Acids, Bases and pH (Chapter 20) Flashcards
Describe the Arrhenius model about acids and base
1) acids - dissociate and release H+ ions in aqueous solutions
2) alkalis - dissociate and release OH- ions in aqueous solutions
3) H+ ions are neutralised by OH- ions to form water
What is an alkali?
A soluble base
Define a Bronsted-Lowry acid
A proton donor
Define a Bronsted-Lowry base
A proton acceptor
What does a conjugate acid-base pair contain?
2 species that can be interconverted by transfer of a proton
Describe an example of a conjugate acid-base pair
- HCl (conjugate acid) and Cl- (conjugate base)
- Forward reaction: HCl releases a proton to form Cl-
- Reverse reaction: Cl- accepts a proton to form HCl
What is the equilibrium between HCl and Cl-?
HCl + OH- <=> Cl- + H2O
What does dissociation require in aqueous solution?
Water bc a proton needs to be transferred from an acid to a base
Describe the dissociation of HCl
- HCl + H2O <=> H3O+ + Cl-
- H2O accepts a proton to form the conjugate base, the hydronium ion (H3O+/H+) which is the active acid ingredient in any aqueous acid
What do the words monobasic, dibasic and tribasic refer to?
The total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction
What is the spectator ion?
The ion in an acid which isn’t H+ (normally not include in the ionic equation)
What are the 4 important equations for acids?
1) acid + metal => salt + hydrogen
2) acid + base/alkali => salt + water
3) acid + carbonate => salt + water + CO2
4) H+ + OH- => H2O
What are bases normally
Metal oxides or metal hydroxides
What is the pH equation?
pH = -log10[H+]
What is the equation for [H+]?
10^-pH
What is the effect of pH being a logarithmic scale?
A change of one pH number is equal to 10x the difference in [H+]
Describe the [H+] in strong monoprotic acids
[H+]=[HA] bc strong acids completely dissociate in water
Describe the [H+] in strong diprotic acids
[H+] = 2[HA]
What is the difference between the dissociation equation for weak and strong acids?
Weak acids are in equilibrium, strong acids are not:
HA => H+ + A- vs HA <=> H+ + A-
What is the Ka equation?
Ka = [H+][A-]/[HA]
What are the units of Ka?
Mol/dm3
What is Ka just a version of?
Kc
What does Ka change with?
Temperature (but normally standard at 25 degrees
What is the magnitude of Ka show?
The larger the value of Ka, the further the equilibrium to the right ∴ the greater the dissociation and the acid strength
What is the equation for pKa?
-logKa
What is the equation of Ka?
10^-pKa
Why is pKa used to compare weak acids instead of Ka?
It is difficult to compare numbers with negative indices ∴ Ka is converted into the negative logarithm pKa
What are Ka and pKa used for?
To measure the strength ([H+]) of weak acids
What happens to Ka and pKa with stronger acids?
The stronger the acid, the larger the Ka value and the smaller the pKa value
What happens to Ka and pKa with weaker acids?
The weaker the acid, the smaller the Ka value and the larger the pKa value
Describe the dissociation of sulfurous acid
Sulfurous acid = H2SO3
- H2SO3 <=> H+ + HSO3-
- HSO3- <=> H+ + SO32-
What does [H+] depend on for a weak acid?
The concentration of the acid - [HA] and Ka
What happens when the weak acid HA dissociates?
H+ and A- are formed in equal quantities
What is the concentration of the weak acid HA at equilibrium?
[HA]start - [H+]eqm
What is the concentration of H+/A- at equilibrium?
[H+]eqm and [A-]eqm
What is the complicated equation for the Ka of a weak acid at equilibrium?
Ka = ([H+]eqm x [A-]eqm)/[HA]start-[H+]eqm
What are the two approximations used to simplify the the Ka equation of a weak acid?
1) [H+]eqm = [A-]eqm - bc, even though there will be a very small amount of H+ ([H+]) from the dissociation of water, this can be neglected as it is so small compared with the [H+] from the acid
2) [HA]eqm = [HA]start - as the dissociation of weak acids is small, we can assume that the [HA]start is significantly larger than [H+]eqm and can ∴ neglect any decrease in the [HA] from dissociation
What is the simplified equation for the Ka of a weak acid?
Ka = [H+]^2/[HA]
How can the Ka of a weak acid be determined?
1) prepare a standard solution of the weak acid of known concentration
2) measure the pH using a pH meter
When does approximation 1 ([H+]eqm = [A-]eqm) for the Ka of a weak acid break down?
For very weak acids (pH>6) or very dilute solutions - bc then [H+] from the dissociation fo water will be significant compared with the dissociation from weak acids
When does approximation 2 ([HA]eqm = [HA]start) for the Ka of a weak acid break down?
For stronger weak acids (Ka>10^-2) or very dilute solutions - bc here, [H+] comes significant and there is a different between [HA]eqm and [HA]start-[H+]eqm
Describe the ionisation of water
- Water ionises very slightly, acting as both an acid and a base, setting up the acid-base equilibrium:
- H2O + H2O <=> H3O+ + OH- (H2O <=> H+ + OH-)
- ∴ treating water as a weak acid,
Ka = [H+][OH-]/[H2O] OR Ka x [H2O] = [H+][OH-} - The dissociation of water is very small ∴ 1dm3 of water is mainly undissociated H2O - [H2O] = 55.6mol/dm3 (constant)
Describe Kw
- Kw = ionic product of water (ions in water multiplied together)
- Kw = [H+][OH-]
- Kw varies with temperature - the value of Kw at 298K is 1x10^-14, this value sets up the neutral point on the pH scale
- Kw controls the [H+] and [OH-] in aqueous solutions
How does the Kw equation give pH 7?
1) [H+]=[OH-]
2) ∴ Kw = [H+][OH-]=[H+]^2
3) ∴ [H+] = rootKw = 1 x 10^-7
4) ∴ pH = -log(1 x 10^-7)=7
When is a solution acidic?
When [H+]>[OH-]
When is a solution neutral?
When [H+]=[OH-]
When is a solution alkaline?
When [H+]
What is try about acidic and alkaline solutions?
An acidic solution still contains OH- ions and an alkaline solution still contains H+ ions
What is a strong base?
An alkali that completely dissociates in solution
e.g. NaOH => OH- + Na+ (monobasic)
What are the two ways to calculate the pH of a base?
1) [H+] = Kw/[OH-]
2) pH = 14-pOH where pOH=-log[OH-]
Show how a weak base (NH3) partially dissociates
NH3 + H2O <=> NH4+ + OH-
What happens to the concentration of something when an equal volume of water is added to it?
The concentration halves
