Standard heats of reaction (Delta H degree) values are given for the first three
ID: 889370 • Letter: S
Question
Standard heats of reaction (Delta H degree) values are given for the first three primary mechanistic steps in the free-radical chlorination of methane. Explain why the -105 kJ/mol overall Delta H degree value for the reaction does not include the delta H degree value for the initiation step, but instead is the sum of the delta H degree values for the two propagation steps. CH4 + Cl2 rightarrow ClCH3 + HCl initiation step: 242 kJ/mol propagation step #1 : 4 kJ/mol propagation step #2: -109kJ/mol For a hypothetical reaction, A + B rightarrow C + D, explain why not every collision between the reactants leads to products.Explanation / Answer
A.
Initiation breaks the bond between the chlorine molecule(Cl2). For this step to occur the step requires an input of the energy as the step is not energitically favorable. It is a +ve value and is an unfavorable contribution to the free energy change.
B.
The reactant molecules mst undergo collisions with each other for the reaction to occur. The molecules which possess a minimum amount of energy to break the bonds of the reactants(activation energy) are successful in the formation of products. Though some reactant molecles possess the activation energy, they must be in proper orientation at the time of collisions for a chemical reaction to occur.
Therefore, the collision theory can be summarized as:
1.The reactant molecules should suffer collisions for reaction to occur.
2. Out of all collisions, only the molecules that meet the activation energy result in the chemical reaction.
3. The molecules having the activation energy must be in a proper orientation at the time of collision.
The molecule must satisfy these criteria for a successful reaction.
Therefore, every collision may not lead to the product formation.
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