Acids, Bases and Buffers

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Define a Brønsted-Lowry Acid.
A species that donates a proton.
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Define a Brønsted-Lowry Base.
A species that accepts a proton.
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What are conjugate acid-base pairs?
Set of two species that transform each other by loss or gain of a proton.
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What are monobasic, dibasic and tribasic acids?
Monobasic acids each release 1 proton, dibasic acids each release 2 protons and tribasic acids each release 3 protons.
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What is the dissociation constant, Ka?
It tells us the extent of dissociation, Ka=[H+(aq)][A-(aq)]/[HA(aq)] (Moldm^-3)
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What does the value of Ka tell us?
A large Ka suggests a strong acid with a high [H+] and a large dissociation with a small pKa. A small value suggests a weak acid with a low [H+] and a small dissociation with a large pKa.
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How do you calculate pKa?
pKa=-logKa (no units)
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How do you calculate Ka?
Ka=10^-pKa (moldm^-3)
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How do you calculate pH?
pH=-log[H+] (no units)
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How do you calculate [H+]?
[H+]=10^-pH (Moldm^-3)
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How do you calculate concentrations of strong acids?
[H+]=[HA]
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How do you calculate concentrations of weak acids?
[H+]=(Ka x [HA])^1/2
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What assumptions do you make when calculating concentrations of weak acids?
[HA] at equilibrium ~ [HA] undissociated and [H+] at equilibrium ~ [A-] at equilibrium.
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What is the ionic product of water, Kw?
Kw=[H+][OH-] where Kw=1.00 x 10^-14 at 298k
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What is the relationship between concentration of H+ and OH- in neutral solutions?
[H+]=[OH-]
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How do you calculate concentration of strong bases?
[OH-]=[base]
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How do you calculate concentration of H+ using Kw?
[H+]=Kw/[OH-]
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How do you calcuate concentrations of OH- or H+ in dibasic bases or acids?
[OH-]= 2[base], [H+]= 2[HA]
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Why can you only use Ka for 'stronger' weak acids?
The approximation [HA] at equilibrium ~ [HA] undissociated would no longer be valid.
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Define a buffer solution.
A system that minimises pH changes on addition of small amounts of an acid or a base.
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What is a buffer solution formed from?
A weak acid and the salt of a weak acid or excess of a weak acid and a strong alkali.
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What happens when you increase [H+] in a buffer solution?
Cojuagate base, A-, reacts with H+ ions and shifts the equilibrium to the left by removing the H+ ions..
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What happens when you increase [OH-] in a buffer solution?
Small concentrations of H+ ions react with OH- ions to form water, so equilibrium shifts to tight by HA dissociating to restore most of H+ ions.
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How to calculate Ka of a buffer solution?
Ka=([H+][A-])/[HA]
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What is the carbonic acid-hydrocarbonate buffer system?
Carbonic acid, H2CO3, acts as week base and hydrogencarbonate, HCO3-, acts as conjugate base, to maintain pH between 7.35 and 7.45.
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How does the carbonic acid-hydrogencarbonate buffer system work?
Increase [H+] removed by HCO3-, moving equilibrium to left and form CO2 through enzyme action. Increase [OH-] removed by H2CO3 dissociating into H+ and OH-, shifting equilibrium to the right to restore [H+] and allow H+ and OH- to form water.
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What is an indicator?
A weak acid that is one colour in its acid form and a different in its conjugate base form.
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How to choose an indicator?
The pH value of the end-point should be as close as possible to the pH value of the equivalence point.
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Define end point.
Point in a titration at which there are equal concentrations of weak acid and conjugate base forms of the indicator.
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Define equivalence point.
Point in a titration at which the volume of one solution has exactly reacted with the volume of the second solution, matching the stoichiometry of the reaction.
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How to set up a pH meter?
Remove probe from storage solution and rinse with deionised water. Blot probe dry and place into known solution, often pH 4. Repeat the process to confirm pH probe accurately measure pH across a range of known values.
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Other cards in this set

Card 2

Front

Define a Brønsted-Lowry Base.

Back

A species that accepts a proton.

Card 3

Front

What are conjugate acid-base pairs?

Back

Preview of the front of card 3

Card 4

Front

What are monobasic, dibasic and tribasic acids?

Back

Preview of the front of card 4

Card 5

Front

What is the dissociation constant, Ka?

Back

Preview of the front of card 5
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