Search
Search

# What’s the difference between thermodynamics and kinetics?

What’s the difference between thermodynamics and kinetics?

Thermodynamics concerns the energy that goes into and out of chemical reactions. Reactions can be exothermic, meaning the system loses energy, or endothermic, meaning the system gains energy. An exothermic reaction would be water freezing. Water in a liquid stage has kinetic energy — its molecules are moving — but as this energy leaves the system in the form of heat transferring into the surrounding environment, the molecules slow down and eventually the water turns to solid. Similarly, heating up an ice cube introduces heat energy into the system of water molecules, causing them to move faster, become liquid, and perhaps even gas.

##### A more complex example is the Calvin cycle which you mentioned earlier.

This reaction is on net endothermic — but with a twist. The Calvin cycle’s reactants (initial ingredients) are water, carbon dioxide, and light energy and its products are sugar and oxygen. In order to make sugar, you have to put energy into the system to break up the H2O and CO2 molecules into individual carbon, hydrogen, and oxygen atoms. Then, you recombine the individual atoms into new, complex sugar molecules. This recombination takes energy out of the system. Because lone, charged ions are unstable and have a lot of potential energy, but once they bond, they stabilize and lose that potential energy. I think that’s what makes thermodynamics interesting; many reactions, like the Calvin cycle, have both endothermic and exothermic properties.

##### Kinetics allow us to find out how fast reactions occur.

We can measure how much the concentration of a solution changes over a set period of time. To determine the rate of the reaction. We can predict these rates using the gas laws we learned in the fall semester:

higher temperatures, lower volumes, and higher pressures mean more particle collisions and subsequently faster reactions.

In time some basic calculus to get detailed information about how that rate of reaction changes over time.

Generally, we expect that reactions “start off quickly” and slow down over time. Because as the reaction progresses, the number of reactants decreases. So the molecules are simply less likely to bump into each other and react. However, on the other hand. As the number of products is increasing, these products start reacting with each other. As a result, producing our initial reactants in a “backwards” reaction. Eventually, the rate of the “forward” reaction in which products become reactants. And the rate of the “backward” reaction in which reactants become products, equalize. Moreover, this is known as equilibrium.