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physical chemistry > C21. Acids, Bases and Buffers (A2) (DONE) > Flashcards

C21. Acids, Bases and Buffers (A2) (DONE) Flashcards

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1
Q

What is a Brønsted acid and base?

A

A species that donates protons (H+) is a Brønsted acid; A species that accepts protons (H+) is a Brønsted base

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2
Q

How do H+ ions exist in water?

A

Form Hydroxonium ions (H3O+)

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3
Q

How are hydroxide ions formed?

A

Mix base with H2O, they react with the H+ ions and form hydroxide ions OH-

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4
Q

Describe the equation of formation of H3O+ ions

A

HA + H2O <=reversible=> H3O+ + A-

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5
Q

Describe the equation of formation of OH- ions.

A

B + H2O <=reversible=> BH(+) + OH(-);

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6
Q

Describe the dissociation of bases and acids.

A

Weak bases and acids dissociate poorly; Strong bases and acids dissociate almost completely

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7
Q

Describe the reactions of weak acids.

A

Carboxylic acids; Backward reaction favoured, not many H+ produced; CH3COOH <=> Ch3COO(-) + H(+)

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8
Q

Describe the reactions of strong acids.

A

HCl, H2SO4, HNO3; Forward reaction favoured strongly, lots of H+ formed; HCl <=> H(+) + Cl(-)

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9
Q

Describe the reactions of strong bases.

A

NaOH, KOH; Forward reaction favoured strongly. Lots of OH- ions produced; NaOH <=> Na(+) + OH(-)

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10
Q

Describe the reactions of weak bases.

A

NH3; Backwards reaction favoured, not many OH- ions formed; NH3 + H2O <=> NH4(+) + OH(-)

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11
Q

Describe a general acid base reaction.

A

HA(aq) + B(aq) <=> BH(+)(aq) + A(-)(aq); (A - acid, B - base); In equilibrium; Protons are exchanged

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12
Q

Describe the reaction between acids and water.

A

H2O acts as a base, accepting proton forming hydronium ions, (H3O+); HA(aq) + H2O(l) <=> H3O(+)(aq) + A(-)(aq)

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13
Q

Describe the ionic properties of water.

A

Exist in equilibrium; 2H2O <=> H3O(+) + OH(-); Simplified, H2O <=> H(+) + OH(-); H2O dissociates into ions very weakly; Insignificant amount of OH- & H+ compared to H2O molecules;

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14
Q

What is the Kc of water?

A

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

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15
Q

What is the Kw?

A

Kw = [H+][OH-]; Ionic product of water; Kw = 1 x 10^-14 mol2dm-6

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16
Q

What are the factors that affect Kw ?

A

Value of Kw is same in a solution at given temp; Value changes if temp changes;

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17
Q

Describe the ionic properties of pure water.

A

Pure H2O has equal conc of H+ and OH- ions; [H+] = [OH-]; Kw = [H+]^2

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18
Q

What is pH?

A

Logarithmic scale that measures the concentration of H+ ions in a solution

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19
Q

What is the equation to calculate pH?

A

pH = -log10[H+]

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20
Q

What is the equation to calculate concentration of H+ ions?

A

[H+] = 10^-pH

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21
Q

How do monoprotic acids behave when dissociating?

A

Dissociate to produce 1 H+ for every acid molecule; Conc of acid = conc of H+ ions; [H+] = [Acid]

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22
Q

How do diprotic acids behave when dissociating?

A

Dissociates to produce 2 H+ ions for every acid molecule; Conc of acid = 2 x conc of H+ ions; 2[H+] = [Acid]

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23
Q

How do you calculate pH of base in aqueous solution?

A

Find [OH-] and Kw of solution at given temperature; Substitute into formula Kw = [H+][OH-]; Find pH using pH = log10[H+]

24
Q

Describe the dissociation of strong bases.

A

Dissociate fully; Strong bases dissociate to produce on OH- for every basic molecule, Base = [OH-] ; NaOH => Na(+) + OH[-]

25
Q

What is Ka?

A

Acid dissociation constant; Ka = [H+][A-] / [HA] start;

26
Q

What is Ka used for?

A

Find the constant for the equilibrium for weak acid dissociation; HA <==> H(+) + A(-)

27
Q

What are the assumptions made during Ka calculations?

A

Small amount of weak acid (HA) dissociates, [HA(aq)] Equilibrium = [HA(aq)] Start; Dissociation of acid is greater than that of water in solution, all H+ ions come from acid, [H+(aq)] = [A-(aq]

28
Q

What’s the formula for finding Ka for weak acids?

A

Ka = [H^2] / [HA]

29
Q

What is pKa?

A

Method of measuring strength of an acid; Lower the value the stronger the acid; pKa = -log10Ka

30
Q

What does the p in pH mean?

A

p = -log10; pH = -log10[H+]

31
Q

Whats the equation for finding Ka from pKa?

