In this lesson, we will learn:
- The meaning of the term spectator ion and to identify them in salt and water interactions.
- To predict hydrolysis reactions and changes in pH of water when salt solutions are made.
- How to use Ka and Kb to determine pH changes when amphiprotic species are produced in solution.
- Recall that strong acids and strong bases completely dissociate in water. Salts that are water-soluble are also assumed to completely dissociate in water. This means that interactions between water and a water-soluble salt MX can be put in an equation:
MX = H2O → M+ + X- + H2O
- Some but not all aqueous ions react with water and this can have a major effect on pH.
Strong acids dissociate completely in water, so their conjugate bases are such poor bases that they can’t go back to being the conjugate acid. Once they become the conjugate base, they stop reacting – they are ‘spectator’ ions. The conjugate pair of any strong acid or strong base is a spectator ion.
There are some general and specific spectator ions that should be remembered when learning about hydrolysis of salts:
- The M+ metal ions of group 1 (alkali metals) and group 2 (alkali earth metals) are spectator ions.
- The ions I-, Br-, Cl-, NO3- and ClO4- are spectator ions as they are the conjugate bases of strong acids.
- Once you have identified any spectator ions formed in solution from a salt and water interaction, they can be ignored in any reaction to water.
- The ions that are not spectator ions can react with water in a hydrolysis reaction. Hydrolysis is the breaking down of a compound (in a chemical reaction) by water. This can cause changes in pH because salts contain oppositely charged ions that dissociate in solution. If one of the ions is a spectator ion but the other (the counter-ion) is not, then the effect on water will be asymmetrical! See below for two examples:
- Ammonium chloride, NH4Cl is soluble in water and the dissociation can be written in an equation:
NH4Cl → NH4+ + Cl-
The two ions that make ammonium chloride have an asymmetric effect on water; chloride, Cl- is a spectator ion but ammonium, NH4+, isn’t! The result is an unopposed hydrolysis reaction of the NH4+ ions with neutral water:
NH4+ + H2O NH3 + H3O+
As the equation shows, H3O+ is a product (the ammonium ion is a weak acid) so the resultant solution is more acidic. The overall effect of adding ammonium chloride to water is a lowering of pH.
- Sodium ethanoate, CH3COONa is soluble in water. When it dissociates, the Na+ ion is a spectator but the ethanoate ion is not. A hydrolysis reaction can be shown with the equation:
CH3COO- + H2O CH3COON + OH-
The ethanoic acid formed is a moderately ‘strong’ weak acid, but because all of this acid will have come from the ethanoate originally formed, the net effect is OH- ions being made, so the resultant solution is more basic. The overall effect of adding sodium ethanoate to water is a rise in pH.
- There will be some instances where the salt dissolved in water gives two spectator ions – in this case, the resultant solution is neutral.
- In some cases, the ions produced when salts dissolve in water will be amphiprotic; molecules that are capable of accepting and donating protons (all amphiprotic molecules are amphoteric molecules):
- For example: hydrogen oxalate, HC2O4- has two carboxylic acid groups. One is still protonated and could donate a proton, behaving as an acid. The other has already been deprotonated; it could be re-protonated, behaving as a base if this happened. Amphiprotic molecules can and will take part in reactions to accept and donate a proton, but will do one more than the other. The Ka and Kb values for your amphiprotic molecule will tell you if it is a stronger base or acid (the Ka or Kb value will be larger).
This greater acid/base behaviour will give you the net effect on the pH of the solution.