Acids & bases

Question 1

What is a Bronsted-lowry acid?

Answer 1

A species that donates a proton

Question 2

What is a Bronsted-lowry base?

Answer 2

A species that accepts a proton

Question 3

Whats a conjugate acid-base pair?

Answer 3

Contains 2 species that can be interconverted by the transfer of a proton (H+)

Question 4

What is the conjugate acid-base pair for:
HCl (aq) <=> H+ (aq) + Cl- (aq)

Answer 4

• In the forward direction, HCl releases a proton to form its conjugate base, Cl-
• In the reverse direction, cl- accepts a proton to form its conjugate acid, HCl

Question 5

What are the conjugate acid-base pairs in:
HCl (aq) + OH- (aq) <=> H2O (l) + Cl- (aq)

Answer 5

Acid 1 - HCl
Base 1 - Cl -
Acid 2 - H2O
Base 2 - OH-

Question 6

What is a monobasic acid?

Answer 6

An acid that can donate a total of 1 ion per molecule of acid in an acid-base reaction

Question 7

What is a dibasic acid?

Answer 7

An acid that can donate a total of 2 hydrogen ions per molecule of acid in an acid-base reaction

Question 8

What is a tribasic acid?

Answer 8

An acid that can donate a total of 3 hydrogen ions per molecule of acid in an acid-base reaction

Question 9

What is a spectator ion?

Answer 9

Ions that do not change during the reaction - they are the same in the products as they were in the reactants

Question 10

2H+ + Zn ->

Answer 10

Zn 2+ + H2

Question 11

2H+ + CuCO3 ->

Answer 11

Cu 2+ + H2O + CO2

Question 12

2H+ + MgO ->

Answer 12

Mg2+ + H2O

Question 13

How can the dissociation of any weak acid be shown?

Answer 13

HA (aq) <=> H+ (aq) + A- (aq)

Question 14

For equation HA (aq) <=> H+ (aq) + A- (aq) : what is the acid dissociation constant calculated at?

Answer 14

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

Question 15

What temperature of Ka values usually standardised at?

Answer 15

25 degrees

Question 16

What does the larger the Ka value mean?

Answer 16

• The further equilibrium is to the right
• The greater dissociation and the greater the acid strength

Question 17

How do you convert between Ka and pKa?

Answer 17

pKa = -log10 Ka
Ka = -10^pKa

Question 18

The stronger the acid, the _________ the Ka value

Answer 18

Larger

Question 19

How do you find pH from the concentration of hydrogen ions?

Answer 19

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

Question 20

What is Kw?

Answer 20

The ionic product of water - it has avlue of 1 x 10^-14 mol^2 dm^-6

Question 21

How do you find the pH of water?

Answer 21

H2O (l) <=> H+(aq) + OH-(aq)

so Ka = [H+] [OH-] / [H2O]
this becomes Ka x [H2O] = [H+][OH-]

the concentration of H2O is constant so
Kw = [H+][OH-]

On dissociation, water is neutral - it produces the same number of H+ and OH- ions

so Kw = [H+]^2
[H+] = √ 1 x 10^-14 = 1 x10^-7
pH = -log 10 [H+] = 7

Question 22

What is the concentration of a strong acid equal to?

Answer 22

Its H+ concentrations

[HA] = [H+]

Question 23

What can the pH of a strong base be calculated from?

Answer 23

• The concentration of the base
• The ionic product of water

[H+] = Kw / [OH-]
pH = -log 10 [H+]

Question 24

What does the concentration of a strong monobasic alkali equal to?

Answer 24

[OH-]

Question 25

How can you calculate the pH of a weak acid?

Answer 25

• The concentration of the acid
• The Ka value of the acid

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

The equilibrium concentrations of [H+] and [A-] will be the same since one molecule of HA dissociates to each ion

Ka x [HA] = [H+]^2
pH = -log 10 [H+] = √ Ka x [HA]

Question 26

What are assumptions that are made when finding concentrations and pHs?

Answer 26

• HA dissociates to produces equilibrium concentrations of H+ and A- that are equal ; issue is that a tiny percentage of H2O molecules dissociate so the concentrations are only roughly equal
• [HA] original = [HA] equilibrium ; issue is this means there is no dissociation but this cant be true as it must dissociate to form an acid