Acid Base Flashcards
What is the Arrhenius acid-base theory
Acids produce H+ ions in water and alkalis produce OH- ions in water
What is the Brønsted-Lowry acid base theory
Acid is a proton donor
Base is a proton acceptor
What is the Lewis acid-base theory
Acid is an electron pair acceptor
Base in an electron paid donor
What is a conjugate base
When an acid loses a proton
What is a conjugate acid
When a base loses a proton
What is an Amphoteric substance
One that can act as both an acid and a base
What is a strong acid
One that fully dissociates in solution
Formula for pH
pH = -log[H+]
Formula for [H+]
[H+] = 10 to the power of negative PH
How to calculate pH for a strong mono protic acid
[H+] = [acid]
Then use pH equation
How to calculate the pH of a diprotic strong acid
Calculate [H+]
Then use pH equation
What is the equation for Kc
Kc = [H+][OH-] / [H2O]
What assumption do you use when using Kc and how does it lead to the Kw equation
Because [H2O] is very large due to small amount of dissociation, you assume it to be constant
Therefore Kw= [H+][OH-]
When does Kw change
With temperature
When is the pH of pure water different to 7 and does this make water acidic/alkali
As temperature changes, the pH of water will also change
However, it is still neutral as [H+] = [OH-]
How does Kw change as temp increases
As temp increases, equilibrium moves right
Why can you use Kw to calculate pH of alkaline solution
Because the [OH-] from H2O is so small we can ignore it and assume that all of the OH- in the solution comes from the alkali
What acids are weak acids and why
All organic acids
Because the H-F bond is strong due to hydrogen bonding, making it hard to fully dissociate
Ka equation
Ka = [H+][A-] / [HA]
What is Ka
Acid dissociation constant- measure of extent of dissociation
Assumptions used when using Ka
[H+] = [A-] ignoring H+ from water
[HA] at equilibrium = [HA] at start due to such small dissociation
Show how Ka used to calculate pH of Ethanoic acid
Ka = [H+] squared / [CH3COOH]
What happens when you dilute a weak acid by 10
Won’t result in a rise of one unit
HA + H2O becomes H3O+ + A-
When water added equilibrium shifts right, producing more H3O+ increasing pH but by less than 1
How can u follow an acid base titration
Using indicators
Using a pH probe
Issues with using indicators
Nah not show when [H+] = [OH]-
Will a stronger acid have a higher or lower Ka
Stronger acid - higher Ka
Will a stronger acid have a higher or lower pKa
Stronger acid - Lower pKa
Steps to calculate pH of a mixture of a strong acid and strong base
Calculate moles of H+ from acid
Calculate moles of OH- from alkali
Calculate excess of H+ or OH-
Calculate conc of excess
Put into pH formula
How to calculate pH of mixture of weak acid and strong base
Calculate moles of HA (NOT H+)
Calculate moles of OH-
Calculate XS of HA/OH-
If OH- in excess- sub into pH formula
If HA in excess- calculate moles of [HA] remaining and [A-] formed
Sub into Ka formula to get pH
When is equivalence for a strong acid and strong base
Equivalence at pH 7
When is the equivalence point for a strong acid and weak base
Equivalence < pH7
Equivalence point for a weak acid and strong base
pH > 7
What is an indicator
Indicators are weak acids or bases that change colour when they lose or gain protons
Why does the colour of litmus change
Adding H+ —> shifts equilibrium left and turns red
Adding OH- —> shift equilibrium right and turns blue
Colour of methyl orange in acid, alkali and neutral
Acid- red
Neutral- orange
Alkali- yellow
Colour of phenolphthalein in acid, alkali and yellow
Acid- colourless
Neutral- very pale pink
Alkali- pink
How to choose an indicator
Choose an indicator that has its pH range within the vertical section of the pH curve
When to use methyl orange
Titration involving Strong acid
When to use phenolphthalein
Titration involving a strong base
Key points in drawing titration curve
Calculate start pH value using conc of acid/alkali in the flask
Calculate equivalence point
For a titration with a weak acid/alkali include kink at start and buffering region
Where does the equivalence point centre round for a weak acid and where is straight section
Centres around pH 8-9
Straight section pH 6.5-11.5
Where does the equivalence point centre round for a weak alkali and where is straight section
Centres around pH 6
Straight section pH 3.5-8.5
How long should a straight section on titration graph be for weak/strong
No longer than 5 pH units long
How long should a straight section on titration graph be for strong/strong
No longer than 7 units long
What is a buffer and what does it contain
A buffer is a solution that resists changes in pH when small amounts of acid/alkali are added
Acid buffers- weak acid and one of its salts
Alkaline buffers- ammonia and ammonium chloride
Why do acid buffets resist changes in pH
Large reservoir of acid and salt
Small changes to [HA] and [A-] means little affect on conc so small amount of pH change
Assumptions in buffer calculations
Because [HA] hardly dissociated we assume [HA]eq = [HA]initial
Because HA hardly dissociated the amount of A- formed from dissociation is negligible [A-]eq = [A-]initial
How to calculate pH of a buffer
[H3O+] = Ka times [acid]/[salt]
Then sub into pH equation pH = -log[H3O+]
Ionic equations for buffers
Adding alkali : HA + OH- becomes A- + H2O
Adding acid: H+ + A- becomes HA
Why does the buffering region occur
Remaining excess of undissociated acid and increasing quantity of salt
Created a buffer that resists further sharp change in pH
How does breathing faster and deeper change blood pH
Breathing faster and deeper speeds up removal of CO2 from the lungs, lowering conc of carbonic acid in blood, making it more alkaline and raising blood pH
How does exercise change blood pH
During exercise cells produce more CO2, increasing the H+ ion conc and thus lowering pH
Moreover, exercise can cause latic acid production, acidifying blood
How to calculate conc of H+ for a strong alkali
Conc of H+ = Kw/ conc of OH-
Steps to calculate pH of a weak acid
Calculate [H+] - normally given or using pH
Ka = [H+] squared / concentration
Rearrange to find [H+]
Sub into pH formula
How to calculate pH of a strong alkali
[H+] = Kw/ [OH-]
Then sub into pH formula
Else use pH = 14 - pOH
How calculate pH and pKa using weak acid NaOH titration
Pipette 25cm3 of weak acid into flask
Add phenolphthalein
Titrate against NaOH until end point reached
Note volume of NaOH added
Add another 25cm3 of acid but no indicator
From burette add half original volume of NaOH
Measure the pH
That pH = pKa
Equivalence point for weak acid strong base
Half equivalence point
Halfway up straight section of curve
Marking points for if region of slow pH growth shown on graph
Mention buffering
Identify species present- HA and A-
How species were formed- CH3CH2COOH + OH- becomes H2O + CH3CH2COOH- excess CH3CH2COOH left
Marking points for “is pure water always pH7”
No
As Temp increases, equilibrium moves to RHS
So [H+] increases
Hence pH > 7
