Module 5: pH Flashcards
Give the 3 reactions of acids.
Acid + Base —> Salt + Water
Acid + Metal —> Salt + Hydrogen
Acid + Carbonate —> Salt + Water + CO2
Define strong acids
Fully dissociate in water. Examples: HCl HNO3 (nitric acid) H2SO4 (sulfuric acid)
What do acids do in water?
Acids split apart in water (dissociate in water) and an equilibrium is set up.
What are acids called that donate 1 proton?
Monobasic acids.
E.g:
HCl —> H+ + Cl-
What are dibasic acids?
Acids that donate 2 protons.
Done in 2 stages. E.g:
H2SO4 —> H+ + HSO4-
HSO4- —> H+ + SO4^2-
What are acids called that donate 3 protons?
Tribasic acids. Done in 3 stages. E.g: H3PO4 --> H+ + H2PO4- H2PO4- --> H+ + HPO4^2- HPO4^2- --> H+ + PO4^3-
Define weak acids.
Partially dissociated.
Tend to be organic acids (ethanoic acid).
The equilibrium position lies massively on the left.
Define conjugate acid-base pairs.
An acid-base pair is a set of two related species that transform into each other by the gain or loss of a proton (H+).
Is an acid a proton donor or acceptor?
What about a base?
An acid is a proton donor.
A base is a proton acceptor.
Can you get a conjugate acid-base pair when 2 acids are mixed?
YES.
When 2 acids are mixed, the stronger acid will donate a proton to the weaker acid.
What is the equilibrium constant when acids from an equilibrium in water?
Ka
Do we include water in the equilibrium constant for acid dissociations?
Water is in excess, so is not included in the equilibrium constant.
What does a large value of Ka represent?
A large value of Ka means a high degree of dissociation.
The bigger the value of Ka, the stronger the acid.
What does a small value of Ka represent?
A small value of Ka means a small level of dissociation.
The smaller the value of Ka, the weaker the acid.
Give the equations for calculating:
1) pKa
2) Ka
pKa = -log10Ka (The bigger the value of pKa, the weaker the acid).
Ka = 10^-pKa
Give the equation for calculating:
- pH
- [H+]
pH = -log[H+]
[H+] = 10^-pH
How do you calculate the pH of strong acids?
As strong acids are fully dissociated into H+, this means the [reactants] = [H+]
Example:
HCl —–> H+ + Cl-
Ka= [H+][Cl-] / [HCl]
[HCl] = [H+]
pH=-log10([HCl]
How do you calculate the pH of weak acids?
A weak acid is partially ionised. We need to have the value of Ka to calculate the pH.
Example:
CH3CO2H —–> CH3CO2- + H+
Ka = [CH3CO2-][H+] / [CH3CO2H]
Ka = [H+]^2 / [CH3CO2H]
[H+] = square root (Ka x [CH3CO2H])
pH = -log[H+]
How do you calculate the pH of a strong base?
To do this we use an equilibrium called “the ionic product of water”
Example:
H2O —–> H+ + OH- [H2O] is regarded as a constant,
so is removed from the Kw
equation.
Kw = [H+][OH-] / [H2O]
Kw = [H+][OH-]
Data sheet: Kw = 1x10^-14 mol^2dm^6 at 298K
1x10^-14 = [H+][OH-]
[H+] = 1x10^-14 / [OH-]
pH = -log[H+]
Describe the neutralisation- titration curves for a strong acid and a strong base.
1. Vertical section covers a large change in pH: start at pH3 end at pH 12 2. Equivalence point at pH 7 3. Starts at approx pH 1
Describe the neutralisation - titration curve for a strong acid and a weak base.
1. Vertical section covers a smaller change in pH: start at pH 2 end at pH 8 2. Equivalence point at pH 7 3. Starts at approx pH 1 Ends at approx pH 9
Describe the neutralisation - titration curve for a weak acid and a strong base.
1. Vertical section covers a smaller change in pH: start at pH 6 end at pH 12 2. Equivalence point at pH 7 3. Starts at approx pH 3
Describe the neutralisation - titration curve for a weak acid and a weak base.
- There is no vertical section on the graph. This means:
no indicator would work
impossible to carry out an accurate titration. - Starts at approx pH 3
Ends at approx pH 9
What does the equivalence point represent?
The same no. of mols of acid is added to the same no. of mols of alkali.
What should you consider when choosing the correct indicator?
Indicator must have a colour change that lies in the pH of the vertical part of the graph.
Name 4 common strong acids.
HCl = Hydrochloric acid H2SO4 = Sulfuric acid HNO3 = Nitric acid HBr = Hydrobromic acid
Why would no indicator work for a weak acid/ weak base titration?
There is no vertical part on the graph.
What is a buffer?
A solution that minimises changes in pH, upon the addition of H+ and OH-
How is a buffer made?
The acid must be in excess.
How does a buffer work?
- Equilibrium of acid dissociating (HA —> A- + H+)
Add acid (H+)
Equilibrium shifts to LHS.
H+ reacts with A- to make HA. H+ + A- –> HA
Reservoir of A-, so solution can absorb a significant amount of H+, minimising change in pH.
Add alkali (OH-)
Equilibrium shits to RHS.
OH- reacts with H+ to make water. H+ + OH- –> H2O
Reservoir of undissociated acid (HA), so solution can absorb significant amount of OH-, minimising changes in pH.
What is the buffer system in blood?
H2CO3 —> HCO3- + H+
(Equilibrium)
Carbonic acid —> Hydrogen carbonate
How do you calculate the ratio of Hydrogen carbonate:Carbonic acid in blood.
Ka [HCO3-]
—– = ———-
[H+] [HCO3]
- Calculate healthy [H+] and Ka.
- Use figures to calculate the ratio.
State a compound that can be added to ethanoic acid to make a buffer solution.
CH3COONa OR Na
