topic 12: acid - base equilibrium

Question 1

what is a bronsted lowry acid

Answer 1

a substance that can donate a proton

Question 2

what is a bronsted lowry base

Answer 2

a substance that can accept a proton

Question 3

what is each acid linked to

Answer 3

linked to a conjugate base on the other side of the equation

Question 4

what is the definition of pH

Answer 4

-log[H+]

Question 5

what do strong acids do

Answer 5

completely dissociate

Question 6

what is the equation for finding [H+] from pH

Answer 6

10^-pH

Question 7

what is the Kw expression

Answer 7

= [H+(aq) ][OH-(aq) ]
=10^-pkw

Question 8

at 25 degrees celsius , what is the value of Kw

Answer 8

1x10^-14 mol2dm-6

Question 9

what are the expression for pKw

Answer 9

=-logKw

Question 10

what are weak acids

Answer 10

only slightly dissociate when dissolved in water, giving an equilibrium mixture

Question 11

what is the weak acids dissociation expression (Ka)

Answer 11

[H+(aq)][A-(aq)]/[HA (aq)]

Question 12

the larger the Ka

Answer 12

the stronger the acid

Question 13

what are the 2 assumption made to simplify the Ka expression

Answer 13

1) [H+(aq)]eqm = [A(aq)] eqm because they have dissociated according to a 1:1 ratio
2) as the amount of dissociation is small we assume that the initial
concentration of the undissociated acid has remained constant.
so [HA (aq) ] eqm = [HA(aq) ]
initial

Question 14

what is the Ka expression

Answer 14

[H+(aq)]^2 /[HA (aq)]initial

Question 15

using one of the assumptions, what is the other expression of pH

Answer 15

pKa

Question 16

what is a buffer solution

Answer 16

one where the pH does not change significantly if small amounts of acid or alkali are added to it

Question 17

what is a acidic buffer solution made from

Answer 17

made from a weak acid and a salt of that weak acid ( made from reacting the weak acid with a strong base)
example : ethanoic acid and sodium ethanoate

Question 18

what is a basic buffer solution

Answer 18

made from a weak
base and a salt of that weak base (made from reacting the weak base with a strong acid)
example :ammonia and ammonium chloride

Question 19

what happens when small amounts of acid is added to the buffer

Answer 19

the above equilibrium
will shift to the left removing nearly all the H+ ions added, CH3CO2 -(aq) + H+ (aq) <—> CH3CO2H (aq)
as there is a large concentration of the salt ion in the buffer the ratio
[CH3CO2H]/ [CH3CO2
-] stays almost constant, so the pH stays fairly
constant

Question 20

what happens when a small amount of alkali is added to the buffer

Answer 20

the OH- will react with H+ ions to form water.
H+ + OH -  H2O
the equilibrium will then shift to the right to produce more H+
ions.
CH3CO2H(aq) <—> CH3CO2-(aq) + H+(aq)
some ethanoic acid molecules are changed to ethanoate ions but as there is a large
concentration of the salt ion in the buffer the ratio [CH3CO2H]/[CH3CO2-] stays almost constant, so the pH stays fairly constant

Question 21

what would happen to pH when the acid is diluted by 10

Answer 21

pH increases by 1 unit

Question 22

H2CO3 (aq) ⇌ H+(aq) + HCO3–(aq)
what is the buffer that controls the blood’s pH

Answer 22

H2CO3/HCO3– buffer

Question 23

what is the value of pH in blood the H2CO3
/HCO3– buffer maintains

Answer 23

7.35 and 7.45

Question 24

what does the H2CO3
/HCO3– buffer do when the alkali is added

Answer 24

adding alkali reacts with H+ with the equation
H+ + OH- —> H2O
so the above equilibrium would shift right forming new H+ and more HCO3–

Question 25

what does the H2CO3
/HCO3– buffer do when the acid is added

Answer 25

adding acid shifts the above equilibrium left.
the reaction is
H+ + HCO3– —> H2CO3

