Acid-base equilibria Flashcards
What is the Bronsted-Lowry acid-base theory?
Bronsted-Lowry acid-base theory is the theory in which:
Acids are defined as proton donors.
Bases are defined as proton acceptors.
What is a monoprotic acid/monobasic
an acid that can donate one proton per molecule (eg. HCl)
What is a diprotic acid?
A diprotic, or dibasic, acid can donate two protons per molecule (eg. H2SO4)
How is pH defined?
-log[H+], usually just through the equation pH = -log[H+]
What do strong acids do when in solution?
Strong acids fully ionise (dissociate) to produce H+ ions when in solution in water.
What do weak acids do in solution?
Weak acids partially ionise (dissociate) to produce H+ ions when in solution in water
What is a conjugate base
it is formed when a base donates a proton
What is a conjugate acid
it is formed when a base accepts a proton
What is the Kw equation
Kw = [H+][OH-] / [H2O]
is H2O included in the Kw equation? why?
No, because it is a liquid in its natural state and so doesn’t match the products that are aqueous, so we don’t include it so the Kw equation is homogeneous
How is pKa defined
mathematically, pKa = -log(Ka)
what is the acid dissociation constant?
Ka = [H+][A-] / [HA], (where the weak acid is denoted as HA, and its salt is denoted as A-)
What is a buffer solution?
a mixture of a weak acid and its salt, that will resist changes in pH when small amounts of acid or alkali are added
What is the enthalpy of neutralisation
enthalpy change when one mole of water is formed by neutralisation between an acid and a base, under standard conditions (298K, 100kP), with all reactants and products in their standard states.
what is a conjugate acid base pair
a pair of species on opposite sides of an equation which are related, and differ only by their number of protons.
What dp is pH given to
2 dp
You have 1.5 M solution of a weak acid, HA. Its pKa is 2.88. Find its pH.
- Use the Ka equation -> Ka = [H+][A-] / [HA]
- 10^-pKa = Ka = 1.32x10^-3
- 1.32 x 10^-3 = [H+]^2 / 1.5
- rearrange to find [H+]^2 = 1.5 x 1.3x10^-3
- Find [H]+
- calculate pH
Answer = 1.35
What is the Ka equation
Ka = [H+][A-] / [HA]
What is the anatomy of a pH curve graph
- Half equivalence point
- equivalence point
- Buffer region (in weak acids)
- At the half equivalence point, pH = pKa
What is the equivalence point on a pH curve?
The straight portion of a graph, appears when the same amount of moles (assuming they act in a 1:1 ratio) of acid and alkali have been added.
It should always pass through pH 7.00
How is half equivalence point determined?
- it is half the quantity of acid/alkali added compared to the equivalence point
What does a STRONG ACID STRONG ALKALI pH curve graph look like
- long straight portion in the middle
- very high pH for alkali and very low pH for acid
What does a STRONG ACID WEAK ALKALI pH curve graph look like?
- Low pH for acid
- Weaker pH for alkali than a strong one
- middle length of straight portion in the middle
What does a WEAK ACID STRONG ALKALI pH curve graph look like?
- higher pH for acid than a strong one
- High pH for alkali
- middle length of straight portion in the middle
What does a WEAK ACID WEAK ALKALI pH curve graph look like?
- High pH for acid, low pH for alkali
- very short straight portion in the middle
What are indicators?
They are usually weak acids that have two colours; one in protonated species ( H+ still attached) and the other in deprotonated species (H+ species removed)
Why do we only add a few drops of indicator?
- Indicators are weak acids themselves, and so they will start to shift the equilibrium depending on what environment they are in (acid or alkali)
