CHEM1100 Lecture Notes - Lecture 17: Standard Molar Entropy, Spontaneous Process, Thermodynamics

Model 1 – Spontaneous Processes Process Description Change in Enthalpy (∆H) Change in Entropy (∆S) Spontaneous? A Two pure → Homogeneous substances mixture ~ 0 Increasing Yes B Solute on Solute on one side of → both sides of membrane membrane ~ 0 Increasing Yes C Polypeptide → Individual chain amino acids Exothermic (negative) Increasing (positive) Yes D C3 H8 + 5O2 → 3CO2 + 4H2 O Exothermic Increasing Yes E 6CO2 + 6H2 O → C6 H12O6 + 6O2 Endothermic Decreasing No F Glucose → Starch Endothermic Decreasing No G Water → Ice Exothermic Decreasing Below 0 °C, yes Above 0 °C, no H Cold water (25 °C) → Hot water (60 °C) Endothermic Increasing Below 60 °C, no Above 60 °C, yes

Some of the processes in Model 1 are spontaneous—that is, they will occur without any additional work being done on the system. For example, a solute will diffuse across a membrane until the concentrations of both sides are equal. However, glucose will not spontaneously form from carbon dioxide and water in the atmosphere. The Second Law of Thermodynamics states that a process will be spontaneous when it results in an increase of total entropy in the universe. In other words, either the system or the surroundings must have an increase in entropy, or both. (Please note that the term “spontaneous” does not imply that the change will happen quickly. Rusting is spontaneous under the right conditions, but it can still occur very slowly.)

13. Predict the change in entropy of the surroundings for an endothermic reaction.

Problem A: This is a very simpllistic approach to protein folding, which ignores solvent and volume effects, but it allows us to practice what we have learned about thermodynamics and kinetics. Enzymes are long chains of amino acids that have to fold in the proper way so as to exhibit enzyme activity. They can catalyze certain reactions. Let us pick a reaction in the human body at 37 C, we will call it enzyme X.

1) What is the expected sign of delta G for the reaction that forms enzyme X starting from the long chain of amino acids at 37 C?

2)What is the value of K(eq) for the reaction at 37C, assuming the system is reversible?

3)Based purely on the drawing (unfolded and folded (native form)) what is the expected sign of delta S for the reaction that forms enzyme X at 37 C?

4)What is the expected sign of delta H for the reaction that forms enzyme X at 37 C?

5) Under what conditions is the folded form NOT thermodynamically favored? always favored, high temperatures, never favored, low temperatures

6)Let's say that enzyme X is the catalyst responsible for breaking starch down to individual glucose molecules. What of the following things can happen to the starch and enzyme system if you rainse the temperature?

a-the rate of the reaction of starch going to glucose will increase

b-nothing should happen since a catalyst does not affect the equilibrium

c-the enzyme could stop working effectively becauset the folded form would stop being made and would start to unravel if the process is reversible

d-the equilibrium constant will get bigger to favor the glucose side more.