Why is the value of ea for a spontaneous reaction less than the ea value for the same reaction running in reverse?

Answer and Explanation:

As the spontaneous reaction (exothermic), the energy of the products is lower than that of the reactants.

The difference in energy between reactants and products is the enthalpy change of the reaction (ΔH).

The activation energy (Ea) of the forward direction is the difference between  the energy of the the activated complex state and the reactants state.

In the reverse direction we need a supply of energy because the reaction is converting from a more stable state (low energy) to a lower stable state (high energy).

The supply in the reverse direction is ΔH, the amount of heat released in the forward direction.

Here, the activation energy of the reverse direction will be (Ea of the forward direction + ΔH).

The difference between the Ea of the reverse and forward direction is ΔH.

Ea reverse = Ea forward + ΔH.

So, in spontaneous reactions, the value of the Ea of forward direction is lower than the same reaction running in the reverse direction.

- lead + phosphoric acid = hydrogen + lead(ii) phosphate

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