Module 5: pH Flashcards
(33 cards)
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.
