Topic 12 Flashcards
Bronsted-Lowry Theory
Acid = proton donor Base = proton acceptor
What does pH measure
The concentration of H+ ions in solution
Logarithmic scale
Equation for pH
pH = -log, base 10, [H+]
Equation for hydrogen ion concentration
[H+] = 10 to the power -pH
Ionic product for water expression
Kw = [H+] [OH-]
In pure water Kw = [H+] squared
Varies with temp.
Equation for finding the concentration of [OH-] ions in a diluted strong base
diluted [OH-] = initial [OH-] x base volume / total volume
Equivalence point
The point at which the reaction has completed, according to the stoichiometry of the balanced equation.
Moles base = moles acid.
End point
The point at which the indicator changed colour, indicating the reaction is complete.
Buffer solution
Resists changes in pH when small quantities of an acid are or alkali are added to it.
Monoprotic acid
Releases 1 H+ ion per molecule when ionised in solution.
Ka expression
Ka = [H+] [A-] / [HA]
Ka value and strength of acid
Larger the Ka value, stronger the acid
pKa value and strength of acid
Larger the pKa value, weaker the acid
Ka expression for a weak acid and water only
Ka = [H+] squared / [HA]
[H+] squared because [H+] = [A-]
Assumption for weak acids in calculations
[HA] equilibrium = [HA] initial
Because degree of ionisation is negligible
[H+] =[A-]
All H+ ions come from the acid (ignore H+ ions from water)
What is the significance of half the equivalence point in a weak acid-strong base titration curve
pH = pKa
What makes an indicator suitable?
A suitable indicator changes colour at the equivalence point
How many pH units should the vertical section of a titration curve be?
Strong acid-strong base: 8
Weak acid-strong base: 4
How does an acidic buffer solution resist pH change when OH- ions are added?
The buffer has large concentrations of HA and A- (in excess)
HA + OH- -> A- + H2O
Ratio of [HA] and [A-] remains almost unchanged
Why does diluting weak acids cause less of a pH increase than diluting strong acids?
HA + H2O (reversible arrow) A- + H3O+
Add water, shift equilibrium right
So decrease in [H+] less than expected
