Activation Energy and Catalysts. Activation energy The activation energy of a re

Question

Activation Energy and Catalysts.

Activation energy

The activation energy of a reaction is the difference in energy between the reactants and the activated complex. Activation energy is always a positive number.

Internal energy change

The internal energy change, E, of a reaction is the difference in energy between the reactants and the products. E may be positive or negative depending upon whether the reaction is endothermic or exothermic. To ensure that you always get the correct sign for E, use the following equation:

E=EproductsEreactants

Enthalpy change

H

Part A

What is the value of the activation energy of the uncatalyzed reaction?

Part B

What is the value of the enthalpy change of the uncatalyzed reaction?

Part C

What is the value of the activation energy of the uncatalyzed reaction in reverse?

Part D

What is the value of the enthalpy change of the uncatalyzed reaction in reverse?

Part E

How does the presence of a catalyst affect the activation energy of a reaction?

Part F

How does the presence of a catalyst affect the enthalpy change of a reaction?

A. A catalyst increases the enthalpy change of a reaction.

B. A catalyst decreases the enthalpy change of a reaction.

C. A catalyst does not affect the enthalpy change of a reaction.

D. It depends on whether you are talking about the forward or the reverse reaction.

A. A catalyst increases the activation energy of a reaction. B. A catalyst decreases the activation energy of a reaction. C. A catalyst does not affect the activation energy of a reaction. D. It depends on whether you are talking about the forward or the reverse reaction.

Explanation / Answer

reactants energy is 225 kj/moles

activation complex energy = 400kj/mole

activation energy = activation complex energy-reactants energy

                         = 400-225 = 175 kj/mole

Part B

What is the value of the enthalpy change of the uncatalyzed reaction?

Enthalpy change = products energy -reactant energy

                        = 100-225 = -125kj/mole

Part C

What is the value of the activation energy of the uncatalyzed reaction in reverse?

Activation energy = energy of products - energy of reactants

                         = 100-400 = -300kj/mole

Part D

What is the value of the enthalpy change of the uncatalyzed reaction in reverse?

enthalpy of reverse = 225-100 = +125Kj/mole

E.

B. A catalyst decreases the activation energy of a reaction.

F

C. A catalyst does not affect the enthalpy change of a reaction.

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