Reactivity 1.4 Entropy and spontaneity (HL)

**Reactivity 1.4.1**

Understandings:

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, Δ
*S*, from standard entropy values,*S*.

- Standard entropy values are given in the data booklet.

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

**Reactivity 1.4.2 and 1.4.3**

Understandings:

Understandings:

**Change in Gibbs energy,****Δ***G***, relates the energy that can be obtained from a chemical reaction to the change in enthalpy,****Δ***H***, change in entropy,****Δ***S***, and absolute temperature,****T (1.4.2)***.*- At constant pressure, a change is spontaneous if the change in Gibbs energy, Δ
*G*, is negative (1.4.2). - Interpret the sign of Δ
*G*calculated from thermodynamic data (1.4.3). - Determine the temperature at which a reaction becomes spontaneous (1.4.3).

- Apply the equation Δ
*G*= Δ*H**− T*Δ*S*to calculate unknown values of these terms (1.4.2). - Δ
*G*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)*.*

- Thermodynamic data values are given in the data booklet.
- Note the units: Δ
*H*kJ mol–1; Δ*S*J K–1 mol–1; Δ*G*kJ mol–1.

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

**Reactivity 1.4.4**

Understandings:

Understandings:

**As a reaction approaches equilibrium,****Δ***G***becomes less negative and finally reaches zero.**

- Perform calculations using the equation Δ
*G*= Δ*G*⦵ +*RT*ln*Q*and its application to a system at equilibrium Δ*G*⦵ =*−RT*ln*K*.

- The equations are given in the data booklet.

- Reactivity 2.3—What is the likely composition of an equilibrium mixture when Δ
*G*⦵ is positive?