Topic 12
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Acid base equation
HA+ H20—> H30+ + A-
Acid base equation 2
B + H20 —> BH+ + OH-
Examples of strong acids
HCl H2SO4 HNO3 H3PO4
Water is amphoteric what does that mean and explain the circumstances
Water behaves as a base when acid is added to it
Ammonia reacts with acids to make ammonium salts but no water
2NH3+H2SO4 —> (NH4)2SO4
Calculate the pH of Diprotic and triprotic acids
H2SO4 is diprotic so pH=-log(2×H+) H3PO4 is triprotic so pH=-log(3H+)
How to find the pH of a base
Use Kw=[H+][OH-] where Kw is 1×10^(-14)
What is the Ionic product of water(Kw)
The value of Kw is the same in a solution Kw= 1×10^(-14) Value changes if the temperature changes
Explain how the carbonic acid-hydrogencarbonate(H2CO3 and HCO3-) controls the pH of blood
CO2 dissolves in blood forming carbonic acid(H2CO3) Equilibrium shift to the right favouring H+ and HCO3- and produces more H+ ions Large reservoir of HCO3- reacts with H+ ions
How to make an Acidic buffer
• React a weak acid with the salt of its conjugate base e.g ethanoic acid and sodium ethanoate • React excess weak acid and a strong base e.g ethanoic acid and NaOH
How to make basic buffers(keep pH above 7)
Made from weak base and its salt e.g ammonia and ammonium chloride
Calculate the pH of the strong base 0.1 mol dm-3 NaOH Assume complete dissociation.
Kw = [H+ (aq)][OH- (aq)] = 1x10-14 H+] = Kw / [OH- (aq)] = 1x10-14 / 0.1 = 1x10-13 mol dm-3 pH = - log[1x10-13 ] =13.00
What are the 2 assumptions when calculating pH of a weak acid
1) H+]eqm = A-] eqm because they have dissociated according to a 1:1 ratio. 2) As the amount of dissociation is small we assume that the initial concentration of the undissociated acid has remained constant. So [HA (aq) ] eqm = [HA(aq) ] initial
Explain the effects of adding H+ on the carbonic acid hydrogencarbonate buffer H2CO3(aq) ⇌ H+(aq) +HCO3-(aq)
Adding alkali reacts with H+ with the equation H+ + OH- → H2O so the above equilibrium would shift right forming new H+(as H+ has decreased) and more HCO3
Explain the effects of adding acid on the carbonic acid hydrogencarbonate equilibrium H2CO3(aq) ⇌ H+(aq) + HCO3–(aq)
Adding acid shifts the above equilibrium left. The reaction is H+ + HCO3– → H2CO3
When should and shouldn’t phenolphthalein be used as an indicator in a titration
Only use when there is strong bases(not weak bases) Colour change: Colourless acid→ pink alkali
Explain how the Carbonic acid hydrogencarbonate buffer system restored pH of the blood after exercise H2CO3→ H+ + HCO3-
There is a large reservoir of HCO3- and H2CO3(HA and A-) When H+ ion increases it reacts with HCO3- to form H2CO3 restoring pH
How to measure Ka practical
Titrate ethanoic acid(WA) with sodium hydroxide(SB) Measure pH at regular intervals Plot pH against volume Use graph to find pH at half equivalence point Half equivalence neutralisation point is when pH=pKa Measure pH of resultant mixture
Explain how the carbonic acid/hydrogencarbonates buffer system restores pH when CO2 is increased
CO2 is dissolved in blood which forms carbonic acid Equilibrium shifts to the right and produces more H+ ions Large excess of HCO3- ions combine with H+ ions to control blood pH
How to measure pH experimentally
Using a pH meter but needs to be calibrated correctly Place pH probe in distilled water first make sure it says 7 as pH Repeat with standard solutions of pH 4 and 10 rinsing with distilled water Wash pH probe with distilled water after each use
What happens when we dilute an acid
It reduces concentration of H+ which increases the pH