This document discusses theories and models in science. It explains that theories are comprehensive models that can be tested through experiments and observations. As an example, it discusses using bond enthalpy models and average bond enthalpy values to calculate and explain enthalpy changes in chemical reactions, like breaking carbon-hydrogen bonds in methane. However, it notes that bond enthalpy values are approximations since they are averages that may not precisely apply to individual compounds.

1. Nature Of Science
Akshat Singh
Aditya Seth

2.  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.

3. 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.

4. Example
Gaseous
molecule
Elements
in
standard
state
Gasous
Atoms

5.  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

6.  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

7. 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

8.  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

9.  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