Salt hydrolysis happens when ions from a dissolved salt react with water to change the concentration of H3O+ or OH− in solution. This is why some salts make water acidic, some make it basic, and some leave it nearly neutral. The idea matters because it connects acid base theory, equilibrium, buffers, and pH prediction.
It also helps explain everyday and laboratory solutions such as ammonium chloride, sodium acetate, and sodium chloride.
Understanding Chemistry: Salt Hydrolysis
When a crystal dissolves, its ions separate and become surrounded by water molecules. Some ions barely disturb those water molecules. Others can transfer a proton.
An ion such as acetate can take a proton from water. This leaves more hydroxide ions in the solution. An ion such as ammonium can give a proton to water.
This forms more hydronium ions. These changes are usually reversible.
The forward and reverse reactions continue until an equilibrium is reached. The final pH depends on where that equilibrium settles, not simply on the amount of salt poured in.
The key idea is conjugate strength. A strong acid gives up protons very easily. Its conjugate base has very little ability to take a proton back from water.
Nitrate and chloride ions are examples of very weak bases, so they have almost no effect on pH. A strong base has the same pattern in reverse. Its conjugate acid has little ability to donate a proton.
Potassium ions and sodium ions are usually spectators in water. This is why a salt can dissolve completely while producing almost no acid base change.
Salts made from a weak acid and a weak base need closer study. Both ions may react with water, but in opposite directions. The acidic effect of the positive ion must be compared with the basic effect of the negative ion.
If the positive ion has a larger acid strength value than the base strength value of the negative ion, the solution is acidic. If the base strength value is larger, the solution is basic. Similar values give a pH near neutral.
Temperature matters here because neutral water has a pH of seven only near twenty five degrees Celsius. Concentration matters too. A more concentrated salt solution often has a larger pH shift, although the direction of the shift stays the same.
In calculations, first identify the parent acid and parent base that formed the salt. Decide whether each parent was strong or weak. Then write the ion that can react with water and state whether it accepts or donates a proton.
For a basic ion, use the acid strength of its parent weak acid to find its base strength. For an acidic ion, use the base strength of its parent weak base to find its acid strength. Students often confuse dissolving with reacting.
Dissolving separates ions, while hydrolysis is the later proton transfer with water. This distinction helps when using indicators, measuring pH in the lab, or explaining why carbonate cleaning products feel slippery and why some ammonium fertilizers can make soil more acidic.
Key Facts
- Salt hydrolysis is the reaction of a salt ion with water to produce H3O+ or OH−.
- A salt made from a strong acid and a strong base is usually neutral, such as NaCl.
- A salt made from a strong acid and a weak base is acidic, such as NH4Cl.
- A salt made from a weak acid and a strong base is basic, such as CH3COONa.
- For the conjugate base of a weak acid, Kb = Kw / Ka.
- For the conjugate acid of a weak base, Ka = Kw / Kb.
Vocabulary
- Salt hydrolysis
- Salt hydrolysis is the reaction between an ion from a dissolved salt and water that can change the pH of the solution.
- Conjugate acid
- A conjugate acid is the particle formed when a base gains a proton.
- Conjugate base
- A conjugate base is the particle formed when an acid loses a proton.
- Spectator ion
- A spectator ion is an ion that stays dissolved but does not significantly react with water or affect pH.
- Hydronium ion
- The hydronium ion, H3O+, is formed when water accepts a proton and is responsible for acidity in aqueous solution.
Common Mistakes to Avoid
- Assuming every salt solution is neutral is wrong because ions from weak acids or weak bases can react with water and shift the pH.
- Treating Na+ or Cl− as hydrolyzing ions is wrong because they come from the strong base NaOH and strong acid HCl, so they are usually spectators in water.
- Forgetting to identify the parent acid and base is wrong because the strength of those parents determines whether each ion is acidic, basic, or neutral.
- Using Ka instead of Kb for a conjugate base is wrong because a basic anion reacts with water to form OH−, so its equilibrium is described by Kb = Kw / Ka.
Practice Questions
- 1 Predict whether 0.10 M NH4Cl is acidic, basic, or neutral. Write the hydrolysis equation for the ion that reacts with water.
- 2 Sodium acetate, CH3COONa, has acetate ion with Ka for acetic acid equal to 1.8 × 10−5. Calculate Kb for CH3COO− using Kw = 1.0 × 10−14.
- 3 A salt contains one cation that is the conjugate acid of a weak base and one anion that is the conjugate base of a weak acid. Explain what information you need to predict whether the solution is acidic, basic, or neutral.