Chemical Thermodynamics



We've discussed aspects of thermodynamics previously. Why cover it again? According to the theoretical physicist Arnold Sommerfied:
"Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, it doesn't bother you anymore."

Energy and the First Law of Thermodynamics

Energy is conserved. It is neither created nor destroyed.


We use this law of thermodynamics all the time. In any chemical reaction, the energy contained in the reactants is equal to the energy contained in the products plus energy released to the environment or absorbed from the environment (as heat or pressure-volume work).

In many reactions, energy released or absorbed is entirely heat energy. Under constant pressure conditions, the change in heat energy in a reaction is called the enthalpy change, H. This is easy to measure by using a calorimeter.

Once we determine the energy change experimentally from several reactions, we can use the First Law to calculate energy changes in some other reactions.


Entropy and the Second Law of Thermodynamics

The entropy of the universe is increasing.


Entropy is a measure of randomness or disorder. Spontaneous changes always go from a more ordered state to a less ordered state. There are many examples of this.
  • If you put a drop of blue food coloring in a beaker of water, the dye tends to spread out and you get a pale blue solution. No matter how long you wait, the colored molecules will never come back together to form a drop of dark blue in colorless water.

  • Similarly, all of the oxygen molecules in this room will never spontaneously arrange themselves in one spot, leaving us to be asphyxiated.

  • A block of shiny iron metal exposed to atmosphere will rust but that rust will never spontaneously convert itself back to pure iron and oxygen.


It is the entropy of the universe that must increase. This is a combination of the entropy of the system we investigate and the entropy of the surroundings.

Suniverse = Ssystem + Ssurroundings


A system can increase order (decrease entropy) but it requires an input of energy from the surroundings, making the surrounding more disordered. For example, you as a living thing are constantly making highly ordered chemical structures within your body. To do this, you require a great deal of energy from your surroundings in the form of food. If you stop taking in this energy, you will ultimately reach a more disordered, decomposed state.

Temperature is proportional to the kinetic energy of molecules and atoms. Increasing temperature increases the overall kinetic energy, the random motion of molecules, and so increases entropy.

When heat is released by an exothermic chemical reaction to the environment, the entropy of the surroundings increases. This means that Ssurroundings will be a positive number.

When heat is absorbed from the surroundings by an endothermic chemical reaction, the entropy of the surroundings decreases. This means that Ssurroundings will be a negative number.

We can calculate the entropy change of the surrounding that is caused by a chemical reaction from the enthalpy change of the reaction and the temperature.

Ssurroundings = - H/T


Within the chemical reaction system, entropy increases when the number of molecules of products is greater than the number of molecules of reactants or when the products have an inherently greater ability to move (stretch and bend) than the reactants. The change in entropy in a chemical reaction can be calculated based on changes in volume, number of particles, and degrees of freedom between reactants and products.

In the forward reaction of the NO2 dimerization reaction, one molecule is formed from 2 molecules and entropy decreases. In the reverse reaction, 2 molecules are formed from 1 molecule and entropy increases.

NO2 + NO2 N2O4



Gibbs Free Energy

Energy is the capacity to do work or release heat. In chemical reactions, we are not usually concerned with the total energy but rather in changes of the usable chemical energy. This is the Gibbs free energy change or G. The free energy is the capacity to do non-mechanical work at a constant temperature and pressure.

Chemical reactions spontaneously proceed in a way that leads to the minimum value of G, that is to the equilibrium condition.
  • When the G of a reaction is a negative number, the reaction will tend to proceed in the forward direction.

  • When the G of a reaction is a positive number, the reaction will tend to proceed in the reverse direction.

  • When the G of a reaction is zero, the system is at equilibrium and there will be no net change.

A spontaneous reaction is not necessarily a fast reaction. Remember that the rate of a reaction depends not on the energy difference between product and reactant, but on the height of the activation barrier in a particular pathway.

The reaction between molecular oxygen and molecular hydrogen doesn't occur at any measurable rate but it is thermodynamically favorable. At the standard temperature and pressure (STP) of 0 deg C and 1 atmosphere, the Gibbs free energy change is -237 kJ/mol. The negative value means that the reaction should be spontaneous in the forward direction.

H2(g) + 1/2 O2(g) H2O(l) G0 = -237 kJ/mol

At STP the Gibbs free energy is G0 but the value changes with temperature (T) an pressure (P) according to the formula below where R is the ideal gas constant.

G = G0 + R T ln P


The Gibbs free energy depends on both enthalpy and entropy. The entropy term is small at low temperature but becomes more important as the temperature increases.

G = H - T S



Equilibrium

Chemical equilibrium is a state in which there is no net change in the concentration of reactants and products because the forward and reverse reactions are occurring at the same rate and the Gibbs free energy value is at its minimum.

The Gibbs free energy change of a reaction tells us what the concentration of reactants and products will be at equilibrium. For a general reaction of a moles of A, b moles of B and c moles of C combining to form n moles of N, m moles of M, and o moles of O...



If the reaction is not at equilibrium, it will proceed in the forward or reverse direction to the minimum value of the Gibbs free energy. At that point, there is no further change in the Gibbs free energy (G = 0) and the reaction is at equilibrium.




Obey the laws of thermodynamics!




Professor Patricia Shapley, University of Illinois, 2012