Free energy(G) is the energy available (or required) to do work in a system. If a given system releases free energy, then it can do work. Conversely, if it absorbs free energy, then work can be done on it.
Let's return to the example of marbles being held in a hand. We will define the system as the person holding the marbles and the marbles themselves. When the marbles are held, they are relatively ordered (they have low entropy) but unstable (simply opening the hand will cause a spontaneous change in the system). The potential energy of the marbles is also relatively high. When the hand is opened, however, several things happen. First, potential energy is converted to kinetic energy. Second, friction is produced and released in the form of heat as the marbles fall through the air. Third, the system becomes disordered as the marbles bounce around (entropy increases). If the person walks around and picks up the marbles, then the thermodynamic state changes again. First, the kinetic energy exerted by the person picking up the marbles is converted to gravitational potential energy as the marbles become elevated above the ground, and second, the marbles become more ordered (less entropic). Importantly, the ordered state can be restored, but energy (in the form of the person picking up the marbles) is required to order matter.
The change in free energy (ΔG) isendergonicif energy enters the system, andexergonicif it leaves the system. Moreover, an exergonic reaction is unstable, has a negative ΔG, and is therefore aspontaneous reaction.
Lastly, in this example one can see why energy flow is not 100% efficient; as the marbles fall through the air there is a production of frictional heat (which, in this example, does no useful work and represents waste). All energy transfers have some inefficiencies, which is why reactions do not transduce 100% of the available energy.
Organisms can only live at the expense of free energy (G). The free energy changes (ΔG) associated with life's metabolic energy involve the movement of matter. This free energy comes from a series of metabolic reactions that result in work being done at the molecular level (the movement of electrons, atoms, or molecules). Recall the relationship above, between free energy and stability; a given reaction (a system) that has the potential to do a lot of work (release a lot of free energy) is inherently unstable; it typically has a low relative entropy and tends to change spontaneously to a more stable, disordered state. In fact, the concept of spontaneity actually defines whether free energy is made available to do work (or if work is required).
Free energy is more than a change in entropic state because each given system has a certain amount of total energy. However, not all of this total energy is available to do work. Free energy is a function of the total energy change of a system and the entropic change.
A reaction actually describes a change (Δ) from one state to another. The change in free energy is denoted as ΔG. If this value is negative, then free energy will leave the system, work can be done, and the reaction will occur spontaneously.
Enthalpy(H) is the total energy in a system. If this value is negative, then some energy (typically heat) will leave the system. If this value is positive, then energy will enter the system (typically heat will be absorbed from outside). An exothermic reaction has a negative -ΔH and will release heat, whereas an endothermic reaction has a +ΔH and will absorb heat.
The entropic state of the system is denoted S. If the reaction results in an increase in entropy, then this value is positive. If the reaction decreases in entropy, then this value is negative.
Free energy is also dependent on temperature. (Recall the relationship between temperature and entropy) The value "T" is given in degrees Kelvin. (To convert Centigrade to Kelvin, add 273.)
The relationship between these terms is expressed by the free energy equation:
The more negative the value of ΔG, the more free energy released by the reaction and the more work that can be done. Conversely, as ΔG becomes progressively more positive, the energy required for the reaction to proceed also increases.
When the energy in a system at the start of a reaction is greater than the energy at the end of the reaction, theΔG is negative and the reaction is an exergonic reaction. This means that energy is released and can be used to do work. For example, the reactions involved in breaking down glucose to retrieve energy during cellular respiration are exergonic. Cellular respiration (which will be discussed in the next tutorials) releases energy that the cell can then use to do work. We can express this reaction as:
As you determined, this reaction has a ?G of -686 kcal/mol. For every 180 gms of glucose (1 mole), 686 kcals become available to do work.
On the other hand, endergonic reactions require an input of energy. Photosynthesis, the process by which plants convert CO2and H2O into sugars, requires an input of energy from the sun, so it is an endergonic process overall. In other words, photosynthesis can be described as:
This reaction has a ΔG of +686 kcal/mole and requires that work be done. This work is done via energy provided by the sun.