3.1.12 Acids and bases Flashcards
What is a Bronsted - Lowry acid?
- Proton donors
- When we mix acids with water , hydrogen ions (H+) are released
What are Bronsted- Lowry bases
- Proton acceptors
- When we mix bases with water they react with H+ ions to form hydroxide ions (OH-)
What do strong bases do
Dissociate (ionise ) almost completely
What do weak bases do
Dissociate poorly
what do strong acids do
Dissociate (ionise) almost completely
What do weak acids do
Dissociate poorly
What is an example of a weak acid and some information about it
- Ethanoic acid and other carboxylic acids
- Backwards reaction favoured so not many H+ (protons) produced
What are some examples of a strong acid and some information about it
- HCl (hydrochloric acid)
- H2SO4 ( Sulfuric acid)
- HNO3 ( nitric acid)
Forwards reaction favoured strongly . Lots of H+ produced
What are examples of a strong base and some information about it
NaOH ( sodium hydroxide )
KOH ( potassium hydroxide)
Forward reaction favoured strongly . Lots of OH- ions produced
What is an example of a weak base and some information about it
NH3 ( ammonia)
Backwards reaction favoured so not many OH- ions produced
What happens when acids and bases react with each other
Protons are exchanged
What does water do when acid is added to it
Acts as a base (accepts a proton)
What is the Kc equation to represent water
[H+][OH-] / [H2O]
What can water dissociate into and what is the equation to show this
Water dissociates into OH- ions and H+ ions
H2O (reversible reaction) H+ + OH-
What is Kw?
Ionic product of water
What are the units for Kw?
mol^2dm-6
what is the expression for Kw
[H+][OH-]
What is the value of Kw
1.00 x10^-14 mol^2dm^-6
When does the value of Kw change
When temperature changes
- Value of Kw is the same in a solution at a given temperature
What concentration of OH- ions and H+ ions does pure water have
The same concentration of H+ and OH- ions.
What is the expression for pure water
Kw = [H+]^2
What is pH
Is a logarithmic scale that measures the concentration of H+ ions in solution
What is the pH equation
pH = -log10[H+]
What is the equation to work out H+ ions from pH?
[H+] = 10^-pH
(press shift then log for the 10)
What are examples of monoprotic acids
HCl
HNO3
What are monoprotic acids
They are acids which dissociate to produce one H+ ion for every acid molecule. This means the he concentration of the acid = the concentration of H+ ions
What are diprotic acids
They dissociate to produce two H+ ions for every acid molecule. This means the concentration of the acid = 2 x the concentration of the H+
ions
What is an example of a diprotic acid
H2SO4 - sulfuric acid
What is the equation to work out the pH of a diprotic acid from H+ ions
2 [H+] = [Acid]
pH = -log10[H+]
What do strong bases do
Dissociate to produce one OH- ion for every base molecule . This means the concentration of the base = the concentration of the OH- ions
How do we calculate the pH of a base
We need the [H+] . To get this we need to use the Kw expression.
To work out [H+] we need to know Kw and [OH-] at a specific temperature . Once we know [H+] we can work out the pH
what decimal place should pH always be in
To 2 decimal places
what is the equation to work out [H+] of diluted solution once water is added
[H+] of original acid solution x old volume / new volume
why do we have to use another constant for weak acids
weak acids only dissociate slightly in aqueous solutions so we have to use another constant to help work out their pH values
what is the equilibrium constant for weak acids
Acid dissociation constant Ka
what’s the equilibrium for weak acids
HA (reversible reaction sign) H+ + A-
What are the two assumptions of weak acids
1) Only a small amount of the weak acid dissociates so we can assume that the concentration of the acid at equilibrium is approximately equal to the acid at the start
Ka = [H+] [A-] / [HA]
• the dissociation of acid is greater than the dissociation of water present in the solution . We can assume all the H* ions come from the acid
Ka = [H+] ^2 / [HA]
what are the units for Ka
moldm-3
what’s the expression for Ka
Ka = [H+] [A-] / [HA]
what can the Ka expression be simplified to
Ka = [ H+] ^2 / [HA]
What is pKa
Another way of measuring the strength of an acid similar to pH. The lower the value , the stronger the acid
What’s the expression for pKa
pKa = -log10Ka
What’s the expression for pKa
pKa = -log10Ka
What’s the expression for Ka from pKa
Ka = 10 ^-pKa
what’s the expression if HA = OH ( half a neutralisation reaction )
pH = pKa
EQ : Ionic product of water , Kw = [H+] [OH-]
Explain why H2O is not shown in the expression
H20 is constant
EQ : Explain why water is neutral at 50 degrees ( example ) (1)
[H+] = [OH-]
EQ: For pure water at 40 degrees , pH = 6.67 . A student thought that the water was acidic . Explain why the student was incorrect (1)
[H+] = [OH-]
How are titration practicals performed
- have an acid or base in burette with known concentrations
- add the chemical in the burette to the conical flask until the indicator changes colour. This is known as the end point
- Add drop by drop near end point
- Have an acid or base with an unknown concentration but known volume in conical flask , add a few drops off indicator too
what is the titration curve of strong acid and. strong base
S shaped
- graph starts at pH 1 as there is excess strong acid
- Ends at around pH 13 as we now have excess strong base
what is the titration curve of strong acid / weak base
smaller s shape
- graph starts at pH 1 as there is excess strong acid
- Ends at around pH 9 as we now have excess weak base
What is the pH curve of weak acid / strong base
S shaped at the top
- graph starts at around PH 5 as there is excess weak acid
- Ends at around pH 13 as we now have excess strong base
what is the titration curve for weak acid and weak base
not a prominent S shaped curve ( quite straight )
- graph starts at around pH 5 as there is excess weak acid
- Ends at around pH as we now have excess weak base
