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[OBSELETE] Edexcel - A Level Chem > Topic 12 > Flashcards

Topic 12 Flashcards

1
Q

What are bronsted-Lowry acids

A
  • Acids are proton donors
  • When mixed with water they dissociate into H+ ions
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2
Q

What are Bronstead-lowry bases

A
  • Bases are proton acceptors
  • Which will mix with water to dissociate into OH- ions
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3
Q

What are strong acids?

A
  • Strong acids/bases will always dissociate completely into OH- or H+ ions
  • Where the forward reaction is favoured strongly
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4
Q

What are weak acids?

A
  • Strong acids/bases will partially completely into OH- or H+ ions
  • Where the backwards reaction is favoured
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5
Q

What are conjugate pairs?

A

Acids and Bases that are linked by the transferring of a proton from one to the other

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6
Q

What is a conjugate base?

A

The conjugate base is a base that has lost a proton

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7
Q

What is a conjugate acid?

A

A conjugate acid is an acid that has gained a proton

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8
Q

How are conjugate pairs formed?

A
  1. HA will lose an electron to A
  2. Meaning that A becomes A-
  3. HA becomes BH+
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9
Q

How do you find the pH of a solution from H+ concentration?

A
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10
Q

How do you find the H+ from pH?

A
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11
Q

What does the p in pH mean?

A
  • p means POTENTIAL
  • if you ever see p its -log(rest of symbol)
  • pKA = -log(Ka)
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12
Q

What is Ka (explanation + formula)?

A
  • Acidic dissociation constant
  • Indicated the extent of dissociation
  • The equilibrium constant for the dissociation of a weak acid at 298K
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13
Q

What is pKa?

A
  • pKa is used to compare the Ka between different acids
  • As Ka values are generally too small
  • -log[Ka]
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14
Q

What does Ka tell you?

A
  • Indicated the extent of dissociation
  • High Ka
    • Acid has almost fully dissociated
    • Meaning its strongly acidic
    • The equilibrium lies to the right (more $H^+$)
  • Low Ka
    • Acid partially dissociated
    • Meaning its a weak acid
    • The equilibrium lies to the left (more $HA_{aq}$)
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15
Q

What assumptions are made when writing the equilibrium for weak acids?

A
  • The concentration of $HA_{aq}$ is the same at the start and is the same at the equilibrium
  • The concentration of HA = $A^-$
    • Due to small amounts of dissociation
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16
Q

What are the steps for calculating Ka?

A
  1. Write the ionic equation for the dissociation of the acid
  2. Write the equilibrium expression
  3. Determine conc of H+,A-, HA
  4. Use the equation to find Ka
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17
Q

How do you calculate Ka from mass and pH?

A
  1. Calculate moles of acid in the solution
  2. Calculate the concentration of acid from moles giving you [HA]
  3. Use 10^{-pH} to calculate [H+] concentration
  4. Use the Ka equation to calculate Ka
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18
Q

How do you calculate Ka from a practical measuring pH?

A
  1. Accurately weigh the acid and dissolve it in a small volume of deionised water in a beaker
  2. Transfer the solution to a 250cm3 volumetric flask. wash the beaker several times and pour the washings into the flask. make up to the mark with deionised water
  3. Invert the flask several times
  4. Take a sample from the solution and place it in a small beaker
  5. Measure the pH of the solution using a calibrated pH metre
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19
Q

What is Kw and what does it tell you?

A
  • Ionic product of water
  • The equilibrium constant for the dissociation of water at 298K
  • 1×10^{-14} mol^2}dm{-6}
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20
Q

What is the Kw expression with the simplified versions?

A
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21
Q

What is pKw?

A
  • pKw is used to compare the Kw between different acids
  • As Kw values are generally too small
  • -log[Kw]
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22
Q

How do you find the pH from Kw?

A
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23
Q

How do you prove the pH of water is 7?

A
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24
Q

How can the relative strengths of acids/bases be determined?

