5.1.3 Acids, bases, and buffers Flashcards
Brønsted-Lowry acid
species that donates a proton
Brønsted-Lowry base
species that accepts a proton
conjugate acid-base pair
can be interconverted by the transfer of a proton
eg. HCl and Cl-
what does monobasic, dibasic and tribasic refer to
the no. hydrogen ions that can be replaced per molecule in an acid-base reaction eg. by metal/ammonium ions
mathematical relationship between pH and [H+] (two equations)
pH = -log[H+] [H+] = 10^-pH
assumptions when calculating the pH of strong acids
acid completely dissociates, therefore [HA] = [H+] and you can calculate the pH directly from the conc of the acid
what’s the difference in [H+] between pH 2 and pH 1?
the [H+] is 10x greater at pH 1 than at pH 2
mathematical relationship between pKa and Ka (two equations)
pKa = -log(Ka) Ka = 10^-pKa
What is Ka
the acid dissociation constant
what are the Ka and pKa values of a strong acid
high Ka
low pKa
assumptions when calculating the pH of weak acids
- negligible dissociation of H₂O
- -> [H+]equilibrium = [A-]equilibrium - no dissociation of HA
- -> [HA]equilibrium = [HA]start
limitations of the two assumptions used when calculating the pH of weak acids
- negligible dissociation of H₂O
- breaks down with very weak acids as dissociation of water becomes significant in comparison to dissociation of acid - no dissociation of HA
- breaks down for stronger weak acids as dissociation of HA becomes significant
Kw (what is it, equation, value at 298K)
ionic product of water
Kw = [H+][OH-]
1.00 x10^-14 at 298K
how to calculate the pH of strong bases
[base] = [OH-]
use Kw to find [H+]
pH = -log[H+]
buffer solution is
a system that minimises pH changes on addition of small amounts of an acid or base
what are the two components of a buffer solution
weak acid and its conjugate base
what could you add to CH3COOH to make a buffer solution
CH3COONa (which dissociates to provide the conjugate base)
or… NaOH (which neutralises some of the weak acid, forming the conjugate base)
how do you calculate the pH of a buffer solution?
rearrange Ka
[H+] = Ka x [HA]/[A-]
explain the changes that take place when you add an acid to a buffer solution
[H+] increases
H+ ions react with conjugate base A-
equilibrium shifts to the left, removing the excess H+ ions
explain the changes that take place when you add an alkali to a buffer solution
[OH-] increases
H+ + OH- → H₂O
HA dissociates, shifting the equilibrium to the right, increasing [H+]
what is the equation for the carbonic acid - hydrogen carbonate buffer system?
H₂CO₃ ⇌ H+ + HCO₃-
what does the carbonic acid - hydrogen carbonate buffer system control?
blood pH
between 7.35-7.45
describe and explain the shape of a pH titration curve
gradual increase- pH increases very slowly as base is added as acid is in great excess
vertical section - pH increases rapidly with a very small addition of base. Acid and base conc. are v similar
gradual increase - pH increases slowly as basic solution is added, base is in excess
what is the equivalence point
the volume of one solution that reacts exactly with another solution. The middle of the vertical section of the pH titration curve.
why does an indicator change colour?
HA (weak acid) is one colour
A- (conjugate base) is another colour
colour change indicates endpoint
how do you measure pH with a pH meter?
electrode dipped in solution and connected to a meter that displays pH reading.
how do you choose an indicator for a reaction?
pH range must coincide with the vertical section of the pH titration curve. Ideally end point = equivalence point.
why is no indicator suitable for a weak acid-weak base reaction?
no vertical section
