5.1.3 - Acids and Bases Flashcards
Bronsted - Lowry Acid
Proton donor
Bronsted - Lowry Base
Proton acceptor
How are acids related to bases
Acid —> proton + conjugate base
How are bases related to acids
Base + proton –> conjugate acid
pH
Conversion of hydrogen ion conc
pH = - log [H^+]
[H^+] = 10 ^-pH
For strong monoprotic acids [H^+] =
[HA] - acid fully dissociates
What does water dissociate to give
H^+ and OH^-
Kw
[H^+][OH^-]
Kw at 25 degrees
1.0 * 10^-14
In neutral solutions at 25 degrees [H^-] =
[OH^-]
How can an aq solution of an acid contain OH-
Water dissociates
When is a solution alkaline
[OH^-] > [H^+]
Finding pH of strong bases using Kw
Rearrange Kw
Finding [OH^-] using pKw
Kw = [H+][OH-] pKw = pH + pOH 14 = pH + pOH 14-pH = pOH [OH-] = 10^-pOH
Assumptions w/ Ka
Little dissociation [HA] eqm = [HA] undis. Less valid for stronger weak acids w Ka > 10-2
[H2O] is constant so [H+]=[A-]. Less valid for v weak acids and dilute sol
Ka
Dissociation constant for a weak acid
Bigger the value, sronger the acid
Finding [H+] from Ka
Ka = [H+][A-]/ [HA] [H+] = [A-] Ka = [H+]^2/ [HA] [H+]^2 = Ka [HA] Square root answer
Dilution of strong base
[OH-] in orig.
[OH-] in diluted ([OH] * orig vol/total vol)
Find [H+] using Kw
Reaction between strong acid and strong base
Calculate MOLES H+ (x2 if dibasic) Calculate MOLES OH- Calculate XS MOLES Calculate XS [H+] or XS [OH-] (using total vol.) If XS [OH] use Kw to find XS [H+]
Example of weak base
NH3
Buffers
A solution that minimises pH changes in addition of SMALL amounts of acid or base
Components of a buffer solution
Weak acid and its conjugate base
How to make buffer solutions
Mixing weak acid and its conjugate base (ethanoic acid and sodium ethanoate)
By partially neutralising a solution of the weak acid to give a mixture of the acid and its salt (React NaOH with an excess of the weak acid)
What does a buffer solution look like
HA only partially dissociates and the salt completely dissociates
How do buffers resist change to pH if an acid is added
HA H+ and A-
[H+] increases
Eqm shifts to left as A- reacts w/ H+
Removes added H+
[H+] and pH are restored
How do buffers resist change to pH if an alkali is added
Ha H+ and A-
[OH- ] increases
OH- reacts w/ H+
HA dissociates to form more H+
Eqm shifts to right to restore pH
HA and conjuate base buffer (solution)
Find moles of HA and A-
[HA] = moles of HA/ Vol 1+2
[A-] = moles of conjugate base / Vol 1+2
Use Ka to find [H+]
HA and conjugate base buffer (solution and solid)
Find moles of HA and A-
[HA] start = [HA] end
[A-] = moles of conjugate base/Vol HA
HA and base buffer (solution)
Find moles of HA and B
nHA end = nHA start - nNaOH
[HA] = n/Vol of base
[A-] = [NaOH]
Weak acid and strong base
Calculate moles HA
Calculate moles OH
Find XS HA or OH-
If XS HA, use HA + OH- –> A- + H2O to find nHA and nA- after
Find [HA] and [A-] using total vol
Find [H+]
If XS OH- use Kw to find [H+]
Acid Base Indicators
HI –> H+ + In-
Weak acid (Ka) Unionised is diff colour to anionic form
Acid base indicators in acid
In acid [H+] inc
Eqm moves to left to give colour A
Acid base indicators in bases
In alkali [OH-] inc
Reacts w/ water
Eqm moves to right
End point of a titre
Ka = [H+] pKa = pH
End point of titration
[HA] and [A-] are the same
Colour is a mix between HA form and A- form
pKa = pH
Indicator for SA and SB
Phenolphthalein
Litmus
Methyl orange
Indicators for SA and WB
Methyl orange
Indicator for WA and SB
Phenolphthalein
Indicator for WA and WB
None suitable because no rapid change in pH during titration and indicator needs a sharp pH rise to change colour quickly
Finding pH of Ba(OH)2 from conc.
Times [OH] by 2
Finding conc. of Ba(OH)2 from pH
Half [OH-]
Finding pH when base is added to a buffer sol
Find [HA] and [A-] Use Ka to find [H+] --> pH Find moles of B Find moles of HA (using c*vol. of buffer sol.) Find excess HA or B
If HA:
Find [HA] using total vol (base and buffer)
Use Ka to find [H+] –> pH
% dissociation of HA
[H+]/[HA] * 100
Experimentally determining the value of Ka
Preparing a standard solution of the weak acid of known concentration
Measuring the pH of the standard solution using a pH meter
Most effective buffers
[HA] = [A-]
pH is same as pKa for HA
Range is approx around two pH values of pKa
Calculating conc ratio of HCO3-/H2CO3
[HCO3-]/[H2CO3] = Ka/[H+]
Using a pH meter
Using a grad. pipette, add a measure volume of acid to a conical flask
Place electrode into flask
Add aq base 1cm3 at a time
Swirl and record pH
Add base drop wise when a rapid increase occurs
Add base 1cm3 at a time until there’s an excess and pH is basic
What does the equivalence point tell you
Exact volume of solution that reacts w the other
Adding metal to buffers
Calculate moles of M
Calculate moles of HA: 1-(charge of M * nM)
Calculate moles of A-: 1 + (charge of M *nM)
[H+] = Ka * HA/A-
Making buffers using HA and metal
Find moles of metal
Find nA-: charge of metal * nM
Find nHA: nHA - nA-
Find [HA] and [A-] using total vol.
