5.1.3 Acids, bases, and buffers

Question 1

Brønsted-Lowry acid

Answer 1

species that donates a proton

Question 2

Brønsted-Lowry base

Answer 2

species that accepts a proton

Question 3

conjugate acid-base pair

Answer 3

can be interconverted by the transfer of a proton

eg. HCl and Cl-

Question 4

what does monobasic, dibasic and tribasic refer to

Answer 4

the no. hydrogen ions that can be replaced per molecule in an acid-base reaction eg. by metal/ammonium ions

Question 5

mathematical relationship between pH and [H+] (two equations)

Answer 5

pH = -log[H+]
[H+] = 10^-pH

Question 6

assumptions when calculating the pH of strong acids

Answer 6

acid completely dissociates, therefore [HA] = [H+] and you can calculate the pH directly from the conc of the acid

Question 7

what’s the difference in [H+] between pH 2 and pH 1?

Answer 7

the [H+] is 10x greater at pH 1 than at pH 2

Question 8

mathematical relationship between pKa and Ka (two equations)

Answer 8

pKa = -log(Ka)
Ka = 10^-pKa

Question 9

What is Ka

Answer 9

the acid dissociation constant

Question 10

what are the Ka and pKa values of a strong acid

Answer 10

high Ka

low pKa

Question 11

assumptions when calculating the pH of weak acids

Answer 11

1. negligible dissociation of H₂O
   - -> [H+]equilibrium = [A-]equilibrium
2. no dissociation of HA
   - -> [HA]equilibrium = [HA]start

Question 12

limitations of the two assumptions used when calculating the pH of weak acids

Answer 12

1. negligible dissociation of H₂O
   - breaks down with very weak acids as dissociation of water becomes significant in comparison to dissociation of acid
2. no dissociation of HA
   - breaks down for stronger weak acids as dissociation of HA becomes significant

Question 13

Kw (what is it, equation, value at 298K)

Answer 13

ionic product of water
Kw = [H+][OH-]
1.00 x10^-14 at 298K

Question 14

how to calculate the pH of strong bases

Answer 14

[base] = [OH-]
use Kw to find [H+]
pH = -log[H+]

Question 15

buffer solution is

Answer 15

a system that minimises pH changes on addition of small amounts of an acid or base

Question 16

what are the two components of a buffer solution

Answer 16

weak acid and its conjugate base

Question 17

what could you add to CH3COOH to make a buffer solution

Answer 17

CH3COONa (which dissociates to provide the conjugate base)

or… NaOH (which neutralises some of the weak acid, forming the conjugate base)

Question 18

how do you calculate the pH of a buffer solution?

Answer 18

rearrange Ka

[H+] = Ka x [HA]/[A-]

Question 19

explain the changes that take place when you add an acid to a buffer solution

Answer 19

[H+] increases
H+ ions react with conjugate base A-
equilibrium shifts to the left, removing the excess H+ ions

Question 20

explain the changes that take place when you add an alkali to a buffer solution

Answer 20

[OH-] increases
H+ + OH- → H₂O
HA dissociates, shifting the equilibrium to the right, increasing [H+]

Question 21

what is the equation for the carbonic acid - hydrogen carbonate buffer system?

Answer 21

H₂CO₃ ⇌ H+ + HCO₃-

Question 22

what does the carbonic acid - hydrogen carbonate buffer system control?

Answer 22

blood pH

between 7.35-7.45

Question 23

describe and explain the shape of a pH titration curve

Answer 23

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

Question 24

what is the equivalence point

Answer 24

the volume of one solution that reacts exactly with another solution. The middle of the vertical section of the pH titration curve.

Question 25

why does an indicator change colour?

Answer 25

HA (weak acid) is one colour
A- (conjugate base) is another colour
colour change indicates endpoint

Question 26

how do you measure pH with a pH meter?

Answer 26

electrode dipped in solution and connected to a meter that displays pH reading.

Question 27

how do you choose an indicator for a reaction?

Answer 27

pH range must coincide with the vertical section of the pH titration curve. Ideally end point = equivalence point.

Question 28

why is no indicator suitable for a weak acid-weak base reaction?

Answer 28

no vertical section