Acids and bases Flashcards
Acids and bases
what is a Brønstead-lowry acid
is a proton donor (H+ ion)
what is a Brønstead-lowry base?
is a proton acceptor
HCL (aq) + H2O (l) –> H3O+ + CL-
Explain what each reactant and product in this reaction are, their roles?
HCl = Brønstead-lowry acid , proton donor
H2O = Brønstead-lowry base, proton acceptor
H3O+ = a conjugate acid of H2O
Cl- = conjugate base of HCl
What is a conjugate acid?
a conjugate acid is a species formed from a bronstead lowry base by the addition of a proton
What is a conjugate base?
a conjugate base is a species formed when a bronstead lowry acid by the loss of a proton
HNO3 (aq) + H20 (l) —> H3O+ + +NO3- (aq)
Explain what each reactant a product is in this reaction, and their roles?
HNO3 = Brønstead-lowry acid , proton donor
H2O = Brønstead-lowry base, proton acceptor
H3O+ = a conjugate acid of H2O
+NO3- = conjugate base of HNO3
what if forced to act as a base even if it is usually neutral?
whatever is least acidic is forced to act as a base, even if it is ‘usually’ neutral e.g water
what is a strong acid?
examples?
a strong acid is a fully dissociated (fully ionised) proton donator
e.g HCL, HNO3, H2SO4, H3PO4
What symbol do we use to represent a fully ionised proton donor?
→ (arrow symbol)
What is a weak acid?
a weak acid is a partially dissociated (ionised) proton donator
e.g all carboxylic acids,
What symbol is used to represent a partially ionised proton donor?
use ⇌, (reversible reaction)
e.g HCOOH + H20 –> HCOO- + H3O+
what is a monoprotic acid?
Give examples?
a monoprotic acid donates one H+ per molecules
e.g HCL, HF, HNO3, HCOOH
what are the two general ways of representing monoprotic acids?
HA and HX
What is a diprotic acid?
Give examples?
a diprotic acid donates two H+ per molecule
e.g H2SO4 and all dicarboxylic acids
the reaction of diprotic acids in water occurs in two steps, what are the two steps? Use H2SO4 + H20 as an example
step 1: lose the first H+
Step 2: loose the second H+
Step 1: H2SO4 (aq) + H2O (l) –> HSO4- (aq) + H3O+ (aq)
step 2: HSO4- (aq) + H2O (l) –> SO42- (aq) H3O+ (aq)
Overall: H2SO4 (aq) + 2H2O (l) –> SO4- (aq) + 2H3O+ (aq)
what is a triprotic acid?
Give examples?
a triprotic acid donates three H+ ions per molecule
e.g H3PO4
explain the reaction of triprotic acids in water, what are the three steps? use H3PO4 + H2O as an example?
Step 1: H3PO4 (aq) + H2O (l) –> H3PO4- (aq) + H3O+ (aq)
step 2: H2PO4- (aq) + H2O (l) –> HPO4^2- (aq) + H3O+ (aq)
step 3: HPO4^2- (aq) + H2O (l) –> PO4^3- + H3O+ (aq)
Overall: H3PO4 (aq) + 3 H20 (l) –> 3H3O+ + PO4^3- (aq)
what is the pH scale?
nemerical scale between -1 and 15, that can be used to measure the pH of any solution
What is the pH?
the pH tells us the exact [H+] , concentration of hydrogen ions in a solution
pH = -log10[H+]
Calculate the pH of strong monoprotic acid of
a) 0.125 mol dm^-3 of HCL
pH = 0.90
Calculate the pH of strong diprotic acid of:
a) 0.100 mol dm^-3 of H2SO4
pH = 0.70
How do we calculate the [H+] concentration from the pH?
us the equation [H+] = 10^-pH
Calculate [H+] of a solution with a pH of 2.14?
0.00724 mol dm-3
Weak Acids
what are weak acids?
weak acids are partially ionised, partially dissociated proton donors
In weak acids, approximately how much of the acid dissociates? what does this mean about [H+] concentration?
in weak acids only a few molecules dissociate, about 1 molecule out of 1000 dissociates, so [H+] concentration is much lower than the concentration of the weak acid [HA]
Write an equation for HA?
