Reversible Reactions

objectives

Understandings

A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal.
The equilibrium law describes how the equilibrium constant (Kc) can be determined for a particular chemical reaction.

Applications and skills:

The characteristics of chemical and physical systems in a state of equilibrium.

Guidance:

Physical and chemical systems should be covered.

Dynamic Equilibria

A Dynamic Equilbrium reaction is the opposite to a Static Equilibrium. In this reaction

"A Chemical Reaction in where the reaction is proceeding in both directions AT THE SAME RATE"

It will not surprise you to know that the model of a chemical reaction where:

A + B --> C + D

This model is now a limiting model and over simple.

In fact - All chemical reactions should be represented with the following symbol:

This set of Double headed arrows represents a reversible reaction - and by definition ALL CHEMICAL REACTIONS ARE REVERSIBLE

A + B ⇌ C + D

In the above reaction this means that A and B react together to make C and D AND C and D react together to make A and B.

Dynamic Equilibria are characterised by:

A reaction that proceeds in both directions
A reaction that never reaches completion
Represented by ⇌
Reactants and Products continue to change chemically but the amounts of each remain constant
A + B ⇌ C + D

Static Equilibrium

Many reactions appear to only go from Reactants to Products because of the Large Amounts of Energy required to Reverse the Reaction.

When Chemical reactions appear to go to completion (i.e. move from reactants to products) we call these Static Equilibrium Reactions

These are represented by Forward arrows and are likely to be how most of the reactions have been presented to you in Chemistry prior to this program. Like this:

A + B --> C + D

In this reaction we can see that Only A and B React to make C and D - C and D do not react back in this representation.

Static Equilibria Reactions are characterised by:

No exchange between Reactants and Products
Reaction has 'gone to completion'
Reaction is said to be irreversible
Represented by →
A + B → C + D
Particles do not change chemically
Example: C(diamond) --> C(graphite)

A chemical reaction is usually written in a way that suggests it proceeds in one direction, the direction in which we read, but all chemical reactions are reversible, and both the forward and reverse reaction occur to one degree or another depending on conditions.
In a chemical equilibrium, the forward and reverse reactions occur at equal rates, and the concentrations of products and reactants remain constant.
If we run a reaction in a closed system so that the products cannot escape, we often find the reaction does not give a 100% yield of products.
Instead, some reactants remain after the concentrations stop changing.
At this point, when there is no further change in concentrations of reactants and products, we say the reaction is at equilibrium. A mixture of reactants and products is found at equilibrium.
For example, when we place a sample of dinitrogen tetroxide (N2O4, a colorless gas) in a glass tube, it forms nitrogen dioxide (NO2, a brown gas) by the reaction

The color becomes darker as N2O4 is converted to NO2. When the system reaches equilibrium, both N2O4 and NO2 are present (Figure 1).

Figure 1. A mixture of NO2 and N2O4 moves toward equilibrium. Colorless N2O4reacts to form brown NO2. As the reaction proceeds toward equilibrium, the color of the mixture darkens due to the increasing concentration of NO2.

The formation of NO2 from N2O4 is a reversible reaction, which is identified by the equilibrium arrow (⇌).
All reactions are reversible, but many reactions, for all practical purposes, proceed in one direction until the reactants are exhausted and will reverse only under certain conditions.
Such reactions are often depicted with a one-way arrow from reactants to products.
Many other reactions, such as the formation of NO2 from N2O4, are reversible under more easily obtainable conditions and, therefore, are named as such.
In a reversible reaction, the reactants can combine to form products and the products can react to form the reactants.
Thus, not only can N2O4decompose to form NO2, but the NO2 produced can react to form N2O4.
As soon as the forward reaction produces any NO2, the reverse reaction begins and NO2 starts to react to form N2O4.
At equilibrium, the concentrations of N2O4 and NO2 no longer change because the rate of formation of NO2 is exactly equal to the rate of consumption of NO2, and the rate of formation of N2O4 is exactly equal to the rate of consumption of N2O4.
Chemical equilibrium is a dynamic process: the numbers of each remain constant, yet there is a flux back and forth between them

In a chemical equilibrium, the forward and reverse reactions do not stop, rather they continue to occur at the same rate, leading to constant concentrations of the reactants and the products.
Plots showing how the reaction rates and concentrations change with respect to time are shown in Figure 1.
We can detect a state of equilibrium because the concentrations of reactants and products do not appear to change.
However, it is important that we verify that the absence of change is due to equilibrium and not to a reaction rate that is so slow that changes in concentration are difficult to detect.
We use a double arrow when writing an equation for a reversible reaction.
Such a reaction may or may not be at equilibrium. For example, Figure 1 shows the reaction:

When we wish to speak about one particular component of a reversible reaction, we use a single arrow.
For example, in the equilibrium shown in Figure 1, the rate of the forward reaction

is equal to the rate of the backward reaction