Question

25) Am enzyme-catalyzed reaction proceeds at a faster rate than an uncatalyzed reaction because the enzyme lowers the difference in free energy between the reactants and the products.
a. True
b. False

Answer 1

25. Answer: (a) false

Reason: To study a chemical reaction, we use thermodynamics and chemical kinetics. Now two things are to be kept in mind, the equilibrium and Gibb's free energy of the reaction.

The difference between free energy of products and free energy reactants give the Gibb's free energy of the reaction.Gibb's free energy of reaction determines the spontaneity of a reaction, i.e if the G of reaction is negative, the reaction is spontaneous and exergonic(release of energy) and if G is positive, the reaction is non-spontaneous and endergonic(absorption) of energy. It depends on the free energy of products and reactants and is independent of the path of reaction. Hence, the Gibb's free energy of catalysed and un catalysed reaction is exactly the same , as a catalyst doesn't change the free energy of reactants and products. So what does an enzyme do? Every reaction in a biological system has an energy constraint,which prevents reactions from proceeding in an uncontrolled or spontaneous manner. Hence energy has to be provided to overcome this barrier, attain the transition state and to get the reaction going. This energy is known as the activation energy of the reaction.Enzymes speed up the reaction by lowering the activation energy and only provide a different reaction pathway to attain the equilibrium sooner. You can understand from the diagram below, notice that the free energy of reactants and products doesn't change hence the Gibb's free energy of both catalysed and uncatalysed reaction is same.

When the Gibb's free energy of the reaction is 0, the reaction is said to have reached an equilibrium.In a state of equilibrium, the rate of forward and rate of backward reaction is same.

Hence , enzymes do not lower the difference in free energy between reactants and products (or G).