A

Ka = 10^-pKa

32
Q

What are titrations used for?

A

Used to work out the concentration of an acid or base

33
Q

What’s the basic method for titrations?

A

Acid or base of known conc in burette; Have acid or base of unknown conc but known volume in conical flask; Add chemical from burette into conical flask till indicator changes colour; Read how much chemical was added from burette to neutralise; Repeat till 2 concordant results, 0.1cm^3 apart

34
Q

What is the endpoint?

A

Point at which indicator changes colour

35
Q

What a precautions do you have to take when reading volume form a burette?

A

Read from bottom of the meniscus

36
Q

Describe a titration graph of a strong acid and strong base

A

pH starts 1, due to excess strong acid; End around 13pH due to excess strong base; Sharp sigmoid shape

37
Q

Describe a titration graph of a strong acid and weak base.

A

pH starts 1, due to excess strong acid; Ends around pH 9 due to excess weak base; Sharp sigmoid shape

38
Q

Describe a titration graph of a weak acid and strong base.

A

Approximately pH 5 start, due to excess weak acid; Ends around pH 13 due to excess strong base; Sharp sigmoid shape

39
Q

Describe a titration graph of a weak acid and weak base?

A

Approximately pH 5 start, due to excess weak acid; Ends around pH 9, due to excess weak base; Softer sigmoid shape

40
Q

What is the equivalence point?

A

Acid has been neutralised fully by base; Sharp vertical rise shows rapid change in pH

41
Q

Why are weak acid - weak base titration curves bad examples for showing equivalence points?

A

Change in pH is small and sigmoid shape is not as well pronounced

42
Q

What is the half neutralisation point?

A

Point halfway between 0 and the equivalence point; Used to calculate pKa of weak acids by taking pH at this point; At this point [HA] (acid) = [A-] (salt formed)

43
Q

What conditional equation can be formed at half neutralization point?

A

Ka = [H+]; -logKa = -log[H+]; pKa = pH

44
Q

What factors are needed for indicators to be suitable for the titration?

A

Indicator must change colour entirely within vertical part of titration; For it to be effective at determining the end point

45
Q

What do you use to measure the endpoint of weak acid - weak base titrations?

A

pH meter as no sharp pH change

46
Q

Describe a diprotic acid titration graph.

A

2 separate curves due to protons being released at different times; 2 equivalence points; Stair shaped graph;

47
Q

Give the definition of buffers.

A

Chemical that resists the change in pH when small amounts of acid or base are added; They do not stop the change in pH, they resist it

48
Q

What are the types of buffers?

A

Acidic and base buffers

49
Q

What do acidic buffers do and what are they formed from?

A

Resist the change in pH in order to keep the solution below pH7 7. They are made from a weak acid and its salt

50
Q

How do acidic buffers work?

A

2 equilibrium equations co-exist in the same beaker; Eq 1, Acid <=> conjugate base + H(+); Weak acid dissociates weakly so eq lies to the left heavily; Eq 2, Salt <=> salt ion + H(+); Salts dissociate strongly so eq lies heavily to the right; When H+ added, reacts with salt ion (high conc of these from salt) from Eq 2 forming acid for Eq 1; Eq shifts to left, more acid produced; OH- added, reacts with H+ ions forming H2O; Low conc of H+ is replenished from high conc of acid to counteract change; Eq shifts to right to replace reacted H+ ions

51
Q

What do basic buffers do and how are they formed?

A

Resist the change in pH in order to keep the solution above pH7 made from weak base and its salt

52
Q

How do basic buffers work?

A

2 equilibriums equations coexist in same beaker; Eq 1, Base + H2O <=> Base ion + OH-; Weak base dissociates weakly, little OH- ions maid, eq lies to left; Eq 2, Salt <=> Conjugate base + Conjugate acid; Salt dissociates strongly, eq lies to right; OH- added, reacts with conjugate base from eq2; High conc of these from salt; More base and H2O produced, eq shifted to left; H+ added, reacts with OH- ions; Low conc of OH- but replenished from high conc of base and H2O; Eq shift right

53
Q

How do you calculate the pH of a buffer?

A

Need to know Ka and concentration of weak acid and salt; Assume [Salt] = [A-] and [HAstart] = [HAequilibrium];

54
Q

What assumptions are made when calculating the pH change of a buffer?

A

When strong acid added assume all H+ ions react with salt ions [A-], to form acid [HA]; Remember to add volumes together

55
Q

What are the uses of buffers?

A

Shampoo, pH 5-6; Hair becomes dry and damaged if exposed to alkaline conditions; Washing powder, biological washing powders contain enzymes only work at specific pH; Blood, pH needs to be kept at pH7.4; CO2 plays the role of buffer

56
Q

What is the equation for finding [H+] from pH?

A

[H+] = 10^-pH

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