Topic 12 Acid Base Equilibria

Question 1

What are brønsted Lowry acids and bases

Answer 1

Proton donators and acceptors

Question 2

What is transferred in acid base reactions

Answer 2

Protons

Question 3

What is pH

Answer 3

-log[H+]

Question 4

What do acids actually form in water

Answer 4

H3O+ / hydroxonium ions

Question 5

What’s the difference between weak and strong acids

Answer 5

Strong acids fully dissociate in water but weak acids dissociate poorly

Question 6

What behaves as a base when acid is added to water

Answer 6

The water

Question 7

What do acids and bases react to form

Answer 7

Salts

Question 8

Is the enthalpy change of neutralisation always exo or endothermic

Answer 8

Exothermic

Question 9

What are the 2 enthalpys involved in neutralisation of a weak acid

Answer 9

1. Enthalpy of dissociation
2. Enthalpy for the reaction of OH- and H+

Question 10

What enthalpy is involved in neutralisation of a strong acid

Answer 10

Enthalpy when OH- and H+ react

Question 11

What is Ka

Answer 11

Acid dissociation constant

Question 12

When working with Ka for a weak acid what 2 assumptions do you have to make

Answer 12

1. [HA] at equilibrium = initial conc
2. [H+]=[A-] at equilibrium

Question 13

What is the simplified Ka expression

Answer 13

Ka = [H+]^2/HA

Question 14

Does water exist in equilibrium or not

Answer 14

Yes

Question 15

Water dissociation equation non simplified

Answer 15

2H2O <—> H3O+ + OH-

Question 16

Water dissociation equation simplified

Answer 16

H2O <—> H+ + OH-

Question 17

Kc expression for water

Answer 17

Kc = [H+][OH-]/[H2O]

Question 18

Formula of Kw

Answer 18

Kw =[OH-] x [H+]

Question 19

What is the value of Kw

Answer 19

1.00 x 10^-14

Question 20

Does Kw change with temp and why

Answer 20

Yes - dissociation of water increases with temperature so there is more OH- and H+

Question 21

What is special about pure water

Answer 21

[OH-] = [H+]

Question 22

What is the simplified definition of Kw when working with pure water

Answer 22

Kw = [H+]^2

Question 23

Define pKw and pKa

Answer 23

pKw = -logKw
pKa = -logKa

Question 24

What do you use to measure pH

Answer 24

pH meter

Question 25

Qualities of strong base into strong acid titration curve

Answer 25

Equivalence point at 7, starts at pH 1 ends at pH 13

Question 26

Qualities of weak base into strong acid titration curve

Answer 26

Equivalence point < 7 , start at pH 1 end at pH 9

Question 27

Qualities of strong base into weak acid titration curve

Answer 27

Equivalence point > 7 , starts at pH 5 ends at pH 13

Question 28

What is the half neutralisation point and what can it be used to calculate

Answer 28

Half way between start and equivalence points, pH at this point can be used to calculate pKa as [HA] = [A-]. Cancels out in the Ka equation so Ka = [H+] which can be found from the pH (pKa = pH)

Question 29

What makes an indicator “suitable”

Answer 29

Change colour completely within the vertical section of a titration curve

Question 30

Methyl orange colour change

Answer 30

Red-Orange acid - base

Question 31

Phenolphthalein colour change

Answer 31

Clear - acid Pink - base

Question 32

Why are no indicators suitable for weak acid/base titrations

Answer 32

There is no sharp pH change

Question 33

What is a buffer

Answer 33

A solution that resists changes in pH when a small amount of acid or base is added

Question 34

What makes up an acidic buffer

Answer 34

A weak acid and a salt of its conjugate base

Question 35

What happens when you add an acid to an acidic buffer (2 steps)

Answer 35

1. H+ ions react with A- (high conc from salt) 2. More HA is produced as equilibrium has shifted left

Question 36

What happens when you add a base to an acidic buffer (2 steps)

Answer 36

1. OH- ions react with H+ ions in solution 2. Equilibrium shifts right so HA dissociates to H+ and A- to restore the concentration of H+ ions

Question 37

What other way can a buffer be formed and where does this happen. Give 2 steps to explain this process

Answer 37

Excess weak acid + strong base - in a titration. 1. All of the base reacts HA + OH-—-> A- + H2O 2.A- and excess HA left in solution which forms a buffer solution

Question 38

What are basic buffers made of

Answer 38

Weak base and its salt

Question 39

What weak acid and ion form the buffer in blood. Give two equilibrium equations for this buffer

Answer 39

Carbonic acid and hydrogencarbonate ions 1. H2CO3 <—> H+ + HCO3- 2. H2CO3 <—-> H2O + CO2

Question 40

What do you have to remember when calculating pH of buffers

Answer 40

[H+] ≠ [A-]

Question 41

What is the Henderson Hasselbalch equation and what is it used for

Answer 41

pH = pKa + log [A-]/[HA] Used to work out how to make a buffer of a specific pH