Topic 12: Acid-Base Equilibria Flashcards
Define a Bronsted-Lowry acid
A proton donor
e.g. HCl is a Bronsted-Lowry acid, as it donates a proton:
HCl —> H+ + Cl-
Which ions make solutions acidic?
Hydroxonium ions
H3O+
Define a Bronsted-Lowry base
A proton acceptor
e.g. OH - is a Bronsted-Lowry base, as it accepts a proton:
OH- + H+ —-> H2O
What are conjugate acid-base pairs?
A pair of reactants and products that are linked to each other by the transfer of a proton
How do you identify acid-base conjugate pairs?
Acid = acid formula
Conjugate acid = gains a proton
Base = base formula
Conjugate base = loses a proton
Define the term pH
the acidity of an aqueous solution depends on the number of H+ ions in solution.
It is defined as -log10[H+]
pH equation
pH = -log10[H+]
Hydrogen ion concentration
[H+] = 10^-pH
What is a strong acid?
A strong acid fully dissociates in solution
e.g. H2SO4, HCl, HNO3
What is a weak acid?
Partially dissociates in aqueous solution
e.g. CH3CH2COOH, HCN
How do you calculate the pH of a strong acid?
-log10[H+]
What is Ka, and when is it used?
Ka is the acid dissociation constant, and is used for weak acids.
What is the Ka expression?
Ka = [H+] [A-]
————
[HA]
What assumptions are made when writing the equilibrium expression for weak acids?
The concentration of hydrogen ions due to the ionisation of water is negligible
What do Ka values indicate about dissociation?
The higher the value of Ka the more dissociated the acid and the stronger it is
The lower the value of Ka the weaker the acid
Equation for pH of a weak acid
-log10(√(Ka ×[weak acid]) )
Define the ionic product of water
An equilibrium exists where some water molecules partially dissociate:
H2O —> OH- + H3O+
H20 —> OH- + H+
Kw = [H+] [OH-]
What is the exact value for Kw
1.00 x 10^-14 mol2dm-6
Equation for pH of a strong base
14 - (-log10[H+])
Define the term ‘pKa’
pKa = -log10 Ka
The lower the value for pKa, the stronger the acid.
The higher the value for pKa, the weaker the acid
Define the term ‘pKw’
pKw = -log10 Kw
The Kw value of 1.00 x 10-14 mol2 dm-6 at room temperature gives you a pKw value of 14
How do you calculate Ka for a weak acid from pH?
1: balanced dissociation equation for the weak acid.
2: ICE Table for the disassociation of the weak acid.
3: Write the equilibrium expression of Ka for the reaction
4: Using the given pH, determine the concentration of hydronium ions present with the formula:
5: Solving for the concentration of hydronium ions gives the x M in the ICE table. Substitute the hydronium concentration for x in the equilibrium expression.
6: Simplify the expression and algebraically manipulate the problem to solve for Ka.
What is the equivalence point?
when you mix the two solutions together in the correct proportions according to the equation and they neutralise.
What is the end point?
When the indicator changes colour
What does the pH curve look like for a strong acid-strong base titration?
(pH/vol. acid)
Curve begins at pH 14
Decreases to pH 12 - very flat
Sharp drop to pH 4
Slow decrease to pH 1
How do you know the volume needed to neutralise the acid, from the pH graph?
Calculate the volume at the equivalence point (pH 7)
Steps for drawing titration curves
- Calculate start and end pH values of the substance in flask
- Calculate the equivalence point via a titration calculation
- Draw the curve following the correct pH points.
Where is the equivalence point for a weak acid-strong base titration?
The volume at halfway up the vertical section
Where does pH=pKa on the weak acid-strong base curve?
Halfway to the volume at equivalence point
What does the pH curve look like for a weak acid-strong base titration?
(pH/vol. alkali)
Curve starts at around pH 3
Small curve upwards
Flat
Curve shoots up sharply to around pH 13
Flattens at top
What values do you centre the pH around for a weak/strong titration?
- A weak acid centres around pH, so draw vertical section from pH 7-11
- A weak alkali centres at pH 6, so draw vertical section from pH 4-8
What values do you centre the pH around for a strong/strong titration?
It will centre around pH 7
A strong acid in the burette finishes at pH 0-1.
A strong alkali in the burette finishes at pH 12-13.
What are indicators?
Weak acids or bases which change colour when they lose or gain protons
What is the equilibrium for Litmus paper?
HLit —> Lit- + H+
red blue
How do you choose an indicator?
Choose an indicator that has its pH range within the vertical section of the pH curve.
The colour change will then happen on addition of a very small volume of solution from the burette.
What are the colour changes for Phenolphthalein?
in Acid - colourless
in Alkali - pink
What are the colour changes for methyl orange?
in Acid - red
in Alkali - yellow
endpoint - orange
Which indicator do you use for a strong acid?
Methyl orange
Which indicator do you use for a strong base?
Phenolphthalein
What is meant by the term ‘Buffer solution’?
A solution that resists changes in pH when small amounts of acid or alkali are added.
Devise an experiment to determine the Ka of a weak acid.
- Titrate the weak acid with the strong base
- Measure the pH at regular intervals
- Plot a graph of pH against the volume of strong base
- Use the graph to find the pH at half equivalence point
- A half neutralisation
pH = pKa
Therefore, Ka = 10^pKa
How does blood act as a buffer?
- CO2 dissolves into the blood forming carbonic acid
- Equilibrium shifts to the right, producing more H+ ions
- The large reservoir of HCO3 - ions combine with H+ ions to prevent a pH change
