5.1.3 Acids, Bases And Buffers

Question 1

Definition of Bronsted-Lowry acid

Answer 1

Proton donor

Question 2

Definiton of Bronsted-Lowry base

Answer 2

Proton acceptor

Question 3

What is a conjugate acid-base pair

Answer 3

Two species that can be interconverted by transfer of a proton

Question 4

Conjugate acid-base pair equation

Answer 4

Acid 1 + base 2 -> acid 2 + base 1

Question 5

What is a mono basic acid

Answer 5

Acid with one hydrogen that can be replaced per molecule in acid-base reaction

Question 6

What is a dibasic acid

Answer 6

Acid with 2 hydrogens that can be replaced per molecule in an acid-base reaction

Question 7

What is a tribasic acid

Answer 7

Acid with 3 hydrogens that can be replaced per molecule in an acid-base reaction

Question 8

Reactions of H+ (acids)

Answer 8

Metals (salt + hydrogen)
Carbonates (salt + CO2 + H2O)
Metal oxides (salt + water)
Alkalis (salt + water)

Question 9

Relationship between pH and [H+]

Answer 9

PH = -log [H+]

Question 10

Relationship between [H+] and pH

Answer 10

[H+] = 10^-pH

Question 11

PH of strong acid

Answer 11

[H+] = [HA]
Then pH = -log [H+]

Question 12

Ka equation (acid dissociation constant)

Answer 12

Ka = [H+] [A-] ÷ [HA]

Question 13

Relationship between Ka and pKa

Answer 13

Ka = 10^-pKa

Question 14

Relationship between pKa and Ka

Answer 14

PKa = -log Ka

Question 15

Strong acid values of Ka and pKa

Answer 15

Large Ka and small pKa

Question 16

Weak acid values of Ka and pKa

Answer 16

Small Ka and large pKa

Question 17

Weak acid equation

Answer 17

HA <≈> H+(aq) + A- (aq)

Question 18

Ka expression for weak acids

Answer 18

Ka = [H+]^2 ÷ [HA]

Question 19

What are the assumptions made when determining pH of a weak acid

Answer 19

[H+] at equilibrium = [A-] at equilibrium
[HA] at equilibrium = [HA] at start (since dissociation of weak acids is small)

Question 20

How can Ka of a weak acid be determined experimentally

Answer 20

Standard solution of weak acid of known concentration
PH measured by pH meter

Question 21

Limitations to assumptions made

Answer 21

If acid is very weak or very dilute, cant assume the dissociation of water is negligible
If Ka is large, [H+] is significant and there’s a difference between [HA] at equilibrium and [HA] at start - [H+] at equilibrium
So doesn’t hold for stronger weak acids (Ka > 10-2) and dilute solutions

Question 22

Ionic product of water equation

Answer 22

Kw = [H+] [OH-]

Question 23

What is a buffer solution

Answer 23

A system that minimises pH changes when small amounts of acid/base are added
Contains weak acids (HA) and its conjugate base (A-)

Question 24

How can buffer solutions be prepared

Answer 24

Weak acid and its salt
Weak acid and aqueous solution of an alkali

Question 25

Why is there a smaller concentration of [H+] in buffer solutions

Answer 25

When conjugate bases are added, equilibrium shifts further to left, more weak acid produced using the H+ Leaving mainly weak acids and conjugate base

Question 26

How does a buffer solution react to added acid

Answer 26

Increased [H+] reacts with conjugate base Equilibrium shits to left, H+ removed

Question 27

How does a buffer solution react to added alkali

Answer 27

Increased [OH-] reacts with small concentration of H+ (H+ + OH- -> H2O) HA dissociates which shifts equilibrium to the right to restore lost H+

Question 28

How to calculate pH of a buffer solution

Answer 28

Ka = [H+] [A-] ÷ [HA] So [H+] = Ka [HA] ÷ [A-] (Kacid over salt)

Question 29

Why does blood pH need to be maintained

Answer 29

Blood plasma needs to be maintained between 7.35-7.45 - below 7.35, acidosis (causes fatigue, shortness of breath, death) - above 7.45, alkalosis (causes muscle spasms, light-headedness, nausea)

Question 30

How is blood pH maintained

Answer 30

Carbonic acid-hydrogencarbonate buffer system H2CO3 <=> H+ + HCO3- More acidic products produced by body, H2CO3 produced, prevented from building up by being converted to CO2 which is exhaled by the lungs

Question 31

Blood pH change when acid is added

Answer 31

[H+] increased, reacts with HCO3- Equilibrium shifts to left to remove H+ More H2CO3 produced

Question 32

Blood pH change when alkali is added

Answer 32

[OH-] increased, small concentration of H+ reacts with OH- ( H+ + OH- -> H2O) H2CO3 dissociates, equilibrium shifted to the right to restore H+

Question 33

How can pH be monitored with a pH meter

Answer 33

- Use a pipette to add acid to conical flask - Place pH meter electrode in the flask - add aqueous base to Burnett and add to acid, 1cm3 at a time - swirl after every addition, record pH and total volume of base added - repeat until pH changes rapidly, then add base dropwise - when pH stops changing rapidly, add at 1cm3 intervals again, until minimal change in pH - plot graph of pH against volume of base added

Question 34

What is the equivalence point of a titration

Answer 34

Volume of solution that exactly reacts with the volume of the other solution Solutions have exactly reacted Centre of vertical section of pH titration curve

Question 35

What demonstrates excess of acid/base on a titration curve

Answer 35

Slow increase/decrease in pH

Question 36

Explain indicator colour changes

Answer 36

Strong base + strong alkali - methyl orange is red due to H+ forcing equilibrium to the left Added OH- - OH- react with H+ in indicator, HA dissociates and shifts equilibrium to the right to replace H+ - orange at end point, yellow as equilibrium shifts Added acid: - H+ reacts with A- - equilibrium shifts to the left - orange at end point, red when equilibrium shifts

Question 37

How to choose an indicator for a titration

Answer 37

Indicator with colour change at vertical section of titration curve

Question 38

Suitable indicator for strong acid with strong base

Answer 38

Phenolphthalein Methyl orange

Question 39

Suitable indicator for strong base with weak acid

Answer 39

Phenolphthalein

Question 40

Suitable indicator for strong acid with weak base

Answer 40

Methyl orange

Question 41

Suitable indicator for weak acid with weak base

Answer 41

No indicators, no vertical section

Question 42

Methyl orange colour change

Answer 42

Red = acid Yellow = alkali End point = orange

Question 43

Phenolphthalein colour change

Answer 43

Colourless = acid Pink = alkali

Question 44

Universal indicator colour change

Answer 44

Red = strongly acidic Purple = strongly alkaline Green = neutral