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### Course: AP®︎/College Chemistry>Unit 8

Lesson 5: Acid–base reactions

# Weak base–strong acid reactions

When a weak base and a strong acid are mixed, they react according to the following net-ionic equation: B(aq) + H₃O⁺(aq) → HB⁺(aq) + H₂O(l). If the acid and base are equimolar, the pH of the resulting solution can be determined by considering the equilibrium reaction of HB⁺ with water. If the base is in excess, the pH can be determined from the concentrations of B and HB⁺ after the reaction. If the acid is in excess, the pH can be determined from the concentration of excess H₃O⁺. Created by Jay.

## Want to join the conversation?

• So did Jay in situation 2 (weak base > strong acid) (at ) did not include the equation:
NH4+ + H2O <=> NH3 + H3O+
because the net gain of H3O+ would be negligible to the net gain of OH- ions?
• With ammonia (the weak base) in excess here that means the solution's pH is going to be dominated by it more so compared to the other chemicals. Ammonia is making so many hydroxide ions that ammonium is more likely to react with those than neutral water.

There is a little of ammonium's acid reaction with neutral water producing hydronium, but again since there's so much hydroxide in solution any hydronium produced just gets neutralized to water. So essentially the pH change from ammonium's acid reaction is so minor that it's not included.

Hope that helps.
• I don't understand anything anymore. This whole topic has been a complete brain ache.
Can someone summarise the key ideas here that I need to understand please?
I feel like all of Jay's videos have just been processes , and I follow the algebra and formulas but I don't feel like I've learnt a single new idea in this unit.

All this strong base weak acid strong acid weak base what is the actual concept and idea I'm meant to be learning here? Please help
(1 vote)
• Strong acids and bases dissociate completely and go to completion. This means that nearly the entirety of your starting strong acid or base will be available for an acid-base reaction. Considering equilibrium, these reactions lie far to the right towards the products and have very large equilibrium constants.

Weak acids and bases dissociate only partially and are mostly unreacted starting material. This means only a smaller fraction of the acid or base will be available for an acid-base reaction. Considering equilibriums, these reactions lie far to the left towards the reactants and have smaller equilibrium constants than strong acids and bases.

Hope that helps.
General acid dissolution: `HA + H₂O <-> H₃O⁺ + A⁻`
Acid dissolution is an equilibrium. Dissolving an acid to water will always increase `[H₃O⁺]`, and the reaction always goes forward; how far it does is what determines the acid's strength. `Ka` measures the extent to which reactants turn into products in the equilibrium.
`Ka = [H₃O⁺][A⁻]/[HA]`
If `Ka > 1`, it follows that `[H₃O⁺][A⁻] > [HA]`, indicating that the majority of `HA` molecules ionise - it's a strong acid.