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Chemsitry > Acid Base > Flashcards

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
Q

Issues with using indicators

A

Nah not show when [H+] = [OH]-

26
Q

Will a stronger acid have a higher or lower Ka

A

Stronger acid - higher Ka

27
Q

Will a stronger acid have a higher or lower pKa

A

Stronger acid - Lower pKa

28
Q

Steps to calculate pH of a mixture of a strong acid and strong base

A

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
Q

How to calculate pH of mixture of weak acid and strong base

A

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
Q

When is equivalence for a strong acid and strong base

A

Equivalence at pH 7

31
Q

When is the equivalence point for a strong acid and weak base

A

Equivalence < pH7

32
Q

Equivalence point for a weak acid and strong base

A

pH > 7

33
Q

What is an indicator

A

Indicators are weak acids or bases that change colour when they lose or gain protons

34
Q

Why does the colour of litmus change

A

Adding H+ —> shifts equilibrium left and turns red

Adding OH- —> shift equilibrium right and turns blue

35
Q

Colour of methyl orange in acid, alkali and neutral

A

Acid- red
Neutral- orange
Alkali- yellow

36
Q

Colour of phenolphthalein in acid, alkali and yellow

A

Acid- colourless
Neutral- very pale pink
Alkali- pink

37
Q

How to choose an indicator

A

Choose an indicator that has its pH range within the vertical section of the pH curve

38
Q

When to use methyl orange

A

Titration involving Strong acid

39
Q

When to use phenolphthalein

A

Titration involving a strong base

40
Q

Key points in drawing titration curve

A

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
Q

Where does the equivalence point centre round for a weak acid and where is straight section

A

Centres around pH 8-9

Straight section pH 6.5-11.5

42
Q

Where does the equivalence point centre round for a weak alkali and where is straight section

A

Centres around pH 6

Straight section pH 3.5-8.5

43
Q

How long should a straight section on titration graph be for weak/strong

A

No longer than 5 pH units long

44
Q

How long should a straight section on titration graph be for strong/strong

A

No longer than 7 units long

45
Q

What is a buffer and what does it contain

A

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
Q

Why do acid buffets resist changes in pH

A

Large reservoir of acid and salt

Small changes to [HA] and [A-] means little affect on conc so small amount of pH change

47
Q

Assumptions in buffer calculations

A

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
Q

How to calculate pH of a buffer

A

[H3O+] = Ka times [acid]/[salt]

Then sub into pH equation pH = -log[H3O+]

49
Q

Ionic equations for buffers

A

Adding alkali : HA + OH- becomes A- + H2O

Adding acid: H+ + A- becomes HA

50
Q

Why does the buffering region occur

A

Remaining excess of undissociated acid and increasing quantity of salt

Created a buffer that resists further sharp change in pH

51
Q

How does breathing faster and deeper change blood pH

A

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
Q

How does exercise change blood pH

A

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
Q

How to calculate conc of H+ for a strong alkali

A

Conc of H+ = Kw/ conc of OH-

54
Q

Steps to calculate pH of a weak acid

A

Calculate [H+] - normally given or using pH
Ka = [H+] squared / concentration
Rearrange to find [H+]

Sub into pH formula

55
Q

How to calculate pH of a strong alkali

A

[H+] = Kw/ [OH-]

Then sub into pH formula

Else use pH = 14 - pOH

56
Q

How calculate pH and pKa using weak acid NaOH titration

A

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
Q

Equivalence point for weak acid strong base

A

Half equivalence point

Halfway up straight section of curve

58
Q

Marking points for if region of slow pH growth shown on graph

A

Mention buffering

Identify species present- HA and A-

How species were formed- CH3CH2COOH + OH- becomes H2O + CH3CH2COOH- excess CH3CH2COOH left

59
Q

Marking points for “is pure water always pH7”

A

No
As Temp increases, equilibrium moves to RHS
So [H+] increases
Hence pH > 7

Chemsitry (15 decks)

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