Topic A7: Acid-base
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What is an acid?
H+ donor/ proton donor
What is a base?
H+ acceptor/ proton acceptor
Is HCl monobasic, dibasic or tribasic?
monobasic - has one donatable H+
Is sulfuric acid monobasic, dibasic or tribasic?
dibasic - has 2 donatable H+
Is phosphoric acid monobasic, dibasic or tribasic?
tribasic - has 3 donatable H+
Metal + acid —>
salt + hydrogen
Metal oxide + acid —>
salt + water
Metal hydroxide (base) + acid —>
salt + water
Metal carbonate + acid —>
salt + water + carbon dioxide
How do you calculate pH?
-log₁₀[H+]
What is the concentration of H+ ions in water?
10^-7
How do you calculate H+ concentration from pH?
10^-pH
How can neutrality be defined from [H+] and [OH-]?
[H+] = [OH-] (always true for pure water)
How is pure water still classified neutral when the pH is 6.5?
Hot water -equilibrium of dissociation of water shifts right (endothermic direction) -causes Kw to increase -Kw = [H+]^2 -[H+] increases -as pH = -log[H+], a higher [H+] value, causes pH to decrease in value -but as [OH] = [H+], it is still classified as neutral
What is a strong acid?
a H+ donor which completely dissociates its H+ ions
Give 6 strong acids
-Hydrochloric acid (HCl) -Sulfuric acid (H2SO4 - only the first H+) -Nitric acid (HNO3) -Perchloric acid (HClO4) -Hydrogen bromide (HBr) -Hydrogen Iodide (HI)
What is a weak acid?
a H+ donor which only partially dissociates its H+ ions
A weak acid dissociation reaction is…
reversible
What are the values of Ka for weak acids?
less than 1 -equilibrium is on the LHS -greater conc. of reactants than products
How do you calculate the pH of a strong base, with a known conc.?
-set Kw = [OH-][H+] -10^-14 = [OH-][H+] -[H+] = 10^-14/ [OH-] -sub value of [H+] into pH= -log[H+]
What are the 2 assumptions made when calculating [H+] for weak acids?
• [H+] from water is negligible • dissociation of the acid is negligible
Why is assuming [weak acid] equilibrium = [weak acid] initial, more valid for weaker acids?
-equilibrium is further to LHS -so fewer weal acid molecules dissociate
When is assuming that [H+] of the acid > [H+] for water, for a weak acid dissociation less valid?
More dilute acid - [H+] value would be smaller (concentration argument) Very weak acid - [H+] value would be smaller (equilibrium argument)
How do you convert from pKa into Ka?
pKa = -log₁₀(Ka)