Reaction pathway diagrams: the hill between reactants and products

Molecules do not react just by bumping into each other politely. They must collide with enough energy to start breaking bonds — like pushing a boulder over a ridge before it can roll down the other side.

A reaction pathway diagram is a picture of that journey:

• the y-axis is enthalpy (energy stored in the chemicals);
• the x-axis is the progress of the reaction (no numbers — it is just "start → finish");
• the curve rises to a maximum and then comes down.

Once you have the picture, almost every question is a subtraction:

1. Is it exothermic or endothermic? Compare the levels. Products lower than reactants = exothermic.
2. What is $\Delta H$? $\Delta H = H_\text{products} - H_\text{reactants}$. Exothermic gives a negative value; endothermic a positive one.
3. What is $E_a$? Peak minus reactants — always positive, because energy must be paid in.

Careful with reverse reactions: the activation energy going backwards is measured from the products up to the same peak, so

$E_{a,\text{reverse}} = E_{a,\text{forward}} - \Delta H$

With an exothermic reaction the reverse activation energy is larger than the forward one — that is why the reverse reaction is harder.