12.2

Question 1

what is the equivalence point

Answer 1

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

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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

Question 4

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

Answer 4

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

Question 5

in a pH titration curve what are the axis labeled as

Answer 5

x axis - volume
y axis - pH

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

there is not steep drop in pH at the equivalence point

Question 11

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

Answer 11

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

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

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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

Question 14

what is the pH range of phenylalanine

Answer 14

8.2-10.0

Question 15

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

Answer 15

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

Question 15

what is the range of methyl orange

Answer 15

3.1-4.4

Question 16

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

Answer 16

phenylalanine

Question 17

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

Answer 17

methyl-orange

Question 18

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

Answer 18

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

Question 19

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

Answer 19

by measuring temperature changes or conductivity changes in a titration

Question 20

what is a buffer solution

Answer 20

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

Question 21

what are the 2 main ways of making a buffer solution

Answer 21

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

Question 22

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

Answer 22

ethanoic acid + sodium ethanoate

Question 23

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

Answer 23

because the salt must be soluble in water

Question 24

how does buffer action work

Answer 24

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

Question 25

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

Answer 25

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

Question 26

what is henderson-hasselbalch equation

Answer 26

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

Question 27

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

Answer 27

ammonia + ammonium ion (often as ammonium chloride)

Question 28

what is the equation for the dissociation of ammonia

Answer 28

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

Question 29

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

Answer 29

weak base + conjugate acid

Question 30

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

Answer 30

weak acid + conjugate base

Question 31

explain the pH buffer in the blood

Answer 31

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

Question 32

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

Answer 32

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

Question 32

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

Answer 32

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

Question 33

explain the mechanism behind maintaining blood pH

Answer 33

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

Question 34

what is the x and y axis on a pH curve

Answer 34

x axis - volume of base added y axis - pH

Question 35

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

Answer 35

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

Question 36

why does pH = pKa in buffer solutions

Answer 36

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

Question 37

what is the experimental method to calculate Ka

Answer 37

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

Question 38

define standard enthalpy of neutralization

Answer 38

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

Question 39

what does HA stand for

Answer 39

a weak acid H+ concentration

Question 40

is the hydration of ions an exo or endothermic process

Answer 40

exothermic

Question 41

is the dissociation of an acid an exo or endothermic process

Answer 41

endothermic

Question 42

what is the equation for the dissociation of a weak acid

Answer 42

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

Question 43

what is the equation for the hydration of a weak acid

Answer 43

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

Question 44

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

Answer 44

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