What are the two requirements for a proposed mechanism to be valid for a given reaction?

Understanding the necessity of a reaction mechanism's alignment with experimental data is fundamental. It's the cornerstone of validating whether a proposed mechanism accurately reflects the realities of a chemical reaction. In essence, chemists must observe that the substances react in a particular ratio and that the rate at which they react is defined by specific concentrations, which is often expressed in the form of a rate law.

For a chemist, the concordance with experimental data is like a detective confirming an alibi - without it, the proposed mechanism lacks credibility. Consequently, when evaluating a mechanism, it's essential to ensure that it not only suggests how reactants become products but does so in a manner corroborated by experimental observations. This includes reproducing the amounts of products and rate of reaction, as well as the detection of any intermediate compounds formed during the process.

Stoichiometry is akin to a recipe for chemical reactions, dictating the precise measure of each reactant that's required to form the desired products. It's the balance sheet for atoms before and after a reaction, ensuring that none disappear or appear out of nowhere. All proposed reaction mechanisms must hold true to this steadfast rule; the stoichiometry must be impeccable.

By tracking atoms through each step of a proposed mechanism, chemists can confirm that the stoichiometry matches the established equation for the reaction. This exercise not only verifies atomic conservation but also serves as a didactic scaffold for students, aiding in their understanding of the complex dance of atoms and how they elegantly transform into new substances.

The rate law represents the rhythm at which reactants convert to products, regulated by how often the reacting molecules meet and the energy required for their interaction to lead to a reaction. It elucidates the dependence of the reaction rate on the concentration of reactants. A mechanism's validity hinges on its rate-determining steps aligning perfectly with the experimentally-derived rate law.

Dissecting the dance of molecules further, the rate law provides insights into the sequence of steps, pointing out which step takes the longest or requires the highest energy peak (the transition state) and therefore governs the rate. It's this meticulous matching of the rate law with the proposed sequence of elementary reactions in the mechanism that adds another layer of affirmation to a hypothesis in the world of chemical kinetics.

Imagine a climber tackling a mountain - each step must be possible with the equipment at hand and within the climber's abilities. Similarly, for a reaction mechanism, each elementary step must be energetically viable; the energy required to reach the transition state - akin to the climber's rest-stop on a ledge - should be within reach given the reaction conditions.

This concept encapsulates the necessity of having a pathway that doesn't demand more energy than what's available. The steps must be plausible without invoking an energy miracle, obeying the fundamental laws of thermodynamics. If any step in the mechanism defies this rule, it's akin to the climber being expected to leap an impossible chasm - the proposed mechanism could then be considered flawed or incomplete.

The interplay of kinetics and thermodynamics is essential when deciphering the choreography of a chemical reaction. Kinetics is concerned with the speed of the reaction, while thermodynamics determines whether the reaction is energetically favorable.

The proposed mechanism should not only depict a route that is kinetically accessible (where reactants can transform into products at a reasonable rate) but also a thermodynamically sound path (where the final products are more stable than the reactants). It's the synergy between these two realms that enforces the natural progression of reaction events, dictating both the feasibility and the spontaneity of the reaction's course. For learners delving into the intricacies of chemistry, grasping the concepts of kinetics and thermodynamics is pivotal in comprehending why certain reactions proceed and others do not.