12. Acids and Bases Flashcards
What is a Bronsted-Lowry acid?
A proton donor
What is a Bronsted-Lowry base?
A proton acceptor
When can protons only be given up by acids?
When there is a base that accept them
What do reactions between acids and bases involve?
Proton transfer
Where do most common acid-base reactions take place?
In aqueous solution
How are acids and bases generally defined? Why is this?
By the way they react with water - as most acid-base reactions take place in aqueous solution
What does how strong an acid/base is depend on?
How much they dissociate in water
How much do strong acids dissociate in water?
Almost completely dissociate
What is another phrase for ‘dissociates’?
Ionises
General equation for strong acids/bases dissociating in water?
HA ⇋ H⁺ + A⁻
How much do weak acids dissociate in water?
Only slightly dissociate
Example of a strong acid dissociating in water?
HCl ⇋ H⁺ + Cl⁻
Example of a strong base dissociating in water?
NaOH ⇋ Na⁺ + OH⁻
Example of a weak acid dissociating in water?
CH₃COOH ⇋ CH₃COO⁻ + H⁺
Example of a weak base dissociating in water?
NH₃ + H₂O ⇋ NH₄⁺ + OH⁻
Is the strength of an acid/base different to its concentration?
Yes
What does the acidity of a solution depend on?
The concentration of H⁺ (aq) ions
What is pH defined as?
-log₁₀[H⁺]
What type of scale is the pH scale? Why is this?
Logarithmic, as the concentration of hydrogen ions in aqueous solution covers a wide range
When there is a smaller pH, what will be true of the concentration of H⁺ ions?
Higher concentration of H⁺
What types of acid are there?
- monoprotic
* diprotic
What is a monoprotic acid?
One that releases one H⁺ ion per molecule
What is a diprotic acid?
One that releases two H⁺ ions per molecule
Is HCl monoprotic or diprotic?
Monoprotic
Is H₂SO₄ monoprotic or diprotic?
Diprotic
In strong acids, what is the moles of a monoprotic acid equal to? Why?
The moles of H⁺, therefore [H⁺] = conc. of acid, because strong acids fully dissociate
In strong diprotic acids, what are the moles of H⁺ equal to?
Double the moles of acid, therefore [H⁺] = double conc. of acid
What effect do diprotic acids have on pH compared to monoprotic?
Results in lower pH (stronger acid)
How many decimal places is pH given to?
2
How can pH be used to calculate the concentration of H⁺?
[H⁺] = 10⁻ᵖᴴ
If pH is used to find [H⁺] of a monoprotic acid, what is the concentration of the acid?
Conc. of H⁺ = conc. of acid
If pH is used to find [H⁺] of a diprotic acid, what is the concentration of the acid?
Conc. of H⁺ must be halved to determine conc. of acid
What effect will diluting a strong acid have?
- reduce its concentration
- hence concentration of H⁺ ions
- increasing pH slightly
How would you calculate the pH of the solution when 100 cm³ water is added to 50 cm³ of 0.1 moldm⁻³ HNO₃
[H⁺] original solution = 0.1
[H⁺] diluted solution = 0.1 x (original volume/total volume)
= 0.1 x 50/150 = 0.0333
pH = -log(0.0333) = 1.48
How would you calculate the pH when an original solution is diluted with water?
[H⁺] of diluted solution = [H⁺] of original solution x (original volume/total volume)
pH = -log(ANS)
What does it mean, that water is slight dissociated?
It exists not just as molecules but in equilibrium
What is the equation for dissociation of water?
H₂O ⇋ H⁺ + OH⁻
Do H⁺ ions exist in water alone?
No
What do H⁺ ions exist as in water?
Hydroxonium ions (H₃O⁺)
Equation for hydroxonium ions?
H₃O⁺
What equation can be used to show how water acts as a base?
2H₂O ⇋ H₃O⁺ + OH⁻
What is the equilibrium expression for the dissociation of water?
Kc = [H⁺] [OH⁻] / [H₂O]
Why is the Kc of water the same as Kw?
The concentration of water is constant
What is the equation for Kw?
Kw = [H⁺(aq)] [OH⁻(aq)]
What is Kw also called?
The ionic product of water
What is the value of Kw at 298K?
1 x 10⁻¹⁴
What is true of the concentration of H⁺ and OH⁻ in pure water?
They must be equal
What does it mean for the Kw equation, that H⁺ and OH⁻ concentrations are equal?
Expression simplified to Kw = [H⁺]²
What type of reaction is the dissociation of water?
Equilibrium
What would happen to the pH of water if the temperature was increased?
H₂O ⇋ H⁺ + OH⁻ (ΔH = 57.3 kJmol⁻¹)
- equilibrium shifts right in endothermic reaction to lower temperature
- which increases concentration of H⁺ ions
- so decreases pH
Why is water always neutral?
The number of H⁺ and OH⁻ ions are always equal, despite pH varying
Is water always neutral?
Yes
Is neutral always at pH 7?
No
What is the pH of water dependent on?
Temperature
What happens if a base is dibasic, when calculating the pH of a strong base?
