20 - Acids, bases and pH

Question 1

What is the Arrhenius model of acids?

Answer 1

-> acids dissociate and release H+ ion in (aq) solution
-> alalis dissociate and release OH- ions in (aq) solutions
-> H+ ions are neutralised by OH- to form water

H+(aq) + OH-(aq) —-> H2O (l)

Question 2

What is the Bronsted-Lowry model for acids and bases?

Answer 2

-> Bronsted-Lowry acid is a proton donor
-> Bronsted-Lowry base is a proton acceptor

Question 3

What is a conjugate acid-base pair?

Answer 3

Are two species that can be interconverted by the transfer of a proton

ie HC- and Cl-

Question 4

In the reaction what is the conjugate acid and base?

HCl(aq) <—-> H+(aq) + Cl-(aq)

Answer 4

-> in the forward reaction HCl releases a proton to form Cl-, its conjugate base
-> in the reverse direction Cl- accepts a proton to form HCl, its conjugate acid

Question 5

HCl(aq) + OH-(aq) <—–> H2O(aq) + Cl-(aq)

What are the conjugate base pairs?

Answer 5

Acid 1 : HCl
Base 1 : Cl-

Acid 2 : H2O
Base 2 : OH-

Question 6

Reaction between HCl and H2O, what are its conjugate pairs?

Answer 6

HCl + H2O <—-> H3O+ + Cl-
A1 B2 A2 B1

Question 7

Why is the hydronium ion important

Answer 7

it is the active ingredient in any aqueous acid

Question 8

Monobasic, dibasic and tribasic

Answer 8

refers to the number of H+ that cen be replaced per molecule in an acid-base reaction

Question 9

acid + metal

Answer 9

-> salt + hydrogen
redox

Question 10

What are carbonates?
Acid + Carbonate

Answer 10

-> they are a base which can neutrlise acids

—> salt + water + CO2

Question 11

mathmatical relationship between [H+] and pH

Answer 11

pH = -log[H+]
[H+] = 10^-pH

Question 12

on a logarithmic scale, what dies a change in one pH represent?

Answer 12

a 10 times difference

Question 13

what happens when a strong acid is in (aq) solution?
what is this equation?

Answer 13

a strong monobasic acid completely dissociates
HA(aq) —> H+(aq) + A-(aq)

Question 14

what is the relationship of [H+] and [HA] for a strong acid

Answer 14

[H+] = [HA]

Question 15

what is a weak acid?
give an equation

Answer 15

partially dissociates
HA <—> H+ + A-

Question 16

What is the acid dissociation constant Ka?
compare Ka pf a strong and weak acid

Answer 16

strong acids have a large Ka
-> the value of [H+][A-] is larger than [HA]

Question 17

mathmatical relationship between Ka and pKa

Answer 17

pKa = -logKa
Ka - 10^pKa

Question 18

what is the Ka equation of a weak acid, why is it this?

Answer 18

Ka= [H+]^2 / [HA]

-> when HA molecules dissociate H+ and A- are formed in equal quantities

Question 19

What are the approximations that have led to the simplification of the Ka equation?

Answer 19

Approximation 1
-> that the [H+] and [A-] are equal. Because the acid is in aqueous solution there is a small concentration due to the dissociation of water. However this is very small and can be neglected
[H+]eqm = [A-]eqm

Approximation2
-> the equilibrium concentration of HA is smaller than the undissociated concentration
[HA]eqm = [HA]start - [H+]eqm
-> as the dissociation of a weak acid is small you can assume that [HA]start> [H] and you can neglect any decrease in concentration of HA from dissociation
(Dissociation of HA is negligible )
[HA]eqm = [HA]start

Question 20

approximation in calculation involving weak acids

Answer 20

1) assumes that dissociation of water is negligible
-> [H+]eqm = {A-}eqm
-> at 25’c dissociation of water can be considered negligible if pH<6.
-> this approximation falls for very weak acids or dilute solutions

2) assumes that the concentrations of acid is much greater than the H= concentration at equilibrium
[HA]start» [H+]eqm
[HA]eqm = [HA]start - [H+]eqm approximate to
[HA]eqm = [HA]start
-> it holds for weak acids with a small Ka value
-> it breaks down wen [H+] becomes significant and there is a real difference between [HA]eqm and [HA]start - [H+]eqm
- it is not justified for stronger weak acids and for very dilute solutions

Question 21

how can the Ka of a weak acid be determined?

Answer 21

• prepare a standard solution of the weak acid of a known concentration
• measuring the pH of the standard solution using a pH meter

Question 22

Explain the ionisation of water
H2O + H2O

Answer 22

• water ionies slightly acting as both an acid and a base

H2O + H2O <—> H3O- + OH-
A1 B2 A2 B1

Question 23

How do find the ionic product or water?

Answer 23

Ka = [H+][OH-] / [H2O]
Ka x [H2O] = [H+][OH-]
Ka x [H2O] = Kw

Kw = [H+][OH-]

Question 24

what is Kw

Answer 24

the ionic product of water
it is the ions in the water multiplied together

Question 25

what is the value of Kw at 298K

Answer 25

1 x 10^-14 mol2dm-6

Question 26

in an acidic solution of pH 3 [H+] = 10-3 moldm-3
what is the [OH-]

Answer 26

10-11