# The Self-Ionisation of Water

Water is the world's most common acid and base - pure water also defines what it means for a solution to be neutral.

When water molecules are next to each other, there is a phenomenon that occurs in which a proton (hydrogen ion) on one water molecule will break off and attach itself to another water molecule, resulting in the formation of a hydronium ion and a hydroxide ion:

In its self-ionisation, water is acting as both an acid and a base - but overall, the water still remains neutral (the concentration of hydronium ions is equal to the concentration of hydroxide ions). This is because hydronium ions and hydroxide ions are being produced in a 1:1 ratio. As a result, there is no possible way for either of them to become greater in concentration than the other.

Since the ionisation of water is a reversible reaction, it has an equilibrium constant. The equilibrium constant of water is given by...

At 25 degrees celsius, the equilibrium constant of the hydronium and hydroxide ions in any solution will always be equal to 1×10^-14 M (or 0.00000000000001 M).

This gives the equilibrium constant for the self-ionisation of water as...

Since the concentrations of hydronium and hydroxide are of equal concentrations, we can deduce that...

This value of 1×10^-7 M defines neutral on the pH scale at 25 degrees celsius. This is because it is the concentration at which [hydronium] = [hydroxide] for any solution at this temperature.

If [hydronium] > 1×10^-7 M, then the solution will be acidic at a temperature of 25 degrees celsius.

If [hydronium] < 1×10^-7, then the solution will be basic at a temperature of 25 degrees celsius.

This scale may be understood more easily by using pH.

# pH

pH is a measure of the concentration of hydronium ions in solution.

It is given by...

This logarithmic relationship can be rearranged into an exponential relationship that calculates [hydronium] when the pH of the solution is known...

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