pH and Acids Flashcards
What is the Bronstead-Lowry acid-base theory?
An acid is a proton donor
A base is a proton acceptor
What are conjugate acid-base pairs?
Once an acid has “donated” a proton it would become able to “accept” a proton back and hence act as a base
Give an example of an acid
HCl
Given an example of a base
Cl^-
What are the acid base pairs in:
HCl + H2O ⇌ H3O^+ + Cl^-
Acid 1 = HCl Base 1 = Cl^-
Acid 2 = H3O^+ Base 2 = H2O
What are the acid base pairs in:
CH3COOH + H2O ⇌ CH3COO^- + H3O^+
Acid 1 = CH3COOH Base 1 = CH3COO^-
Acid 2 = H3O^+ Base 2 = H2O
What do strong acids do?
Completely dissociate
How do you calculate the pH of a strong acid?
-log[H+]
Give an example of a monobasic acid
HCl
Give an example of a dibasic acid
H2SO4
Give an example of a tribasic acid
H3PO4
What do you do if you’re calculating pH when you have a di/tribasic acid
Multiple the [H+] by 2 or 3
How do you calculate [H+] for strong acids?
10 ^ -pH
How do you calculate pH changes on dilution?
Calculate fraction for volume
Divide conc by fraction
The calculate pH using -log[H+]
Calculate pH 25cm3 of 0.05 diluted with 75cm3 of water
75/25 = 1/3
0.05/ 3 = 0.0167
pH = -log(0.0167) = 1.78
How do you calculate the pH of a dilute acid?
[H+] old X old vol/ new vol
New vol = (original + added vol)
What do weak acids do?
Only partially dissociate
What is the equation for a strong acid?
HA —-> H+ + A-
What is the equation for a weak acid?
HA ⇌ H+ + A-
What is the Ka equation?
[HA]
What can the Ka equation be shortened to?
[HA]
What assumptions must be made for Ka?
1) [H+] = [A-]
2) [HA] start = [HA] eqm
What alters Ka?
Temperature
What does the larger the Ka value tell you?
Equilibrium lies to the right
= pH lower as more [H+]
Why is it easier to give Ka as its negative logarithm?
Because Ka very small so difficult to compare
How do you calculate pKa?
-logKa
How do you calculate Ka from pKa?
10 ^ -pKa
What does the higher the value of pKa tell you?
The weaker the acid
What does the lower the value of pKa tell you?
The stronger the acid
How do you calculate the pH of a weak acid?
[H+] = √Ka X [HA] pH = -log[H+]
What are the steps for calculating pH of a weak acid?
Write out Ka expression
Calculate [H+]
Calculate pH
What are the problems with Ka calculations approximations?
1) At pH values >6 water dissociation is significant
= doesn’t work for very weak acids/ dilute solutions
2) If [H+] significant there will be a difference between [HA} start and [HA] eqm
= doesn’t work for stronger acids with Ka> 10-2 / very dilute solutions
What does the [H+] of a weak acid depend on?
Value of Ka and [HA]
What happens if a di/tribasic weak acid dissociates twice in 2 following reactions?
The first reaction will always have the stronger acid (lower pKa value)
What does water do?
Ionises only slightly, acting as both an acid and base - setting up equilibrium
How much does water dissociate?
Very small amount
How do we know what is dissociates only a very small amount?
Conducts electricity
What is the equilibrium equation for water?
H2O ⇌ H+ + OH-
Where does equilibrium lie in the equation for water?
Left
What is the constant for “ionic product of water”?
Kw
What is the equation for Kw?
Kw = [H+] [OH-]
What is the K equation for water?
K = [H+][OH-]
—————
[H2O]
What is Kw dependent on?
Temperature
How do you calculate the pH of 0.4 moldm-3 of NaOH?
Using Kw
Kw = [H+] [OH-] [H+] = 1X10-14 / 0.4 = 2.5X10-14 pH = -log[H+] -log(2.5X10-14) = 13.60
Why is it useful to know the value of Kw?
Can rearrange the equation give [H+] or [OH-]
What are the concentrations of H+ and OH- in a solution of pH 3.25 at 25 degrees?
[H+] = 10 ^-pH = 10^ -3.25 = 5.62X10-4 [OH-] = 1X10-14 / 5.62X10-13 = 1.78X10-11
Why is it easy to calculate pH value that are whole numbers?
As [H+] and [OH-] indices add up to 14 eg. Acid solution = 3 [H+] = 10^ -3 14-3 = 11 [OH-] = 10^ -11
What does the value of Kw control?
Concentration of each ion
What is the solution if [H+] > [OH-]?
Acidic
What is the solution if [H+] < [OH-]?
Alkaline
What is the solution if [H+] = [OH-]?
Neutral
What is a neutral solution?
When there is an equal number of moles of H+ and OH- ions
NOT when it has a pH of 7
What can K w also tell us?
The pH of pure water
What assumption can we make when H2O splits into equal concentrations of OH- and H+?