HA (aq) ⇌ H+ (aq) + A- (aq)
what constant is used for weak acids?
Ka constant
what is the Ka constant?
HA (aq) ⇌ H+ (aq) + A- (aq)
Ka = [H+] [A-] / [HA]
units always = mol dm^-3
the greater the value of Ka the..?
stronger the acid
strong acids ka> 1,weak acids ka < 1
how can we compare Ka values?
using pKa values
How do we calculate pKa and Ka?
if the pH is higher than 0, then the acid is..?
pKa = -log10[Ka]
Ka = 10^-pKa
strong acid pKa < 0, weak acid pKa > 0
write an expression for Ka for the following reaction:
CH3COOH (aq) ⇌ H+ (aq) + CH3COO- (aq)
Ka = [H+] [CH3COO-]/CH3COOH]
Simplify the acid equation for Ka:
HA (aq) ⇌ H+ (aq) + A- (aq)
- it can be assumed that [H+] = [A-]
- so Ka can be simplified to Ka = [H+]^2 / [HA]
Calculate the pH of a 0.025 mol dm^3 solution of nitrous acid (HNO2) (Ka = 4.266x10^-4)
HNO2 (aq) ⇌ H+ + NO2-
ph = 2.49
Exam q’s
the ionic product of water
Give the equilibrium equation for water?
2H2O (l) ⇌ H3O+ (aq) + OH- (aq)
write the simplified version of this equation?
H2O (l) ⇌ H+ (aq) + OH- (aq)
what type of acid does water acts as?
a Br⌀nsted-Lowry acid and a Br⌀nsted-Lowry base
what is the constant for the ionic product for water?
Kw
give the Kw equation?
Kw = [H+] [OH-]
what are the units for Kw?
mol^2dm^-6
why is H2O not included in this equation?
because H2O is essentially constant, as [H2O] is very high
H2O (l) ⇌ H+ (aq) + OH- (aq)
what is the forward dreaction?
the forward reaction is endothermic
what happens when we increase the temperature of this reaction?
if we increase the temperature, equillibrium will shift to the right as the forward reaction is endothermic. so [H+] increases, Kw increases, pH decreases
what happens to the pH of water as temperature increases?
at high temperatures, the pH decreases but the water remains neutral as [H+] = [OH-]
what is the value of Kw all the time?
1.00x10^-14
At 25°, Kw = 1.00 x 10^-14 mol^2dm^-6. calculate the pH of pure water.
Kw = [H+] [OH-]
pure water has [H+] = [OH-]
so Kw = [H+^2]
[H+] = √(1.oo x 10^-14) = 1 x 10^-7
pH = 7.00
Calculating pH of strong bases
what is a base?
a proton acceptor
what is a monobasic acid?
donates one OH- per molecule
e.g NaOH
what is a dibasic acid?
donates 2 OH- per molecule
e.g Ba(OH)2, Ca(OH)2
what are strong bases?
what are some examples of strong bases?
strong bases fully dissociate/ionise in water. (They donate one mole of OH- per one mole of base)
e.g NaOH, KOH
this means that…?
the concentration of of OH- ions equals the concentration of the base
e.g 0.02 moldm^-3 of NaOH, [OH-] = 0.02 moldm^3
to calculate the pH of a base what must you know?
you must know the [H+]
How is the equation rearranged to calculate the [H+] concentration?
[H+] = Kw/[OH-]
find the pH of 0.100 moldm^-3 NaOH. At 25°, Kw = 1.00 x 10^-14.
pH = 13.00
Calculate the pH of strong base, o.250 moldm^-3 KOH, given Kw = 1.00 x 10^-14 at 25°
pH = 13.40
Strong Acid - Strong Base
When can the pH be calculated during Titration
At any time
what are the three stages involved in the calculation?
- calculate the number of moles present of whichever component (H+ or OH-) is in excess
when will equivalence occur?
when stoichiometrically equivalent amounts of acid and base have been added
When a base is being added to an acid, what is in excess in relation to equivalence?
if a base is being added to acid, the acid will be in excess before equivalence, and the base after equivalence
what is step 2 and 3 to calculating Ph of strong acid strong base titration?
