Reversible reactions: What are they?
Reversible reactions (shown with a reversible arrow) do not go to completion. In a closed system, reversible reactions will instead reach a state known as dynamic equilibrium. The equilibrium is dynamic because there are changes occurring at the microscopic level, even though the system undergoes no change at the macroscopic level.
An example of a reversible reaction includes ammonium chloride breaking down when heated, producing ammonia and hydrogen chloride. Upon cooling, these gasses can react to form ammonium chloride again. This can be modeled as: NH4Cl(s) NH3(g) + HCl(g).
On the other hand, irreversible reactions (shown with a forward arrow →) are those that go to completion to reach a static equilibrium.
An example of an irreversible reaction includes combustion. This is because in complete combustion a fuel and oxygen are the reactants, and the products are carbon dioxide and water, therefore, the products cannot reform back to the reactants. This can be modeled as: Fuel(g) + O2(g) → CO2(g) + H2O(g).
It should be noted the following types of systems can exist:
- Open System → It is when energy and matter can enter and leave.
- Closed System → It is when energy and matter cannot enter and leave, however energy transfer can still take place through the surroundings.
- Isolated System → It is when energy and matter cannot enter and leave.
Static and Dynamic Equilibrium
– Equilibrium for a closed chemical system refers to when:
- The concentrations of both reactant and products do not change with time.
- The rate of the forward reaction is equal to the rate of the reverse reaction.
Specifically, irreversible reactions will reach a static equilibrium upon completion, whereas reversible reactions will reach a dynamic equilibrium upon completion.
A static equilibrium refers to when no more macroscopic changes occur as well as no microscopic changes occur within the chemical system. On the other hand, at a dynamic equilibrium there are changes occurring at the microscopic level, even though the system undergoes no change at the macroscopic level.
An important indicator to look out for when performing experiments in the lab is colour change, which is a macroscopic property used to see if a reversible reaction reaches equilibrium.
Non-Equilibrium Chemical Systems
– Non-equilibrium chemical systems include those reactions which are irreversible. This includes combustion as well as photosynthesis.
– Non-Equilibrium chemical systems exist due to:
- Products are no longer in the system, such as in an open system.
- Products do not react with each other.
- Forward reaction has a negative enthalpy change and positive entropy change.
– Combustion reactions are exothermic; that is, they release heat. Also, they are associated with a positive change in entropy because of greater production of product molecules compared to reactant molecules; that is, greater disorder within the system. For example, this can be modeled by 2C3H8(g) + 7O2(g) → 8H2O(g) + 6CO2(g). As a result, because the forward reaction has a negative enthalpy change and positive entropy change, the reverse reaction is always nonspontaneous. Thus, it is classified as a non-equilibrium chemical system, as it is an irreversible reaction.
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