topic 12 - acid-base equilibria

Question 1

a bronsted-lowry acid is…

Answer 1

an acid is a proton donor

Question 2

a bronsted-lowry base is…

Answer 2

a base is a proton acceptor

Question 3

how does an acid act as a proton donor

Answer 3

-when the hydrogen atom is bonded to a more electronegative atom the hydrogen is slightly delta positive
-the bond can break heterolytically, H+ breaks off with no electrons so if positively charged
-so the acid loses a H+ ion (proton)

Question 4

how does a base act as a proton acceptor

Answer 4

-a base must have a lone pair of electrons
-the lone pair of electrons forms a dative covalent bond with the H+ ion (proton)

Question 5

what does diprotic mean

Answer 5

an acid which can donate 2 protons
or a base that can accept 2 protons

Question 6

what does monoprotic mean

Answer 6

an acid which can donate 1 proton
or a base that can accept 1 proton

Question 7

is sulfuric acid monoprotic or diprotic

Answer 7

diprotic because it can donate 2 protons as it has 2 hydrogen atoms

Question 8

a part from monoprotic and diprotic how can acids and bases be classified

Answer 8

acids - monobasic, dibasic, tribasic
bases - monoacidic, diacidic, triacidic

Question 9

what is the definition of amphoteric

Answer 9

a substance that can act both as an acid and base

Question 10

what is the definition of a strong acid

Answer 10

fully dissociates in aqueous solution

Question 11

what is the definition of a weak acid

Answer 11

partially dissociates in aqueous solution

Question 12

why is the concentration of a strong acid equal to the concentration of hydrogen ions it will produce in a solution

Answer 12

-strong acids fully dissociate into H+ ions in an aqueous solution

Question 13

define PH

Answer 13

PH = -log [H3O+]

Question 14

what is the equation used to calculate H+ ions concentration

Answer 14

[H+] = 10^-PH
or [H3O+]
this can only be used for strong acids

Question 15

what is Ka

Answer 15

the acid dissociation constant
it’s used to find the concentration of H+ ions

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

the concentration of products and reactants is used in moldm-3
the units need to be worked out

Question 16

how do you work out the H+ ion concentration of weak acids and strong acids

Answer 16

for weak acids use the Ka expression
for strong acids use the equation [H+] = 10^-PH

Question 17

define the ionic product of water

Answer 17

Kw = [H3O+] [OH-]

Question 18

why is water not included in the expression for the acid dissociation constant

Answer 18

concentration of water is constant because it’s in a big excess
so water is incorporated into Ka and Kw

Question 19

what affects the value of Ka and Kw

Answer 19

temperature only

Question 20

define pKa

Answer 20

pKa = -log(Ka)

Question 21

define pKw

Answer 21

pKw = -log(Kw)

Question 22

what is meant by a substance being amphoteric

Answer 22

it acts either as an acid or base depending on the reaction

Question 23

Predict, with a reason whether water is acidic, alkaline or neutral at 310k
(2 marks)

Answer 23

Neutral
Because the concentration of H3O+ and OH- are equal

Question 24

what is the general equation for an aqueous strong acid

Answer 24

HA + H2O = H3O+ + A-

Question 25

what is the general equation for an aqueous weak acid

Answer 25

HA + H2O ⇌ H3O+ + A-

Question 26

is the equation PH = -log [H3O+] used for strong or weak acids

Answer 26

strong only

Question 27

what is the difference between equivalence point and end-point

Answer 27

equivalence point - when the acid and base have reacted together in the same proportions as indicated by the molecular equation end-point - when the indicator changes colour in a titration

Question 28

When a weak acid is in water what do we assume

Answer 28

[H3O+] = [A-]

Question 29

when given a titration curve, how do you decide which indicator is best to use

Answer 29

the PH range of the indicator must fall on the straight vertical section of the graph

Question 30

what is the definition of a buffer solution

Answer 30

a solution of known PH which resists changes in PH when small amounts of acid or alkali are added

Question 31

how is a buffer made

Answer 31

mix a weak acid with a salt of that acid e.g. ethanoic acid + sodium ethanoate

Question 32

explain why ethanoic acid has a lower value for the standard enthalpy of neutralisation than hydrochloric acid

Answer 32

propanoic acid is a weak acid whereas HCl is a strong acid some energy is used up to fully ionise the propanoic acid

Question 33

when calculating the PH of a weak acid what 2 assumptions are made

Answer 33

[H3O+] = [A-] [HA]initial = [HA]equilibrium