Chapter 21- Acids, bases and buffers Flashcards
What is a Bronsted-Lowry acid?
A substance that donates protons
What is a Bronsted-Lowry base?
A substance that accepts protons
What happens when a Bronsted-Lowry acid is added to water?
Release H+ ions which react with water
HA + H2O –> H3O+ + A-
What happens when a Bronsted-Lowry base is added to water?
Bond to H+ ions in water molecules
B + H2O –> BH+ + OH-
What is a strong acid?
An acid that completely dissociates in solution
What is a weak acid?
An acid that only slightly dissociates when in solution so only a small number of H+ ions are released
What is a strong base?
A base that completely ionises in water
What is a weak base?
A base that only slightly ionises in water
What is pH?
A measure of the acidity or alkalinity of a solution
How do you find pH given [H+]?
pH = -log[H+]
How do you find [H+] given pH?
[H+] = 10^-pH
How does the pH scale work?
A logarithmic scale from 0-14 which is a measure of [H+] in a solution, showing their acidity or alkalinity
pH<7 is acidic, pH>7 is alkaline
How many decimal places is a pH given to?
2 dp
What is Kw?
The ionic product of water, which is the equilibrium constant for the dissociation of water
What is the equation and value for Kw?
Kw= [H+][OH-]
At 298K, Kw = 1 x 10^-14
Where does Kw come from?
H2O ⇌ H+ + OH-
Kc = [H+][OH-]/[H2O]
As H2O barely dissociates, [H2O] is very large so approximation means Kc (Kw) = [H+][OH-]
How does Kw change with temperature?
Forward reaction in equilibrium of water is endothermic so as temp increases, forward reaction is favoured so [H+] increases and Kw increases
What is Ka and how does it show the strength of an acid?
The dissociation constant for a weak acid. If Ka is large, equilibrium lies to the right so [H+] is greater meaning the larger the Ka, the stronger the acid
What is the equation for Ka and what is important when it is used for weak acids?
HA ⇌ H+ + A-
Ka = [H+][A-]/[HA]
important: [A-] = [H+] when dissociation occurs
[HA] assumed to be the same before and after dissociation as the change is so small
What are some equations involving Ka?
pKa = -logKa
Ka = 10^-pKa
What is special when [HA] = [A-] for a weak acid?
Ka = [H+]
so pKa = pH
What is a pH curve?
A graph of pH against volume of acid/base added to a solution
What are some key features of a pH curve?
Start pH, end pH
End point pH
Equivalence point
What is the equivalence point?
The point where full neutralisation occurs, the point where [H+] = [OH-]
What is the end point?
The point during titration where the indicator changes colour. A suitable indicator should change colour near the equivalence point
What is the end point pH for a strong acid v strong base titration?
pH= 7
What is the end point pH for a strong acid v weak base titration?
pH<7 (more acidic)
What is the end point pH for a weak acid v strong base titration?
pH>7 (more alkaline)
What is the end point pH for a weak acid v weak base titration?
normally pH=7 but hard to determine
What is an indicator?
A chemical solution that will change colour when there is a change in pH, they are weak organic acids
How does an indicator work?
HIn ⇌ H+ + In-
HIn: colour A (acidic conditions), in acid equ shifts to left to decrease [H+] producing colour A
In-: colour B (alkaline conditions), conjugate base, in alkaline equ shifts to right to increase [H+] as they react with OH-, producing colour B
What is the pH for colour change and what is it for methyl orange?
Colour change red to yellow at pH 3-5
What is the pH for colour change and what is it for phenolphthalein?
Colour change colourless to pink at pH 8-10
What is the pH for colour change and what is it for bromothymol blue?
Colour change yellow to blue at pH 6-8
How do you find the point at which an indicator changes colour?
Using Ka, Ka = [H+][In-]/ [HIn], for change point [In-]=[HIn] so Ka = [H+] so change pH is pKa=pH
What is a buffer solution?
A solution that maintains a nearly constant pH when small amounts of a acids or alkalis are added
What is an acidic buffer?
A buffer containing a weak acid and salt of the weak acid (conjugate base)
What is an alkaline buffer?
A buffer containing a weak base and salt of the weak base (conjugate acid)
How do you find pH of a buffer?
HA ⇌ H+ + A-
Use Ka to find [H+] then find pH
What are the proportions of reactants in an acid buffer?
Lots of weak acid, lots of conjugate base (due to it from salt as well) and little H+
