The ChemCollective   NSDL and CMU

Entropy Activity


-Introduction

-The reaction coordinate

-The energy landscape

-Population distributions

-Motion at constant temperature, and exchange of energy with the heat bath

-Thermally activated processes

-Energy and temperature determine the populations

-Entropy and free energy

-Mathematical derivation of free energy




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The energy landscape

In this activity, we explore the factors that establish the energy landscape.

Virtual Activity

    - The energy landscape (Java Required) [Opens in a separate window. Please read through all content below the simulation before returning to the following text.]

This activity illustrates that the energy landscape is established by the structure of the system. For a chemical system, it is established by the molecular structure. For instance, if we replace the chlorines of the molecule shown on the previous page with flourines, we alter the energy landscape for the cis-trans isomerization. The van der Waals ratio (i.e. size) of Flourine is less than that of Chlorine.

Comprehension Question: How do you think the energy landscape changes when you replace Cl with F?

Answer: Since F is smaller than Cl, the steric interaction between F atoms is less than that between Cl atoms. The metastable state for the fluorine substituted compound is then more stable, i.e. has lower energy, than that of the Cl compound, as illustrated below.


Changing Cl to F is analogous to changing the aspect ratio of our box. For the box, we can choose any aspect ratio we want. For molecular systems, we often have discrete choices, such as choosing one of the halogens (F, Cl, Br, or I) or replacing an amino acid of a protein with another amino acid.


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   Page Last Updated: 11.07.2016