What is the activation energy of the forward reaction?

In summary, activation energies in units of J/mole represent the energy required for a specific reaction pathway and are not specific to any particular substance in the reaction. Delta H is also a property of the reaction and is not specific to any substance. To convert from energies per molecule to energies per mole, multiply by Avogadro's number.
  • #1
FCMS34
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In a typical General Chemistry textbook, quantitative treatments of activation energies begin with the Arrhenius equation, which is given (not derived). When R and T values are properly inserted with the given units, the activation energy comes out with the unit of J/mole.

My Question 1: Suppose that the reaction in question is mA + nB -> pC + qD (two reactants and two products). If the activation energy involved in this reaction is determined to be xJ/mole, what does it mean? xJ per mole of which substance(s)?

Typically, a schematic energy diagram accompanies such calculations, indicating an energy “mount” that represents the activation energy; and the same diagram usually indicates that the difference between the energy levels of the reactants and products is equal to the enthalpy change (delta H) of the reaction.

Delta H of a reaction always carries the unit of J (not J/mole) – this is usually found in a different chapter of a book.

My Question 2: I ran into a question that asks the following about a given reaction:
(i) What is the activation energy of the forward reaction? (This was easily calculated; the answer came out with the unit of J/mole).
(ii) What is the heat of reaction (delta H)? (This was also easily calculated from a table of standard heat of formation, by multiplying them with the number of moles, which yields a final answer bearing the unit of J (not J/mole)
(iii) What is the activation of the reverse reaction? Here, it appears that the person who made the question is expecting students to simply add the forward activation energy and delta H, but I am having difficulty with the unit (and the meaning of “per mole” which I mentioned above).

Clarification by an expert will be very much appreciated. Thank you.
 
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  • #2
FCMS34 said:
In a typical General Chemistry textbook, quantitative treatments of activation energies begin with the Arrhenius equation, which is given (not derived). When R and T values are properly inserted with the given units, the activation energy comes out with the unit of J/mole.

My Question 1: Suppose that the reaction in question is mA + nB -> pC + qD (two reactants and two products). If the activation energy involved in this reaction is determined to be xJ/mole, what does it mean? xJ per mole of which substance(s)?

Typically, a schematic energy diagram accompanies such calculations, indicating an energy “mount” that represents the activation energy; and the same diagram usually indicates that the difference between the energy levels of the reactants and products is equal to the enthalpy change (delta H) of the reaction.

Activation energies make sense only when considering elementary reaction steps and not overall reaction equations. Typically elementary steps involve only a small amount of species, so we'll consider the reaction A + BC -> AB + C (similar to a SN2 reaction in organic chemistry where A reacts with BC to displace C from BC).

In this case, the activation energy represents the energy needed to break the B-C bond, offset by the energy gained from the partial A-B bond. Thus the activation energy is a property of the specific reaction, and not any particular substance in the reaction. (In fact, the activation energy is specific to the pathway the reaction takes. If, for example, you change the pathway from reactants to products by adding a catalyst, the activation energy will change.)

Delta H of a reaction always carries the unit of J (not J/mole) – this is usually found in a different chapter of a book.

My Question 2: I ran into a question that asks the following about a given reaction:
(i) What is the activation energy of the forward reaction? (This was easily calculated; the answer came out with the unit of J/mole).
(ii) What is the heat of reaction (delta H)? (This was also easily calculated from a table of standard heat of formation, by multiplying them with the number of moles, which yields a final answer bearing the unit of J (not J/mole)
(iii) What is the activation of the reverse reaction? Here, it appears that the person who made the question is expecting students to simply add the forward activation energy and delta H, but I am having difficulty with the unit (and the meaning of “per mole” which I mentioned above).

Clarification by an expert will be very much appreciated. Thank you.

Energies given in units of joules actually represent energies per molecule. For example, in the reaction A + B -> C, a ΔH of 5J would mean that 5J of heat are absorbed when one molecule of C is produced. Therefore, to convert from energies per molecule to energies per mole, all you need to do is multiply by Avogadro's number (6.02x1023 mol-1).
 

1. What is activation energy and why is it important in a forward reaction?

Activation energy is the minimum amount of energy required for a chemical reaction to occur. In a forward reaction, it is the energy needed for the reactants to overcome the energy barrier and form products. It is important because it determines the rate of the reaction and whether or not it will occur at all.

2. How is the activation energy of a forward reaction determined?

The activation energy of a forward reaction can be determined experimentally by measuring the rate of the reaction at different temperatures and using the Arrhenius equation. It can also be calculated using theoretical models and computational methods.

3. How does the activation energy affect the rate of a forward reaction?

The higher the activation energy, the slower the rate of the reaction. This is because a higher energy barrier means that fewer reactant molecules will have enough energy to overcome it and form products. On the other hand, a lower activation energy will result in a faster reaction rate.

4. Can the activation energy of a forward reaction be changed or manipulated?

Yes, the activation energy of a forward reaction can be changed or manipulated by altering the reaction conditions such as temperature, pressure, and the presence of catalysts. Increasing the temperature or using a catalyst can lower the activation energy, resulting in a faster reaction rate.

5. How does the activation energy of a forward reaction differ from the activation energy of a reverse reaction?

The activation energy of a forward reaction is the energy needed for the reactants to overcome the energy barrier and form products. On the other hand, the activation energy of a reverse reaction is the energy needed for the products to overcome the energy barrier and reform the reactants. These energies may be different due to the different molecular configurations and pathways involved in each reaction.

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