Unit 3 - Chapter 8 Flashcards
Bronsted-Lowry theory
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
Amphiprotic substances
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
Acids
when added to water, acids ionize to form aqueous ions
when ionized, solution is a strong electrolyte
Strong acids
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
Weak acids
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
Ionic bases
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
Molecular bases
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
Acid/base equilibrium
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
Water and kw
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
Acids/bases and water
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
Polyprotic acids
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
pH and pOH
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
pH of strong acids/bases
concentration of acid = concentration of hydronium ions
concentration of base = concentration of hydroxide ions
Salts effect on pH
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
Strong acid and strong base
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
Weak acid and strong base
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
Strong acid and weak base
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
Acidic metal ions
metal ions from metal hydroxides of low solubility (weak base) will behave as acids in water
therefore, a metal hydroxide produces an acidic salt
Buffers
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
Preparation of a buffer
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)
How a buffer works
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
pH of a buffer
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
Response of a buffer to stress
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
Equivalence point
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
Endpoint
the point during a titration when the indicator changes colour
for a successful titration, the endpoint equals/is very close to the equivalence point
Indicators
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
Standard solution
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
Standardization
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
Acid/base indicators
indicators are themselves weak acids that differ in colour between their molecular and ionized form
phenolphthalein (pink)
bromothymol blue (blue)
How indicators work
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
pH at endpoint
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
Titration curves
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
Titration curves and indicators
strong acid and strong base = 4
Solubility
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
Solubility and equilibrium
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
Soluble salts
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
Salts with lower solubility
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
Ksp
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
Ksp and equilibrium
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
Common ion effect
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
Will a precipitate form
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
