Acids and Bases

Question 1

what is a Bronsted-Lowry acid?

Answer 1

• a proton donor

Question 2

what is a Bronsted-Lowry alkali?

Answer 2

• a proton acceptor

Question 3

what is a weak acid/base?

Answer 3

• an acid/base that only slightly dissociates into ions in aqueous solutions

Question 4

Describe and Explain how temperature affects Kw

Answer 4

Kw increases with temperature
- equilibrium shifts to the endothermic direction to oppose the increase in temperature
- this leads to higher hydroxide and hydrogen ion concentrations which equal a higher Kw value

Question 5

what is a strong acid/base?

Answer 5

• an acid/base that completely dissociates into ions in aqueous solutions

Question 6

how does the strength of an acid/base link to equilibrium?

Answer 6

• the weaker the acid/base, the less it dissociates and the more the equilibrium lies to the left

Question 7

what is the equation for Kw in pure water and why?

Answer 7

Kw = [H⁺]²
- in pure water, the concentration of hydrogen ions is equal to the concentration of hydroxide ions

Question 8

what is an amphoteric substance

Answer 8

• a substance that can act as both an acid and a base

Question 9

what is Kw and what is its value at 298K

Answer 9

• Kw is the ionic product of water
• at 298K, Kw = 1x10⁻¹⁴mol²dm⁻⁶

Question 10

what equation links pH and the concentration of hydrogen ions?

Answer 10

pH = -log₁₀[H⁺]

Question 11

How to work out [H⁺] from pH

Answer 11

[H⁺] = 10⁻ᵖᴴ

Question 12

what is the equation for Kw

Answer 12

Kw = [OH⁻][H⁺]

Question 13

how to calculate the dilution of a strong acid

Answer 13

• calculate the moles of H⁺
• convert to conc by dividing by volume
• pH = -log₁₀[H⁺]

Question 14

how to calculate the dilution of a strong base

Answer 14

• calculate moles of OH⁻
• convert to conc by dividing by new volume
• Kw/[OH⁻]=[H⁺]
  -pH = -log₁₀[H⁺]

Question 15

how to calculate the pH of mixtures of strong acids and bases

Answer 15

• calculate the moles of H⁺
• calculate moles of OH⁻
• work out whats in excess
• convert to concentration by dividing by volume
• use Kw = [OH⁻][H⁺] equation if OH in excess
• -pH = -log₁₀[H⁺]

Question 16

what is the equation for Ka and what are its units?

Answer 16

• Ka = [H⁺][A⁻]/[HA]
• moldm⁻³

Question 17

how does the Ka value correlate with equilibrium

Answer 17

• larger the Ka value, the further the equilibrium is to the right

Question 18

how does the Ka value correlate with the strength of the acid

Answer 18

• higher the Ka value , the stronger the acid

Question 19

what equations links pKa with Ka

Answer 19

• pKa = -logKa
• Ka = 10⁻ᵖᴷᵃ

Question 20

what is the expression for Ka for most weak acids and why

Answer 20

• Ka = [H⁺]²/[HA]
• since the acid barely dissociates, HA barely changes at eq and [H⁺] = [A⁻] as the molar ratio is 1:1

Question 21

describe how to calibrate a pH meter

Answer 21

• rinse probe with deionised water, dry and then place in a certain pH solution
• wait for reading to remain steady then record pH
• take probe out, rinse , dry and place in another pH solution, then record for pH again
• plot calibration curve ( pH of buffer on x axis, measured pH on y axis)

Question 22

describe how to use a pH meter to plot a pH curve

Answer 22

• measure pH of known volume of acid before any base is added from burette
• add the base from burette in small portions and then stir
• record pH after each addition
• after the endpoint (colour change) add base from the burette until it is in excess
• plot a graph: vol of NaOH added on x axis against measured pH on y axis

Question 23

what is the equivalence point of a pH curve

Answer 23

• where moles of alkali added is equal to moles of acid

Question 24

why is it hard to draw a pH curve for a weak acid weak base titration

Answer 24

• there is no vertical section on the curve
• there is no sharp end point so no equivalence point

Question 25

what is the half equivalence point on a pH curve

Answer 25

• when the conc of acid = conc of base

Question 26

what is true at the half equivalence point

Answer 26

pH = pKa
- moles of base is half moles of acid

Question 27

what is a buffer solution

Answer 27

• solutions that maintain a constant pH despite the addition of acid, base or water

Question 28

describe two ways to make an acidic buffer

Answer 28

• adding a weak acid to one of its salts
• adding a strong base to an excess of a weak acid

Question 29

explain why adding an acid to an acidic buffer doesn’t change the pH

Answer 29

• HA ⇌ H⁺ + A⁻
• added H⁺ reacts with A⁻
• eqm shifts to left to decrease H⁺

Question 30

describe two ways to make a basic buffer

Answer 30

• adding a weak base to one of its salts
• adding an excess of weak base to a strong acid

Question 31

explain why adding an acid to a basic buffer doesn’t change the pH

Answer 31

• NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
• added H⁺ reacts with OH⁻
• eqm shifts to right to replace OH⁻

Question 32

explain why adding a base to a basic buffer doesn’t change the pH

Answer 32

• NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
• added OH⁻ reacts with H⁺
• eqm shifts to left to decrease OH⁻

Question 33

what happens when a strong base (A⁻) is added to a buffer

Answer 33

• moles of HA decreased
• moles of A⁻ increased

Question 34

what happens when a strong acid (H⁺) is added to a buffer

Answer 34

• moles of HA increased
• moles of A⁻ decreased