Topic 12: Acid Base Equilibria Flashcards
Bronsted-Lowry Acid
Proton Donor
Bronsted-Lowry Base
Proton Acceptor
Bronsted Lowry Acid-Base Reaction
Reaction involving the transfer of a proton
Monoprotic Acid
Acid that releases 1 proton per molecule
Diprotic Acid
Acid that releases 2 protons per 1 molecule
What is the value of Kw in standard conditions?
10^-14
How do you calculate the pH of a strong base?
Kw/[OH] = [H+] then you use -log[H+] to calculate pH
How do you calculate the pH of a strong acid ?
-log[H+] = pH
How do you calculate the pH of a strong acid base reacting?
Use the XS [OH-] or [H+] to calculate the pH of the final solution
Strong acid
All the molecules of acid break apart to form ions
Weak Acid
Small fraction of the molecules of acid break to form ions
Ka (Acid dissociation constant)
Ka = ([H+][A-])/[HA]
What are the assumptions made when weak acid is in just water?
- [A-] = [H+]
- initial [HA] = [HA] at equilibrium
What happens to Kw as temperature increases?
- More [H+] & [OH-] formed
- pH increases
- Solution remains neutral as [H+] = [OH-]
What does amphoteric mean?
- A substance that can act as a base or an acid
When acid solutions are extremely dilute (10^-8) why is the pH not in the alkaline territory?
- The pH is extremely close to 7 as we need to now take into account the [H+] disociation from water
What is a buffer solution?
- A solution that resists changes in pH when small amounts of acid or alkali are added
Position of the Equivalence point when you react : ( Weak acid + Strong base)
- pH greater than 7
Position of the Equivalence point when you react : Strong acid + Weak base
- pH less than 7 (equivalence point)
What and Why is the standard enthalpy change of neutralisation almost the same for all Strong acid + Strong base?
- The same reaction H+ + OH- - > H2O for all strong acids and bases
- -57.6 kJ mol^-1
Why is the enthalpy change on neutralisation more endothermic for a weak base?
- A weak base is only partially ionised
- Energy must be taken in from he environment to ionise the weak base
Why is the enthalpy change of neutralisation more exothermic than -57.6 kJmol^-1 for the weak acid HF ?
- Hydration of HF release more energy than the dissociation
Why is the pH at the equivalence point greater than 7?
- The weak base reacts with the H+ from water
- This forms OH-
- Meaning the pH at equivalence is higher
How do buffers work?
- Significant Quantities of quantities of the acid and its conjugate base is present
- Therefore the pH doesn’t change as much
