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Chemistry > Unit 1: Section 9 - Acids, Bases and pH > Flashcards

Unit 1: Section 9 - Acids, Bases and pH Flashcards

1
Q

Define 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

What ion causes a solution to be acidic? ( answers) Name and formula

A

H+, or more accurately H3O+ (oxonium ion), as protons react with H2O to form it

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4
Q

What ion causes a solution to be alkaline?

A

-OH (hydroxide ion)

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5
Q

Write an equation for the ionisation of water (2)

A

2H2O(l) ⇌ H3O+ (aq) + -OH (aq)
OR
H2O (l) ⇌ H+(aq) + -OH(aq)

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6
Q

Derive Kw using the equation for the ionisation of water

A

Keq = [H+] [OH-] / [H2O]
[H2O]Keq = [H+] [OH-]
[H2O] is so large compared to [H+] and [Oh-] that [H2O]Keq can be considered to be constant.
[H2O]Keq = Kw
∴ Kw = [H+] [OH-]

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7
Q

What is the value of Kw at 298K?

A

1.0 x 10^-14

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8
Q

What physical factors affect the value of Kw? How do they affect it?

A

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

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9
Q

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

A

[H+] = [OH-]

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10
Q

Give an expression for pH in terms of H+

A

pH = -log[H+]

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11
Q

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

A

Lower pH = higher concentration of H=

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12
Q

If 2 solutions have a pH difference of 1, what is the difference in [H+]?

A

A factor of 10

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13
Q

How do you find [H+] from pH?

A

[H+] = 10^-pH

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14
Q

How do you find [OH-] from pH?

A

Find [H+], use Kw to calculate [OH-]

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15
Q

What is the difference when finding [H+] from the concentrations of diprotic and tripotic acids?

A

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

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16
Q

How do you calculate the pH of a strong alkaline solution?

A

Use Kw to calculate [H+] from [OH-]
Use pH = -log[H+]

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17
Q

Define the term strong acid

A

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

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18
Q

Define the term strong base

A

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

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19
Q

What is the difference between concentrated and strong?

A

Concentrated means many mol per dm^3, strong refers to amount of dissociation

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20
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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21
Q

Give some examples of strong acids

A

HCl, H2SO4 and H3PO4

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22
Q

Give some examples of strong bases

A

NaOH, CaCO3 and Na2CO3

23
Q

Give some examples of weak acids

A

CH3COOH (ethanoic), any organic acid

24
Q

Give some examples of weak bases

25
Q

What is Ka? (expression)

A

For acid HA, HA ⇌ H+ + A-
Ka = [H+] [A-] / [HA]

26
Q

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

A
  • Use the equation for Ka, subbing in values for [A-] and [HA]
  • Use pH = -log[H+] equation to find pH
27
Q

What is a titration?

A
  • The addition of an acid/base of known titration to a base/acid of unknown titration to determine the concentration
  • An indicator is used to show neutralization has occurred, as is a pH meter
28
Q

Draw the titration curve for a strong acid with a strong base

29
Q

Draw the titration curve for a strong acid with a weak base added

30
Q

Draw the titration curve for a weak acid with a weak base added

31
Q

Draw the titration curve for a weak acid with a strong base added

32
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 vice-versa

33
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

34
Q

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

A
  1. Calculate moles of one reactant
  2. Used balanced equation to work out moles of other
    Use conc = mol/vol to calculate concentration
35
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
36
Q

What are the properties of a good indicator for a reaction? (3)

A
  • Sharp colour change (not gradual - no more than one drop of acid/alkali needed for colour change
  • End point must be the same as the equivalence point, or titration gives wrong answer
  • Distinct colour change so it is obvious when the end point has been reached
37
Q

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

A

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

38
Q

What indicator would you use for a strongacid-weak base titration?

A

Methyl orange

39
Q

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

A

Phenolphthalein

40
Q

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

A

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

41
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
Approx same pKa value

42
Q

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

A

Colourless in acid; red in alkali
Changes at about pH 9-10
Approx same pKa values

43
Q

What is half-neutralisation point?

A

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

44
Q

Define a buffer solution

A

A solution that resist changes in pH when a small amount of acid/alkali are added

45
Q

What do acidic buffer solutions contain in general terms?

A

A weak acid and a soluble salt of that acid that fully dissociates

46
Q

Write a reaction for an acidic buffer with added acid

A

A- + H+ -> HA, opposes addition of H+

47
Q

Write a reaction for an acidic buffer with added alkali

A

HA + OH- -> H2O + A-

48
Q

How else can you achieve an acidic buffer solution other than just mixing the constituents?

A

Neutralise half of a weak acid (meaning the acid must be in excess) with an alkali - this forms a weak acid/soluble salt mixture

49
Q

What do basic buffer solutions contain in general terms?

A

Weak base and soluble salt of that weak base

50
Q

How can you calculate the pH of buffer solutions?

A

Use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] -> pH

51
Q

How can you calculate the new pH of a buffer solution when acid or base is added?

A

Calculate number of moles of H+ and A- and HA before acid or base is added
Use equations to work out new moles of A- and HA -> find [H+] ->pH

52
Q

Which buffer system maintains blood at 7.4? What happens when acid/alkali is added?

A

H+ + HCO3- ⇌ CO2 + H2O
Add OH- -> reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost
Add H+ -> equilibrium shifts to the right, removing excess H+

53
Q

What products are buffers found in?

A

Shampoos, detergents -> important to keep pH right to avoid damage to skin, hair, fabrics

Chemistry (24 decks)

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