Biochemistry 2280A Lecture : Topic 10
Document Summary
Get access
Related Documents
Related Questions
As entropy increases there will be more useful energy to doΓ’ΒΒworkΓ’ΒΒ. True or false?
False. As entropy increases, there will generally be less usefulenergy to do work. We can think of entropy as usually being presentin the form of heat energy. Imagine a car motor, which attempts toturn chemical potential energy into kinetic energy (movement). Lessefficient motors will get hotter for the same amount of fuelconsumed. When you drive a car you hope that most of the fuelenergy is converted into kinetic energy and not lost as heat(entropy). The hotter the car (and thus the more entropy) the lessefficient the ability to do Γ’ΒΒworkΓ’ΒΒ (the moving of the vehicle). |
True. As entropy increases, there will generally be more usefulenergy to do work. We can think of entropy as usually being presentin the form of heat energy. In most biological and mechanicalsystems it is heat energy which is captured to do useful 'work'.Heat energy, the random movement of molecules, and increases inentropy are central to doing useful work. |
Biological systems sometimes appear on the surface to break theSecond Law of Thermodynamics Γ’ΒΒ by seemingly increasing the order inthe overall system. They donΓ’ΒΒt. Why?
a. Earth (and living things on it) are not a closed system. Thesun's constant input of energy in the form of photons is harnessedby living things to create internal order within themselves. Butwithout such a constant input of energy, most biological systems onearth would break down. |
b. The Second Law tells us that no natural process can occurunless it is also associated with an increase in the entropy of theuniverse. A living organism (such as an animal or plant) brings inmatter and energy from its environment and uses this to creategreater order within its body: the processes of life. But it isimportant to keep in mind that all animals and plants are also partof a larger system of the environment around them. While creating asmall Γ’ΒΒpocketΓ’ΒΒ of order, the processes of life (metabolism,movement, etc..) releases heat into the environment Γ’ΒΒ thusincreasing the entropy of the universe and following the2nd law. |
c. Living things, by harnessing energy to create order (withinand around themselves) actually create a paradox in questionssurrounding the first and second laws of thermodynamics andseemingly violate many previously held assumptions. |
d. Both a and b above are correct. |
When plants capture energy in the photons of sunlight andconvert that energy into the chemical bonds of organic molecules,are they creating new energy?
No, the energy is being transferred from one form into another.But no new energy is being created. |
No, the overall total energy is decreasing over time. |
Yes, by capturing photons, the plants are creating newenergy. |
Yes, by capturing photons, the plants are creating new energy -of a different wavelength. |
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 Γ’ΒΒ O2 + CH2O
The table lists the enthalpy of formation, ΓΒHf and the entropy S for 1 mol of each of the substances involved in this reaction at 298 K and 105 Pa.
ΓΒHf (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?
ΓΒH = ____ 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?
ΓΒS = ____ 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?
ΓΒG = _____ kJ
The Gibbs free energies of formation, ΓΒGf, 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?
ΓΒGf (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.