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Home » Biology Homework Help » Biotechnology » Chemical Energetics
Chemical Energetics
All chemical reactions are theoretically reversible, i.e. they proceed in both the directions, although at different rates. For example, the chemical reaction



utilizes reactants A and B to yield the products C and D (forward reaction). The reverse reaction, i.e. formation of A and B from C and D, will also occur simultaneously. The rates of forward and reverse reactions depend on the concentrations of the reactants (e.g. A and B) and the products e.g. C and D), and on the “rate constants” for the forward and backward reactions. (The rate constants differ for every reaction). Therefore, the rate of forward reaction is the highest at the start of a reaction when the concentration of the reactants is the highest and that of the products is zero. As the reaction proceeds, reactant concentrations progressively decline, while those of the products increase correspondingly. As a result, the rate of forward reaction declines gradually, while that of the reverse reaction increases till both the rates become equal, i.e. the reaction reaches an ‘equilibrium’. At equilibrium the forward and reverse reactions proceed at identical rates there is no net formation of either the products or the reactants.

All chemical reactions possess two major sets of properties:
   
Thermodynamic properties

Thermodynamic properties are based on the stabilities of the products and reactants, and indicate the extent to which the reactants will be converted into products. The stability of a compound depends on its free energy; a lower free energy is more stable and thermodynamically favored state. The most useful thermodynamic value in the determination of whether a reaction will occur or not is the change in Gibbs free energy denoted by ΔG. The value and sign of ΔG for a reaction changes with concentrations of the reactants and the products, and with temperature, a change in any one of them alters ΔG. If the value of ΔG is negative, the reaction will occur, but if it is positive, the reverse reaction will take place. The value of ΔG indicates the direction and the extent of a reaction; it does not reveal anything about the rate of reaction, which depends on kinetic properties of the reaction.

In general, the free energy of reactants is higher than that of the products. Therefore, the products of a reaction are more stable and at equilibrium their concentrations are much higher than those of the reactants.
   
Kinetic properties

According to transition state theory, for chemical reaction to occur between two reactant molecule, their free energy level must be raised above a threshold level to take them to very unstable, high energy state called transition state. The free energy take them to very unstable, high energy state called transition state. The free energy needed to elevate a molecule from its stable ground state to the unstable transition state known as activation energy. The rate of a chemical reaction depends on the number of reactant molecules that have enough energy to reach the transition state of the slowest step in the reaction (the rate determining step). However, only one-half of all the molecules reaching the transition state react to form products, the other half going back to the low energy ground state reactant molecules. As a rule, very few molecules possess enough energy to reach the transition state. Therefore, the magnitude of  is the major factor limiting the rate of a chemical reaction: the higher the value of the lower the rate of reaction. Enzymes markedly reduce the value of for a reaction, thereby phenomenally increasing the rate of reactions.

It may be noted that the energy levels and the stabilities of the products and reactants in a chemical reaction and those in the same reaction catalyzed by an enzyme are identical. Further, the rate of reverse reaction is enhanced by the same order of magnitude as that of the forward reaction. Therefore, enzymes, like other catalysts, do not change the equilibrium ratio of the reactants and the products.

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