Reactivity 1.4 Entropy and spontaneity (HL) 
Reactivity 1.4.1
Understandings:
  • 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.
Learning outcomes:
  • 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.
Additional notes:
  • Standard entropy values are given in the data booklet.
Linking questions:
  • Structure 1.1 Why is the entropy of a perfect crystal at 0 K predicted to be zero?
This video covers entropy. 
This video covers predicting entropy changes. 
This video covers calculating entropy changes. 
This video covers entropy and spontaneity. 
Reactivity 1.4.2 and 1.4.3
Understandings:
  • 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 (1.4.2).
  • At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative (1.4.2).
  • Interpret the sign of ΔG calculated from thermodynamic data (1.4.3).
  • Determine the temperature at which a reaction becomes spontaneous (1.4.3).
Learning outcomes:
  • Apply the equation ΔG = ΔH − TΔS to calculate unknown values of these terms (1.4.2).
  • ​ΔG takes into account the direct entropy change resulting from the transformation of the chemicals and the indirect entropy change of the surroundings resulting from the transfer of heat energy (1.4.3).
Additional notes:
  • Thermodynamic data values are given in the data booklet.
  • Note the units: ΔH kJ mol–1; ΔS J K–1 mol–1; ΔG kJ mol–1.​
Linking questions:
  • Reactivity 3.2 How can electrochemical data also be used to predict the spontaneity of a reaction?
This video covers how to calculate the Gibbs energy change for a reaction. 
This video covers the effect of temperature,  ΔH and ΔS on the spontaneity of a reaction. 
This video covers calculating Gibbs energy change using Gibbs energy of formation values. 
Reactivity 1.4.4
Understandings:
  • As a reaction approaches equilibrium, ΔG becomes less negative and finally reaches zero.
Learning outcomes:
  • Perform calculations using the equation ΔG = ΔG⦵ + RT lnQ and its application to a system at equilibrium ΔG⦵ = −RT lnK.
Additional notes:
  • The equations are given in the data booklet.
Linking questions:
  • Reactivity 2.3—What is the likely composition of an equilibrium mixture when ΔG⦵ is positive?
This video covers the relationship between the Gibbs energy change and equilibrium.