What is Life?:How chemistry becomes biology by Addy Pross Page A

stable
materials are transformed into
more stable
materials. A ball rolling down a slope is a useful analogy. Chemical reactions proceed in a ‘downhill direction’, where downhill signifies toward more stable products, products that are characterized by what is termed lower ‘free energy’. Since the free energy of water is lower than the free energy of a mixture of hydrogen and oxygen gases, the two gases react to form water, and the energy that was stored in the higher-energy hydrogen and oxygen molecules is released as heat. The reverse reaction in which water would betransformed into hydrogen and oxygen gases cannot take place spontaneously because that would be equivalent to a ball rolling uphill.

     
    Fig. 2. Diagram illustrating the free energy change for the reaction of hydrogen and oxygen gases (H 2 + O 2 ) to give water (H 2 O).
     
    The relative free energies of a hydrogen and oxygen mixture compared with that of water are shown schematically in Fig. 2. The hydrogen and oxygen molecules on the left side of the diagram (H 2 +O 2 ) are located at higher energy than the water product (H 2 O) on the right side of the diagram.
    The diagram also reveals another important point—the hydrogen and oxygen reactants are separated from the water product by a barrier. Even though the hydrogen and oxygen gas mixture is higher in free energy than water, the path leading from reactants to products does not go downhill smoothly. It climbs uphill to some extent before it begins to descend, which means that before the reaction can proceed, the barrier must first be overcome. That’s why a sparkor catalyst is needed to get the reaction going. The spark provides the initial energy boost in order to get the reactants over the barrier, after which the downhill trajectory of the reaction profile takes care of the rest. A catalyst may obviate the need for a spark by reducing the barrier height so that no activation is needed and the reaction can proceed without that energy boost.
    Two important lessons can be learnt from the above example. First, reactions will only take place if the reaction products are of lower free energy than the reactants. That determines the direction of any chemical reaction and is called the thermodynamic consideration. Accordingly, the Second Law of Thermodynamics indicates beforehand which reactions are possible and which are not. Once a reaction mixture has reached the lowest possible free energy state for that particular combination of materials, the system is said to be at equilibrium and no further reaction will take place. Like balls at the bottom of a valley, they have nowhere lower to roll. But the fact that a reaction mixture is not at equilibrium, i.e., not in that lowest possible free energy state, does not mean it will necessarily react. If that reaction system is trapped in a local minimum, that is, behind a barrier, it may not be able to overcome the barrier that separates that local minimum from the deeper, product minimum, much like a ball that is trapped in a hollow halfway down some slope. That’s why hydrogen and oxygen gases may be mixed without any reaction taking place if neither catalyst nor spark are provided. These simple notions can now be expressed in the language of chemistry: a reaction that is allowed thermodynamically may or may not proceed, depending on kinetic factors (the barrier height). However, a reaction that is forbidden thermodynamically
cannot
proceed.
Entropy and the Second Law
     
    We have seen that chemical reactions will only proceed if they are in accord with the Second Law. But it will help subsequent discussion to introduce another important concept—entropy. Understanding entropy is important because it is a key component of stability and, in fact, the Second Law can be expressed entirely in terms of entropy.
    Entropy can be thought of intuitively as the degree of disorder in a system. If you throw a number of building blocks onto a surface, they are