R1.4.1—Entropy, S, is a measure of the dispersal or distribution of matter and/or energy in a system. The more ways the energy can be distributed, the higher the entropy. Under the same conditions, the entropy of a gas is greater than that of a liquid, which in turn is greater than that of a solid.

• Predict whether a physical or chemical change will result in an increase or decrease in entropy of a system.

• Calculate standard entropy changes, ΔS⦵, from standard entropy values, S⦵.

R1.4.2—Change in Gibbs energy, ΔG, relates the energy that can be obtained from a chemical reaction to the change in enthalpy, ΔH, change in entropy, ΔS, and absolute temperature, T.

• Apply the equation ΔG⦵ = ΔH⦵ − TΔS⦵ to calculate unknown values of these terms.

R1.4.3—At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative.

• Interpret the sign of ΔG calculated from thermodynamic data.

• Determine the temperature at which a reaction becomes spontaneous.

Structure 1.1—Why is the entropy of a perfect crystal at 0 K predicted to be zero?

Reactivity 3.2—How can electrochemical data also be used to predict the spontaneity of a reaction?