Acidity & Basicity

Question 1

The Acidity/Basicity of a compound will strongly influence:

Answer 1

• Kinetics (speed) of a rxn
• Stability of a compound
• Solubility

Question 2

Bronsted-Lowry Acid

Bronsted-Lowry Base

Answer 2

Proton donor

Proton acceptor

Question 3

What is needed for a substance to act as an acid or base?

Answer 3

-For something to act as an acid it must possess a
hydrogen atom which it can release as H+ (a proton).

-For something to act as a base it must be capable of
accepting H+

Question 4

Amphoteric

Answer 4

Can act as an acid or a base

Question 5

The pH of a solution relates to the concentration of the hydronium ion once a substance (acid) has been added to water.
What does the conc. of hydronium ions in a solution depend on?

Answer 5

• The strength of the acid added- how well it dissociates in water
• The concentration of acid added in solution

Question 6

pKa relates to the strength of an acid. It is a physical constant for a given substance (in water)
What is pKa an indicator of?

Answer 6

It is an indicator of the degree to which a given molecule will release a proton. Strong acids more readily dissociate in to their H+ and OH- ions.

Question 7

pKa equation:

Answer 7

K= products/reactants.

Question 8

Two ways to predict acid strength:

• Quantitative Approach
• Qualitative Approach

Answer 8

Quantitative Approach:
-Use of published pKa values
-Molecule with the higher pKa is the weaker acid
-Equilibrium will always favour the formation of the
weaker acid

Question 9

Qualitative Approach:

Answer 9

• Only small number of published pKa values for > 20 million organic compounds
• Qualitative Approach includes comparison of acids and bases without use of tables
• Uses known chemistry principles to predict relative acidity

Question 10

In order to assess the acidity of a molecular species we must examine the structure of the conjugate base.

The more stable the conjugate base the
stronger the acid was to begin with.

Answer 10

The ability of a molecule to behave as an acid is determined by the stability of the conjugate base which it forms.
The more stable the conjugate base, the stronger is the acid from which it formed

Question 11

How to determine relative acidity?

Answer 11

Must compare the stability of the two conjugate bases. More stable base has the strongest acid.

Question 12

Stability of bases depends on:

Answer 12

• Formal charge
• Identity of atom
  
  • electronegativity
  • size of atom
• Induction
• Resonance
• Hybridisation

Question 13

Formal Charge: Removal of H+ decreases the formal charge by 1.

Answer 13

This is easiest when an atom bears a charge of +1 to begin with. Becomes more difficult as overall charge decreases (become more negative)