Acids And Bases Flashcards
Bronsted – Lowry acid
Bronsted – Lowry acid proton donor
Bronsted – Lowry base
Bronsted – Lowry base proton acceptor
pH
Measure of acidity by measuring concentration of H+ ions
Low pH means
Acidity
High pH means
Alkalinity
Calculating pH equation
pH = -log[H+]
Calculating [H+]
-pH
[H+] = 10
What does an acid/base reaction involves
Transfer of the H+ ions from the acid to the base
Strong acid ful..
Strong acid fully dissociates in water
Weak acid
Only very Slightly dissociates in water
Ka weak acid dissociation constant
Ka= [H+] [A-]
[HA]
The more a weak acid dissociates…
The more H+ ions are formed
the stronger the acid the larger the Ka volume
Calculating pKa
pKa = -log (ka)
Calculating Ka from pKa
-pKa
Ka=10
Dissociation of water
H2O (reverse sign) H+/H3O+OH-
When water boils what happens to its pH
Water dissociating is endothermic and the reverse reaction is exothermic
Le chatliers principal
equilibrium shifts to rights to lower temperature more H+ ions So pH lower
Oxonium ion
H3O+
The ionic product of water at 298 Kelvin
Kw = [H+][OH-] = 1.0 x 10^-14
Calculating pH of a base
[H⁺] = Kw
[OH⁻]
Buffer
Buffer
Solution that maintains an almost constant pH on addition of small amounts of strong acid or strong base.
What does an acidic buffer consist of? Weak acid (HA) with its salt (Na⁺A⁻)
What does an acidic buffer consist of? Weak acid (HA) with its salt (Na⁺A⁻)
What does a basic buffer consist of
What does a basic buffer consist of?
Weak base with its salt.
How can an acidic buffer be made?
How can an acidic buffer be made?
Mix HA with Na⁺A⁻.
Part-neutralise HA with NaOH.
How can a basic buffer be made?
How can a basic buffer be made?
Mix weak base with its salt (e.g. NH₃ with NH₄Cl).
Part-neutralise weak base with HCl.
When H⁺ added to acidic buffer…
When H⁺ added to acidic buffer... Unbalances equilibrium. Higher concentration of H⁺. Le Chatelier's principle. H⁺ + A⁻ → HA
When OH⁻ added to acidic buffer…
OH⁻ + H⁺ → H₂O Unbalances equilibrium. Lower concentration of H⁺. Le Chatelier's principle. HA → H⁺ + A⁻
When H⁺ added to basic buffer…
OH⁻ + H⁺ → H₂O Unbalances equilibrium. Higher concentration of H₂O. Le Chatelier's principle. NH₃ + H₂O → NH₄⁺ + OH⁻
When OH⁻ added to basic buffer…
Unbalances equilibrium.
Higher concentration of OH⁻.
Le Chatelier’s principle.
NH₄⁺ + OH⁻ → NH₃ + H₂O1.
How is a pH titration carried out?
How is a pH titration carried out?
- Calibration curve of pH meter and electrode using buffer solutions.
- Prepare graph of pH (y-axis) against volume of alkali added (x-axis).
- Pipette 25cm³ 0.1M acid into 250cm³ beaker. Measure pH and record as initial pH.
- Add 0.1M alkali from burette in appropriate portions, measuring and plotting pH after each addition.
- Repeat up to 50cm³.
- Plot curve of best fit.
Equivalence volume/endpoint
Equivalence volume/endpoint
Mi-point of “vertical section”.
At half equivalence…
At half equivalence…
[A⁻] = [HA]
Ka = [H⁺]
pKa = pH
Buffer region
Buffer region
Around pa where pH changes very little, despite acid/base being constantly added.
Acid-base indicators
Weak acids.
Acid-base indicators
Weak acids.
Conjugate acid and base forms have different colours
An indicator colour change must…
An indicator colour change must…
Be rapid
Occur at exactly the same time as the equivalence point.
What colour changes does methyl orange show?
What colour changes does methyl orange show?
Yellow - pH5
Orange - pH4
Red - pH3
What colour changes does phenolphthalein show?
What colour changes does phenolphthalein show?
Pink - pH10
Pale pink - pH9
Colourless - pH8
When can you NOT use phenolphthalein?
When can you NOT use phenolphthalein?
With a weak base.
When can you NOT use methyl orange?
When can you NOT use methyl orange?
With a weak acid.
