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8.5.5: Equilibrium and Steady State

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    Now here is an interesting point: imagine a situation in which reactants and products are continually being added to and removed from a system. Such systems are described as open systems, meaning that matter and energy are able to enter or leave them. Open systems are never at equilibrium. Assuming that the changes to the system occur on a time scale that is faster than the rate at which the system returns to equilibrium following a perturbation, the system could well be stable. Such stable, non-equilibrium systems are referred as steady state systems. Think about a cup with a hole in it being filled from a tap. If the rate at which water flows into the cup is equal to the rate at which it flows out, the level of water in the cup would stay the same, even though water would constantly be added to and leave the system (the cup). Living organisms are examples of steady state systems; they are open systems, with energy and matter entering and leaving. However, most equilibrium systems studied in chemistry (at least those discussed in introductory texts) are closed, which means that neither energy nor matter can enter or leave the system.


    In addition, biological systems are characterized by the fact that there are multiple reactions occurring simultaneously and that a number of these reactions share components—the products of one reaction are the reactants in other reactions. We call this a system of coupled reactions. Such systems can produce quite complex behaviors (as we’ll explore further in Chapter 9). An interesting coupled-reaction system (aside from life itself) is the Belousov–Zhabotinsky (BZ) reaction in which cesium catalyzes the oxidation and
    bromination of malonic acid.172 If the system is not stirred, this reaction can produce quite complex and dynamic spatial patterns, as shown in the figure. The typical BZ reaction involves a closed system, so it will eventually reach a boring (macroscopically-static) equilibrium state. The open nature of biological systems means that complex behaviors do not have to stop; they continue over very long periods of time. The cell theory of life (the theory that all cells are derived from preexisting cells and that all organisms are built from cells or their products), along with the fossil record, indicates that the non-equilibrium system of coupled chemical reactions that has given rise to all organisms has persisted, uninterrupted, for at least ~3.5 billion years (a very complex foundation for something as fragile as life).

    The steady state systems found in organisms display two extremely important properties: they are adaptive and homeostatic. This means that they can change in response to various stimuli (adaptation) and that they tend to return to their original state following a perturbation (homeostasis). Both are distinct from Le Chatelier’s principle in that they are not passive; they are active processes requiring energy. Adaptation and homeostasis may seem contradictory, but in fact they work together to keep organisms alive and able to adapt to changing conditions.173 Even the simplest organisms are characterized by great complexity because of the interconnected and evolved nature of their adaptive and homeostatic systems.

    Questions to Answer

    • What does it mean when we say a reaction has reached equilibrium?

    • What does the magnitude of the equilibrium constant imply about the extent to which acetic acid ionizes in water?

    • Write out the equilibrium constant for the reaction H3O+ + AcO ⇄ AcOH + H2O.

    • What would be the value of this equilibrium constant? Does it make sense in terms of what you know about acid-base reactions?

    • If the pH of a 0.15-M solution of an acid is 3.6, what is the equilibrium constant Ka for this acid? Is the acid a weak or strong acid? How do you know?

    • Calcium carbonate (CaCO3) is not (very) soluble in water. Write out the equation for the dissolution of CaCO3. What would be the expression for its Keq? (Hint: recall pure solids and liquids do not appear in the expression.) If Keq for this process is 6.0×10-9, what is the solubility of CaCO3 in mol/L?

    • What factors determine the equilibrium concentrations for a reaction?

    • For the reaction N2 (g) + 3 H2 (g) ⇄ 2 NH3 (g) (ΔH = -92.4 kJ/mol), predict the effect on the position of equilibrium, and on the concentrations of all the species in the system, if you:

      • add nitrogen

      • remove hydrogen

      • add ammonia

      • heat the reaction up

      • cool it down

    • Draw a reaction energy diagram in which the reverse reaction is much faster than the forward reaction (and vice versa).

    • As a system moves towards equilibrium, what is the sign of ΔG? As it moves away from equilibrium, what is the sign of ΔG?

    • Explain in your own words the difference between ΔGo and ΔG.

    • Imagine you have a reaction system A ⇌ B for which Keq = 1. Draw a graph of how ΔG changes as the relative amounts of [A] and [B] change.

    • What would this graph look like if Keq = 0.1? or Keq= 2?

    • If ΔGo is large and positive, what does this mean for the value of Keq?

    • What if ΔGo is large and negative, how does the influence Keq?

    Questions for Later

    • Why is Keq temperature-dependent?
    • Explain mechanistically why random deviations from equilibrium are reversed.

    • If the value of Q is > Keq, what does that tell you about the system? What if Q is < Keq?

    Questions to Ponder

    • The acid dissociation constant for ethanol (CH3CH2OH) is ~10–15. Why do you think acetic acid is 10 billion times more acidic than ethanol? (Hint: draw out the structures and think about the stability of the conjugate base.)
    • If ΔG for a system is = 0, what does that mean?



    173 This type of adaptation is physiological and occurs within individual organisms; it is distinct from, but based on, evolutionary processes that act on populations of organisms.

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