chapter 20

Question 1

What is a Brønsted Lowry acid?

Answer 1

a species that can donate a proton

hydrogen chloride (HCl) is a Brønsted acid as it can lose a proton to form a hydrogen (H+) and chloride (Cl-) ion

Question 2

What is a Brønsted Lowry base?

Answer 2

a species that can accept a proton

A hydroxide (OH-) ion is a Brønsted base as it can accept a proton to form water

Question 3

What is a conjugate acid-base pair?

Answer 3

A conjugate acid-base pair is two species that are different from each other by an H+ ion.

CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq)

Question 4

What does it mean for an acid to be classified as mono basic/dibasic or tribasic?

Answer 4

It means the number of bases that they can donate protons to in a reaction (this depends on how many H+ per molecule that they can give up in a reaction)

Question 5

Example of mono basic acid + what this means

Answer 5

HCL, It contains one ionisable hydrogen atom in each molecule.

Question 6

Example of dibasic acid + what this means

Answer 6

H2SO4, It contains two ionisable hydrogen atoms per molecule. Ionisation occurs in two steps.

Question 7

Example of tribasic acid + what this means

Answer 7

H3PO4, It contains 3 ionisable hydrogen atoms per molecule. Ionisation occurs in 3 steps.

Question 8

Acid + metal =

Answer 8

acid + metal → salt + hydrogen

2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g)

Question 9

acid + metal oxide =

Answer 9

acid + metal oxide → salt + water

2HCl (aq) + CaO (s) → CaCl2 (aq) + H2O (l)

Question 10

acid + metal carbonate =

Answer 10

acid + metal carbonate → salt + water + carbon dioxide

2HNO3 (aq) + CuCO3 (s) → Cu(NO3)2 (aq) + H2O (l) + CO2 (g)

Question 11

acid + alkali =

Answer 11

acid + alkali → salt + water

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

Question 12

What is a weak acid?

Answer 12

A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions.

eg. ethanoic acid/ carbonic acid

(equilibrium is more to the left- there are lots more weak acid molecules than H+ and A- Ions.)

Question 13

What is the equilibrium constant for a weak acid?

Answer 13

Ka

Question 14

What is the equilibrium constant expression for Ka?

Answer 14

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

this can be simplified to [H]^2 / [HA]

Question 15

What are the units of Ka?

Answer 15

mol dm-3

Question 16

A high Ka value means…?

Answer 16

The higher the value of Ka the more dissociated the acid and the stronger it is

Question 17

A low Ka value means….?

Answer 17

The lower the value of Ka the weaker the acid

Question 18

Why do we use pka instead of Ka?

Answer 18

Its difficult to compare numbers with negative indices, so we use pka as it is whole numbers.

Question 19

How do you go from Ka to pKa?

Answer 19

pKa = -log(Ka)

Question 20

How do you go from pKa to Ka?

Answer 20

Ka = 10^-(pKa)

Question 21

A high pKa value means…?

Answer 21

The acid is weak.

Question 22

A low pKa value means…?

Answer 22

The acid is strong and more dissociated.

Question 23

How do you work out pH?

Answer 23

pH= -log[H+]

H+ = conc of hydrogen atoms in moldm-3

Question 24

How do you calculate [H+] with the pH?

Answer 24

[H+] = 10^-pH

Question 25

How much does each pH value differ by?

Answer 25

The pH scale is a logarithmic scale with base 10
This means that each value is 10 times the value below it.

pH 1 = 10^-1
PH 2 = 10^-2

Question 26

What is the ionic product of water?

Answer 26

Kw

Question 27

What’s the value of Kw?

Answer 27

1 x 10^-14 mol^2dm^-6 @ 298k

Question 28

What’s the equation for Kw?

Answer 28

Kw = [H+][OH-] / [H20]

Kw x [H20] = [H+][OH-]

Question 29

What approximations are made with Ka?

Answer 29

HA dissociates to create concentrations of H+ and A-
this is what allows us to write [H+]^2. We also ignore the very small conc of H+ from water dissociating.

As the dissociation of weak acids is small we assume that the concentration of the weak acid is the same at the start and the end.

Question 30

How do you work out pH with Ka?

Answer 30

square root of Ka x [HA] = H+
so you then do -log[H+]

Question 31

How do you calculate the pH of a strong acid?

Answer 31

Strong acids completely ionise within solution.

So [H+] = [HA]

Question 32

How do you calculate the pH of a strong base?

Answer 32

Strong bases completely ionise within solution.

So H+ = Kw (1x10^-14) / [OH-]

You then use the H+ to work out pH.

Kw = [OH-] x [H+]