acid-base equilibria p1

Question 1

bronsted-lowry acid

Answer 1

proton donor

Question 2

bronsted-lowry base

Answer 2

proton accepter

Question 3

acid-base reaction involve the transfer of…

Answer 3

PROTONS

Question 4

what ion causes a solution to be acidic?

Answer 4

H+ or more accurately H3O+ (oxonium ion)

Question 5

what ion causes a solution to be alkaline?

Answer 5

OH-

Question 6

ionisation of water equation

Answer 6

2H2O(l) ⇌ H3O+(aq) + OH-(aq)
OR
H2O(l) ⇌ H+(aq) + OH-(aq)

Question 7

derive Kw using the equation for the ionisation of water

Answer 7

Keq = [H+][OH-]/[H2O]
[H2O]Keq = [H+][OH-]
[H2O] so large compared to [H+] and [OH-] that [H2O]Keq can be considered to be constant
therefore…
Kw = [H+][OH-]

Question 8

what is the value of Kw at 298K?

Answer 8

1 x 10-14

Question 9

what physical factors affect the value of Kw? how?

Answer 9

temperature - if increased the equilibrium moves to the right so Kw increases and the pH of pure water decreases

Question 10

what is pKw?

Answer 10

sometimes pKw is used instead of Kw to make numbers more manageable
pKw = -logKw
Kw = 10-pKW

Question 11

why is pure water still neutral even in pH does equal 7?

Answer 11

[H+]=[OH-]

Question 12

expression for pH in terms [H+]

Answer 12

pH=-log[H+]

Question 13

what is the relationship between pH and concentration of H+?

Answer 13

lower pH = higher concentration of H+

Question 14

if two solutions have a pH difference of 1, what is the difference in [H+]?

Answer 14

a factor of 10

Question 15

how do you find the [H+] from pH?

Answer 15

[H+] = 10-pH

Question 16

how do you find [OH-] from pH (at 298K)?

Answer 16

find [H+], use Kw = [H+][OH-] to calculate [OH-]

Question 17

what is different when finding [H+] from the concentration of diprotic and triprotic acids?

Answer 17

need to multiply the concentration of the acid by the number of protons to find [H+]

Question 18

how do you calculate the pH of a strong alkaline solution?

Answer 18

use Kw to calculate [H+] from [OH-]
use pH = -log[H+] (pOH = -log[OH-])

Question 19

strong acid

Answer 19

one which fully dissociates in water
HA –> H+ + A-

Question 20

how do you calculate the pH of a strong acid?

Answer 20

pH = -log[H+]

Question 21

strong base

Answer 21

one which fully dissociates in water
XOH –> X+ + OH-

Question 22

what is the difference between concentrated and strong?

Answer 22

concentrated means many mol per dm3
strong refers to dissociation

Question 23

what is a weak acid and a weak base?

Answer 23

do not fully dissociate in water
only partially dissociate into their ions

Question 24

examples of strong acids

Answer 24

HCl, H2SO4, H3PO4

Question 25

examples of strong bases

Answer 25

NaOH, CaCO3, Na2CO3

Question 26

examples of weak acids

Answer 26

CH3COOH, any organic acid

Question 27

example of a weak base

Answer 27

NH3

Question 28

Ka expression

Answer 28

HA ⇌ H+ + A- Ka = [H+][A-]/[HA]

Question 29

how do you work out the pH of a weak acid?

Answer 29

use equation for Ka, subbing in values for [A-] and [HA] use pH = -log[H+]

Question 30

what is pKa?

Answer 30

sometimes used to make Ka values more manageable pKa = -logKa Ka = 10-pKa

Question 31

what is a titration?

Answer 31

the addition of an acid/base of a known titrate to a base/acid of an unknown titrate to determine the concentration an indicator is used to show the neutralization has occurred, as is a pH meter

Question 32

TITRATION CURVE GRAPH!

Answer 32

look up pics! strong acid weak base etc ---- ) | | ( ------- y - pH x - volume of acid/base added

Question 33

equivalence point

Answer 33

the point at which the exact volume of base has been added to neutralise the acid or vice-versa

Question 34

what happens to the pH of the solution around the equivalence point?

Answer 34

large and rapid change in pH, except in weak-weak titration

Question 35

how do you calculate the concentration of a reactant if you know the volume and conc of the other reactant and the volume of the reactant added?

Answer 35

calculate moles of one reactant use balanced equation to work out mols of the other use conc = mol/vol to calculate concentration

Question 36

what is the end point?

Answer 36

the volume of acid or alkali added when the indicator just changes colour if the right indicator is chosen, equivalence point = end point

Question 37

what 3 properties do a good indicator have?

Answer 37

sharp colour change (not gradual) - no more than one drop of acid/alkali needed for colour change end point must be the same as the equivalence point, or titration gives wrong answer distinct colour change so it is obvious when the end point has been reached

Question 38

what indicator would you use for a strong acid-strong base titration?

Answer 38

phenolphthalein or methyl orange, but phenolphthalein is usually used as clearer colour change

Question 39

what indicator would you use for a strong acid-weak base titration?

Answer 39

methyl orange

Question 40

what indicator would you use for a strong base-weak acid titration?

Answer 40

phenolphthalein

Question 41

what colour is methyl orange in acid? in alkali? what pH does it change?

Answer 41

acid - red alkali - yellow changes at 4-5 approx same as pKa value

Question 42

what colour is phenolphthalein in acid? in alkali? what pH does it change?

Answer 42

acid - colourless alkali - red/pink changes at 9-10 approx same as pKa value

Question 43

what is the half-neutralisation point?

Answer 43

when volume = half the volume that has been added at the equivalence point

Question 44

buffer solution

Answer 44

a solution that resists change in pH when small amount of acid/alkali are added

Question 45

what do acidic buffer solutions contain in general terms?

Answer 45

a weak acid and a soluble salt of that acid that fully dissociates

Question 46

acidic buffer with added acid reaction

Answer 46

A- + H+ --> HA

Question 47

acidic buffer with added alkali reaction

Answer 47

HA + OH- --> H2O + A-

Question 48

how can you achieve an acidic buffer solution other than just mixing the constituents?

Answer 48

neutralise half of a weak acid (meaning the acid must be in excess) with an alkali - this forms a weak acid/soluble salt mixture

Question 49

what do basic buffer solutions contain in general terms?

Answer 49

weak base and soluble salt of that weak base

Question 50

how can you calculate the pH of buffer solutions?

Answer 50

use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] --> pH

Question 51

how can you calculate the new pH of a buffer solution when acid or base is added?

Answer 51

calculate number of moles of H+ and A- and HA before acid or base is added use equations to work out new moles of A- and HA find [H+] find pH

Question 52

which buffer system maintains blood pH at 7.4? what happens when acid/alkali is added?

Answer 52

H+ + HCOs- CO2 + H2O add OH- which reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost add H+ equilibrium shifts to the right removing excess H+

Question 53

what products are buffers found in?

Answer 53

shampoos, detergents -> important to keep pH right to avoid damage to skin, hair, fabrics

Question 54

why is there a difference in enthalpy changes of neutralisation values for strong and weak acids?

Answer 54

enthalpy changes of neutralisation are always exothermic this value is similar for strong acids and alkalis because the same reaction is occurring H+ + OH- --> H2O weak acids have less exothermic enthalpy change of neutralisation because energy is absorbed to ionise the acid and break the bond to the hydrogen in the un-dissociated acid