Text pages Gibbs Free Energy Free Energy – Enthalpy and Entropy Combine Entropy and enthalpy both determine if a process is spontaneous (thermodynamically favourable). Sometimes they both work together; however, they can oppose each other and the net effect of these two factors can be determined through a quantity called Gibbs Free Energy, G. For any process at constant T and P, the equation is: Temperature is an important factor in determining whether a change will occur spontaneously, especially if the change in enthalpy and change in entropy have the same signs.

Text pages Gibbs Free Energy Free Energy – Enthalpy and Entropy Combine

Text pages Gibbs Free Energy Free Energy – Enthalpy and Entropy Combine Exergonic and endergonic are commonly applied in biological contexts (catabolic processes are exergonic and anabolic processes are endergonic).

Text pages Gibbs Free Energy Free Energy – Work and Equilibrium Spontaneous chemical reactions perform useful work – burning fuels in engines, chemical reactions in batteries or the catabolic portion of metabolism in our bodies. Some energy is lost as heat. The maximum amount of energy from a reaction that we can harness as work is equal to ∆G. This will determine whether a given reaction will be an effective source of energy. Reactions with a negative free energy change will be spontaneous (thermodynamically favourable), while those with a positive free energy change will non-spontaneous (reverse reaction is spontaneous). At equilibrium ∆G = 0. Therefore, at equilibrium, ∆H = T∆S and therefore... If we know ∆H and ∆S, we can calculate the equilibrium temperature (i.e. change in state).

Text pages Gibbs Free Energy Standard Free Energies Helmholtz derived the free energy equation at a similar time as Gibbs, hence the Gibbs- Helmholtz equation is named for: We can calculate the standard free energy change (at 1 atm, 1M) if we have the standard enthalpy change, the standard entropy change and the temperature, T is in Kelvin:

Text pages Gibbs Free Energy Standard Free Energies Since enthalpy and entropy are state functions, Gibbs free energy is also. We can use Hess’s law and calculate the standard free energies of formation: For an element at its normal state at 298 K and 1 atm, Refer to sample problems 3.2.1(a) and (b) to review how to solve for Gibbs free energy using the Gibbs-Helmholtz equation and using the free energies of formation.

Text pages Gibbs Free Energy Standard Free Energies It is significant that ∆S° values are usually very small compared to ∆H°. Only multiplication by T causes subtraction of the T∆S° term from ∆H° to have a significant impact on the value of ∆G°

Text pages Gibbs Free Energy Calculation of Free Energy at Non-Standard Temperatures

Text pages Gibbs Free Energy Experimental Manipulation of the Gibbs-Helmholtz Free Energy Equation We can re-arrange the equation, G = H – TS to the form G = -ST + H. A plot of G vs. T will be a straight line with a slope of –S and a y-intercept of H... Since all chemical species have a positive value for entropy, G decreases with an increase in T (at constant P and state). The graph shows free energies of gases are more temperature dependent than liquids, which are more temperature sensitive than solids.