what is the equivalence point
At this point the acid has been neutralised fully by the base . The sharp vertical rise shows a rapid change in pH
when is the change in pH the smallest
When using a weak acid and weak base together in a titration
what is the half neutralisation point
this is the point half way between zero and the equivalence point . It can be used to calculate pKA of a weak acid by taking the pH at this point
How can we work out pKa using a titration curve
[HA] = [A-]
Therefore Ka = [H+]
-logKa = -log[H+]
so pka = pH
what are the two most common types of indicators
methyl orange
phenolphthalein
what is the colour change for methyl orange and when do you use it
red at low pH and yellow at high pH
It can be used for :
- strong acid / strong base
- strong acid / weak base
what is phenolphthalein used in and what is the colour change
Colourless at low pH and pink at high pH
It can be used for :
- Weak acid / strong base
why can’t you use a pH indicator for weak acids and weak base reactions
They have no sharp pH change . No indicator is suitable so we have to use a pH meter
why do diprotic acids have 2 pH curves
- when reacting with a base it’s neutralised in 2 steps
- This is because protons are released from acid molecules separately
• first equivalence point shows how OH- ion is reacting with the first proton
• second equivalence point shows how OH- ion is reacting with the second proton
What is a buffer
A chemical that resists the change in pH when small amounts of acid or base are added
what are the 2 types of buffer
- acidic
- basic
what is an acidic buffer
they resist the change in pH in order to keep the solution below pH 7. They are made from a weak acid and its salt
In an acidic buffer , where does the equilibrium lie in the weak acid
- 2 equilibrium equations co-exist in the same beaker
• Weak acids dissociate weakly so equilibrium lies well over to the left
In an acidic buffer , where does equilibrium lie in salts
- 2 equilibrium equations co-exist in the same beaker
- Salts dissociate fully so equilibrium lies well over to the right
what are basic buffers
Resist the change in pH in order to keep the solution above pH 7.
They are made from a weak base and its salt
in a basic buffer , where does equilibrium lie with its weak base
weak based produce very little OH- ions so equilibrium lies well over to the left
In a basic buffer where does equilibrium lie in its salts
Salts dissociate strongly so equilibrium lies well over to the right
what do you need to know to calculate the pH of a buffer
The Ka value and the concentration of the weak acid and its salt
what can we assume in calculations of butters
Assume salts dissociate fully and weak acids dissociate poorly so
salt = [A-]
and
[HA start] = [HA equilibrium]
what are the uses of buffers in shampoo
Shampoo are acidic with a pH if 5.6
Hair becomes dry and damaged if exposed to alkaline conditions
The buffer helps to resist changes in the pH to keep hair soft and strong
what are the uses of buffers in washing powder
Biological washing powders contain enzymes which can only work efficiently at a specific pH
Buffers help to resist changes in pH to allow them to work in their optimum pH
what are the uses of buffers in blood
It’s vital to make sure blood pH is maintained a close to pH 7.4 as possible
Our body systems rely on this so a buffer is present in our blood to help. CO2 plays a big role here
EQ : what is meant by the term strong when describing an acid (1)
Completely ionises to give H+ ions in water
what is the end point
stage in a titration where there’s a colour change
EQ : state why calibrating a pH meter just before it is used improved the accuracy of the pH measurement (1)
Over time meter does not give accurate readings
EQ : Describe how you would obtain the pH curve for the titration (5)
- measuring starting pH of acid
- Add alkali from burette in 1cm3 portions ( 1-2cm3)
- Stir or swirl flask
- Measure pH after each addition
- Repeat until alkali in excess
- add alkali in smaller increments, 0.1cm3 as you near the end point
EQ : Suggest a suitable piece of apparatus that could be used to measure out the sodium hydroxide solution . Explain why this apparatus is more suitable than a pipette for this purpose (2)
Burette
Can deliver variable volumes
EQ : Explain why chloroethanoic acid is a stronger acid than ethanoic acid (2)
- Chlorine is more electronegative so withdraws electrons
- Weakens O-H bond
EQ : Explain why data books do not usually contain values of Ka for strong acids (2)
- Strong acids completely dissociated .
- Ka value for strong acids is very large
EQ : Explain why the expression for Kw does not include the conc for water (2)
- Conc of H2O is very high / very few H+ and OH- ions
- Conc of H2O is constant
EQ : Explain why the value of Kw increases as the temp increases (2)
- Breaking bonds is endothermic
- Equilibrium shifts to the right to oppose increase in temp
EQ : HCO3- (reverse sign) CO3 2- + H+
Explain how a solution containing sodium hydrogencarbonate and sodium carbonate can act as a buffer when small amounts of acid or small amounts of alkali are added . (3)
acid :
• Increase in concentration of H+ ions , equilibrium moves to the left .
alkali :
• OH- reacts with H+ ions , equilibrium moves to the right to replace the H+ ions
- Concentration of H+ remains constant
EQ : explain why [H2O] is not shown in the Kw expression
[H2O] is constant
EQ : Suggest why the pH prove is washed with distilled water between each of the calibration measurements (1)
Different solutions must not contaminate each other
EQ : The calibrated pH meter is used to monitor the pH using a titration of HCl and NaOH.
Explain why the volume of NaOH solution added between each pH measurement is smaller as the end point of the titration is approached (1)
To avoid missing the end point
EQ : A buffer solution has a constant pH even when diluted . Use a mathematical expression to explain this . (1)
[HX] / [X-]
