# Thermodynamics

Thermodynamics seeks to explain reactions. To do so requires the use and understanding of some of the properties of reactions.

# In biological reactions enthalpy can increase or decrease, the change being dependent upon the specific reaction.

It is hard to understand the term enthalphy but luckily, in biological terms the enthalphy of a reaction is, for the most part, the heat of the reaction since biological reactions occur under conditions of constant pressure and volume. The heat of a reaction involves such things as the vibration and rotation of the atoms of the molecules, the energy of the bonds that hold molecules together and the nonbonding energy of the molecules in a reaction.

# In biological reactions entropy can increase or decrease, the change being dependent upon the specific reaction.

The entropy of a reaction is a measure of the state of randomness of the molecules in a reaction. The greater the disorder, the greater the entropy.

# Gibb's free energy (G)

For biologist, the most important thermodynamic property is one termed the free energy of a reaction since it is related to the ability to do biological work.

In addition, understanding the free energy of a reaction also allows one to determine under what conditions a biological reaction will proceed, in what direction the reaction will occur and the position of equilibrium and to compute the equilibrium constant (Keq) of the reaction.

# The free energy, enthalpy and entropy of reactions are related by the equation:

G = H -TS where T is the temperature in degrees Kelvin.

When a reaction occurs, a change occurs. Thus, during a reaction the free energy changes. The symbol that indicates a change is an isoceles triangle:

# The standard free energy of a reaction

The free energy of a reaction is affected by temperature, pressure, pH and the concentrations of all the reactants and products of the reaction.

The standard free energy of a reaction is computed under the following conditions: 25 C; 1 atmosphere (atm) of pressure; pH 7; and initial concentrations of reactants and products of 1M for each, except for H+ ions that are kept at pH 7.

The symbol for the standard free energy of a reaction is

# free energy under nonstandard reaction conditions

consider the following reaction:

the free energy yield of the reaction can be given by the following equation

In this case the free energy yield of the reaction is equal to the standand free energy (X) R (a constant) (X) T (temperature) (X) the natural log of the ratio of the concentration of substance-2 and substance-1.

The implication of this relationship is that the free energy yield of the reaction is dependent upon the concentration of the substrate and product of an enzyme catalyzed reaction.

That is, enzyme driven reaction are reversable.

They can go in the forward or reverse direction depending on the standard free energy of the reaction and the concentration of substrate and product.