[OH⁻] is twice the concentration of the base
When calculating the concentration of a strong base given the pH, what is the concentration of OH⁻ ions equal to?
- the concentration of the monobasic base
* dibasic base - concentration of OH⁻ needs to be halved to determine concentration of base
What are weak acids and bases?
Those that are only partially dissociated when dissolved in water
What does it mean, that weak acids and bases only partially dissociate in water?
Not all of the molecules split up into ions
Is the extent of dissociation of a substance linked to concentration?
No
Why is it possible to have a high concentration of a weak acid/base?
The extent of dissociation is not linked to concentration
What is the equilibrium equation for when a weak acid dissociates?
HA ⇋ H⁺ + A⁻
For a weak acid, where will equilibrium lie?
To the left
What is the equilibrium constant for a weak acid called?
The acid dissociation constant (Ka)
What is Ka?
The acid dissociation constant
What is the acid dissociation constant (Ka)?
The equilibrium constant for a weak acid
How is Ka written?
[H⁺] [A⁻] / [HA]
or
[H⁺]² / [HA]
How are the units for Ka worked out?
The same as other equilibrium constants - cancelling using the equation
What will a stronger acid mean for Ka?
Larger value of Ka
What will a larger value of Ka mean for the strength of an acid?
Stronger acid
Why is an acid stronger with a larger value of Ka?
More H⁺ is dissociated and equilibrium lies further to the right
In weak acids, is the concentration of acid equal to the [H⁺]?
No
What does it mean, that in weak acids concentration of acid ≠ [H⁺]?
[H⁺] must be calculated
For weak acids, how is [H⁺] calculated?
- looking closer at Ka, for every mole H⁺ dissociated, one mole A⁻ is also dissociated
- so equation becomes Ka = [H⁺] / [HA]
Why does the equation for Ka become [H⁺] / [HA]?
In a weak acid, for every one mole H⁺ dissociated, one mole of A⁻ is also dissociated
What type of acid is ethanoic acid?
Weak acid
What can be assumed about the concentration of ethanoic acid initially and at equilibrium?
As it is a weak acid, it can be assumed that concentrated at equilibrium = initial concentration
When is pKa more convenient to use?
When Ka is very small
How is pKa calculated from Ka?
pKa = -log₁₀Ka
What can pKa measure?
How strong or weak an acid is
What does a smaller value of pKa mean for the strength of an acid?
Stronger acid
Are there units for pKa?
No
How is Ka calculated from pKa?
Ka = 10⁻ᵖᴷᵃ
Are pH calculations involving weak bases required at A-level?
No
Are carboxylic acids strong or weak?
Weak
Examples of strong acids?
HCl, HNO₃ and H₂SO₄
Example of a weak base?
NH₃
Examples of strong bases?
NaOH, KOH and Ba(OH)₂
In an acid-base titration, what is the acid added from?
A burette
What is added to what in an acid-base titration?
Acid is added from a burette to a measured volume of alkali using a pipette
Steps to calculate the pH of a mixture of strong acids and strong bases?
- Calculate moles H⁺
- Calculate moles OH⁻
- Determine which in XS (biggest moles)
- Calculate moles H⁺/OH⁻ in XS
- Calculate [H⁺] or [OH⁻] in XS
- Calculate pH
When a weak acid and strong base react, what is true for every mole of OH⁻ added?
For every mole OH⁻ added, one mole HA is used up and one mole A⁻ is formed
Steps to calculate pH of a mixture of weak acids with strong bases, when HA is in excess?
- Calculate moles HA (it is still HA not H⁺ as it’s weak)
- Calculate moles OH⁻
- Determine which in XS
- Calculate moles HA/OH⁻ in XS
- Calculate moles HA left and A⁻ formed
- Calculate leftover [HA] and [A⁻] fomed
- Use Ka to find [H⁺]
- Find pH
Steps to calculate pH of a mixture of weak acids with strong bases, when OH⁻ is in excess?
- Calculate moles HA (it is still HA not H⁺ as it’s weak)
- Calculate moles OH⁻
- Determine which in XS
- Calculate moles HA/OH⁻ in XS
- Calculate [OH⁻]
- Use Kw to find [H⁺]
- Find pH
When a weak acid reacts with a strong base, when does [HA] = [A⁻]?
When half of the HA molecules have reacted with OH⁻
What is the half equivalence point?
When a weak acid and strong base react, and half the HA molecules react with OH⁻ so that [HA] = [A⁻]
What does Ka equal at the half equivalence point?
Ka = [H⁺]
What does it mean, that at the half equivalence point Ka = [H⁺]?
pH at half equivalence = pKa of the weak acid
What can a pH meter be used to measure?
Changes in pH at points before and after the end point in a titration
What can be done after a pH meter measures the change in pH at points before and after the endpoint?
A graph of pH against volume of solution can be plotted
Is the shape for a pH curve of a titration linear?
No
Suitable indicator for strong acid - strong base reaction?
Phenolphthalein
Suitable indicator for strong acid - weak base reaction?
Methyl orange
Suitable indicator for weak acid - strong base reaction?
