Non-equilibrium Systems

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, the formation of these products release heat (exothermic) and increase the disorder-ness of the system (entropy). The Gibbs free energy of these reactions is negative at all temperatures meaning that they are always spontaneous; 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 `kJmol^{–1}` and entropy of +269.91 `Jmol^{–1}K^{–1}`.

 

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

Although photosynthesis can easily be summarised by a single equation, it 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 the 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.

• For a reaction to be considered as an (dynamic) equilibrium system, it needs to be spontaneous and reversible. 

• Photosynthesis is a non-equilibrium system because it is a process that consists of a series of non-spontaneous chemical reactions. The overall process is endothermic (∆H > 0) and involves a decrease in entropy of the system (∆S < 0), resulting in a positive Gibbs free energy (∆G > 0).

• Another reason why it is a non-equilibrium system is 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. 

 

Previous section: Introduction to Equilibria

Next section: Collision Theory

 

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