✅12 - Acid Base Equilibria Flashcards
What is a Bronsted-Lowry acid?
A proton donor
What is a Bronsted-Lowry base?
A proton acceptor
What is a conjugate acid?
A species formed by the reception of a proton (H+) by a base in other words, it is a base with a hydrogen ion added to it.
What is a conjugate base?
What is left after an acid has donated a proton in a chemical reaction.
What is a diprotic/dibasic acid?
One which can donate two protons, such as H2SO4
What is a diprotic/diacidic base?
One which can accept two protons, such as CO3 2-
What are amphoteric substances?
One which can act either as an acid or a base, such as water
What is a strong acid?
One which is almost completely dissociated in aqueous solution
How are strong acids represented?
With a single arrow in the dissociation equation, not reversible
What is a weak acid?
One that is only partially dissociated in aqueous solution
How are weak acids represented?
With a reversible arrow in the dissociation equation
How are hydrogen ion concentration and and concentration linked for strong acids?
Directly, so HCl of 0.100 moldm-3 will have an H+ concentration of 0.100 moldm-3
How would you calculate pH from H+ concentration?
pH = -log [H+]
How would you calculate [H+] from pH?
[H+] = 10^-pH
What is Ka?
The acid dissociation constant
What is the equation to work out Ka of a weak acid?
[H+] x [A-]
_______
[HA]
What can the expression for Ka be simplified to?
[H+]^2
_____
[HA]
How can you work out [H+] for a weak acid from Ka?
By rearranging the equation to find a value for [H+]^2, then square rooting the value.
How would you work out pKa?
-log Ka
The larger the value of Ka…
…the stronger the acid
The larger the value of pKa…
…the weaker the acid
How would you calculate the pH of a dibasic acid?
Ka HSO4 = 0.0100
In 0.500 moldm-3 solution of H2SO4, contribution to [H+] from H2SO4 will be 0.500moldm-3. If we assume that contribution to [H+] from HSO4- is (x)moldm-3:
Ka(HSO4-) = 0.0100 = (0.500- x) x/0.500
Solving this equation gives x = 0.0098
So total [H+] = 0.05098 with a pH of 0.293
What is Kw?
The ionic product of water
What is the equation to calculate Kw?
[H+] x [OH-]
_________
[H2O]
What can the equation for Kw be simplified to?
[H+] x [OH-], as pure water is neutral
How would you calculate pKw?
-log Kw
Why do even the most alkaline solutions have some hydrogen ions?
Because water ionises to form H+ and OH-
How would you calculate the pH of a strong base?
Find the concentration of the base
Divide the Kw by the base concentration
This will give you [H+], so you can then calculate pH
How can the relative strengths of different acids/bases be determined?
By measuring the pH of equimolar aqueous solutions of the acids/bases, at the same temperature
What is the pH of a salt of a strong acid and strong base?
7
What is the pH of a salt of a weak acid and strong base?
> 7, alkaline
What is the pH of a salt of a strong acid and weak base?
<7, acidic
How does dilution affect the pH of solutions of strong acids?
pH increases by one for each 10-fold dilution
How does dilution affect the pH of solutions of weak acids?
pH increases by 0.5 for each 10-fold dilution
How can Ka of a weak acid be determined experimentally?
By dissolving a known mass of acid in a small volume of water and making it up to a known volume, then measuring its pH
Calculate concentration of solution
Work backwards to calculate Ka
What is the end point of a titration?
When the acid and base are mixed in exactly the right proportions to react in equivalent amounts
What is the equivalence point of a titration?
When the acid and base have reacted together in the exact proportions dictated by the stoichiometric equation
What does the titration curve for strong acid strong base look like?
Starts at high pH, sudden drop as equivalence point reached, ends at low pH
What does the titration curve for weak acid strong base look like?
Starts at high pH, equivalence point around pH 8/9, shorter middle section, ends at pH 3/4
What does the titration curve for strong acid weak base look like?
Starts at pH 11, slight decrease in pH before equivalence point, around pH 5/6, ends around pH 1
What does the titration curve for weak acid weak base look like?
Starts around pH11, no steep section, short, equivalence point at pH 7, ends around pH 4/5
What is the point of inflexion?
The point on a weak acid/base titration curve where the equivalence point lies, no steep section
For a strong acid/weak base titration, why does the pH fall sharply at first but then level off?
Because a buffer solution is formed
What is the equation for K(in)?
K(in) = [H+] x [In-]
________
[HIn]
What is the rule of thumb for when indicators change colour?
The red colour of methyl orange will first predominate when [HIn] is ten times [In-], and the yellow colour will predominate when [In-] is ten times [HIn].
How can the approximate pH for when each colour predominates be calculated?
When [HIn] = 10[In-]:
[H+] x [In-]
________ = 2.00 x 10^-4 mol dm-3 (K(In))
10[In-]
[H+] + 2.00 x 10^-3
A good indicator shows a colour change when…
…one drop of acid is added from the burette to accurately determine the end point of titration
What is a buffer solution?
A solution whose pH remains the same when small amounts of acid are base are added
What are the two most common ways of making a buffer solution?
Mixing a weak acid with its conjugate base
Mixing a weak base with its conjugate acid
What is the simplest example of a buffer solution?
Ethanoic acid and sodium ethanoate
How can the pH of a buffer solution be calculated?
Find Ka
Then find [H+] by rearranging the equation for Ka with the acid or base and the conjugate ion
Use [H+] to calculate pH
How does a buffer solution work when acid is added?
When small amounts of acid are added, majority of H+ react with the CH3COO- ions to form CH3COOH molecules.
How does a buffer solution work when base is added?
Majority of OH- added react with CH3COOH to form CH3COO- and H2O
What is the Henderson-Hasselbalch equation?
[H+] = Ka x [acid]
____
[salt]
What does the Henderson-Hasselbalch equation show?
The pH of a buffer solution
How would you make a buffer solution with a pH less than 7?
A mixture of a weak acid and its conjugate base
How would you make a buffer solution with a pH more than 7?
A mixture of a weak base and its conjugate acid
How can Ka be determined from a pH titration curve?
At the half equivalence point, pH = pKa
What are the two processes involved in neutralisation?
Dissociation (endothermic) and hydration (exothermic)
