Unit 3 - Chapter 8

Question 1

Bronsted-Lowry theory

Answer 1

acid base reaction = reversible system involving conjugate acid/base pairs
strength of acid or base is defined by the extent of forward reaction at equilibrium
acid = proton donor
base = proton acceptor

Question 2

Amphiprotic substances

Answer 2

acts like acid or base depending on the reaction that it is in
therefore it can either accept or donate protons
to be amphiprotic, it needs an ionizable hydrogen, and a lone pair to be able to accept a hydrogen ion in a coordinate bond

Question 3

Acids

Answer 3

when added to water, acids ionize to form aqueous ions

when ionized, solution is a strong electrolyte

Question 4

Strong acids

Answer 4

forward reaction is virtually 100%
the concentration of aqueous ions is high, and is therefore a strong electrolyte
forms very weak conjugate bases that are unable to mount an effective reverse reaction
no equilibrium is reached
strong acids = HCl, HNO3, H2SO4, HBr, HI, HClO4

Question 5

Weak acids

Answer 5

forward reaction occurs very little before equilibrium is reached
at equilibrium, the concentration of aqueous ions is low, and is therefore a weak electrolyte
forms reasonably strong conjugate bases that are able to mount a significant reverse reaction

Question 6

Ionic bases

Answer 6

metal hydroxides
strength of these bases depends on their solubility in water
strong ionic bases will dissociate 100% (limited to alkali metal hydroxides and barium and strontium)
strong bases do not reach an equilibrium

Question 7

Molecular bases

Answer 7

all weak bases
weak bases have a very small forward reaction to reach equilibrium
concentration of aqueous ions are very low, and is therefore a weak electrolyte
strong enough conjugate acid to mount an effective reverse reaction

Question 8

Acid/base equilibrium

Answer 8

the status of acid equilibrium can be defined by an equilibrium constant (ionization constant) known as ka
strong acid = large ka, weak acid = small ka
the status of base equilibrium can be defined by an equilibrium constant (ionization constant) known as kb
strong base = large kb, weak base = small kb

Question 9

Water and kw

Answer 9

water is amphiprotic and reacts with itself in an acid/base reaction
at equilibrium, the concentration of hydronium ions is equal to the concentration of hydroxide ions
the equilibrium constant for this system is known as kw

Question 10

Acids/bases and water

Answer 10

when dealing with adding acid/base to aqueous solutions, we can assume concentration of hydronium and hydroxide ions contributed from water to be insignificant
when acid is added to water, reverse reaction is favoured, acidic pH
when base is added to water, reverse reaction is favoured, basic pH

Question 11

Polyprotic acids

Answer 11

some acids have more than one ionizable hydrogen ion
these acids do not release all their hydrogen ions at once, but do so in a sequence of steps
progressively weaker acids as hydrogen ions are removed
the concentration of hydronium ions added to solution from the first step acts to stress reactions 2 and 3, favouring their reverse reactions
pH of a polyprotic acid therefore depends only on the concentration of hydronium ions from the first step

Question 12

pH and pOH

Answer 12

quantitative measure of acidity or alkalinity of an aqueous solution
pH is based on concentration of hydronium ions
pOH is based on concentration of hydroxide ions

Question 13

pH of strong acids/bases

Answer 13

concentration of acid = concentration of hydronium ions

concentration of base = concentration of hydroxide ions

Question 14

Salts effect on pH

Answer 14

salt = ionic compound that can be produced by a reaction between an acid and a base
salt gains its anion from the acid, cation from base
anion = negative ion, cation = positive ion

Question 15

Strong acid and strong base

Answer 15

the conjugate base of the strong acid is weak and cannot act as a base, therefore it is a neutral ion
the cation of a strong base is a neutral ion
the overall reaction therefore produces a neutral salt

Question 16

Weak acid and strong base

Answer 16

the conjugate base of the weak acid is strong enough to act as a base in the reaction, therefore it is a basic ion
the strong base is still a neutral ion
the overall reaction therefore produces a basic salt

Question 17

Strong acid and weak base

Answer 17

the strong acid produces a neutral ion
the weak base produces a conjugate acid that is strong enough to act as an acid in the reaction, therefore it is an acidic ion
the overall reaction therefore produces an acidic salt

Question 18

Acidic metal ions

Answer 18

metal ions from metal hydroxides of low solubility (weak base) will behave as acids in water
therefore, a metal hydroxide produces an acidic salt

Question 19

Buffers

Answer 19

a buffer is a solution prepared to have a defined pH

the buffer is able to withstand the addition of acid or base without significantly changing its pH

Question 20

Preparation of a buffer

Answer 20

two ways to produce a buffer:
weak acid and a salt containing its conjugate base (acetic acid and sodium acetate)
weak base and a salt containing its conjugate acid (ammonia and ammonium nitrate)

Question 21

How a buffer works

Answer 21

the weak acid determines the pH of the buffer based on its percent ionization
the salt will dissociate 100% and produce a high concentration of the conjugate base
when the weak acid reacts with water, it will produce even more of the conjugate base
this shifts the acid’s equilibrium to the left, reducing the percent ionization, decreasing the hydronium ion concentration and increasing the pH of the buffer

