Causes of Change

Order and SpontaneityGibbs Energy and Predicting Spontaneity

Spontaneity

• You know that the tendency of a reaction to occur depends on both ______ and _______.

• If change in enthalpy, or ____, is negative and change in entropy, or _____, is positive for a reaction, the rxn WILL occur.

• How can we predict whether or not a reaction will occur if both are positive or both neg??

Gibbs Energy

• Josiah Willard Gibbs solved this issue by proposing another thermodynamic property.

• Gibbs Energy is represented by the symbol G and is defined by the following equation:

G = H – TS

• Another name for Gibbs energy is free energy.

A Spontaneous Reaction

• A spontaneous reaction is one that will occur or is likely to occur without outside assistance

• Outside assistance could be the input of energy.

• A nonspontaneous reaction will NEVER occur without some sort of help.

• Ex: An Avalanche may or may not happen but is always ABLE to occur at any time. But, snow from the bottom of the mountain will never make it back to the top without help.

Gibbs Energy Determines Spontaneity

• A reaction is spontaneous if the Gibbs energy change is negative.

• If a reaction has a ΔG greater than 0, the reaction is nonspontaneous.

• If the reaction has a ΔG of exactly zero, then the reaction is at equilibrium.– What is equilibrium?

Calculating Gibbs Energy

• Reactions that have a large negative ΔG values often release energy and increase in disorder.

• The reactions of K and H2O is an example.

2K(s) + 2H2O(l) K+(aq) + 2OH-

(aq) + H2(g)

ΔH = -392 kJ ΔS = 0.047 kJ/K

The change in Gibbs energy can be calculated from the following equation (at standard thermodynamic temp):

ΔG = ΔH – TΔS

ΔG = (-392 kJ) – (298.15 K)(0.047 kJ/K) = -406 kJ

Potassium and Water

• Thinking about this reaction (K and H2O):– Notice that it increases in disorder (ΔS > 0) and the

change in enthalpy tells us it is ____________.– Notice also that this reaction is occuring at

standard thermodynamic temperature.• This should always be assumed unless otherwise stated

• This is one way of calculating Gibbs Free Energy.

Yet Another Route

• We can calculate Gibbs Energy another way because we have tables of Standard Gibbs Energies of Formation (From a Table of Givens)

• The standard Gibbs Energy of Formation (ΔGf0) is the

change in energy that accompanies the formation of 1 mol of the substance from its elements at 298.15 K, or _________________.

• We can use these values the same way we did for H and S. What equation have we been using?

Hess’s Law and Gibbs Energies

• Hess’s Law also applies when calculating change in Gibbs Energy of a reaction.

ΔGreaction = ΔGproducts – ΔGreactants

Practice:

• Given that the changes in enthalpy and entropy are -139 kJ/mol and 277 J/K respectively for the reaction given below, calculate the change in Gibbs energy (**Careful with the units**). Then, state whether the reaction is spontaneous at 25°C.

C6H12O6(aq) 2C2H5OH(aq) + 2CO2(g)

Practice, The Other Way

• Use the table online or in your books to calculate ΔG for the following water-gas reaction.

C(s) + H2O(g) CO(g) + H2(g)

• Is this reaction spontaneous?

Relating Enthalpy and Entropy Changes to Spontaneity

• Complete the following table from page 366

ΔH ΔS Is the reaction spontaneous?

Negative Positive Yes, at all temperatures

Negative Either positive or negative

Positive Only if T > ΔH / ΔS

Positive Positive Never

Predicting Spontaneity

• Does temperature matter in a reaction?

• Yes, the T in our Gibbs equation makes a difference in determining whether a reaction will occur or not.

• Let’s think about it for a second, what is the equation?

• The previous table helps explains all temperature possibilities you may encounter.

Nonspontaneous Reactions

• Can a nonspontaneous reaction ever occur?

• A nonspontaneous reaction cannot occur unless some form of energy is added to the system.

• Photosynthesis is not a spontaneous reaction but occurs every day.

• Where does this reaction get the needed energy from to take place?

End of Chapter 10

Test on tomorrow!Everything from this chapter (Thermodynamics). Temp, heat, specific heat capacity, enthalpy, entropy, Gibbs, spontaneity, Hess’s Law, etc.

Study MORE tonight, should have already begun