- calculate the [H+] - divide the excess moles by total volume of solution
- Calculate the pH -
pH = -log10[H+]
Calculate the pH in the titration of 10.0cm^3 of a 0.15 moldm^3 solution of HCL at the point when 10.0cm^3 of 0.10 moldm^3 solution of NaOH have been added.
moles of H+ = 0.0015
Moles of OH = 0.0010
Moles of H+ in excess = 0.0005
total volume = 20cm^3
[H+] = 0.025 mol dm^3
Ph = 1.60
Calculate the titration of 16.0 cm^3 of 0.16 moldm^3 solution of NaOH at point when12.0cm^3 of a 0.20moldm^3 solution of HCL have been added
- moles of OH = 0.00256
- moles of H = 0.00240
- moles of OH in excess = 0.00016
- total volume = 28.0 cm^3
- [OH-] = 0.00571 mol dm^3
- Kw equation rearranged to [H+]
- ## pH = 11.76
Weak Acid - Strong Base Calculations
when a weak acid and a strong base are reacted together what is formed?
salt and water
e.g CH3COOH + NaOH –> Na+ CH3COO- + H2O
what equation is needed for WASB?
Ka = [H+] [A-] / [HA]
when excess weak acid is present, then ..?
all thre NaOh is neutralised and turned into a salt [A-]
Moles of the salt formed = …?
Moles of the NaOH neutralised
what are the steps to WASB calculations?
- calculate the moles of acid and moles of base
- calculate whether acid or base is in excess
- calculate [A-]
- calculate [H+]
- calculate the pH
Calculate the pH in the titration of 25cm^3 of a 0.100 moldm^-3 solution of CH3COOH at the point when 2.0cm^3 of a 0.100 moldm^-3 solution of NaOH have been added. ka = 1.78 x 10^-5
pH = 3.69
Calculate the pH in the titration of 25cm^3 of 0.100 mol dm^-3 solution f CH3COOH at the point when 25.1cm3 of a 0100moldm^-3 solution of NaOH has been added?
pH = 10.30
Calculate the pH in the titration of 16.0cm^3 of a 0.16 moldm^-3 solution of NaOH at the point when 12.0cm^3 of 0.20 moldm^-3 solution of HCL has been added
what is half equivalence?
- at half equivalence, exactly one-half of the equivalence volume of strong base has been added to the weak acid
- half of the acid has been deprotonated and half is still in the protonated form hence [HA] = [A-]
The half equivalence point represents the point at which exactly half of the acid in the buffer solution has reacted with the titrant.
explain how half equivalence is calculated?
[H+] = ka[HA]/ [A-]
so, [H+] = ka
so, pH = pKa = -log10[ka]
calculate the pH in the titration when 25.0cm^3 of 0.100moldm^-3 ethanoic acid (Ka = 1.76 x 10^-5) was titrated against 12.5cm^3 of 0.100 mol dm-3 solution of NaOH.
pH = 4.75
pH probe calculation
why should a probe be calibrated?
in order to have confidence in the readings of the pH probe it should be calibrated
how do we carry out pH Probe calibration?
- have ready ideally 3 standard buffer solutions of known pH (e.g pH= 4, pH = 7, pH = 9
- the pH probe should be thoroughly rinsed with deionised water and the pH of the standard buffer solutions measured using the pH probe, making sure that the probe is rinsed between each measurement
- reading are recorded in a table
- plot a graph of recorded pH reading (x axis) against the pH of the buffer solution (y axis)
- use your calibration curve/line to correct tour measured pH values
what line is the calibration curve?
can be a straight line or a curve
Buffers
what is a buffer?
a buffer is a solution which resists changes in pH upon addition of small amounts of acid, alkali and upon dilution
what are the two types of buffers?
- acidic
- alkali
what pH values does acidic buffers have?
pH values below 7
what are acidic buffers?
acidic buffers are solutions of weak acid and it’s salt
which two ways can an acidic buffer be made?