Question 26

what happens to the moles if a small amount of alkali is added to a buffer

Answer 26

• if a small amount of alkali is added to a buffer then the moles of the buffer acid would reduce by the number of moles of alkali added and the moles of salt would increase by the same amount so a new
  calculation of pH can be done with the new values
  CH3CO2H (aq) +OH- —> CH3CO2-(aq) + H2O(l)

Question 27

what happens to the moles if a small amount of acid is added to a buffer

Answer 27

if a small amount of acid is added to a buffer then the moles of the buffer salt would reduce by the number of moles of acid added and the moles of buffer acid would increase by the same amount so a
new calculation of pH can be done with the new values
CH3CO2-(aq) + H+ —> CH3CO2H (aq)

Question 28

PRACTICAL: how do you construct a pH curve

Answer 28

1. transfer 25cm3 of acid to a conical flask with a volumetric
   pipette
2. measure initial pH of the acid with a pH meter
3. add alkali in small amounts (2cm3) noting the volume added
4. stir mixture to equalise the pH
5. measure and record the pH to 1 d.p.
6. repeat steps 3-5 but when approaching endpoint add in smaller volumes of alkali
7. add until alkali in excess

Question 29

describe the curve of strong acid - strong base

Answer 29

equivalence point is 7
long vertical part around 3 to 9
initial and final pH
volume at neutralisation
general shape (pH at neutralisation)

Question 30

describe the weak acid - strong base graph

Answer 30

equivalence point >7
steep part of curve >7 (around 7 to 9)
pH starts near 3
WHEN A STRONG BASE IS ADDED

Question 31

describe the strong acid - weak base graph

Answer 31

vertical part of curve<7 (around 4 to 7)
equivalence point<7
starts at 1
WHEN ALKALI IS ADDED

Question 32

what is the end - point of a titration

Answer 32

the point when the colour of the indicator changes colour

Question 33

what happens when the end - point is reached

Answer 33

when [HIn] = [In-]

Question 34

when would you use phenolphthalein

Answer 34

with strong bases but not weak bases
colour change: colourless acid —> pink alkali

Question 35

when would you use methyl orange

Answer 35

with strong acids but not weak acids
colour change: red acid —> yellow alkali (orange end point)

Question 36

what is the standard enthalpy change of neutralisation

Answer 36

the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

Question 37

why do weak acids have a less exothermic enthalpy change of neutralisation

Answer 37

because energy is absorbed to ionise the
acid and break the bond to the hydrogen in the un-dissociated acid

Question 38

when will an indicator work

Answer 38

if the pH range of the indicator lies on the vertical part of the titration curve
- in this case the indicator will change colour rapidly and the colour change will correspond to the neutralisation point

Question 39

describe and explain the behaviour of the solution formed in the region
circled on the sketch graph

Answer 39

any mention of buffer / buffering
(both) propanoic acid and potassium
propanoate present
CH3CH2COOH + OH− → H2O + CH3CH2COO−
On addition of OH− (in small quantities)
H+ ions react with (the added) OH−and
CH3CH2COOH ⇌ CH3CH2COO− + H+
shifts to the right

Question 40

explain why the pH at the equivalence point of this titration is greater than 7

Answer 40

Propanoate (ions) react with water
Forming hydroxide ions
[OH−] > [H+]
CH3CH2COO− + H2O → OH− + CH3CH2COOH

Question 41

the student made the following statement:
‘The pH of pure water is always 7.0’
is the student correct? use the following information to justify your answer.
- H2O(l) U H+(aq) + OH–(aq)
- Kw = 1.0 x 10–14 mol2dm–6 at 298 K
- ǻH is positive for the forward reaction in the equilibrium

Answer 41

No,
as T increases eqm moves to RHS / Kw
increases / ‘favours RHS’ / ∆Stotal increases
So [H+] ions increases / more H+ ions
[H+] > 1 x 10−7
Hence pH < 7 / pH decreases

Question 42

state the three assumptions you have made in your calculations for weak acids

Answer 42

[H2S]equilibrium = [H2S]initial
[H3O+] = [HS−]
Ka2 very much smaller than Ka1
Measurements at 298 K / standard temperature