A
  • Can be determined by measuring pH
  • Of equimolar aqueous solutions of acids/bases
  • At the same Temperature
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25
Q

How can you work out the pH of NaCl from the data given?

A
  • HCL = Strong acid
  • NaOH = Strong Base
  • So there would be neutralisation
  • Meaning that NaCl would have a pH of 7
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26
Q

What is the summary table for aqueous solutions at 298K

A
27
Q

What is the experiment for determining the Ka of a weak monobasic Acid?

A
  1. weight the acid and dissolve in deionised water in a beaker
    Warm if necessary as some acids wont dissolve
  2. Transfer the solution to a volumetric flask and add more deionised water
  3. Mix the solution (Invert Flask)
  4. Sample the solution and measure the pH of a solution
28
Q

How do you work out Ka from the pH and Mass of an acid?

A
29
Q

What is a stoichiometric equation?

A

An equation that identifies the maximum amount of product that can be obtained

30
Q

What is an equivalence point?

A
  • Acid and base have reacted together
  • Exact proportions from the stoichiometric equation
  • Where 100% of the monoacidic acid will react with 100% of the monoacidic base
31
Q

What determines the pH of the equivalence point

A
  • A combination of the acid and base used
  • Strong acid + Strong Base = pH 7
  • Strong Acid + Weak Base = pH 7>
  • Weak Acid + Strong Base = pH <7
32
Q

What is a titration curve?

A
  • Shows the change in pH of the solution formed
  • The pH of the solution against the volume of acid
  • This explains why you can use specific indicators for specific reactions
33
Q

What is the titration curve for a string acid on a strong base?

A

Phenolphaliene + Methyl Orange

34
Q

What is the titration curve for a weak acid on a strong base?

A

Phenolphthalein

35
Q

What is the titration curve for a strong acid and a weak base?

A

Methyl Orange

36
Q

What is the titration curve for a weak acid on a weak base?

A

Neither Indicator is Suitable

37
Q

What are indicators made up of?

A
  • A weak acid or a weak base
    • Weak acid dissociation in aqueous solution
    • Hln is a conjugate base where Ln ion will have different colours in an aqueous solution
  • The amount of H+ ions will determine what is being formed and what colour is being represented
  • Hence at different pH the colour changes
38
Q

What are the pKln values and pH ranges of these indicators

A
39
Q

How to find the pH at which the indicator will change colour?

A
40
Q

What is a buffer solution?

A
  • A solution that can resist changes in pH by a factor of 1
  • When small amounts of bases or acids are used
  • Needed as many experiments require aqueous solutions with a constant pH
41
Q

How can you make pH buffers?

A
  • Mixing a weak acid with its conjugate base (Acidic buffer)
  • Mixing a weak base with its conjugate acid (Basic buffer)
42
Q

How do you make a buffer from ethanoic acid and sodium ethanoate?

A
  • Ethanoic acid = Weak acid
  • Salt of weak acid has to be soluble in water
    • Meaning sodium and potassium salts are
      commonly used
  • This produces a buffer solution with pH <7
43
Q

What happens when you add an acid to an acidic buffer?

A
  • H+ ions react with negative ions in the solution
  • Creating a high concentration of negative ions from the dissociation of the acid
  • In response, more acid is produced meaning that the equilibrium shift to the left [HA]
44
Q

What happens when you add a base to an acidic buffer?

A
  • OH- ions react with positive ions in the solution
  • Low conc of OH- ions but are reproduced from the high concentration of acid in the buffer
  • Causing an overall decrease in H+ meaning that the equilibrium will shift to the right to replace them
45
Q

What are basic buffers?

A
  • Resist changes in pH keeping the pH above 7
  • Made from a weak base and its salt
  • Weak base dissociates weakly producing low amounts of OH- (left)
  • Salt dissociates strongly producing lots of H+ ions (right)
46
Q

What happens when you add a base to a basic buffer?

A
  • OH- ions react with positive ions from the salt in the buffer solution
  • High concentrations of H+ react with the new OH- to form water and a named molecule
  • Shifting equilibrium to the left [HA]
47
Q

What happens when you add an acid to a basic buffer?

