12.2 Flashcards
pH curve and indicators
what is the equivalence point
when the acid and base have reacted together in the exact proportions as dictated by the stoichiometric equation
what product is produced and what is the pH at equivalence when aqueous sodium hydroxide and dilute hydrochloric acid react together
pH - 7 as they are a strong base and a strong acid
sodium chloride salt is produced
what product is produced and what is the pH at equivalence when ethanoic acid and sodium hydroxide react together
pH - >7 as ethanoic acid is a weak acid and sodium hydroxide is a strong base
sodium ethanoate salt is produced
what product is produced and what is the pH at equivalence when hydrochloric acid and aqueous ammonia react together
pH - <7 because hydrochloric acid is a strong acid and aqueous ammonia is a weak base
ammonium chloride salt is produced
in a pH titration curve what are the axis labeled as
x axis - volume
y axis - pH
describe the pH titration curve for a strong acid and a strong base
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
describe the pH titration curve for a weak acid with a strong base
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
describe the pH titration curve for a strong acid with a weak base
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
describe the pH titration curve for a weak acid with a weak base
there are no steep defined sections to the curve it remains mainly flat dipping slightly at the equivalence point (usually pH 7)
why is it difficult to titrate a weak acid and a weak base using an indicator
there is not steep drop in pH at the equivalence point
what is an acid-base indicator and what equation can be used to represent that
weak acid or weak base
Hln(aq) <=> H+(aq) + ln-(aq)
(Hln is the conjugate base and is a different colour to ln)
what is the colour change of methyl orange represented on the indicator equation
Hln(aq) <=> H+(aq) + ln-(aq)
red <=> yellow
what is Kln for methyl orange and how can it be used to determined the pH at which methyl orange turns orange
2X10^-4 mol/dm^3
therefor [H+(aq)] = 2X10^-4 when [Hln(aq) = [In(aq)]
which gives a pH of 3.7
what is the pH range of phenylalanine
8.2-10.0
how do you decide which indicator is most appropriate for a titration
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
what is the range of methyl orange
3.1-4.4
what indicator is best for a weak-acid strong-base titration
phenylalanine
what indicator is best for a strong-acid weak-base titration
methyl-orange
what indicator is best for a weak-acid weak-base titration
the end point of the titration cannot be determined using a titration as there is no prominent steep section to the curve
what other ways can the end point of a weak-acid weak-base titration be found
by measuring temperature changes or conductivity changes in a titration
what is a buffer solution
a solution that minimizes the change in pH when a small amount of either acid or base is added
what are the 2 main ways of making a buffer solution
mix a weak acid with its conjugate base
mix a weak base with its conjugate acid
give an example of a weak acid and its conjugate base that form a buffer
ethanoic acid + sodium ethanoate
why are sodium and potassium salts often used in creating buffer solutions
because the salt must be soluble in water
how does buffer action work
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
what equation can be used to calculate the pH of a buffer
pH = pKa + log( [salt]/[acid])
what is henderson-hasselbalch equation
[H+(aq)] = Ka X [acid] / [salt]
give an example of a weak-base and its conjugate acid that form a buffer
ammonia + ammonium ion (often as ammonium chloride)
what is the equation for the dissociation of ammonia
NH4+(aq) <=> NH3(aq) + H+(aq)
how can a buffer with a pH greater than 7 be made
weak base + conjugate acid
how can a buffer solution with a pH lower than 7 be made
weak acid + conjugate base
explain the pH buffer in the blood
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
what could cause in increase in H+ ions in the blood and how is it corrected
production of lactic acid
equilibrium shifts to the left and H+ ions react with HCO3- ions
what could cause the pH of the blood to increase and how is it corrected
increased amounts of urine excreted contains large amounts of acid
equilibrium shifts to the right and the H2CO3 molecules ionise to increase H+ concentration
explain the mechanism behind maintaining blood pH
equilibrium shifts to the left
more H+ ions are produced
CO2 concentration increases
carbon dioxide leaves the body maintaining pH
what is the x and y axis on a pH curve
x axis - volume of base added
y axis - pH
how do you calculate Ka from a weak acid strong base pH curve
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
why does pH = pKa in buffer solutions
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
what is the experimental method to calculate Ka
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
define standard enthalpy of neutralization
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
what does HA stand for
a weak acid H+ concentration
is the hydration of ions an exo or endothermic process
exothermic
is the dissociation of an acid an exo or endothermic process
endothermic
what is the equation for the dissociation of a weak acid
HA(aq) -> H+(g) + A-(g)
endothermic
what is the equation for the hydration of a weak acid
H+(g) + A-(g) -> H+(aq) + A-(aq)
exothermic
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
larger because the hydration of ions releases more heat energy than the dissociation of molecules taken in
