AS.3 Acids, Bases And Buffers

Question 1

Acid

Answer 1

Proton donor

Question 2

Base

Answer 2

Proton acceptor

Question 3

Strong acid

Answer 3

Proton donor that dissociates fully

Question 4

Weak acid

Answer 4

Proton donor that dissociates partially

Question 5

Alkali

Answer 5

Soluble base that contains OH- ions

Exception -> NH3 + H2O ——> NH4 + + OH-

Question 6

Acid base pair

Answer 6

Two molecules which interconvert between each other with the loss or gain of a proton

Question 7

Acid base pair example

Answer 7

HA + B ————-> A- + HB+

HA - acid 1
A- - base 1
Conjugate

B - base 2
HB+ - acid 2
Conjugate

Question 8

Acid + metal ( ionic equation )

Answer 8

Salt + H2

Na + H+ ———> Na+ + H2

Question 9

Acid + metal oxide (ionic equation)

Answer 9

Salt + water

MgO + 2H+ ————> Mg2+ + H2O

Question 10

Acid + alkali (ionic equation)

Answer 10

Salt and water

OH- + H+ ——-> H2O

Question 11

Acid and carbonate

Answer 11

Salt + water + carbon dioxide

Group I carbonate - soluble
CO3 2- + 2H+ ———-> CO2 + H2O

Group II carbonate - insoluble
MgCO3 + 2H+ ———> Mg2+ + CO2 + H2O

Question 12

pH equation

Answer 12

-log [H+]

Question 13

[H+] equation

Answer 13

10^-pH

Question 14

pH of strong acids

Answer 14

[monobasic acid] = [H+]

Question 15

pH of weak acids

Answer 15

HA <———-> H+ + A-

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

Question 16

Ka approximates

Answer 16

[HA] eq = [HA] undissociated
Amount of HA considered constant because equilibrium is so far to left

[H+] = [A-]
assume only source of ions is from dissociation of HA

Question 17

Rearrange Ka to calculate pH

Answer 17

[H+] = square root of (Ka x [HA])

Question 18

pKa

Answer 18

-log(Ka)

Question 19

The larger the Ka, the ——- the pKa

Answer 19

The larger the Ka, the more dissociated the acid is, therefore the stronger the weak acid

The larger the Ka, the smaller the pKa

The smaller the pKa, more dissociated the acid is and therefore the stronger the weak acid

Question 20

pH of pure water

Answer 20

Kw = [H+] [OH-]

Question 21

Kw at 298K

Answer 21

1 x 10^ -14

Question 22

Rearranging Kw

Answer 22

Square root of Kw = H+

Question 23

pH of a strong base

Answer 23

Kw
——— = [ H+]
[NaOH]

Kw = [H+] x [OH-]

[OH-] = [NaOH]

Question 24

Buffer solution

Answer 24

A solution that resists the change to its pH despite the addition of small amounts of acid or alkali

Question 25

How to make a buffer - method 1

Answer 25

Weak acid + salt of weak acid CH3COOH + CH3COONa Don’t react but coexist

Question 26

How to make a buffer - method 2

Answer 26

Excess of weak acid + limited amount of alkali

Question 27

Buffer - add small amount of acid

Answer 27

Acid introduces more H+ Weak acid equilibrium will oppose this change and shift to left The CH3COO- reacts with added H+ to restore amount [H+] maintained and pH remains unchanged

Question 28

Buffer - adding small amount of alkali

Answer 28

OH- from alkali will react with H+ ions in the buffer Weak acid equilibrium will oppose this change and shift to right Weak acid will dissociate more to restore amount of H+ [H+] maintained and pH remains unchanged

Question 29

Buffer calculation

Answer 29

Ka = [H+] [A-] ————- [A-]

Question 30

Buffers in blood

Answer 30

Carbonic acid acts as weak acid Hydrogencarbonate acts as conjugate base H2CO3 HCO3-

Question 31

Blood buffers - increase in H+

Answer 31

Weak acid equilibrium shifts to left

Question 32

Blood buffers - increase in OH-

Answer 32

Removed by reaction with H+ from equilibrium Equilibrium then shifts to right to restore [H+]

Question 33

Blood buffers - production of more H2CO3

Answer 33

Removed by action of enzyme H2CO3 ——-> H2O + CO2 (aq) CO2 transported to lungs CO2 (aq) ————> CO2 (g)

Question 34

Normal blood pH

Answer 34

7.35 - 7.45

Question 35

Graph line shape of neutralisation

Answer 35

pH increase slightly - large excess of H+ Point of neutralisation - rapid as small conc of H+ remaining is used up and replaced by OH-

Question 36

Selecting suitable indicator

Answer 36

Indicator - Very weak acid Colour change must be within pH range of equivalence point of titration

Question 37

Suitable indicator for SA/SB

Answer 37

3———->11 Methyl orange Phenolphthalein

Question 38

Suitable indicator for WA/SB

Answer 38

7 ———> 11 Phenolphthalein only (8.4 - 10)

Question 39

Suitable indicator for SA/WB

Answer 39

3————->7 Methyl orange only (2.8 - 4)

Question 40

Suitable indicator for WA/WB

Answer 40

No vertical section of curve No suitable indicator