reversible reaction

(noun)

one that results in an equilibrium mixture of reactants and products

Related Terms

  • chemical equilibrium

Examples of reversible reaction in the following topics:

  • Equilibrium

    • Chemical equilibrium is the state in which the forward reaction rate and the reverse reaction rate are equal.
    • In theory, all chemical reactions are in fact double reactions: for every forward reaction, there is a subsequent reverse reaction.
    • In a chemical reaction, chemical equilibrium is the state in which the forward reaction rate and the reverse reaction rate are equal.
    • When a system consists of competing forward and reverse reaction rates, the reaction will proceed until chemical equilibrium is reached.
    • Recall the relationship between the forward and reverse reaction rates when a reaction is at equilibrium
  • The Effect of a Catalyst

    • Reactions can be sped up by the addition of a catalyst, including reversible reactions involving a final equilibrium state.
    • Recall that for a reversible reaction, the equilibrium state is one in which the forward and reverse reaction rates are equal.
    • In the presence of a catalyst, both the forward and reverse reaction rates will speed up equally, thereby allowing the system to reach equilibrium faster.
    • By lowering the energy of the transition state, which is the rate-limiting step, catalysts reduce the required energy of activation to allow a reaction to proceed and, in the case of a reversible reaction, reach equilibrium more rapidly.
    • A catalyst speeds up a reaction by lowering the activation energy required for the reaction to proceed.
  • Reaction Quotients

    • The reaction quotient is a measure of the relative amounts of reactants and products during a chemical reaction at a given point in time.
    • By comparing the value of Q to the equilibrium constant, Keq, for the reaction, we can determine whether the forward reaction or reverse reaction will be favored.
    • The reaction quotient can be used to determine whether a reaction under specified conditions will proceed spontaneously in the forward direction or in the reverse direction.
    • If Q > Keq, the reaction will move to the left (in the reverse direction) in order to reach equilibrium.
    • Calculate the reaction quotient, Q, and use it to predict whether a reaction will proceed in the forward or reverse direction
  • Electrocyclic Reactions

    • An electrocyclic reaction is the concerted cyclization of a conjugated π-electron system by converting one π-bond to a ring forming σ-bond.
    • The reverse reaction may be called electrocyclic ring opening.
    • The sterospecificity of this reaction is demonstrated by closure of the isomeric trans,cis,cis-triene to trans-5,6-dimethyl-1,3-cyclohexadiene, as noted in the second example.
    • This mode of reaction is favored by relief of ring strain, and the reverse ring closure (light blue arrows) is not normally observed.
  • Changes in Temperature

    • Changes in temperature can affect the equilibrium state of a reversible chemical reaction.
    • Le Chatelier's Principle states that when changes are made to a reversible chemical reaction in equilibrium, the system will compensate for that change with a predictable, opposing shift.
    • Reactions can be classified by their enthalpies of reaction.
    • A diagram of the reaction coordinate for an exothermic reaction is shown in .
    • Le Chatelier's Principle predicts that the addition of products or the removal of reactants from a system will reverse the direction of a reaction, while the addition of reactants or the removal of products from a system will push the reaction towards the formation of products.
  • The Reverse TCA Cycle

    • However, there are numerous organisms that undergo reverse TCA or reverse Krebs cycles.
    • The chemical reactions that occur are the reverse of what is seen in the TCA cycle .
    • The reverse TCA cycle is a series of chemical reactions by which organisms produce carbon compounds from carbon dioxide and water.
    • This process requires a number of reduction reactions using various carbon compounds.
    • The enzymes, unique to reverse TCA, that function in catalyzing these reactions include: ATP citrate lyase, 2-oxoglutarate:ferredoxin oxidoreductase, and pyruvate:ferredoxin oxidoreductase.
  • Ene Reactions

    • The reverse process is called a retro ene reaction.
    • This is the same bond bookkeeping change exhibited by electrocyclic reactions, but no rings are formed or broken in an ene reaction unless it is intramolecular.
    • The following examples illustrate some typical ene reactions, with equation 3 being an intramolecular ene reaction.
    • Reaction 4 is drawn as a retro ene reaction, although this has not been demonstrated to be general for all reactions of allylic alcohols with thionyl chloride.
    • A similar acid-catalyzed reaction of simple aldehydes with alkenes to give allylic alcohols, 1,3-diols or 1,3-dioxanes is known as the Prins reaction.
  • Pericyclic Reactions

    • An important body of chemical reactions, differing from ionic or free radical reactions in a number of respects, has been recognized and extensively studied.
    • The four principle classes of pericyclic reactions are termed: Cycloaddition, Electrocyclic, Sigmatropic, and Ene Reactions.
    • All these reactions are potentially reversible (note the gray arrows).
    • The reverse of a cycloaddition is called cycloreversion and proceeds by a ring cleavage and conversion of two sigma-bonds to two pi-bonds.
    • The reverse electerocyclic ring opening reaction proceeds by converting a sigma-bond to a pi-bond.
  • Zero-Order Reactions

    • Unlike the other orders of reaction, a zero-order reaction has a rate that is independent of the concentration of the reactant(s).
    • For a zero-order reaction, the half-life is given by:
    • The reverse of this is known, simply, as the reverse Haber process, and it is given by:
    • The reverse Haber process is an example of a zero-order reaction because its rate is independent of the concentration of ammonia.
    • The reverse of this process (the decomposition of ammonia to form nitrogen and hydrogen) is a zero-order reaction.
  • Changes in the Entropy of Surroundings

    • Irreversible reactions result in a change in entropy to the surroundings.
    • A change is said to occur reversibly when it can be carried out in a series of infinitesimal steps.
    • The reversible expansion of a gas can be achieved by reducing the external pressure in a series of infinitesimal steps; reversing any step will restore the system and its surroundings to their previous state.
    • Similarly, heat can be transferred reversibly between two bodies by changing the temperature difference between them in infinitesimal steps, each of which can be undone by reversing the temperature difference.
    • Distinguish whether or not entropy of surroundings changes in various reactions
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