the potential of H+ - poor indicator of acid strength - on a scale of -1-14

the acid dissociation constant - larger Ka = greater acid dissociation - better indicator of acid strength than pH

Topic 12 - Acid-based Equilibria Flashcards by Bethany Matthews

According to Bronsted-Lowry, what is an acid?

a proton donor

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According to Bronsted-Lowry, what is a base?

a proton acceptor

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What is a conjugate base?

the product of an acid donating a proton which can return to its original species by acting as a base

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What is a conjugate acid?

the product of a base accepting a proton which can return to its original species by acting as an acid

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What is pH?

the potential of H+

poor indicator of acid strength
on a scale of -1-14

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What is a strong acid?

an acid that fully dissociates

eg. HCl → H+ + Cl-

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What is a weak acid?

an acid that partially dissociates

eg. CH3COOH ⇌ CH3COO- + H+

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pH =

-log[H+]

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[H+] =

10 ^-pH

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How do you calculate pH of a strong acid?

use pH = -log[H+]

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How do you calculate pH of a weak acid?

use Ka rearranged to calc [H+]

then use pH = -log[H+]

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How do you calculate pH of a strong base?

either use pOH = -log[OH-]
pH = 14-pOH

or use Kw rearranged to find [H+]
and then pH = -log[H+]

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How do you calculate pH of a weak base?

use Kb rearranged to find [OH-]
then use use pOH = -log[OH-]
pH = 14-pOH

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What is Ka?

the acid dissociation constant

larger Ka = greater acid dissociation
better indicator of acid strength than pH

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Ka =

[H+][A-]
______
[HA]

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How can Ka be calculated experimentally?

make a standard solution of a known conc
calc pH (using pH meter)
use pH to find [H+] (pH = -log[H+])
use Ka expression to calc Ka

What is Kw?

the equilibrium constant for the dissociation of water

-used to calc pH of strong bases

Kw =

[H+][OH-]

What is the value of Kw?

1x10^-14

How does pH of water vary with temperature?

dissociation of water is endothermic
so when temp increases, eqm shifts to right
so [H+] increases
so pH decreases

What are acid-base titration curves?

graphs of volume against pH which show how pH changes as an acid is added to a base (or vice versa) during a titration

What is the equivalence point?

the half way point on a titration curve where there are equal concentrations of acid and base
-during the vertical section of the titration curve

Why is the equivalence point not always 7 on a titration curve?

when a strong acid is titrated with a weak base, a slightly acidic product is formed, which lowers the equivalence point
when a weak acid is titrated with a strong base, a slightly alkali product is formed, which raises the equivalence point

How can you tell what indicator to use by using a titration curve?

the indicator should have a colour change (normally a range) during the vertical section of the titration curve (aka the end point)

pH at half equivalence point =

pKa at half equivalence point

Why is pH equal to pKa at half equivalence point?

-at the equivalence point, the vol of acid that has reacted is the same as the vol of base that has reacted -so at the half equivalence point, half of the acid remains but half of the salt has been produced -so at the half equivalence point, the concs of acid and salt are equal so can be cancelled out in the Ka expression ∴ we are left with Ka = [H+] so pKa must = pH

How do you find the Ka of a weak acid using a titration curve?

-find the equivalence point (half way on vertical section of graph) -find the half equivalence point (half the vol of the equivalence point, and then use to find pH) pKa = pH so Ka = 10^-pKa

What is a buffer?

a solution which minimises changes in pH when a small amount of acid or base is added

What is in an acidic buffer?

- weak acid - its conjugate base (aka a salt) the salt fully dissociates and the weak acid only partially dissociates (which is suppressed by the ion from the salt) ∴ the buffer has a large reservoir of the undissociated weak acid and a large reservoir of the conjugate base (as well as some H+)

How does a buffer work when an acid is added?

-the [H+] increases in the buffer solution -this causes eqm to shift to the left (to lower [H+]) -BUT the concs of the weak acid and conjugate base don't change significantly ∴ pH does not change significantly

How does a buffer work when a base is added?

-the OH- ions react with the H+ ions (producing water) -this causes [H+] to decrease in the buffer solution -this causes eqm to shift to the right (to increase [H+]) -BUT the concs of the weak acid and conjugate base don't change significantly ∴ pH does not change significantly

How do you calculate the pH of a buffer?

- rearrange the Ka of the acid to find [H+] | - use [H+] to calc pH (using pH = -log[H+])

What is the Henderson-Hasselbalch equation?

pH = -logKa -log [acid]/[salt]

What is in an alkali buffer?

- weak base | - its salt

How can the Henderson-Hasselbalch equation be used to calculate an acid:base ratio needed to be mixed to obtain a buffer with a particular pH?

-plug numbers you do have into equation -do 10^x to remove log should get left with a value equal to [A]/[B] the [A] = value and [B} = 1 -these can then be placed as a ratio

How is buffer action seen on a titration curve?

the curve levels off after a quick, small pH change before it eventually increases again

How is the pH of the blood controlled?

- by carbonic acid (which dissociates into H+ and CO3-) - when [H+] rises in blood, HCO3- ions react with excess H+ (shifts eqm to left) to reduce [H+] (prevents blood pH dropping) - when [H+] decreases in blood, H2CO3 dissociate (shifts eqm to right) to increase [H+] (prevents blood pH rising)