Phenolphthalein
Suitable indicator for weak acid - weak base reaction?
Phenolphthalein
What is the equivalence point?
When sufficient base has been added to just neutralise the acid
What does the end-point refer to?
The indicator
What is the end-point?
The volume at which there are exactly the same moles of H⁺ ions as OH⁻ ions
Is the pH at the end-point exactly 7?
No
In terms of equivalence points, what do diprotic acids result in? Why is this?
They have two equivalence points, due to released two protons but at different times
What would an inappropriate indicator result in?
An end point being observed when equivalence has not yet been reached - or comes after equivalence
When is the indicator for a particular titration classed as suitable?
- when there’s a sharp colour change at end point
- when end point of titration is provided by colour change of indicator
- when indicator gives a distinct colour change
What are indicators?
Weak acids where HA and A⁻ are different colours
HA ⇋ H⁺ + A⁻
What colour is HA in methyl orange?
Red
What is the pH range of colour change for methyl orange?
3.2 - 4.4
What colour is A⁻ in methyl orange?
Yellow
What colour is HA in phenolphthalein?
Colourless
What is the pH range of colour change for phenolphthalein?
8.2 - 10.0
What colour is A⁻ in phenolphthalein?
Pink
In a titration, what happens when the last drop of acid/alkali is added?
The pH rapidly changes at equivalence
What must be true for an indicator to change colour?
Moles of acid is equal to moles of base
Why is universal indicator not suitable for a titration?
It is a mixture of indicators, so shows many colours at different pHs and gradual colour change
What is a buffer solution?
A solution that maintains an approximately constant pH despite dilution or addition of small amounts of acid/base
What are buffer solutions designed to do?
Keep the concentration of hydrogen and hydroxide ions in a solution almost unchanged
What do buffer solutions rely on to do their job?
An equilibrium reaction - which will move in the direction to remove hydrogen and hydroxide ions added
Does adding an acid/alkali in small amounts change the pH of a buffer solution?
Yes, but the pH will only change slightly
What do basic buffers contain?
• a weak base and the salt of that weak base
or
• an excess of weak alkali with a strong acid
What pH do basic buffers maintain?
A pH higher than 7
Example of substances to form a basic buffer?
Ammonia (NH₃) and ammonium chloride (NH₄Cl)
What happens when NH₃ and NH₄Cl are used as a basic buffer?
- salt fully dissociates: NH₄Cl ⇋ NH₄⁺ + Cl⁻
* ammonia reacts with water: NH₃ + H₂O ⇋ NH₄⁺ + OH⁻
What happens to the weak base and its salt in a basic buffer?
- the salt fully dissociates
* the weak base reacts with water to form a weak alkaline solution
What happens when alkali is added to an ammonia and ammonium chloride base?
[NH₃ + H₂O ⇋ NH₄⁺ + OH⁻]
- increase concentration of OH⁻ ions
- causing equilibrium to shift left to remove added OH⁻ ions
- resulting in concentration of weak alkali (ammonia) to increase
What happens when acid is added to an ammonia and ammonium chloride base?
[NH₃ + H₂O ⇋ NH₄⁺ + OH⁻]
- it will react with OH⁻ ions
* so equilibrium shifts right to counteract the change
Example of a buffer than contains an excess of weak alkali with a strong acid?
NH₃ and HCl
Equation for an NH₃ and HCl buffer?
NH₃ + HCl ⇋ NH₄⁺ + Cl⁻
What is the key in a basic buffer solution?
The concentration of the base and salt are much higher than the concentration of OH⁻ ions
What do acidic buffers contain?
• a weak acid and the salt of that weak acid
or
• an excess of weak acid with strong alkali
What pH do acidic buffers maintain?
One that is lower than 7
What happens to a weak acid in solution?
Slightly dissociates:
HA ⇋ H⁺ + A⁻
Overall, what will an acidic buffer contain?
A lot of HA (undissociated weak acid), a lot of A⁻ ions and very little H⁺ ions
Example of an acidic buffer?
Ethanoic acid and sodium ethanoate
What happens when alkali is added to an acidic buffer?
[HA ⇋ H⁺ + A⁻]
- OH⁻ ions will react with H⁺ forming water
- causing equilibrium to shift right to replace reacted H⁺
- therefore decreasing the concentration of the undissociated acid
What happens when acid is added to an acidic buffer?
[HA ⇋ H⁺ + A⁻]
- equilibrium shifts left to remove acid
* therefore concentration of HA↑ and A⁻↓
What is the function of the salt component of the acidic buffer?
To act as a source of A⁻ ions (-ve ion of the weak acid) which can remove any added H⁺ ions
Why can an acidic buffer be formed from an excess of weak acid and strong alkali?
As the acid is in excess, the solution contains some of the salt of the weak acid
What is the key in an acidic buffer solution?
The concentration of the acid and salt are much higher than the concentration of H⁺ ions
If half the acid in an acidic buffer is neutralised, what will the resulting solution be?
A buffer with a pH equal to pKa
What is the half equivalence point?
When half the acid is neutralised
