LECTURE 4 (Acids and bases) Flashcards
What are acids?
Substances that produce hydrogen ions (H+) when they dissolve in water
What are bases?
Ionic compounds that dissociate into a metal ion and hydroxide ion (OH-) when dissolved in water
What is an Arrhenius base?
An Arrhenius base produces a cation and an OH- anion in an aqueous solution
What does the neutralisation reaction of an acid with a base yield?
Water + salt
[salt = an ionic compound composed of the cation from the base and the anion from the acid]
What are the limitations of the definitions of acids and bases?
- Refer only to reactions that take place in aqueous solution
- H+ is so reactive it does not exist in water -> H+ reacts with H2O to make a HYDRONIUM ION H3O+
What does the Bronsted-Lowry theory state?
A conjugate acid-base pair consists of molecules or ions related by the loss or gain of one H+
Explanations: That’s why every acid-base reaction contains two conjugate acid-base pairs because protons are transferred in both the forward and the reverse reactions
What happens when a Conjugate acid-base reaction happens?
When the acid HA donates H+, the conjugate A- forms. When the base B accepts the H+, it forms the conjugate BH+.
What is the strength of an acid and base determined by?
STRENGTH OF ACID = determined by the moles of H3O+ that are produced for each mole of acid that dissolves
STRENGTH OF BASE = determined by moles of OH- that are produced for each mole of base that dissolves
How can you differentiate strong acids and bases to weak acids and bases?
STRONG ACIDS and STRONG BASES dissociate completely in water
WEAK ACIDS and WEAK BASES dissolve mostly as molecules with only a few dissociating into ions
What is Kw?
The Kw expression is the equilibium constant for water. It can be calculated by multiypling the concentration of hydronium ions by the concentration of hydroxide ions
Why is the acid dissociation constant (Ka) derived from Kw?
Since water is a solvent as well as a participant for the reaction, its concentration is constant and has no effect on equilibrium
How do you calculate the acid dissociation constant?
Hydronium ion concentration [H3O+] X Conjugate base concentration [A-] divided by the undissociated acid concentration [HA]
What are buffers?
Combinations of substances that act together to prevent a drastic change in pH
What does the effective pH range of a buffer depend on?
- pKa of the acid HA
- Relative concentrations of HA and conjugate base A-
What are the conditions of the most effective buffers?
- pKa for the weak acid should be close to the desired pH of the buffer solution
- Ratio of [HA] to [A-] should be close to 1 -> neither additional acid nor additional base changes the pH of the solution dramatically
- Molar amounts of HA and A- should be 10 times greater than molar amounts of either acid or base you expect to add -> ratio of [A-]/[HA] doesn’t undergo a large change
