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You must be hungry 😋 > Acid-base Equilibria > Flashcards

Acid-base Equilibria Flashcards

1
Q

Describe a Bronsted-Lowry acid

A

Proton donor

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2
Q

Define a Bronsted-Lowry base

A

Proton acceptor

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3
Q

Acid base reactions involve the transfer of what

A

Protons

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4
Q

In the reaction HNO3 + HNO2 ⇌ NO3- + H2NO2 + what are the conjugate acid-base pairs?

A

Acid - HNO3
Conjugate base - NO3-
Base - HNO2
Conjugate acid - H2NO2+

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5
Q

Identfiy the conjugate acid-base pairs in HCOOH + CH3(CH2)2COOH ⇌ HCOO- + CH3(CH2)2COOH2+

A

Acid - HCOOH
Conjugate base - HCOO-
Base - CH3(CH2)2COOH
Conjugate acid - CH3(CH2)2COOH2+

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6
Q

What ion causes a solution to be acidic (2 marks)

A

H+ ions release in water combining with H2O to form H3O+ oxonium ions.

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7
Q

What causes a solution to be alkaline

A

OH- (hydroxide ions)

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8
Q

What is the equation for the ionisation of water

A 
2H2O (l) ⇌ H3O+ (aq) + OH- (aq)
Or H2O (l) ⇌ H+ (aq) + OH- (aq)
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9
Q

What us the value of Kw at 298K

A

1.0 x 10^-14

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10
Q

What physical factors affect the value of Kw and how do they affect it

A

Temperature only - if temperature is increased, equilibrium moves to the right so Kw increases and the ph of the pure water decreases

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11
Q

What us pKw

A

Sometimes pKw is used instead of Kw to make numbers more manageable
pKw = -log Kw
Kw = 10^-pKw

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12
Q

Why is pure water still neutral even if pH does not equal 7?

A

[H+] = [-OH]

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13
Q

Give an expression for Ph in terms of H+

A

ph = -log10[H+]

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14
Q

Whay is the relationship between pH and concentration of H+?

A

Lower pH = higher concentration of H+

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15
Q

If two solutions have a pH difference of 1, what is the difference in [H+]?

A

A factor of 10

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16
Q

How do you find [H+] from pH

A

[H+] = 10^-pH

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17
Q

How do you find [-OH] from pH? (298k)

A

Find [H+] and then use Kw = [H+][OH-] which is equal to 1.0 x 10^-14 at 298k to calculate [-OH]

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18
Q

What is the difference when finding [H+] of diprotic and triprotic acids

A

Need to multiply the concentration of acid by the number of protons to find [H+]

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19
Q

How do you calculate the pH of a strong alkaline solution

A

Use Kw to calculate [H+] from [OH-]

Then use pH = -log[H+]

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20
Q

Define the term strong acid

A

One which fully dissociates in water (HX → H+ + X-)

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21
Q

How do you calculate the pH of a strong acid

A

pH = -log [H+]

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22
Q

Define the term strong base

A

One which fully dissociates in water (XOH → X+ + OH-)

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23
Q

What is the difference between concentrated and strong

A

Concentrated means many mol per dm3, strong refers to amount of dissociation

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24
Q

What is a weak acid and a weak base?

A

Weak acids and bases do not fully dissociate in water. They only partially dissociate into their ions

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25
Q

Give some examples of strong acids

A

HCl, H2SO4, H3PO4

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26
Q

Give some examples of strong bases

A

NaOH, CaCO3, Na2CO3

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27
Q

Give some examples of weak acids

A

CH3COOH (ethanoic), any organic acid

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28
Q

Give some examples of weak bases

A

NH3, Zn(OH)2

29
Q

What is Ka? (expression

A

For acid HA, HA ⇌ H+ + A-

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

30
Q

How do you work out the pH of a weak acid?

A

Use the equation for Ka, subbing values for [A-] and [HA]

Use pH = -log[H+] to find pH

31
Q

What is pKa

A

Used to make Ka values more manageable
pKa = -log Ka
Ka = 10^-pKa

32
Q

What is a titration

A

The addition of an acid/base of known titration to acid/base of unknown titration to determine the concentration. An indicator is used to show that neutralisation has occured, as is a pH meter

33
Q

Define the term equivalence point

A

The point at which the exact volume of base has been added to just neutralise the acid or the other way around

34
Q

What generally happens to the pH of the solution around the equivalence point?

A

There is a large and rapid change in Ph, except in the weak-weak titration

35
Q

How would you calculate the concentration of a reactant if you know the volume and concentration of the other reactant and the volume of that reactant

A
  • Calculate moles of one reactant
  • Use balanced equation to work out the moles of the other
    Use concentration = mol/volume to calculate concentration
36
Q

What is the end point?

A

The volume of acid or alkali added when the indicator just changes colour. If the right indicator is chosen, equivalence point = end point.

37
Q

What are the properties of a good indicator reaction

A

Sharp colour change - no more than one drop of acid/alkali needed for change

  • End point must be the same as the equivalence point or titration gives the wrong answer
  • Distinct colour change so it is obvious when the end point has been reached
38
Q

What indicator would you use for a strong acid-base titration

A

Phenolphthalein or methyl orange, but phenolphthalein is usually used as it gives a clearer colour change

39
Q

What indicator would you use for a strong acid-weak base titration

A

Methyl orange

40
Q

What indicator would you use for a strong base-weak acid titration

A

Phenolphthalein

41
Q

What indicator would you use from a weak acid-weak base titration?

