the ionic product of water

• the definitions of acids and bases state that an acid is a proton donor and a base is a proton acceptor
• acid

  proton donor
• base

  proton acceptor
• when an acid donates a proton, the species that is left is called a conjugate base
• when a base accepts a proton the species that is left is called a conjugate acid
• acid donates proton

  conjugate base
• base accepts proton

  conjugate acid
• H+ = H3O+
• water dissociates very slightly according to the the equation

  H2O (L) + H2O (L) -><- H3O+ (aq) + OH- (aq)
• in pure water the concentrations of hydroxide and hydronium ions are equal and the pH is 7 at 25 degrees
• water can be called amphoteric as IT can behave as either an acid or as a base
• in water and aqueous solutions, there's an equilibrium between the water molecules and the hydronium and hydroxide ions
• water acts as both a proton donor and acceptor to make a conjugate base and conjugate acid
• the dissociation constant for the ionisation of water can be written as

  H2O (acid) + H2O (base) -><- H3O+ (conjugate acid) + OH- (conjugate base)
• the equilibrium constant for the ionisation of water is known as the ionic product of water
• the ionic product of water is represented by Kw
• Kw = [H3O+][OH-]
• the reactants aren't included in Kw equation because water is a pure liquid and has a value of 1
• Kw is temperature dependant and the value is approx

  1 X 10-14 at 25 degrees
• because the Kw reaction is endothermic, an increase in temperature moves the equilibrium to the right, while a decrease moves it to the left
• the pH scale gives a measure of the concentration of hydrogen ions in an aqueous solution
• [H+]>[OH-]

  acidic
• [H+]<[OH-]

  alkaline
• [H+]=[OH-]

  neutral
• the greater the [H3O+] ion concentration, the lower the pH
• the concentrations of these ions are 0.000000101 in units of mol l-1

  which is 1.01 X 10-7 mol l-1 in scientific notation
• the relationship between pH and the hydrogen ion concentration is given by the equation

  pH = -log[H3O+] and [H3O+]=10-pH
• the pH scale is logarithmic, which means any change in pH unit requires a factor of 10 change in the hydrogen ion concentration
• if the concentration of one ion is known the other can be found by

  Kw or pH+pOH=14