- mixing ethanoic acid CH3COOH (aq) and sodium ethanoate H3COONa (aq)
- mixing excess ethanoic acid CH3COOH (aq) with sodium hydroxide NaOH (aq)
what happens during the second way?
some of the ethanoic acid will neutralise all the NaOH and convert it to the salt sodium ethanoate (like WASB)
how can the pH of an acidic buffer be changed?
- altering the ratio of the weak acid to salt ratio
- using different weak acid with a different Ka value
explain how an acidic buffer resists the change in pH when a little HCL is added?
- HA ⇌ H+ + A-
- adding acid H+ reacts with A- (salt)
- H+ + A- –> HA
- so [H+] and pH remain approximately constant
explain how an acidic buffer resists change in pH when a little NaOH (alkali) is added.
- adding alkali, Oh - reacts with H+ forming water, so more weak acid dissociates to replace the H+ used up
- HA –> H+ + A-
- so [H+] and pH remain approximately constant
explain how an acidic buffer resists change in pH on dilution?
- adding water does not change the moles of weak acid, HA or salt A-
- so [HA]/ [A-] ratio stays the same
- so [H+] and pH remain approximately constant
NOTE: [H+] = Ka x [HA] / [A-]
Alkaline buffers
what is the pH of alkalin buffers?
have pH values over 7
What are alkaline buffers?
are solutions of weak bases and it’s salt
which two ways can an alkaline buffer be made?
- mixing ammonium solution (NH3) with ammonium chloride (NH4CL)
- mixing excess ammonia solution with hydrochloric acid (HCL)
explain what happens to in the seond way?
some of the NH3 will neutralise all the HCL and convert it to the salt NH4CL (like SAWB)
how can the pH of an alkaline buffer be changed by?
- altering the ration of the weak base to salt
- using different weak base with a different Kb value
explain how a basic buffer resist change in pH when a little HCL is added
B + H2O ⇌ BH+ + OH-
- adding acid, H+ reacts with OH- to form water, more weak base reacts with water to replace the OH- used up
- NH3 + H2O —> NH4+ + OH-
- so [H+] and pH remain approximately constant
explain how a basic buffer resists change in pH when a little NaOH is added?
- B + H2O ⇌ BH+ + OH-
- adding alkali, OH- reacts with salt NH4+
- NH4+ + OH- –> NH3 + H20
- so [H+] and pH remain approximately constant
explain how a basic buffer resists change in pH on dilution?
- adding water (dilution) does not change the moles of NH2, weak base or salt (NH4+)
- so [NH3]/[NH4+] ratio stays the same
- so [OH-], [H+] and pH remain approximately constant
give 4 uses of buffers?
- in fermentation
- in making dyes
- intravenous injections
- to calibrate pH meters
Buffer pH calculations
what can the pH of an acidic buffer be calculated using?
Ka expression for a weak acid, BUT REARRANGED to calculate [H+]
Ka = [H+] [A-] / [HA] –>
[H+] = Ka [HA]/ [A-]
what steps do we need to go through to calculate the pH of an acidic buffer?
- same method for weak acid strong base calculations
Calculate the pH of a buffer made by mixing 14.0 cm^3 of 2.0 mol dm^3 solution of ethanoic acid (Ka = 1.74 x 10^-5) with 15.0 cm^3 of a solution of 1.50 mol dm^3 sodium ethanoate
answer = 4.66
what happens to the position of equilibrium when we add acid?
- HA ⇌ H+ + A-
- addition of acid will shift equilibrium to the left, as the salt (A-) will react with H+ to form HA
So how do we calculate the change in pH when acid is added?
- need to calculate the moles of H+ added and then the new number of moles of the weak acid and salt
- then use Ka expression to calculate [H+] and pH
How will the moles after the addition of acid change?
- moles of A_ will have decreased
- Moles of HA will have increased
Calculate the change in pH of the buffer solution (from example 1) after the addition of 10cm^3 of 0.10 mol dm^3 solution of HCl.
change in pH = -0.03
How does the position of equilibrium change when we add an alkali to a buffer solution?
- addition of an alkali will shift equilibrium to the right, as the weak acid (HA) will dissociate yo replenish the H+ used by the addition of OH-
how do we calculate the change in Ph of a buffer when an alkali is added?
