pH Flashcards
What does Kw equal?
Kw = [OH-][H+]
What is the value of Kw at standard conditions?
1x10^-14
How do you work out pKw from Kw?
pKw = -lg(Kw)
What is a Bronsted-Lowry acid?
Proton donor
What is a Bronsted-Lowry base?
Proton acceptor
What is the difference between a base and an alkali?
Bases that dissolve in water are alkali. All alkalis are bases but not all alkalis are bases
What is a conjugate base?
When an acid donates a proton, the species formed/left over is the conjugate base
What is a conjugate acid?
When a base accepts a proton, the species formed is the conjugate acid
How do we work out the pH of strong acids?
pH = -lg [H+]
What is Ka?
The acid dissociation constant
What assumption is made in the calculation of Ka?
The concentration of both dissociation products is the same
What is the equation for finding Ka?
[H+]^2 / [HA] = Ka
If we are given the value of Ka and the concentration of the weak acid, how can we work out the pH?
Conc of acid x Ka = [H+]^2
Square root to get [H+]
-lg x [H+] = pH
What is H2SO4 an example of?
A diprotic acid
What is a diprotic acid?
An acid that when it dissociates releases 2 H+ ions
How do you work out the concentration of H+ from pH?
10^-pH
How do you work out pKa?
pKa = -lg (Ka
How do you work out Ka from pKa?
Ka = 10^-pKa
What must you use to work out the pH of a base?
Kw
How do you use Kw to find the pH of a base?
Once you find [OH-]:
Kw / [OH-] = [H+]
Why is it difficult to find the end point of a titration between a weak acid and a weak base?
Because the starting and finishing pH are close together, making it difficult to identify an end point.
How do you pick an indicator for your titration?
You must pick an indicator that changes colour at a pH within the end point
What is the pH range of methyl orange?
3.1-4.4
What is the pH range of phenolphthalein?
8.2-10
What is the colour change for phenolphthalein acid -> alkali?
Colourless -> Pink
What is the colour change for methyl orange acid -> alkali?
Red -> Yellow
What indicator should be used for strong acid-strong base titrations?
Phenolphthalein or methyl orange can be used
What indicator should be used for weak acid-strong base titrations?
Phenolphthalein
What indicator should be used for strong acid-weak base titrations?
Methyl orange
What is the end point of a titration on a graph?
The steep section
What is a buffer?
A buffer solution is one which resists changes in pH when small quantities of an acid or an alkali are added to it
What are acid buffers?
Buffers with a pH below 7
What are alkaline buffers?
Buffers with a pH above 7
How are acidic buffers made?
Acidic buffer solutions are commonly made from a weak acid and a salt of its conjugate base
What is the common example of an acidic buffer?
A mixture of ethanoic acid and sodium ethanoate in solution
How are alkaline buffers made?
Alkaline buffers are commonly made from a weak base and a salt of its conjugate acid
What is the common example of an alkaline buffer?
A mixture of ammonia solution and ammonium chloride solution.
What 2 assumptions are made when calculating the pH of an acidic buffer made up of carboxylic acid and sodium ethanoate?
The weak carboxylic acid doesn’t dissociate (so initial conc remains the same)
The sodium ethanoate (salt) dissociates fully (so conc of salt = conc of ethanoate ion)
What is the buffer range in a titration?
The part of a titration where changes in pH is slow and gradual
What happens if you add H+ to an acidic buffer (e.g. ethanoic acid + sodium ethanoate)?
The H+ will react with the carboxylate ion (CH3COO-) to form more carboxylic acid (CH3COOH)
What if you add OH- to an acidic buffer (e.g. ethanoic acid + sodium ethanoate)?
The OH- will react with H+ ions, and the CH3COOH will counteract this change by dissociating more and producing more H+ ions (le Chatelier’s principle)
How else can you make an acidic buffer (without a salt)?
Acidic buffers can be made from mixing an EXCESS of a weak acid with a strong base
What is the Henderson-Hasselbalch equation?
pH = pKa + lg (salt/acid)
What is the effect of a ten-fold dilution on a strong acid?
pH increases by one for each 10-fold dilution
1 moldm-3 = pH 0
0.1 moldm-3 = pH 1
0.01 moldm-3 = pH 2
What is the effect of a ten-fold dilution on a weak acid?
pH increases by 0.5 for each 10-fold dilution
1 moldm-3 = pH 2.38
0.1 moldm-3 = pH 3.88
0.01 moldm-3 = pH 3.38
What is the half neutralisation point?
The point of a titration at which half of the acid has been neutralised
At the half neutralisation point, what do we know about the concentrations of the acid and conjugate salt?
The conc of the acid = the conc of the salt
Half of the acid has been neutralised to the salt
What can we work out from pH at the half neutralisation point?
At the half neutralisation point, pH = pKa
Why does pH = pKa at the half neutralisation point?
Henderson’s equation is pH = pKa + log(salt/acid)
The conc of the salt and the acid is the same, so salt/acid = 1
log(1) = 0
So essentially the equation is pH = pKa + 0
