5-Acid-Base Equilibria Flashcards
What did Arrhenius think about acids and bases?
Acids had common properties because they all produce H+ ions when dissolved in water
HA H+ + A-
B + H2O BH+ + OH-
What evidence is there for the existence of the oxonium ion?
The rate at which positive ions carry electrons is consistent with their size
You’d expect the H+ to move much faster
What is the problem with Arrhenius’ definition for acids and bases?
Water must be present
Reaction between NH3 and Hal doesn’t need any solvent
By his definition, not an acid-base reaction without water
What are the Brønsted-Lowry definitions of acids and bases?
An acid is a proton donor
A base is a proton acceptor
Makes the dissociation of an acid and acid-base reaction
What are the conjugate pairs of the dissociation of water?
HA + H2O A- + H3O+
A B B A
HA + A-
H2O + H3O+
Does water act as an acid or a base?
Both
When a weak acid dissolves, the acid and H3O+ compete to donate a proton
From electrical conductivity we can tell that H3O+ is a better proton donor than ethanoic acid therefore it is a stronger acid
What do strong and weak Brønsted acids show about their conjugate bases?
Strong Brønsted acids have weak conjugate bases
Weak Brønsted acids have strong conjugate bases
What is pH defined by?
pH = -log[H+(aq)]
Measured using universal indicator or a pH meter
What do strong acids and weak acids indicate about the dissociation of ions?
Strong acids - all molecules dissociate into ions
Weak acids- only some of the acid molecules dissociate into ions
What do strong bases and weak bases indicate about the dissociation of ions? Neutral solutions
Strong bases - all molecules dissociate into ions
Weak bases- only some of the acid molecules dissociate into ions
Neutral solutions - the concentrations of hydrogen ions and hydroxide ions are equal
How does a 10 fold dilution affect the pH of acids?
Strong acid- increases pH by 1 unit
Weak acid- decrease concentration of hydrogen by factor of 10. Addition of water produces more H+ ions, causing the pH to decrease
What is the ionic product of water?
Kw
Equilibrium between water molecules and the ions they dissociate into
[H+][OH-] = 1 x 10^-14 mol2dm-6 (at 25C)
How does temperature affect Kw?
As temperature increases, Kw decreases
What is Ka?
Direct measure of the strength of an acid
Larger the value of Ka the more dissociated the acid, and the greater its strength as a proton donor
What is Ka affected by?
Unaffected by concentration but affected by temperature
What is Ka for the reaction:
HA H+ + A-
[H+] [A-]
[HA]
How can pKa be worked out?
pKa = -logKa
Stronger acid has a smaller pKa
What assumptions are made when calculating the pH of a methanoic acid?
Ignore [H+] due to the dissociation of water [HCOO-] = [H+]
As HCOOH is a weak acid, its concentration remains largely unchanged
How do you find the pH of a solution of base?
Find the concentration of OH- ions then use Kw to find [H+]
For a strong base, assume the base is 100% dissociated
How do you find the pH of a strong acid?
Assume that the acid is 100% dissociated
[H+] = [HA]
How do you find Ka for a weak acid?
weight, volume and pH
Calculate number of moles used Calculate concentration Use pH to calculate [H+] Use Ka equation (assume [H+]=[A-]) Assume no dissociation took place
What is the end-point of a titration?
When the appropriate colour changes take place
What is the equivalence point of a titration?
Two reactants are mixed in exactly the same proportions indicated by the equation
(Look at the ratio of the acid to base)
Why isn’t the equivalence point necessarily 7?
Sometimes a slightly alkaline/acidic product is formed
What is the titration curve of a strong acid- strong alkali?
Begins low pH in y axis
Initially acts as a buffer solution
Rapid change of pH at the equivalence point
Finished at high pH
What is the titration curve of a strong acid- weak alkali?
Begins low pH in y axis
Equivalence point
Finishes around pH 9
What is the titration curve of a weak acid- strong alkali?
Curve initially where there is buffer action and mid low pH
Equivalence point
Finishes at high pH
What is the titration curve of a weak acid- weak alkali?
Not suitable for a titration as no area of rapid change
Gentle curve
What is the titration curve when running a strong acid into a weak base?
Initially there is buffer action
What titration curve is produced when an acid is run into an alkali?
Generally produces a similar graph flipped
Except strong acid into weak base
What is an indicator and what can the dissociation of ions be represented as for a weak acid?
Generally weak acids or weak bases
HIn (aq) H+ (aq) + In- (aq)
How does a weak acid indicator work?
Either weak acid and/or its conjugate base is coloured
If pH increases (hydrogen ions are removed) the equilibrium moves to the right to produce more In-, which causes a colour change
How is a suitable indicator selected?
pKin should be equal (or as close as possible) to the pH of the equivalence point of the titration
Good indicator would show dramatic colour change to easily detect end-point
Why is there no suitable indicator for a weak acid-weak base titration?
No vertical portion
No indicator would change dramatically enough so another method would have to be used
How can Ka be found for a weak acid from a pH titration?
Weak acid in a conical flask and a standard solution of NaOH in a burette
Measure the pH after each addition and plot this on the graph
From the graph you can find the equivalence point and the half equivalence point and estimate the pH at this point
What is the half-equivalence point?
When half the total volume of NaOH has been added
At this point the concentrations of acid molecules and conjugate base ions are equal
pKa = pH
What is a buffer solution?
Resist a change in pH
Made from a weak acid and its conjugate base
How does a buffer solution resist the change in pH?
Check mark scheme
How can the proportions needed to make a buffer solution of a certain pH be calculated?
pH = pKa - log([A]/[B]) Rearrange the equation to find [A]/[B] Therefore [B] = 1 To make a buffer solution of 100cm3 mol/total mol x 100cm3
How does the haemoglobin act as a buffer?
HHb (aq) + O2 (g) H+ (aq) + HbO2- (aq)
If pH falls, [H+] increases and the equilibrium adjusts to the left, displacing oxygen from haemoglobin
If pH rises, [H+] decreases and the equilibrium adjusts to the right, causing more oxygen to bing to the haemoglobin
Why is carbon dioxide a threat to the blood’s pH?
Forms carbonic acid which dissociates to produce hydrogen ions
Body has backup system to ensure no dramatic change in pH such as with the kidneys
What does buffer capacity describe?
The amount of acid or base required to change the pH of the food significantly
More protein there is in food the higher its blood capacity
What buffer systems do processed foods such as jams contain?
Citric acid and sodium citrate
Helps to maintain the pH within a range where growth of microogansims is very slow of non-existent
