Summary A LEVEL CHEMISTRY NOTES - Acid Base Equilibria
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Course
Unit 12 - Acid-base Equilibria
Institution
PEARSON (PEARSON)
- Detailed A - Level Chemistry Notes
- Clear, detailed with diagrams for ease of understanding
- Covers all specifications points to ensure all content is covered
Includes :
- titration curves
- strong and weak acids/bases
- pH calculations
- titration curves and indicators
- acidic and b...
When acids are mixed with water, hydrogen ions are released. These H+ ions form hydroxonium
ions (H₃O+) which make the solution acidic
HA(aq) + H₂O(l) ⇌ H₃O+(aq) + A-(aq)
HA is the acid and H₂O is the base in the forward reaction. In the reverse reaction H₃O+ is the acid
and A- is the base
HA and A- are a conjugate acid base pair
H₂O and H₃O+ are a conjugate acid base pair
Water acts as a base when an acid is added to it
When bases are mixed with water, hydroxide ions are released.
B(aq) + H₂O(l) ⇌ BH+ + OH-
When a solution is a base this means it has a larger concentration of hydroxide ions than hydrogen
ions.
When acids and bases react with water, the reaction is reversible
Acid-base reactions involve transfer of protons
Conjugate pairs
linked by transfer of a proton
HA + B ⇌ BH+ + A-
HA is the acid in the forward reaction as it is donating a proton and A- is the base in the reverse
reaction as it accepts a proton from BH+ to form HA.
When an acid donates a proton, the species formed is its conjugate base
When a base accepts a proton, the species formed is its conjugate acid
Acid base reactions
,When acids react with bases, salts are formed which are pH neutral.
Overall ionic equation : H+ + OH- ⇌ H2O
Salts are made from the metal from the base (or ammonium ion) and the non metal (other than
hydrogen) from the acid
HCl + LiOH → LiCl + H2O
Ammonia doesn’t have hydroxide ions so needs to react with water rst. It accepts a proton to
produce ammonium ions and hydroxide ions.
NH3 + H2O ⇌ NH4+ + OH-
2NH3 + H2SO4 ⇌ (NH4)2SO4
Note that with ammonia’s neutralisation reaction, water is not produced
fi
, pH
a measure of hydrogen ion concentration
Lower the pH, higher the concentration of hydrogen ions
Strong acid
a large amount of the strong acid dissociates
Equilibrium lies well over to the right favouring the forward reaction
Lots of H+ ions are formed
Concentration of monoprotic acid = concentration of H+ ions as we assume all the H+ ions produced
have come from the acid
Calculating pH of a strong acid (assume they fully dissociate)
Work out pH of 0.1 mol dm-3 hydrochloric acid. HCl is a strong acid so nearly 100% dissociation. 1
mole of HCl has 1 mole of hydrogen ions so if the concentration of the acid 0.1 mol dm-3 then the
concentration of hydrogen ions is 0.1 mol dm-3
-log10[H+] = 1
pH = -log10[H+]
[H+] = 10-pH
Monoprotic and polyprotic acids
Polyprotic acid - acids that donate more than one proton
Nitric acid is monoprotic as 1 mole of HNO₃ will produce 1 mole of H+ ions
Sulfuric acid is diprotic as 1 mole of H₂SO₄ will produce 2 moles of H+ ions.
Phosphoric acid is triprotic as 1 mole to H₃PO₄ will produce 3 moles of H+ ions
Diprotic acids dissociate to produce two H+ ions for every acid molecule.
concentration of 2 x [HA] = [H+]
pH of diprotic acid example
Calculate the pH of 0.25 mol dm-3 of sulfuric acid
[H+] = 2[H₂SO₄ ]
0.25 mol dm-3 of acid produces 0.50 mol dm-3 of H+ ions.
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