A

Neither methyl orange or phenolphthalein is suitable as neither gives a sharp change at the end point

42
Q

What colour is methyl orange in acid? In alkali? At what pH does it change?

A

Red in acid; yellow in alkali. Changes at about pH = -4.5. Approximately the same as the pKa value

43
Q

What colour is phenolpthalein in acid? In alkali? At what pH does it change?

A

Colourless in acid, red in alkali. Changes at about pH = 9-10. Approximately the same as pKa value

44
Q

What is the half neutralisation point

A

When volume = half the volume that has been added at the equivalence point

45
Q

Why is there a difference in enthalpy changes of neutralisation values for strong and weak acids

A

Enthalpy changes of neutralisation are always exothermic. This value for enthalpy change is similar for strong acids and alkalis becuase the same reaction occurs H+ + OH- → H2O
Weak acids have a less exothermic enthalpy change of neutralistation since the energy absorbed to ionise the acid is used to break the bond to hydrogen in the un-dissociated acid

46
Q

pH defintion

A

The pH of an aqueous solution is defined as the reciprocal to the base 10 of the hydrogen ion concentration measured in moles per cubic decimetre, pH=-log[H+]

47
Q

Ka defintion

A

The acidic dissociation constant which measures the strength of an acid in solution, Ka = [H+][A-]/[HA]
Re-arranging for pH gives, pH = pKa + log[A-]/[HA]

48
Q

Amphoteric

A

A substance that can act as an acid or a base.

For example HCO3- can accept a proton and form H2O and CO2, or donate a proton and form CO3^2-

49
Q

Monoprotic acid

A

Can release only one H+ upon dissociation

50
Q

Polyprotic acid

A

Can release more than one H+ upon dissociation

51
Q

Why are acid-base indicators used?

A

To detect when a reaction reaches its equivalence point. The indicator should be chosen so that its end point matches the equivalence point

52
Q

Why does a pH probe need to be calibrated

A

So that for each pH reading the pH value is accurate

53
Q

How do you calibrate a pH probe

A

Submerge pH probe in buffer solutions of three different pHs including pH7 and usually pHs around 4 and 10 pressing the calibrate button each time

54
Q

What is accuracy

A

The more accurate the data the closer it is to the actual value

55
Q

What equipment is used to carry out a titration

A
  • A pipette and pipette filler are used to accurately measure out the volume of a reactant before transferring it into the conical flask
  • A burette is a controlled way to add small volumes of one reaction into the other reactant
56
Q

How do you carry out a titration

A
  • Once the pipette has been used to place one reactant into the conical flask, fill the burette with the other reactant and record the intial volume
  • Add a few drops of indicator to the conical flask
  • Open the burette tap and allow the reactant to flow into the conical flask, swiriling it
  • Close the burette tap once the expected colour change occurs, use white tile to make the colour change easier to identify
  • Record final burette volume
  • Repeat until you get concordant results and then calculate the mean titre
57
Q

How do you carry out a titration to calculate pH

A
  • Add 25cm3 of 0.1moldm-3 ethanoic acid solution into a conical flask with a few drops of phenolphthalein
  • Sodium hydroxide goes into burette
  • Titrate the solutions together until the mixture turns pale pink
  • Add another 25cm3 of 0.1 moldm-3 ethanoic acid solution into the conical flask using a pipette
  • Record the pH of the resulting solution
58
Q

How to calculate Ka from titration

A

Using the restulting solution, you know only half the acid has been titrated so [HA] = [A-]

  • You can then cancel [A-] and [HA] in the Ka equation as they equal the same and so Ka = [H+]
  • Convert the resulting solutions pH to [H+] to give a value for the acid dissociation constant, Ka ([H+] = 10^-pH)
59
Q

What are some sources of uncertainty in CP9

A
  • Innacuracy of burette readings

- Difficulty identifying the exact end point

60
Q

What are ways to overcome uncertainties in CP9

A
  • Read the value from the bottom of the mensicus

- Use a white tile to see colour change more clearly

61
Q

Define a neutral solution

A

A solution where the [H+] concentration is equal to the [OH-] and this is the case for pure water. Bear in mind this can occur at any pH

62
Q

Conjugate acid

A

When a base accepts a proton, the species formed becomes the conjugate acid

63
Q

Conjugate base

A

When an acid donates a proton, the species formed becomes the conjugate base

64
Q

Can pure water conduct electricty?

A

Yes, it has slight electrical conductivity as it self ionises, the ionic product of water given as Kw = [H+(aq)][OH-(aq)]

65
Q

The dissacotation constant of water, Kw, increases with water. When temperature increases what happens to water?

A

It remains neutral

66
Q

An aqueous solution of ethanoic acid is gradually diluted, what does this mean?

A

The pH increases

67
Q

What is the conjugate base of the acid HCO3-?

A

CO3^2-

68
Q

Why are aqeous solutions of sodium ethanoate slightly alkaline?

A

The ethanoate ions react with water to give OH- ions

69
Q

A solution of HCl has pH3. When it is made ten times more dilute, pH is?

A

4

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