Kw = [H+] [OH-] = 1X10-14 [H+]^2 = 1X10-14 - This is squared as [H+]=[OH-] [H+] = √1X10-14 pH = -log(1X10-7) = 7
What happens to the value of Kw when temp is increased?
Kw increases
What does the pOH scale measure?
OH- conc
What is the value for the ionic product of water at 25 degrees?
1X10^-14 mol^2dm^-6
What are buffer solutions?
Systems that minimise pH changes on the addition of small amounts of an acid or base
When is the buffer most effective?
When [HA]=[A-]
When is the buffer no longer effective?
When one of the components are completely used up
Why shouldn’t a buffer be considered a constant?
As its pH will vary slightly when reacting with any added species
What happens to a buffer if there is an addition of an acid?
[H+] increases
Conjugate base (A-) reacts
Equilibrium shifts left
[H+] removed
What happens to a buffer if there is an addition of an alkali?
[OH-] increases
H+ reacts
Equilibrium shifts right
HA dissociates restoring [H+]
What pH must blood maintain?
7.4 +/-0.05
What is the most important buffer in the body (blood)?
Carbonic acid - hydrogencarbonate
What happens to a person if their blood becomes too acidic?
Fatigue
Shortness of breath
Shock/death
What happens to a person if their blood becomes too alkaline?
Spasms
Light-headed
Nausea
What is called when the blood becomes too acidic?
Acidosis
What is called when the blood becomes too alkaline?
Alkalosis
What happens when the body produces more acidic products?
[H+] increases
Conjugate base (HCO3-) will react with H+
Equilibrium shifts left to remove H+ from blood
What happens if acid levels continue to rise in the body?
pH would fall leading to acidosis
Symptoms would show
As [HCO3-] could be used up
So buffer system will not function
What will the body do to prevent the build up of CO2?
Body convert H2CO3 to CO2 to be expelled from lungs
What are the two ways to form a buffer?
Adding solutions of weak acid + its conjugate base
Partial neutralisation of weak acid
How can adding solution of weak acid + its conjugate form a buffer?
Dissociation of weak acid = very low = assume conc of acid remains same
As salts are ionic = completely dissociate in solution to provide conc for conjugate base
How can partial neutralisation of a weak acid form a buffer?
Adding an alkali to an excess of weak acid
= neutralise some of acid to form conjugate base + leave some acid over
What the equation you would use for buffers?
[H+] = [HA]
Ka ————
[A-]
How do you calculate the pH of a buffer?
Write the Ka expression
Rearrange expression for [H+]
Convert [H+] to pH
What must you do if you need to calculate the pH of a buffer but are not given the concs?
Use conc = moles X1000 / vol
Use ice table to calculate final conc
Place in [H+] = Ka[HA]/[A-]
Calculate pH = -log
What does the shape of titration curve depend on?
Substance being titrated = base or acid
Type of acid or base = strong or weak
Why is the equivalence point of a strong acid-strong base titration pH 7?
H+ completely neutralised by OH- so only water + salt present = pH 7
Why is the equivalence point of a strong acid-weak base titration pH <7?
Weak base (NH3)will have strong conjugate base (NH4+), which will react with water to produce H3O+
Why is the equivalence point of a weak acid-strong base titration pH >7?
Weak acid, Ha, will have a strong conjugate base, A-, which reacts with water to produce OH-
What is an acid-base indicator?
Weak acids that have different coloured conjugate bases
What happens in acidic conditions to the indicator?
Indicator equilibrium shifted towards right
What happens in basic conditions to the indicator?
Equilibrium shifts towards conjugate base
When is the endpoint reached?
When equal HIn and In- are present
What should happen to the colour when the end point is reached?
Colour lie between 2 extremes
Why will each indicator have a different pH value for the end point?
Because they each have different Ka values
Which indicator can be used for a strong acid-strong base titration?
Methyl orange/ phenolthanein
Which indicator can be used for a strong acid-weak base titration?
Methyl orange
Which indicator can be used for a weak acid-strong base titration?
Phenolthanein
What would be used in a strong acid-base titration?
pH meter to monitor the change in pH as an acid is added to a base
Describe the method for the titration of strong acid- base
Pipette 10cm3 of 0.5M NaOH into conical flask
Place pH probe into flask + record pH
Fill burette with 0.5M HCl
Add 1cm3 of acid to flask, then swirl + record pH
Repeat step 4 until pH changes rapidly
Now add acid dropwise till pH changes less rapidly
Continue adding 1cm3 at a time until acid in excess
Plot data on graph of acid vol to pH, including line of best fit
LEARN GRAPHS FOR TITRATIONS
DO IT
Describe the graph of a strong acid-base
Excess of pH = pH decreases as acid added
Vertical section = acid-base conc similar, pH alters rapidly
Excess of acid = pH decreases slightly as acid added
Where is the equivalence point on the titration graphs?
Halfway up the vertical section of the graph
