How can I draw a reaction coordinate in a potential energy diagram?
The graph of reaction co-ordinate vs potential energy for standard exothermic and endothermic reactions are known.
Reaction co-ordinates represent the way the reactant molecules "evolve" to give products.These plots can be computed by using softwares like Gaussian.(I use it regularly)
example of how reaction proceeds
As one can see from the gif that the overall geometry,the bond distances change .Every point on the graph represents this change with respect to the potential energy.As the reaction proceeds we reach a state where the potential energy is highest.This point is called the transition state.Some reactions can have more than one transition state.
Note:- the following part is not necessary for school students.
How to get such a plot?
An IRC calculation allows one to map out a reaction pathway by integrating the intrinsic reaction coordinate.This In the input the molecular geometry is of the transition state molecule. An IRC can go in the forward direction or in the reverse direction. By default BOTH reaction pathways are followed. An IRC calculation requires initial force constants. These can be calculated by computing a frequency on the transition state and saving the checkpoint file. Finally the same checkpoint is then used for the IRC calculation with the key word RCFC. If a frequency computation was not preformed, one can simply calculate the force constants before the IRC by using the keyword CALFC.