Chapter 2, Problem 31P

Interpretation Introduction

Interpretation: The effect observed upon total enthalpy associated with oven and room, total entropy, total free energy should be determined and whether this process is thermodynamically feasible or not should be determined.

Concept introduction: Theexpression that determines spontaneity of the reaction is,

  $\Delta G=\Delta H-T\Delta S$

Where,

• $\Delta G$is the change in the Gibbs free energy.
• $\Delta H$is the change in the enthalpy.
• $\Delta S$is the change in the entropy.

Explanation of Solution

Total enthalpy of “oven and room” remains unchanged because as pizza is brought out system (oven) may have lost energy but an equivalent amount of heat goes inside from surroundings. This makes overall enthalpy as constant.

When the oven door is opened to cool down the hot molecules present earlier turn cold and less random in motion because of lowered temperature. Thus entropy change is negative as randomness reduces.

The expression that determines spontaneity of the reaction is,

  $\Delta G=\Delta H-T\Delta S$

In accordance with above expression at constant enthalpy change and negative entropy change $\Delta G$ is positive as shown below:

  $\begin{array}{c}\Delta G=\Delta H-T\Delta S\\ =0-\left(-\text{ve}\right)\\ =+\text{ve}\end{array}$

The positive value of $\Delta G$ make overall phenomenon as non-spontaneous. Thus, process is not thermodynamically feasible.