Acids, bases & buffers
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How do you work out a conjugate acid-base pair?
The acid loses a H+ ion, the base accepts it
What is the definition of a strong acid?
An acid that fully dissociates, releasing a large amount of H+ ions
What is the definition of a weak acid?
An acid that partially dissociates, releasing a small amount of H+ ions
What is the equation to calculate pH?
-log [H+]
What is the equation to calculate [H+] when given pH?
10^-pH
What is the equation for Kw?
Kw = [H+][OH-] This equation can be used to calculate conc. of H+ ions if given OH- and vise versa. Kw value is given on the data sheet.
What equation allows you to calculate the conc of H+ ions in water at any given temperature when given a Kw value?
Square root of Kw = [H+]
How to calculate the pH of a strong base using Kw?
• The conc of the OH- ions will be given. Rearrange the Kw equation so that it equals H+. • Equation = [H+] = Kw/[OH-] • Calculate [H+] and then use -log[H+] equation
What is the expression that represents Ka?
Ka = [H+][A-]/[HA]
In acid-base notation, what do [A-] and [HA] represent?
• [A-] = the base • [HA] = The compound before dissociation occurs HA -> [H+] + [A-]
What is the equation to calculate pKa when given Ka?
pKa = -log (Ka)
What is the equation to calculate Ka when given pKa?
10-^pKa
What is the equation to calculate the H+ concentration of a weak acid?
[H+] = √initial acid concentration x Ka value
What is the equation to calculate the new H+ concentration when a weak acid is diluted?
[H+] = (Initial volume/final volume) x concentration
What is a buffer made out of?
A weak acid and either a strong base or a conjugate salt
What is the definition of a buffer?
A system that minimises pH changes when small amounts of acid or base are added
In what conditions is a buffer most effective?
When the concentration of the weak acid (HA) is equal to the concentration of the conjugate base (A-)
How does a buffer maintain pH levels when an acid is added to the system?
• Concentration of H+ ions increases • The conjugate base, A-, reacts with the H+ ions • The position of the equlibrium shifts to the left • Added H+ ions are removed • pH remains constant
How does a buffer maintain pH levels when an alkaline solution is added to the system?
• Concentration of OH- ions increases • H+ ions react with the OH-, making water • Equlibrium shifts to the right • The weak acid dissociates, restoring the H+ ions • pH remains constant
What is the dissociation equlibrium for carbonic acid?
H₂CO₃ ⇌ H+ + HCO₃-
In the body, how is a carbonic acid buffer used to regulate an excess of acid in the blood?
• Equlibrium shifts to the left to form more H₂CO₃ • Concentration of H+ ions falls to a safe level
In the body, how is a carbonic acid buffer used to regulate an excess of alkali in the blood?
• H+ ions react with excess OH- ions to form water • Equlibrium shifts to the right, causing H₂CO₃ molecules to dissociate • H+ concentration is increased back to a safe level
What is the process to form a buffer from a weak acid and it’s conjugate salt?
• As the weak acid dissociation is very low, the concentration of the acid is assumed to have not changed • As a salt is ionic, it completley dissociates into the buffer and forms the base in the buffer
What is the process to form a buffer from an acid and alkali solution?
• Add a strong base to an excess of weak acid • Some of the acid is neutralised, the base is formed and some is leftover to form the buffer solution