A
  • H+ ions react with OH- in the solution
  • Low conc of OH- ions but are reproduced from the high concentration of a named molecule and water
  • Causing an equilibrium shift to the right to replace the OH- ions
48
Q

How do you calculate the pH of the buffer solution?

A
49
Q

How do you calculate the concentrations in a buffer?

A
  • to make a buffer with a specific pH we need to calculate the conc. of salt, weak acid and base
    • use the Henderson-Hasselbalch equation
    • this assumes that [HA]=[HA] at the start of the
      reaction
    • also assumes [A]=[A-] at the the start
  1. calculate pKa
  2. substitute the figures into the Henderson-Hasselbalch equation to find the ratio of [A]:[HA]
  3. to simplify we have to remove log10 by doing the inverse→ 10^ratio
    1. just press shift then ‘log’
  4. we know salts fully dissociate so [salt]=[A-]. we can calculate [HA] at equilibrium as its equal to [HA] at the start
50
Q

What is the enthalpy of neutralisation?

A

Enthalpy Change of Neutralisation

We get a change in enthalpy when neutralisation occurs

  • The enthalpy change
  • To form 1 mole of water
  • Under standard conditions
51
Q

What are neutralisation reactions?

A
  • Where an acid and a base react together
  • Always exothermic
  • Enthalpy changes of neutralisation are always negative
52
Q

What is the neutralisation between weak acids and bases?

A
  • These dissociate weakly so Oh- and H+ ions are used up quickly
  • As there is only a small number in the solution
  • Since lots of OH- and H+ ions would be replaced due to la Chatillers principle
  • Enthalpies of dissociation + Enthalpy when OH- and H+ ions react
  • Enthalpy of dissociation varies depending on the acid and base combination
  • Enthalpy of neutralisation of weak acids and bases varies
53
Q

What is polyprotic acids?

A
  • AKA polybasic acids
  • Acids that can donate more than one proton
54
Q

What are monoprotic acids?

A
  • AKA monobasic
  • Example = HNO3
  • 1 mole of HNO3 will produce 1 mole of H+ ions
55
Q

What are dipotic acids?

A
  • AKA diabasic
  • Example = H2SO4
  • 1 mole of sulfuric acids will produc 2 moles of H+ ions
56
Q

What are triprotic acids?

A
  • AKA Tribasic
  • Example = H3PO4
  • 1 mole of phosphoric acid will produce 3 moles of H+ ions
57
Q

How do you calculate the pH of strong monoprotic acids?

A
  • Assumption that ther is full dissociation
  • Monoprotic acids dissociate to produce one H+ ion for every acid molecule
58
Q

How do you calculate the pH of strong dioprotic acids?

A
  • Assumption that there is full dissociation
  • Durprotic acids will dissociate into two H+ ions for every 1 acid molecule
59
Q

How do you calculate the pH of strong bases?

A
  • Assumption on bases fully dissociating
  • Most bases dissociate to produce the OH- ion
  • Use Kw to find H+ and then pH
60
Q

How do you calculate the pH of weak acids?

A
  • Only dissociate partially so assumption is that the HA at equalibrium = HA at the start
  • Second assumption = dissociation of acid is greater than the dissociation of water present in the solution [H+] = [A-]
  • Requires Ka
61
Q

What are acidic buffers?

A
  • Made from a weak acid and salt of its conjugate base
  • 2 Equalibrium equations in play co-exsisting
62
Q

How are buffers used in blood?

A
  • Blood must be contained at as close to pH 7.4
  • Relys on a buffer like CO2
63
Q

How do you calculate the concentrations in a buffer?

A
  • Buffers with specific pH require concentrations of salt, weak ac\cid and a base
  • Calculate pKa
  • Substituate figures into the henderson-hasselback equation
  • Once you find salt=A- you can caulculate HA at equalibrium (=[HA])

[OBSELETE] Edexcel - A Level Chem (27 decks)

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