4. Which of the following energy diagrams corresponds to a one-step exothermic reaction with a high...

Energy diagram that corresponds to one step exothermic reaction with highest activation energy :

Exothermic reaction is exergonic process where energy is released. The initial line of the energy diagram indicates reactants and heap is the process of converting the reactants to products i.e. intermediate or transition step. The last line indicates products.

For a one step exothermic reaction there will be only one heap whereas two step exothermic reaction there will be two heaps.

For a exothermic reaction, energy is released. Hence enthalpy of reactants is greater than the enthalpy of products.

The excess energy is released. Hence reactants energy line is higher in position than the products.

Whereas in endothermic reaction, energy is absorbed. Hence enthalpy of reactants is lesser than the enthalpy of products. The excess energy is absorbed. Hence reactants energy line is lower in position than the products.

Activation energy is the difference of energy of reactant and transition state.

From the above graphs :

The exothermic graphs are A, B and E.

But E is two step exothermic reaction.

Among A and B option the one with highest activation energy is Graph B because the energy difference between the reactants (initial line) and transition state (heap - highest point) is higher than the Graph A.