Nature Of ScienceAkshat Singh

Aditya Seth

Models and theories—measured energy changes can be explained based on the model of bonds broken and bonds formed. Since these explanations are based on a model, agreement with empirical data depends on the sophistication of the model and data obtained can be used to modify theories where appropriate.

Theories

Theories are themselves integrated, comprehensive models of how the universe, or parts of it, work. A theory can incorporate facts and laws and tested hypotheses. Predictions can be made from the theories and these can be tested in experiments or by careful observations. Examples are the germ theory of disease or atomic theory.

Example

Gaseous molecule

Elements in

standard state

Gasous Atoms

We can use this cycle to calculate the bond enthalpy of the C-H bond

What we calculated is an average value for the four bonds

This does not mean that in

CH4 CH3 +H

The enthalpy change is +414kJ mol-1

Usually the enthalpy associated with a particular covalent bond is almost independent of the bonding in the rest of the molecule

Hence, the C-H bond enthalpy is approximately +414kJ mol-1 in both methane and ethanol

Hence, it is useful to assign average bond enthalpy values to particular bonds

Average bond enthalpy

It is defined as the average of the enthalpy required to break a particular covalent bond in a range of molecules determined using methods similar to that we used for Methane

This is a great example of when even though the model doesn’t work precisely it is useful in provided that the approximate nature of results and the reasons for the lack of precision are not remembered

The concept of enthalpy change being measured as

Sum of enthalpies of bonds broken- sum of enthalpies of bonds formed

Can only be used when the substances involved are in the same state,usually gases as change of state also involve enthalpy changes

This, coupled with the fact that bond enthalpy values are the average of that bond in a range of compounds,

They are only approximate for a particular compound

Hence, values calculated using bond enthalpy are less accurate than using other methods.

Still, other than Benzene (resonance stablised), the values are within 10% of more accurate values and is a useful way to calculate app. Enthalpy changes