CHEM 1032 Lecture Notes - Lecture 25: Spontaneous Process

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1 Apr 2017

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Entropy: measure of energy randomization or energy dispersal in a system. A process that increases the energy of the universe if spontaneous (continuous without external interaction) Entropy helps us understand how endothermic processes can be spontaneous. How can a reaction be spontaneous if it appears that the energy is decreasing: h2o below 0 degrees: It seems that the entropy is decreasing (random (cid:0) ordered) The criteria for spontaneous is based on an increase in universe entropy (2nd law) Even though entropy of water decreases, there must be some increase in the universe entropy for this to be spontaneous. How is (cid:0) s surroundings positive for h2o(l) (cid:0) Releasing energy to surroundings (energy is being dispersed around = why it is a positive spontaneous reaction) So freezing of h2o is a spontaneous process below 0 degrees. Above 0 degrees water freezing is no longer spontaneous. The higher the temperature of the system and surroundings, the lower the amount of energy dispersed.

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

Some of the processes in Model 1 are spontaneousâthat 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 âspontaneousâ 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.

calculate the value of delta G at 25

Photosynthesis is the process by which plants effectively capture energy from the sun and store it for later use by organisms. It's the fundamental process that enables the entire food chain. Actual photosynthesis is a highly complex process involving many steps and many different chemicals, but we can get an idea of what's happening to thermodynamic quantities (enthalpy, Gibbs free energy, and entropy) using simpler reactions.

Let's consider a toy model in which an input of energy (for example from the sun) is be used to combine water and carbon dioxide to create an organic molecule (formaldehyde) and oxygen, effectively "storing" that energy for use later. The simplest possible such process is given by:

CO₂ + H₂O â O₂ + CH₂O

The table lists the enthalpy of formation, ÎH_f and the entropy S for 1 mol of each of the substances involved in this reaction at 298 K and 10⁵ Pa.

	ÎH_f (kJ)	S (kJ/K)
CH₂O	-115.9	0.2189
H₂O	-241.82	0.1888
CO₂	-393.51	0.2138
O₂	0	0.2051

Answer the following questions assuming that 1 mol of formaldehyde molecules are formed.

1. What is the change in enthalpy of this reaction?
ÎH = ____ kJ

Does this change in enthalpy represent energy that must be added to the system, or energy that is released from the system?
-Energy must be added or Energy is released?

2. What is the change in entropy for this reaction?
ÎS = ____ kJ/K

Does the entropy increase or decrease?
-Entropy increases or Entropy decreases?

At 298 K, what is the heat transfer that corresponds to this change in entropy?
Q = ____ kJ
Does this heat transfer represent energy that is added to the system, or energy that is released from the system?
--Energy is added Energy is released ?

3. What is the change in Gibbs free energy for this reaction?
ÎG = _____ kJ

The Gibbs free energies of formation, ÎG_f, are: -228.57 kJ for water; -394.36 kJ for CO₂; and 0 kJ for O₂. What is the Gibbs free energy of formation for formaldehyde?
ÎG_f (formaldehyde) = _______ kJ

4. Using the formation of formaldehyde as a model, comment on photosynthesis and the formation of sugars using what we know about entropy and free energy.

CHEM 1032 Lecture Notes - Lecture 25: Spontaneous Process

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