Kinetic and thermodynamic control of reactions pdf

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kinetic and thermodynamic control of reactions pdf

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Because thermodynamics deals with state functions, it can be used to describe the overall properties, behavior, and equilibrium composition of a system. It is not concerned with the particular pathway by which physical or chemical changes occur, however, so it cannot address the rate at which a particular process will occur.

Kurchatov Sq. Rovisco Pais, Lisbon , Portugal E-mail: max mail. Therefore, a very rare and unexpected example of full kinetic and thermodynamic control in the Diels—Alder reaction is reported in this paper.

14.3: Kinetic vs. Thermodynamic Control of Reactions

Because thermodynamics deals with state functions, it can be used to describe the overall properties, behavior, and equilibrium composition of a system. It is not concerned with the particular pathway by which physical or chemical changes occur, however, so it cannot address the rate at which a particular process will occur.

Although thermodynamics provides a significant constraint on what can occur during a reaction process, it does not describe the detailed steps of what actually occurs on an atomic or a molecular level. Thermodynamics focuses on the energetics of the products and the reactants, whereas kinetics focuses on the pathway from reactants to products. Table Most reactions that we encounter have equilibrium constants substantially greater or less than 1, with the equilibrium strongly favoring either products or reactants.

In many cases, we will encounter reactions that are strongly favored by thermodynamics but do not occur at a measurable rate. In contrast, we may encounter reactions that are not thermodynamically favored under standard conditions but nonetheless do occur under certain nonstandard conditions.

In such cases, mixing the reactants results in only a physical mixture, not a chemical reaction. In comparison, the analogous reaction of SiCl 4 with water to give SiO 2 and HCl, which has a similarly large equilibrium constant, occurs almost explosively.

Although the two reactions have comparable thermodynamics, they have very different kinetics! For example, consider the reaction of lead sulfide with hydrogen peroxide.

One possible reaction is as follows:. Yet when lead sulfide is mixed with hydrogen peroxide, the ensuing vigorous reaction does not produce PbO 2 and SO 2. Instead, the reaction that actually occurs is as follows:. Thermodynamically, such reactions do not occur spontaneously under standard conditions. Nonetheless, these reactions can be made to occur under nonstandard conditions. An example is the reduction of chromium III chloride by hydrogen gas:.

Although the reaction is not thermodynamically spontaneous under standard conditions, it becomes spontaneous under nonstandard conditions. There are also cases in which a compound whose formation appears to be thermodynamically prohibited can be prepared using a different reaction.

The reaction for the preparation of chlorine monoxide from its component elements, for example, is as follows:. Nonetheless, Cl 2 O is easily prepared using the reaction. Consider, for example, the formation of metallic lithium from molten lithium chloride:. Hence in the industrial preparation of metallic lithium, electrical energy is used to drive the reaction to the right, as described in Chapter 19 "Electrochemistry". In the preparation of chlorine monoxide from mercuric oxide and chlorine Equation We can see how the chemical coupling works if we write Equation Hence the overall reaction is indeed thermodynamically spontaneous as written.

By coupling reactions, a reaction that is thermodynamically nonspontaneous can be made spontaneous. Bronze Age metallurgists were accomplished practical chemists who unknowingly used coupled reactions to obtain metals from their ores. Realizing that different ores of the same metal required different treatments, they heated copper oxide ore in the presence of charcoal carbon to obtain copper metal, whereas they pumped air into the reaction system if the ore was copper sulfide.

Assume that a particular copper ore consists of pure cuprous oxide Cu 2 O. A Write the balanced equilibrium equation for each reaction. B Substitute appropriate values into Equation A The chemical equation for the decomposition of cuprous oxide is as follows:.

B Rearranging and substituting the appropriate values into Equation This is a very small number, indicating that Cu 2 O does not spontaneously decompose to a significant extent at room temperature. B We can find the corresponding value of K p :. Although this value is still less than 1, indicating that reactants are favored over products at room temperature, it is about 24 orders of magnitude greater than K p for the production of copper metal in the absence of carbon.

At temperatures of a few hundred degrees Celsius, the reaction occurs spontaneously, proceeding smoothly and rapidly to the right as written and producing metallic copper and carbon monoxide from cuprous oxide and carbon.

Thermodynamics describes the overall properties, behavior, and equilibrium composition of a system; kinetics describes the rate at which a particular process will occur and the pathway by which it will occur. Whereas thermodynamics tells us what can occur during a reaction process, kinetics tells us what actually occurs on an atomic or a molecular level. What have you overlooked in your scheme?

What can you do to drive the reaction to increase your product yield? Previous Section. Table of Contents. Next Section. Note the Pattern Thermodynamics focuses on the energetics of the products and the reactants, whereas kinetics focuses on the pathway from reactants to products. Equation Note the Pattern By coupling reactions, a reaction that is thermodynamically nonspontaneous can be made spontaneous. Example 15 Bronze Age metallurgists were accomplished practical chemists who unknowingly used coupled reactions to obtain metals from their ores.

Summary Thermodynamics describes the overall properties, behavior, and equilibrium composition of a system; kinetics describes the rate at which a particular process will occur and the pathway by which it will occur. Key Takeaway Thermodynamics describes the overall properties, behavior, and equilibrium composition of a system, whereas kinetics describes the particular pathway by which a physical or a chemical change actually occurs.

For all intents and purposes, the reaction does not proceed in the forward direction: only reactants are present at equilibrium. Both forward and reverse reactions occur: significant amounts of both products and reactants are present at equilibrium. For all intents and purposes, the forward reaction proceeds to completion: only products are present at equilibrium.

Thermodynamic and Kinetic Products

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Like nonconjugated dienes, conjugated dienes are subject to attack by electrophiles. In fact, conjugated electrophiles experience relatively greater kinetic reactivity when reacted with electrophiles than nonconjugated dienes do. Upon electrophilic addition, the conjugated diene forms a mixture of two products—the kinetic product and the thermodynamic product—whose ratio is determined by the conditions of reaction. A reaction yielding more thermodynamic product is under thermodynamic control, and likewise, a reaction that yields more kinetic product is under kinetic control. The reaction of one equivalent of hydrogen bromide with 1,3-butadiene gives different products at under different conditions and is a classic example of the concept of thermodynamic versus kinetic control of a reaction. The horizontal axis is a reaction coordinate, and the vertical axis represents Gibbs energy. The delocalized carbocation intermediate A is the protonated form of 1,3-butadiene first step of the reaction of 1,3-butadiene with HBr.

Thermodynamics vs kinetics

Reactions of Dienes: 1,2 and 1,4 Addition. More On 1,2 and 1,4 Additions To Dienes. First, an explanation. My dad died.

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Experimental Results

The potential outcome of a reaction is usually influenced by two factors:. This will be the faster reaction since it has a lower energy more stable transition state, and therefore a lower activation barrier. Therefore, product 1, P1 is the kinetic product the product that forms the fastest. Reaction 2 via pathway 2 blue generates product 2 P2 via transition state 2 TS2. P2 is the more stable product since P2 is at a lower energy than P1.

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Thermodynamic versus kinetic reaction control

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