Acids, Bases and Buffers Flashcards
what is a bronzed-lowry acid?
a proton (H+) donor
What is the definition of a strong acid?
A proton donor that fully dissociates into it’s ions in a solution
e.g. HA –> H+ + A-
What are examples of common strong acids?
Hydrochloric acid (HCl)
Sulphuric acid (H2SO4)
Nitric acid (HNO3)
Phosphoric acid (H3PO4)
What is the definition of a weak acid?
A proton donor that partially dissociates into its ions in a solution
What are examples of common weak acids?
methanol acid (HCOOH)
Ethanoic acid (CH3COOH)
E.G. HA ⇌ H+ + A-
What is concentration?
the measure of how much substance is present in a particular volume of solution, in moldm-³
What is the strength of acid?
a measure of the degree of dissociation of the acid into its ions in solution. We can have a dilute solution of a strong acid or a concentrated solution of a weak acid
What is a bronty-lowry base?
is a proton (H+) acceptor
what are examples of common bases?
any metal oxide (MgO), metal hydroxide, ammonia solution
what is the definition of an alkali?
an alkali is a base that dissolves in water forming OH- ions (not all alkali bases are soluble)
what are examples of common alkalis?
NaOH, KOH, NH3, Ca(OH)2
what is the definition of a salt?
a salt is an acid that has had its H+ ion replaced by a metal ion or another positive ion.
what are examples of salts?
KCl, NaCl
what do the terms mono, di and tribasic acids mean?
the total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction
what are conjugate acid-base pairs?
a set of two species that transform into each other by loss or gain of a proton.
i.e. they consist of the same species apart from a H+
how do you identify conjugate acid-base pairs?
- identify acid on left, and conjugate base on right (by seeing which loses a H+)
- identify base on the left and conjugate acid on right (by seeing which gains a H+)
- assign a number: acid 1/2 base 1/2
how do you find out the pH?
pH = -log10 [H+]
how do you calculate the H+ concentration?
[H+] = 10 -^pH
what is the background behind the pH scale
the more manageable pH scale of approx 1-14 was introduced by Soren sorenson to replace the use of hydrogen ion concentration ranging form 10-¹ to 10-¹⁴
how do you calculate the pH of strong acids?
[acid] = [H+] due to the acid fully dissociating
therefore we can work out [H+] using the normal pH equation (pH = -log10 [H+])
what does amphoteric mean, and what molecule is this?
it can act as an acid and a base, pure water is amphoteric
what is the ionic product of water?
Kw
what is the Kw of pure water?
1 x 10-¹⁴ mol²dm-⁶
what is the only change that affects Kw?
temperature
how do you calculate the pH of water?
[H+] =√kw
pH = -log10 [H+]
what does the Kw tell you about the enthalpy change of the dissociation of water?
- as the temp increases, the pH decreases
- this means that the forward reaction is favoured so equilibrium shifts right
- increasing temp always favours the endothermic reaction which means the endothermic reaction is forward
How to we calculate the pH of a strong base?
strong bases fully dissociate in solution,
if we know the conc. of a strong base we know the conc of the OH- ions
what is the expression to work out the pH from a strong base?
Kw = [H+] [OH-]
Kw / [OH-] = [H+]
pH = -log10 [H+]
What is the Ka?
Ka is the acid dissociation constant for a weak acid
What is the expression for Ka?
ka = [H+] [A-] / [HA]
what is the only change that affects Ka?
temperature
the larger the numerical value of ka…
the more the equilibrium lies to the right, the greater the conc. of the dissociated ions H+ & A- (numerator) and the stronger the acid
the smaller the numerical value of ka…
the more the equilibrium lies to the left, the smaller the conc. of the dissociated ions H+ & A- (numerator) and the weaker the acid
how do you compare numbers with negative indices in Ka?
by converting the Ka into a negative log called Pka
How do you work out pKa?
pKa = - log10 ka
How do you work out ka from pKa?
Ka = 10 -pKa
what is the strong acid pKa / Ka ?
large Ka / small pKa
What is the weak acids pKa / Ka ?
small Ka / large pKa
how can the change in pH be measured in a titration?
by using both indicators and pH meters to monitor the change
what is a pH meter?
a pH meter consists of an electrode that is dipped into a solution, and connected to a meter that displays a pH reading
What is pH paper?
pH paper is dipped into a solution and the colour compared to a colour chart
what is the equivalence point on a pH titration curve?
is the volume of one solution that exactly reacts with the volume of the other solution, neutralisation point
what does the centre of the pH titration curve mean?
an extreme increase in pH rapidly when addition of a small volume of base
what does the vertical line on the titration curve determine?
the indicator that must be chosen to change colour within this vertical lie region, or we will not accurately and precisely measure the equivalence point in the reaction
what are the suitable indicators for a strong - strong titration?
