Topic 12 - Acid-based Equilibria
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According to Bronsted-Lowry, what is an acid?
a proton donor
According to Bronsted-Lowry, what is a base?
a proton acceptor
What is a conjugate base?
the product of an acid donating a proton which can return to its original species by acting as a base
What is a conjugate acid?
the product of a base accepting a proton which can return to its original species by acting as an acid
What is pH?
the potential of H+ • poor indicator of acid strength • on a scale of -1-14
What is a strong acid?
an acid that fully dissociates eg. HCl → H+ + Cl-
What is a weak acid?
an acid that partially dissociates eg. CH3COOH ⇌ CH3COO- + H+
pH =
-log[H+]
[H+] =
10 ^-pH
How do you calculate pH of a strong acid?
use pH = -log[H+]
How do you calculate pH of a weak acid?
use Ka rearranged to calc [H+] then use pH = -log[H+]
How do you calculate pH of a strong base?
either use pOH = -log[OH-] pH = 14-pOH or use Kw rearranged to find [H+] and then pH = -log[H+]
How do you calculate pH of a weak base?
use Kb rearranged to find [OH-] then use use pOH = -log[OH-] pH = 14-pOH
What is Ka?
the acid dissociation constant • larger Ka = greater acid dissociation • better indicator of acid strength than pH
Ka =
[H+][A-] ______ [HA]
How can Ka be calculated experimentally?
• make a standard solution of a known conc • calc pH (using pH meter) • use pH to find [H+] (pH = -log[H+]) • use Ka expression to calc Ka
What is Kw?
the equilibrium constant for the dissociation of water -used to calc pH of strong bases
Kw =
[H+][OH-]
What is the value of Kw?
1x10^-14
How does pH of water vary with temperature?
• dissociation of water is endothermic • so when temp increases, eqm shifts to right • so [H+] increases • so pH decreases
What are acid-base titration curves?
graphs of volume against pH which show how pH changes as an acid is added to a base (or vice versa) during a titration
What is the equivalence point?
the half way point on a titration curve where there are equal concentrations of acid and base -during the vertical section of the titration curve
Why is the equivalence point not always 7 on a titration curve?
• when a strong acid is titrated with a weak base, a slightly acidic product is formed, which lowers the equivalence point • when a weak acid is titrated with a strong base, a slightly alkali product is formed, which raises the equivalence point
How can you tell what indicator to use by using a titration curve?
the indicator should have a colour change (normally a range) during the vertical section of the titration curve (aka the end point)