Introduction
Entropy is the degree of disorder in a system. This refers to the random behaviour of the particles in terms of the number of particles and the distribution of all energy on the particles.
Entropy is said to be nature'a arrow, in that universal entropy can only increase.
Reactivity 1.4.1 - Entropy, S, is a measure of the dispersal or distribution of matter and/or energy in a system. (HL)
- 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⦵.
Guidance
- Standard entropy values are given in the data booklet.
Tools and links
- Structure 1.1 - Why is the entropy of a perfect crystal at 0 K predicted to be zero?
Reactivity 1.4.2 - 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. (HL)
- Apply the equation ΔG⦵ = ΔH⦵ − TΔS⦵ to calculate unknown values of these terms.
Guidance
- 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.
Tools and links
Reactivity 1.4.3 - At constant pressure, a change is spontaneous if the change in Gibbs energy, ΔG, is negative. (HL)
- Interpret the sign of ΔG calculated from thermodynamic data.
- Determine the temperature at which a reaction becomes spontaneous.
Guidance
- 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.
Tools and links
- Reactivity 3.2 - How can electrochemical data also be used to predict the spontaneity of a reaction?
Reactivity 1.4.4 - As a reaction approaches equilibrium, ΔG becomes less negative and finally reaches zero.
- Perform calculations using the equation ΔG = ΔG⦵ + RT lnQ and its application to a system at equilibrium ΔG⦵ = −RT lnK. (HL)
Guidance
- The equations are given in the data booklet. Reactivity
Tools and links
- Reactivity 2.3 - What is the likely composition of an equilibrium mixture when ΔG⦵ is positive?
In Reactivity 1.4 - Entropy and spontaneity