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Acid Base Flashcards

1
Q

What is the Arrhenius acid-base theory

A

Acids produce H+ ions in water and alkalis produce OH- ions in water

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2
Q

What is the Brønsted-Lowry acid base theory

A

Acid is a proton donor
Base is a proton acceptor

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3
Q

What is the Lewis acid-base theory

A

Acid is an electron pair acceptor
Base in an electron paid donor

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4
Q

What is a conjugate base

A

When an acid loses a proton

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5
Q

What is a conjugate acid

A

When a base loses a proton

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6
Q

What is an Amphoteric substance

A

One that can act as both an acid and a base

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7
Q

What is a strong acid

A

One that fully dissociates in solution

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8
Q

Formula for pH

A

pH = -log[H+]

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9
Q

Formula for [H+]

A

[H+] = 10 to the power of negative PH

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10
Q

How to calculate pH for a strong mono protic acid

A

[H+] = [acid]

Then use pH equation

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11
Q

How to calculate the pH of a diprotic strong acid

A

Calculate [H+]

Then use pH equation

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12
Q

What is the equation for Kc

A

Kc = [H+][OH-] / [H2O]

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13
Q

What assumption do you use when using Kc and how does it lead to the Kw equation

A

Because [H2O] is very large due to small amount of dissociation, you assume it to be constant

Therefore Kw= [H+][OH-]

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14
Q

When does Kw change

A

With temperature

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15
Q

When is the pH of pure water different to 7 and does this make water acidic/alkali

A

As temperature changes, the pH of water will also change

However, it is still neutral as [H+] = [OH-]

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16
Q

How does Kw change as temp increases

A

As temp increases, equilibrium moves right

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17
Q

Why can you use Kw to calculate pH of alkaline solution

A

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

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18
Q

What acids are weak acids and why

A

All organic acids

Because the H-F bond is strong due to hydrogen bonding, making it hard to fully dissociate

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19
Q

Ka equation

A

Ka = [H+][A-] / [HA]

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20
Q

What is Ka

A

Acid dissociation constant- measure of extent of dissociation

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21
Q

Assumptions used when using Ka

A

[H+] = [A-] ignoring H+ from water

[HA] at equilibrium = [HA] at start due to such small dissociation

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22
Q

Show how Ka used to calculate pH of Ethanoic acid

A

Ka = [H+] squared / [CH3COOH]

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23
Q

What happens when you dilute a weak acid by 10

A

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

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24
Q

How can u follow an acid base titration

A

Using indicators

Using a pH probe

25
Issues with using indicators
Nah not show when [H+] = [OH]-
26
Will a stronger acid have a higher or lower Ka
Stronger acid - higher Ka
27
Will a stronger acid have a higher or lower pKa
Stronger acid - Lower pKa
28
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
29
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
30
When is equivalence for a strong acid and strong base
Equivalence at pH 7
31
When is the equivalence point for a strong acid and weak base
Equivalence < pH7
32
Equivalence point for a weak acid and strong base
pH > 7
33
What is an indicator
Indicators are weak acids or bases that change colour when they lose or gain protons
34
Why does the colour of litmus change
Adding H+ —> shifts equilibrium left and turns red Adding OH- —> shift equilibrium right and turns blue
35
Colour of methyl orange in acid, alkali and neutral
Acid- red Neutral- orange Alkali- yellow
36
Colour of phenolphthalein in acid, alkali and yellow
Acid- colourless Neutral- very pale pink Alkali- pink
37
How to choose an indicator
Choose an indicator that has its pH range within the vertical section of the pH curve
38
When to use methyl orange
Titration involving Strong acid
39
When to use phenolphthalein
Titration involving a strong base
40
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
41
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
42
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
43
How long should a straight section on titration graph be for weak/strong
No longer than 5 pH units long
44
How long should a straight section on titration graph be for strong/strong
No longer than 7 units long
45
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
46
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
47
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
48
How to calculate pH of a buffer
[H3O+] = Ka times [acid]/[salt] Then sub into pH equation pH = -log[H3O+]
49
Ionic equations for buffers
Adding alkali : HA + OH- becomes A- + H2O Adding acid: H+ + A- becomes HA
50
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
51
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
52
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
53
How to calculate conc of H+ for a strong alkali
Conc of H+ = Kw/ conc of OH-
54
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
55
How to calculate pH of a strong alkali
[H+] = Kw/ [OH-] Then sub into pH formula Else use pH = 14 - pOH
56
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
57
Equivalence point for weak acid strong base
Half equivalence point Halfway up straight section of curve
58
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
59
Marking points for “is pure water always pH7”
No As Temp increases, equilibrium moves to RHS So [H+] increases Hence pH > 7

Chemsitry (15 decks)

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