Acids and Bases Flashcards
what is a Bronsted-Lowry acid?
- a proton donor
what is a Bronsted-Lowry alkali?
- a proton acceptor
what is a weak acid/base?
- an acid/base that only slightly dissociates into ions in aqueous solutions
Describe and Explain how temperature affects Kw
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
what is a strong acid/base?
- an acid/base that completely dissociates into ions in aqueous solutions
how does the strength of an acid/base link to equilibrium?
- the weaker the acid/base, the less it dissociates and the more the equilibrium lies to the left
what is the equation for Kw in pure water and why?
Kw = [H⁺]²
- in pure water, the concentration of hydrogen ions is equal to the concentration of hydroxide ions
what is an amphoteric substance
- a substance that can act as both an acid and a base
what is Kw and what is its value at 298K
- Kw is the ionic product of water
- at 298K, Kw = 1x10⁻¹⁴mol²dm⁻⁶
what equation links pH and the concentration of hydrogen ions?
pH = -log₁₀[H⁺]
How to work out [H⁺] from pH
[H⁺] = 10⁻ᵖᴴ
what is the equation for Kw
Kw = [OH⁻][H⁺]
how to calculate the dilution of a strong acid
- calculate the moles of H⁺
- convert to conc by dividing by volume
- pH = -log₁₀[H⁺]
how to calculate the dilution of a strong base
- calculate moles of OH⁻
- convert to conc by dividing by new volume
- Kw/[OH⁻]=[H⁺]
-pH = -log₁₀[H⁺]
how to calculate the pH of mixtures of strong acids and bases
- 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⁺]
what is the equation for Ka and what are its units?
- Ka = [H⁺][A⁻]/[HA]
- moldm⁻³
how does the Ka value correlate with equilibrium
- larger the Ka value, the further the equilibrium is to the right
how does the Ka value correlate with the strength of the acid
- higher the Ka value , the stronger the acid
what equations links pKa with Ka
- pKa = -logKa
- Ka = 10⁻ᵖᴷᵃ
what is the expression for Ka for most weak acids and why
- Ka = [H⁺]²/[HA]
- since the acid barely dissociates, HA barely changes at eq and [H⁺] = [A⁻] as the molar ratio is 1:1
describe how to calibrate a pH meter
- 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)
describe how to use a pH meter to plot a pH curve
- 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
what is the equivalence point of a pH curve
- where moles of alkali added is equal to moles of acid
why is it hard to draw a pH curve for a weak acid weak base titration
- there is no vertical section on the curve
- there is no sharp end point so no equivalence point
what is the half equivalence point on a pH curve
- when the conc of acid = conc of base
what is true at the half equivalence point
pH = pKa
- moles of base is half moles of acid
what is a buffer solution
- solutions that maintain a constant pH despite the addition of acid, base or water
describe two ways to make an acidic buffer
- adding a weak acid to one of its salts
- adding a strong base to an excess of a weak acid
explain why adding an acid to an acidic buffer doesn’t change the pH
- HA ⇌ H⁺ + A⁻
- added H⁺ reacts with A⁻
- eqm shifts to left to decrease H⁺
describe two ways to make a basic buffer
- adding a weak base to one of its salts
- adding an excess of weak base to a strong acid
explain why adding an acid to a basic buffer doesn’t change the pH
- NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
- added H⁺ reacts with OH⁻
- eqm shifts to right to replace OH⁻
explain why adding a base to a basic buffer doesn’t change the pH
- NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
- added OH⁻ reacts with H⁺
- eqm shifts to left to decrease OH⁻
what happens when a strong base (A⁻) is added to a buffer
- moles of HA decreased
- moles of A⁻ increased
what happens when a strong acid (H⁺) is added to a buffer
- moles of HA increased
- moles of A⁻ decreased
