M5-S2: Non-equilibrium Systems

  • Analyse examples of non-equilibrium systems in terms of the effect of entropy and enthalpy, for example:

– combustion reactions
– photosynthesis
 
  • Many chemical processes are irreversible, which means that when products are formed, they do not revert back into reactants. This could be due to many reasons:
  1. Products are no longer in the system e.g. an open system.
  2. Products do not react with each other due to their highly stable nature.
  3. Forward reaction has a negative enthalpy change and positive entropy change.

 

 

Enthalpy Change

 `DeltaH>0` (endothermic)

 `DeltaH<0` (exothermic)

Entropy Change

`DeltaS>0`

(increase)

Reversible

Irreversible (reverse reaction is not spontaneous)

`DeltaS<0`

(decrease)

Forward reaction is not spontaneous, does not proceed

Reversible

 

 

  • Irreversible, non-equilibrium reactions are characterised by a negative change in enthalpy and positive change in entropy. In other words, these formation of products release heat (exothermic) and increases the disorder-ness of the system. The Gibbs free energy of these reactions is negative at all temperatures; therefore, the reverse reaction is always non-spontaneous.

 

Combustion

  • Combustion reactions are exothermic (`DeltaH<0`) and associated with a positive change in entropy due to production of greater number of gas molecules compared to reactants.

 

$$2C_3H_{8(g)} + 7O_{2(g)} \rightarrow 8H_2O_{(g)} + 6CO_{2(g)}$$

 

  • For example, the combustion of propane gas has a standard enthalpy of -2220.0 kJ/mol and entropy of +269.91 J/mol/K.

 

Complete combustion of propane yields 14 moles of gas for every 9 moles of gas consumed. The increase in gas molecules means the entropy of the system is increased (`DeltaS>0`).

 

  • Combustion of liquid and solid fuel has greater increases in entropy. For example, complete combustion of octane (liquid). This means these reactions are more spontaneous as they have a more negative change in Gibbs free energy.

 

$$2C_8H_{18(l)} + 25O_{2(g)} \rightarrow 18H_2O_{(g)} + 16CO_{2(g)}$$

 

  • In an open system, combustion reactions are always irreversible because carbon dioxide and water leave the system.
  • In a closed system, combustion reactions are also irreversible because carbon dioxide and water do not react due to the reaction having a positive change in Gibbs free energy at all temperatures (`DeltaG>0`). 
  • This is because the reverse reaction is endothermic and entails a decrease in entropy. These two conditions combined means the reaction is never spontaneous.

 

Photosynthesis

Photosynthesis, although can be summarised by one chemical equation, actually consists of numerous chemical reactions taking place at different time and locations in plants. 

 $$6CO_{2(g)} + 6H_2O_{(l)} \rightarrow 6O_{2(g)} + C_6H_12O_{6(s)}$$

  • Photosynthesis is an irreversible reaction because the system in which it occurs is an open system. The products of one reaction (component of photosynthesis) may be quickly consumed in another reaction. In particular, the final product, oxygen, is produced in the chloroplast but quickly leaves the plant through its stomata (pores in leaves)
  • Photosynthesis is a biological process which requires enzymes (biological catalysts) to take place. These enzymes only catalyse forward reaction.
  • The reverse reaction of photosynthesis does not take place because the activation energy is too high in the absence of a suitable enzyme.

     

    Figure shows numerous reactions part of photosynthesis. The knowledge of this diagram is not required for HSC Chemistry. The diagram's aim is to illustrate the complexity and step-wise nature of photosynthesis. Reversal of this process would require every reactant and product to remain in contact in a closed system. Credit: Encyclopedia Britannica.

     

    Why is photosynthesis irreversible?

    • While photosynthesis as a chemical equation is the reverse of aerobic respiration, the biological process is distinctively different. The synthesis of glucose from carbon dioxide and water is a highly unfavourable process because it is endothermic and decreases in entropy. It is not possible as a single-step process.
    • Instead, photosynthesis is broken into many parts and requires specific enzymes, biological apparatuses such as the chloroplast to occur. See figure above.
    • Photosynthesis is a non-equilibrium system because it occurs in an open system within plant cells. Products generated from one part of the process can be transported to another part of the cell. 
    • Photosynthesis is irreversible because the enzymes are highly specific for their substrate (reactants). Many enzymes are unable to facilitate the reverse reaction.

     

    Previous section: Introduction to Equilibria

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