Nature And H,C,N And O

Question 1

Why do H, C, N and O make up most of biological molecules?

Answer 1

Readily available
Form stable covalent bonds
N and O atoms can ionise by loss of H
Can form hydrogen bonds

Question 2

Describe the thermodynamic stability of organic and biological molecules

Answer 2

They are thermodynamically unstable in air
This means the energy of products is lower than reactants
They break down in air spontaneously

Question 3

What is needed for the synthesis of organic and biological molecules

Answer 3

Require an input of energy to make them as they

They spontaneously break down in air so need to input energy to go the other way in the reaction

Question 4

Describe the bonds these atoms form

Answer 4

Strong covalent bonds

Question 5

We are made up of organic and biological molecules. Why do we not combust at normal temperatures?

Answer 5

The activation energy to change the structure of such elements is large- therefore they are stable
Large kinetic barrier to their breakdown

Question 6

Describe through SN1 and SN2 reactions why organic and biological molecules containing these atoms do not breakdown normally in air

Answer 6

SN1- Ea is high due to the small size of atoms- high charge density
SN2- transition state has high Ea due to small size of atoms
Also difficult close packing 5 ligands around central atom

Question 7

What is required to break a bond and what is bond strength related to?

Answer 7

Bond breaking requires thermal energy

Strength of bond is related to average vibrational temp

Question 8

Describe the bonds between these atoms and why they are so strong

Answer 8

CHNO sigma bonds are very strong

This is due to the fact they are small atoms- smaller MO so therefore greater overlap