- methyl orange
- bromophenol blue
- methyl red
- bromothymol blue
- phenolphthalein
methyl orange
- the colours at low/high pH
- pH range
- end point
- red (low), yellow (high)
- 3.2-4.4
- 3.7
bromophenol blue
- the colours at low/high pH
- pH range
- end point
- yellow (low), blue (high)
- 2.8 - 4.6
- 4.0
methyl red
- the colours at low/high pH
- pH range
- end point
- red (low), yellow (High)
- 4.2 - 6.3
- 5.1
bromothymol blue
- the colours at low/high pH
- pH range
- end point
- yellow (low), blue (high)
- 6.0 -7.6
- 7.0
phenolphthalein
- the colours at low/high pH
- pH range
- end point
- colourless (low), pink (high)
- 8.2 - 10.0
- 9.3
what is the suitable indicator of a weak base titration?
methyl orange
weak base titration findings?
change in pH is more gradual initially, rapid change starting between pH 3-7
suitable indictor for strong base titration?
phenolphthalein
strong base titration findings?
start pH of approx 3-4 and the pH rises slowly on addition of the sodium hydroxide, until we reach the equivalence point where there is a rapid rise from 7-11.
what is the suitable indicator for a weak acid - weak base titration?
none, no vertical section to the pH curve. Use pH meter
what is the difference between the weak - weak titration?
titration is rarely carried out, as the equivalence point is difficult to spot due to a gradual pH change throughout
what are the similarities and differences between the 4 graphs/curves (where they start and finish pH?
If you are asked for a sketch – weak acids start at a higher pH than strong acids and
Strong bases have a higher pH than weak bases for the same concentration
what are the similarities and differences between the 4 graphs/curves (equivalence point / straight line)?
if both acid and alkali are the same concentration AND they are in a 1:1 mole ratio, it will be when they have the same volume .
what Is the pH range of the vertical line of:
- strong acid - strong base
- strong acid - weak base
- weak acid - strong base
- weak acid - weak base
- 4 to 10
- 4 to 8
- 6 to 10
- no line
choice of indicators?
The working range/ pH range of the indicator must coincide with the vertical region of the curve/graph.’
what is a buffer?
a system that resists changes in pH where moderate amounts of acid or base are added
what does a weak acid buffer contain?
a weak acid & the salt of a weak base
how of you make a buffer solution: method 1?
- Simply mixing together
- the weak acid e.g. ethanoic acid
and one of its salts e.g. sodium ethanoate
how of you make a buffer solution: method 2?
- Adding an alkali, e.g. NaOH
- to an excess of the weak acid e.g. ethanoic acid.
how do buffers work?
For a buffer to be effective, there needs to be a large excess of the dissociated weak acid HA AND a large excess of its conjugate base, A-.
on addition of the acid, the conjugate base removes the acid?
H+ from the acid reacts with the conjugate base
The Equilibrium shifts to the left
H+ ions being removed
Addition of alkali (OH- ions)
The weak acid removes the added alkali
Added OH- reacts with H+
Equilibrium shifts to the right
H+ ions being replaced
what is the role of the weak acid in a buffer?
The role of the weak acid HA, and its conjugate base, A-, can be summarised by
how do you calculate the pH of a buffer?
Henderson-Hasselbalch equation derived from rearranged Ka expression
[H+] = Ka [HA] / [A-]
Using –log10 ..
pH = pka + log 10 [SALT]/[ACID]
What is the equation for making buffer with a different pH?
Ka = [H+] [CH3COO-] / [CH3COOH]
[H+] ≠[CH3COO-]
what is the rearranged [H+] to make a buffer with a different pH?
Ka x ([CH3COOH]/[CH3COO-])
if [HA] = [A-] Then [CH3COOH] = [CH3COO-]
Then [H+] = Ka and using the –log10
Then pH = pKa
the pH of the buffer solution is…
the same as the pKa value of the weak acid
what happens by adjusting the conc of the weak acid and salt?
the pH of the buffer can be adjusted by 1 pH value higher or lower.