Question 22

pH of a buffer

Answer 22

high concentration of acid, low concentration of salt = favours forward reaction in acid equilibrium, decreasing the pH
low concentration of acid, high concentration of salt = favours reverse reaction in acid equilibrium, increasing the pH
when a buffer is produced from equal concentration of acid and salt, pH = pKa

Question 23

Response of a buffer to stress

Answer 23

stress = addition of an acid or a base
adding a strong acid = favours reverse reaction in buffer as acid increases concentration of hydronium ions, pH is only slightly lower
adding a strong base = favours forward reaction in buffer as base reduces concentration of hydronium ions, pH is only slightly higher

Question 24

Equivalence point

Answer 24

the point during a titration when the number of moles of hydrogen ions from acid equals the number of moles of hydroxide ions from the base
this is the theoretical point where we hope to stop the titration
at this point, the solution consists of an aqueous salt solution

Question 25

Endpoint

Answer 25

the point during a titration when the indicator changes colour
for a successful titration, the endpoint equals/is very close to the equivalence point

Question 26

Indicators

Answer 26

indicators change colours in response to the pH of the solution
for a successful titration, the chosen indicator must have an endpoint which closely matches the pH at equivalence point
at equivalence point we have a salt solution, therefore the choice of indicator depends on the properties of the salt

Question 27

Standard solution

Answer 27

solution of accurately and precisely known concentration used to titrate unknown
standard solution will be a strong acid or a strong base
however, strong acids and bases they tend to be very reactive and unstable, so we cannot prepare an accurate and precise molarity based on the mass
standard solution must be standardized prior to use

Question 28

Standardization

Answer 28

titration strong acid/base against a primary standard compound
primary standard = compound of high stability, high purity, does not obtain water from air
the mass therefore accurately reflects the number of moles and the concentration of the solution can be accurately and precisely determined from mass

Question 29

Acid/base indicators

Answer 29

indicators are themselves weak acids that differ in colour between their molecular and ionized form
phenolphthalein (pink)
bromothymol blue (blue)

Question 30

How indicators work

Answer 30

phenolphthalein is a very weak acid
when added to a solution containing a stronger acid, the reverse reaction is favoured, leaving the phenolphthalein colourless
as long as the solution contains hydronium ions from the stronger acid, Hph is prevented from ionizing
the drop of base that consumes the last hydronium from the stronger acid will allow Hph to begin to ionize
the next 1-2 drops of base will turn solution pink

Question 31

pH at endpoint

Answer 31

at halfway point of the Hph reaction (when the solution turns pink), the number of moles of the acid equal the number of moles of the salt in the total volume of solution
therefore we have a buffer where the concentration of the acid equals the concentration of the salt (pH = pKin)
the endpoint colour change pH range is estimated at +/- 1 pH unit around pKin

Question 32

Titration curves

Answer 32

plots pH vs the volume of standard solution added to unknown during titration
prior to the equivalence point, the solution contains both the remaining weak acid and the salt of that acid, producing a buffer
at equivalence point, a salt is produced
once the acid/base is consumed, there is no more buffer and the pH depends only on the hydronium/hydroxide ions
at the halfway point of the titration pH = pKa

Question 33

Titration curves and indicators

Answer 33

strong acid and strong base = 4

Question 34

Solubility

Answer 34

the solubility of a substance defines the maximum amount of the substance that can be dissolved in a defined volume of solvent at a given temperature
ionic compounds must dissociate into aqueous ions to dissolve and therefore their solubility is inversely proportional to strength of ionic bonds

Question 35

Solubility and equilibrium

Answer 35

in a saturated solution it appears as if no more solute can dissolve
in reality, it is in a state of equilibrium between the rate of dissolution and the rate of recrystallization

Question 36

Soluble salts

Answer 36

soluble salts contain alkali metal ions and nitrate ions
they have a very high solubility and therefore rarely exist as a saturated solution
assume 100% forward reaction, and are not dealt with as equilibrium

Question 37

Salts with lower solubility

Answer 37

most likely to be found as a saturated solution
must be dealt with as equilibrium
there is therefore an equilibrium constant for aqueous solutions of ionic compounds

Question 38

Ksp

Answer 38

equilibrium constant = Ksp
Ksp = solubility product = concentration of anion multiplied by concentration of cation
as Ksp increases, solubility increases, since it is a measure of the concentration of ions able to remain in solution

Question 39

Ksp and equilibrium

Answer 39

Ksp sets a limit on the concentration of a pair of ions that can coexist in solution
if the ion concentrations exceed Ksp, they will combine to form a precipitate, decreasing the concentrations of the ions until equilibrium is reduced

Question 40

Common ion effect

Answer 40

by adding in a more soluble salt with a common ion to an aqueous salt solution the reverse reaction is favoured
this then reduces the amount of the less soluble salt that must dissolve to reach equilibrium
adding a common ion therefore reduces solubility of the less soluble salt

Question 41

Will a precipitate form

Answer 41

upon mixing two solution, there is a combination of ions in the mixture that would form a salt of low solubility
ksp sets a limit on the concentration of ions that may remain in solution
therefore whether or not a precipitate forms depends on the concentration of ions in the resulting solution and the Ksp
Kt > Ksp = precipitate will form
Kt