Chapter 21 - Acids, bases and buffers 1 Flashcards
What are the 3 types of acids
Monoprotic - 1 hydrogen ion
Diprotic - 2 hydrogen ions
Triprotic - 3 hydrogen ions
What is the definition of an acid
it is a proton donor (hydrogen ion donor)
What is the definition of a base
It is a proton acceptor (hydrogen ion acceptor)
What is the definition of an acid base reaction
It is a reaction which involves the transfer of protons
Is water an acid or a base
It is amphoteric so both
What is a strong acid
It is one that fully dissociates in water
What is pH
It is a measure of the concentration of H+ ions in a solution
What is the definition of pH
pH = -log (H+)
Give to 2 decimal places
What is the equation to find concentration of H+ ions
(H+) = 10 -pH
Give to 2 decimal places
What happens when water is added to solution of a strong acid
The number of moles stays the same but the volume and concentration change
What equation do you use for a dilution of a strong acid
C1V1 = C2V2
What equilibrium is established in water and aqueous solution
H20 <-> H+ + OH-
What is the Kc equation for water
Kc = [H+][OH-] / [H2O]
Kc x [H2O] = [H+][OH-]
What is Kw
It is equivalent of Kc x [H2O]
Kw = [H+][OH-]
Kw = 1x10-14 at 298K
What happens to Kw as temperature rises
As temperature rises so does Kw
What are the steps needed to calculate the pH of a strong acid and strong base
Moles H+
Moles OH-
XS moles of H+ or OH-
XS concentration of H+ or OH-
pH
Whats the equation of a strong acid dissociating
HX -> H+ + X-
Whats the equation for a weak acid dissociating
HX <-> H+ + X-
What is Ka
The acid dissociation constant
Used for weak acids
Ka = [H+][A-] / [HA]
What happens to Ka as acidity increases
it increases
What happens to Ka as temperature increases
It increases
What is the equation for pKa
= - log (Ka)
Whats the equation for Ka from pKa
Ka = 10 -pKa
What happens to pKa as acidity increases
It decreases
What is assumed for calculations of a monoprotic weak acid in water
the concentration of H+ is the same as the concentration of A-
What equation do we use for weak acids with water
Ka = [H+]2 / [HA]
What can’t be used for a weak acid strong base calculation
That the concentration of H+ is the same as A-
What steps do you use for a weak acid strong base calculation if HA is in XS
Moles HA
Moles OH-
XS HA
Moles of HA left and A- formed
Concentration of HA and A-
Use Ka to find [H+]
pH
What steps do you use for a weak acid strong base calculation if OH- is in XS
Moles HA
Moles OH-
XS OH-
Use Kw to find [H+]
pH
What is a half neutralisation of a weak acid strong base
It is when the moles of HA = A-
This means Ka = [H+]
pKa = pH
What is the equivalence point
When the same moles of acid and base are mixed together
What is the pH at equivalence point for a strong base strong acid mixture
7
What is the pH at equivalence point for a weak acid weak base mixture
approximately 7
What is the pH at equivalence point for a weak base strong acid mixture
Below 7
Because a salt of a weak base which is acidic is formed
What is the pH at equivalence point for a weak acid strong base mixture
Above 7
Because a salt of a weak acid which is basic
What are indicators
They are water soluble weak acids
What is the general formula of a indicator
Hln
What makes a indicator work
Hln and ln- have different colours
The rapid change in pH at equivalence causes the equilibrium to shift from one side to the other
What is the universal indicator
It is a mixture of several indicators so shows lots of colours
What is the equivalence point
The point at which equimolar amounts of acid and alkali are present
What makes an indicator suitable for a specific reaction
Have an end point that coincides with the rapid change of pH at the equivalence point
Change colour quickly upon the addition of a drop of acid or alkali
Give a distinct colour change
Range of pH values matches up with equivalence point range
What is the best indicator for a strong acid strong base reaction
Phenolphthalein or methyl orange
What is the best indicator for a strong acid weak base reaction
Methyl orange
What is the best indicator for a weak acid strong base reaction
Phenolphthalein
What is the best indicator for a weak acid weak base reaction
No suitable indicator
What is a buffer solution
It is one that resists change in pH upon addition of small amounts of acid and alkali
What is an acidic buffer
It is a solution with a pH less than 7 and contains a weak acid and salt of the weak acid
How can you make an acidic buffer
Mixing a weak acid and its salt
Half / partially neutralising a weak acid with a strong base
How does an acidic buffer work
The weak acid partially dissociates
The salt fully dissociates when it dissolves
HA <-> A- + H+
What happens when you add H+ and OH- ions to a acidic buffer
H+ ions move the equilibrium left as there are XS H+ ions
OH- ions move the equilibrium right as they react with the H+ ions
What is a basic buffer
A solution with a pH greater than 7 and contains a weak base and the salt of that weak base
How can basic buffers be made
Weak base and its salt
Half/Partially neutralise a weak base with a strong acid
How do basic buffers work
The weak base reacts partially with water
The salt fully dissociates when it dissolves
HA +H2O <-> A- + OH-
What happens when you add H+ and OH- to a basic buffer
H+ ions cause the equilibrium to shift right as it reacts with OH_ ions
OH- ions cause the equilibrium to shift left as there is a higher concentration of OH- ions
When does an acidic buffer work best
When its pH is equal to the pKa value of the weak acid as it will be equally good at resisting change in pH when acid and alkali are added
What equation is used to find the pH of acidic buffer solutions
Ka = [H+][A-] / [HA]
What steps do you do for acidic buffer calculations
mol acid/HA and salt/A-
Concentration acid and salt
Concentration of H+
pH
What steps do you do for acidic buffer calculations when acid or base’s are added
mol acid/HA and salt/A-
Concentration acid and salt
Concentration of H+
pH of buffer
mol of added acid or base
ICE table - moles of HA, A- or OH-
Concentration of HA, A- or OH-
Concentration of H+
pH
What does a gradual pH change mean for the end point
The indicator would change over a range of pH so end point is hard to determine
What equation explains why buffers pH doesn’t change when diluted
ratio of [HX] / [X-] remains almost constant
What is different about diprotic acid titration curves
They have 2 different vertical sections
As the second H+ is only dissociated once all the first H+ ions are reacted
Why is [H2O] not shown in the Kw equation
[H2O] is almost constant
[H2O] is incorporated into Kw
Why is a pH probe washed with distilled water between measurements
To wash away and remaining solution which could interfere with the reading
Why is the volume of base added smaller nearer the end point
To avoid missing the end point
When is an indicator suitable from a titration curve
When it has a pH range within the steep part of the titration curve
