Flashcards in Module 5: 20 & 21 Deck (67)
What is a Bronsted-Lowry acid?
A proton donor
What is a Bronsted-Lowry base?
A proton acceptor
What is a conjugate acid-base pair?
Contains 2 species that be interconverted by transfer of a proton
HCl + H2O ⇌ H3O+ + Cl-
NH3 + H2O ⇌ NH4+ + OH-
What are some common acids and their conjugate bases?
HCl -> Cl-
HNO3 -> NO3-
H2O -> OH-
NH4+ -> NH3
H3O+ -> H2O
Why is water special?
Can act as an acid or a base (accept and donate protons)
What is the type of acid linked to?
The number of hydrogen ions that can be replaced per molecule in an acid-base reaction
Monobasic acid: HCl & CH3COOH
Dibasic Acid: H2SO4
Tribasic acid: H3PO4
What is the general redox reaction involving acids?
Acid + metal -> salt + hydrogen gas (MASH)
What are the 3 neutralisation reactions involving acids?
Acid + alkali -> salt + H2O
Acid + carbonate -> salt + H2O + CO2
Acid + metal oxide -> salt + H2O
How can molecules accept protons?
Lone pairs of electrons -> dative covalent bonds
What is a strong acid?
HA -> H+ + A-
- strong tendency to donate H+ ions
- fully dissociate in aqueous solution
- equilibrium lies to the right
What does the pH scale show?
the relationship between pH and hydrogen ion concentration [H+(aq)]
How is pH calculated?
pH = -log[H+(aq)]
- give all answers to 2 decimals places
How can the pH of a strong acid be calculated?
- directly from the conc of the acid because the acid completely dissociates into ions
- H+ ion conc is equal to the conc of the strong acid in question
HA(aq) -> H+(aq) + A-(aq)
[HA(aq)] = [H+(aq)]
How is the pH changes on dilution calculated (1)?
diluted concentration = original moles / volume after dilution
How is the pH changes on dilution calculated (2)?
diluted concentration = original conc x (volume before dilution / volume after dilution)
What is a weak acid?
HA ⇌ H+ + A-
- weaker tendency to donate H+
- partially dissociate in aqueous solution
- equilibrium lies on the left
- strength of a weak acid is given by the acid dissociation constant (Ka)
How is Ka calculated?
Ka = [H+][A-] / [HA]
What is Ka?
The acid dissociation constant
What do the Ka values represent?
- The larger the value of Ka, the stronger the acid
- The smaller the value of Ka, the weaker the acid
When does Ka change?
Changes with temperature (standard temp is used, 298K)
How is pKa calculated?
pKa = -log(Ka)
How is Ka calculated with pKa values?
Ka = 10^-pKa
How is [H+(aq)] calculated from Ka?
What do the pKa values indicate about the acid?
- the larger the pKa value, the weaker the acid
- the smaller the pKa value, the stronger the acid
What happens if 2 acids are in the same reaction?
- Weaker acids act as a base and accept the proton
- Stronger acid acts as an acid and donates the proton
What 2 approximations does the weak acid pH calculation require?
1. [H+] = [A-] are equal at equilibrium as the dissociation of water is negligible (produces a very small amount of H+ ions)
2. [HA]start = [HA]eqm since dissociation eqm lies on the left, little HA dissociates
- the conc of the undissociated acid is much greater than the [H+] at equilibrium
What happens to the Ka equation when the 2 approximations are applied to it?
Ka = [H+]^2 / [HA]
- concentrations are equal
- little water dissociates so we can ignore any decrease from dissociation
How can Ka of a weak acid be determined experimentally?
- preparing a standard solution of the weak acid of a known concentration [HA]
- measuring the pH of the standard solution using a pH meter [H+] = 10^-pH
When do the approximations break down in validity?
1. breaks down for very weak acids or very dilute solutions (pH>6) as the dissociation of water starts to become significant
2. breaks down for stronger acids with Ka>10^-2