- need to calculate the moles of OH- added and then the new number of moles of weak acid and salt
- then use Ka expression to calculate the [H+] and pH
How will the addition of alkali change the moles?
- moles of A- will have increased
- moles of HA will have decreased
Calculate the change in pH of the buffer solution (from example 1) after the addition of 10cm^3 of 0.10 mol dm^3 solution of NaOH.
answer = change in pH = 0.02
Indicators and pH curves
What is an indicator?
an indicator is a coloured substance that changes colour over a narrow pH range. Different indicators change colour over different pH ranges
what type of acids are indicators?
What determines the colour change?
indicators are weak acids where the protonated form HIn is a different colour from the deprotonated form In-
Draw an equation to show the reaction of how an indicator works?
HIn (aq) (colour one) ⇌ H+ + In- (colour 2)
explain the colour change?
at HIn (protonated form) major species at low pH (acid) and at In- (deprotonated form) major species are at high pH (alkali)
which two indicators can be used for titration?
phenolpthalein and methylorange
what is the colour of Phenolpthalein and Methylorange at HIn?
phenolpthalein - colourless
Methylorange - red
what is the colour of Phenolpthalein and methylorange at In-?
phenolpthalein - pink
Methyl orange - yellow
What is the pH range for both methylornage and phenolpthalein
phenolpthalein = approximately 9 (pH -> 8.2 - 10.0)
methylorange = approximately 4 (pH = 3.2 - 4.4)
where are indicators originally obtained from?
from plant materials e.g red cabbage
where are modern indicators now obtained from?
most modern indicators are made by organic synthesis just like most modern dyes.
what do most indicators now contain?
most indicators are arenes, contain benzene rings and have delocalised electrons
in order to obtain accurate titration results, when must the indicator change colour?
in order to obtain accurate titration results, the indicator must change colour on the vertical part of the pH curve for the titration
what indicator should be used for strong acid - strong base (SASB) titration?
phenolpthalein or methyl organge
what indicator should be used for weak acid - strong base (WASB) titration?
phenolpthalein
what indicator should be used for strong acid - weak base (SAWB) titration?
methyl organge
what indicator should be used for a weak acid - weak base titration?
use a pH meter - can not use an indicator
pH curves
what is the definition for equivalence?
when stiochiometrically amounts of acid and base have been added
How many moles of H+ and OH- has been added? And how many moles have reacted?
- equal amounts.
1 mole of HCl has reacted with 1 mole of NaOH
Give the definition for Half equivalence (for WASB) ?
at half equivalence, exactly one-half of the equivalence volume of strong base has been added to the weak acid
Give the definition for half equivalence when [HA] = [A-]
Half of the acid has been deprotonated and half is still in it’s protonated form this means that [HA] = [A-]
equation for half-equivalence?
- Ka = [H+] [A-]/[HA]
- so at half equivalence Ka = [H+]
- so pKa = pH
Draw a pH curve for a SASB titration, and annotate the graph?
describe:
a) the pH at equivalence
b) the pH of phenolpthalein indicator
c) the pH of methylorange indicator
a) pH = 7
b) Ph = 9
c) pH = 4
Draw a pH Curve for WASB titration, and annotate the graph?
describe:
a) the pH at equivalence
b) the pH of phenolpthalein indicator
a) pH at equivalence > 7
b) 9
What indicator can we use for SASB titration?
For SASB titration pH at equivalence is 7 so we can use either phenolpthalein or methylorange
What indicator can we use for WASB?
for WASB titration pH at equivalence is greater than 7, so we must use phenolpthalein indicator which changes at Ph ≈ 9
Draw a pH curve for strong acid - weak base titration, plus annotations?
what is the pH at equivalence
pH at equivalence > 7 (lessthan)
this means that…?
methyl orange indicator must be used which changes colour at Ph = 4
Draw a Ph curve for weak acid - weak base titration, include annotations?
what is the pH at equivalence?
pH at equivalence is approximately 7 nut there is no vertical line - so not really any equivalence
this means that…?
we use a pH meter, and can not use an indicator because there is no vertical part on the pH curve
FINISH
