[kapital]

Why does stability of hydrocarbons decreases when the length is 20 carbons and more? Thanx for answers?

[Enthalpy]

I don't understand. What do you call stability: resistance to thermal decomposition, heat of formation...?

[kapital]

Mostly let say the ability to not decompose over room temperature

[Enthalpy]

I don't know the answer for sure. Longer alkanes also show a lower autoignition temperature. I believe this is not linked with bond strength nor electron affinity and the like. Some possibilities:

Longer alkanes have more vibration modes and this permits to concentrate more heat on a given atom or bond.

Too short alkanes need a higher temperature to excite the vibration modes.

Longer alkanes have more possibilities to deform into cycles and make aromatic compounds.

I too would be interested in models, interpretations, explanations.

[orgopete]

Why does stability of hydrocarbons decreases when the length is 20 carbons and more? Thanx for answers?

What data are you referring to? Can you post it?