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Chemistry > 12.2 > Flashcards

12.2 Flashcards

pH curve and indicators

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1
Q

what is the equivalence point

A

when the acid and base have reacted together in the exact proportions as dictated by the stoichiometric equation

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2
Q

what product is produced and what is the pH at equivalence when aqueous sodium hydroxide and dilute hydrochloric acid react together

A

pH - 7 as they are a strong base and a strong acid
sodium chloride salt is produced

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3
Q

what product is produced and what is the pH at equivalence when ethanoic acid and sodium hydroxide react together

A

pH - >7 as ethanoic acid is a weak acid and sodium hydroxide is a strong base
sodium ethanoate salt is produced

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4
Q

what product is produced and what is the pH at equivalence when hydrochloric acid and aqueous ammonia react together

A

pH - <7 because hydrochloric acid is a strong acid and aqueous ammonia is a weak base
ammonium chloride salt is produced

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5
Q

in a pH titration curve what are the axis labeled as

A

x axis - volume
y axis - pH

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6
Q

describe the pH titration curve for a strong acid and a strong base

A

the pH falls very slowly until close to the equivalence point (usually around pH 7) where there is a steep plunge and the pH drops below the equivalence point before leveling off

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7
Q

describe the pH titration curve for a weak acid with a strong base

A

the pH falls very slowly until close to the equivalence point (usually pH 8/9) where there is a steep plunge but it levels off earlier as it acts as a buffer solution resisting any large changes in pH

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8
Q

describe the pH titration curve for a strong acid with a weak base

A

the pH dips at the start then levels off until close to the equivalence point (usually pH 5/6) there is a steep plunge but it levels off earlier as a buffer solution has been formed

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9
Q

describe the pH titration curve for a weak acid with a weak base

A

there are no steep defined sections to the curve it remains mainly flat dipping slightly at the equivalence point (usually pH 7)

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10
Q

why is it difficult to titrate a weak acid and a weak base using an indicator

A

there is not steep drop in pH at the equivalence point

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11
Q

what is an acid-base indicator and what equation can be used to represent that

A

weak acid or weak base
Hln(aq) <=> H+(aq) + ln-(aq)

(Hln is the conjugate base and is a different colour to ln)

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12
Q

what is the colour change of methyl orange represented on the indicator equation

A

Hln(aq) <=> H+(aq) + ln-(aq)
red <=> yellow

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13
Q

what is Kln for methyl orange and how can it be used to determined the pH at which methyl orange turns orange

A

2X10^-4 mol/dm^3
therefor [H+(aq)] = 2X10^-4 when [Hln(aq) = [In(aq)]
which gives a pH of 3.7

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14
Q

what is the pH range of phenylalanine

A

8.2-10.0

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15
Q

how do you decide which indicator is most appropriate for a titration

A

the range of the indicator must fall within the steep section of the pH curve, the best indicator will have a pKIn value as close as possible to the pH at equivalence

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15
Q

what is the range of methyl orange

A

3.1-4.4

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16
Q

what indicator is best for a weak-acid strong-base titration

A

phenylalanine

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17
Q

what indicator is best for a strong-acid weak-base titration

A

methyl-orange

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18
Q

what indicator is best for a weak-acid weak-base titration

A

the end point of the titration cannot be determined using a titration as there is no prominent steep section to the curve

19
Q

what other ways can the end point of a weak-acid weak-base titration be found

A

by measuring temperature changes or conductivity changes in a titration

20
Q

what is a buffer solution

A

a solution that minimizes the change in pH when a small amount of either acid or base is added

21
Q

what are the 2 main ways of making a buffer solution

A

mix a weak acid with its conjugate base
mix a weak base with its conjugate acid

22
Q

give an example of a weak acid and its conjugate base that form a buffer

A

ethanoic acid + sodium ethanoate

23
Q

why are sodium and potassium salts often used in creating buffer solutions

A

because the salt must be soluble in water

24
Q

how does buffer action work

A

when small amounts of acid are added to the buffer they react with the negative ions to form molecules
and when small amounts of OH- ions are added they react with the molecule to produce a negative ions and a H+ ion
this must establish a new equilibrium but the changes to the pH are minimal

25
Q

what equation can be used to calculate the pH of a buffer

A

pH = pKa + log( [salt]/[acid])

26
Q

what is henderson-hasselbalch equation

A

[H+(aq)] = Ka X [acid] / [salt]

27
Q

give an example of a weak-base and its conjugate acid that form a buffer

A

ammonia + ammonium ion (often as ammonium chloride)

28
Q

what is the equation for the dissociation of ammonia

A

NH4+(aq) <=> NH3(aq) + H+(aq)

29
Q

how can a buffer with a pH greater than 7 be made

A

weak base + conjugate acid

30
Q

how can a buffer solution with a pH lower than 7 be made

A

weak acid + conjugate base

31
Q

explain the pH buffer in the blood

A

it is controlled by a mixture of buffers but the main one is carbonic acid-hydrogen carbonate buffer
H2CO3(aq) <=> HCO3-(aq) + H+ (aq)
H2CO3 is a weak acid and HCO3- is the conjugate base

32
Q

what could cause in increase in H+ ions in the blood and how is it corrected

A

production of lactic acid
equilibrium shifts to the left and H+ ions react with HCO3- ions

32
Q

what could cause the pH of the blood to increase and how is it corrected

A

increased amounts of urine excreted contains large amounts of acid
equilibrium shifts to the right and the H2CO3 molecules ionise to increase H+ concentration

33
Q

explain the mechanism behind maintaining blood pH

A

equilibrium shifts to the left
more H+ ions are produced
CO2 concentration increases
carbon dioxide leaves the body maintaining pH

34
Q

what is the x and y axis on a pH curve

A

x axis - volume of base added
y axis - pH

35
Q

how do you calculate Ka from a weak acid strong base pH curve

A

find the volume at the equivalence point and half it to find the half equivalence point
find the pH at half equivalence point
do 10^pH to find Ka

36
Q

why does pH = pKa in buffer solutions

A

because
pH = pKa + ln[salt]/[acid]
and at half equivalence point [salt] = [acid] so [salt]/[acid] = 1
then the ln of 1 is 0
so pH = pKa

37
Q

what is the experimental method to calculate Ka

A

pipette 25cm^3 of an aqueous solution of a weak acid into a conical flask and add phenolphthalein and titrate with aqueous sodium hydroxide
pipette another 25cm^3 of the aqueous solution of weak acid
titrate half the volume of sodium hydroxide from the first step without the indicator and measure the pH
pH=pKa

38
Q

define standard enthalpy of neutralization

A

the enthalpy change measured at 100kPa and a specified temperature when one mole of water is produced by the neutralization of an acid with an alkali

39
Q

what does HA stand for

A

a weak acid H+ concentration

40
Q

is the hydration of ions an exo or endothermic process

A

exothermic

41
Q

is the dissociation of an acid an exo or endothermic process

A

endothermic

42
Q

what is the equation for the dissociation of a weak acid

A

HA(aq) -> H+(g) + A-(g)
endothermic

43
Q

what is the equation for the hydration of a weak acid

A

H+(g) + A-(g) -> H+(aq) + A-(aq)
exothermic

44
Q

if the enthalpy of hydration for a weak acid is larger then enthalpy of dissociation will the enthalpy of neutralization be larger or smaller than enthalpy of neutralization of strong acids and why

A

larger because the hydration of ions releases more heat energy than the dissociation of molecules taken in

Chemistry (46 decks)

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