Activation Energy As Prerequisite For Activated Complex
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Activation Energy As Prerequisite For Activated Complex

There is an energy hill all chemical reactions must climb inorder for the reactants in the chemical reaction to produce the desired products. The activation energy is the energy required to climb to the peak of this hill. The peak is the activated complex. It is a short-lived high-energy (excitations due to absorption of activation energy) zone where energetic collisions cause changes in the electron cloud of the colliding molecules and allow bonding rearrangement. Consequently, the reaction is able to slide down the hill as the products are being formed.

(a) Figures 11.1(a) and 11.1(b) are energy diagrams. Which diagram represents an endothermic reaction and which diagram represents an exothermic reaction?

(b) Compare the activation energy required for exothermic reaction with that required for endothermic reaction.

Mechanical processes also need activation energy. Consider figure 11.1(c). A is a rectangular prism in a vertical position. Its weight is W and its center of gravity is C1 when in a vertical position. Suppose the prism is pushed slightly such that it tilts and its center of gravity changes to C2 and thereafter, falls on its own to a horizontal position where its center of gravity is C3. The vertical distance between C1 and C2 is y1. The vertical distance between C1 and C3 is y2.

(c) What is the activation energy required by the prism inorder to fall to its horizontal position?

The strings:

S7P3A32 (Force - Push)

The math:

Pj Problem of Interest is of type force. Energy is the capacity for work. It is force that is the doer of the work. So energy and work problems are of type force. The collisions occuring at the activated complex are force-pushes. Also the push that tilts the rectangular prism in fig 11.1(c) is a force-push.

(a) The energy diagram in figure 11.1(a) represents an exothermic reaction (energy is released). The energy diagram in figure 11.1(b) represents an endothermic reaction (energy is absorbed).

(b) Generally, an exothermic reaction will require less activation energy than an endothermic reaction. However, there are some chemical reactions expected to be exothermic that require very high activated energy.

(c) Activation energy that must be supplied to the rectangular prism to activate the fall:
= Wy1.
The net energy change = -Wy2 (energy is released).
The negative net energy change (energy released) is due to the difference in the gravitational potential energy (G.P.E) of the prism in its vertical position relative to its horizontal position. The prism falls from a metastable position at a higher G.P.E to a stable position at a lower G.P.E.

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