reversible reaction

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.

• Predicting Spontaneous Direction of a Redox Reaction

  • Reactions for which Eo is positive, therefore, have equilibrium constants that favor the products of the reaction.
  • What happens to the standard electrode potential when the reaction is written in the reverse direction?
  • For example if we turn the zinc oxidation half-reaction around ($Zn^{2+} + 2e^- \rightarrow Zn \ E^o = -0.76 V$), the cell potential is reversed.
  • Half-reaction equations can be combined if one is reversed to oxidation in a manner that cancels out the electrons.
  • Predict the direction of electron flow in a redox reaction given the reduction potentials of the two half-reactions

• 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