Module 3  

Lesson 2.2  Energy Transfer



Key Concepts


Recall from the Lesson 1 that a chemical system contains potential energy (Ep). This is due to the fact that energy is stored in the bonds between atoms and in the bonds between entities. Some bonds have more potential energy than others. During a chemical reaction, some bonds are broken and some are formed. When a bond is broken, energy is absorbed. When a new bond is formed, energy is released.


Bond energy, therefore, is the energy needed to break a chemical bond or the energy released when a bond is formed.

In a chemical reaction, it is improbable that the energy required to break the bonds will exactly equal the energy released when new bonds are formed. Because of this, energy will either be absorbed from or released to the surroundings as the reaction proceeds.

Notice that the enthalpy change for a chemical reaction represents the net effect from breaking and making bonds, as evidenced by the following reaction:


Fig. 1 Breaking and Making Bonds

Whether a reaction absorbs energy or releases energy depends upon the combined bond energies of the substances involved in the reaction. An exothermic reaction releases energy; an endothermic reaction absorbs energy.

Endothermic Reaction

Energy required to break existing bonds > Energy released when new bonds form


Exothermic Reaction

Energy required to break existing bonds < Energy released when new bonds form


Endothermic

Exothermic

Absorbs energy Releases energy
ΔrH is positive ΔrH is negative
Energy is a reactant Energy is a product
Temperature of the surroundings will usually decrease. Temperature of the surroundings will usually increase.
Products have more potential energy than reactants have. Reactants have more potential energy than the products have.


Endothermic and exothermic reactions can be depicted graphically, as you will see in Lesson 3, "Communicating Enthalpy Change".

Learning Tip:

Do not confuse activation energy with bond energy. Activation energy is the energy that must be added to a chemical system to initiate a chemical reaction. It does not determine whether a reaction is endothermic or exothermic!

 To review the concepts covered in Lesson 2.1 and 2.2, read pages 524 to 533 and answer the self-check questions below.

Check Your Understanding


Complete the following questions in the textbook. Click on the link below to check your answers.

Practice Questions 1 and 2 on page 526

Section 12.1 Questions  2, 3, 4 and 5 on page 531

Section 12.2 Questions 1, 2, and 3 on page 534

Section 12.3 Questions 1, 2, 3, 4 and 5 on page 542

Page 526 Practice Question 1

The molecules must have sufficient energy and be in the proper orientation when they collide. If these two conditions are met, a reaction can occur.


Page 526 Practice Question 2

Scientific: A better understanding of how chemical reactions occur might improve the ability to predict when chemical reactions might occur, including the effect that catalysts and other substances have on the progress of a reaction.

Technological: A better understanding of how chemical reactions occur may lead to the development of new processes that improve efficiency or reduce the generation of by-products.



Page 531 Section 12.1 Question 2

The hypothesis that reactions have an activation energy is suggested by experiments that show energy may be required to initiate a reaction even if the reaction is self-sustaining once started. An example is the combustion of hydrogen. The fact that similar reactions occur at different rates suggests that the reason for the difference in reaction rates may be differing activation energies. The reactions of alkali metals with water illustrate this. Furthermore, if there were no activation energies, then spontaneous reactions would all occur almost instantaneously. In addition, experiments demonstrating the effect of catalysts are explained readily by hypothesizing the existence of an activation energy.


Page 531 Section 12.1 Question 3
  1. The reaction is endothermic.
  2. (i) represents the activation energy of the forward reaction and (ii) represents the enthalpy change for the reaction.
  3. As the reactant molecules approach each other, their combined kinetic energy is at a maximum and the relative potential energy is at a minimum. As they collide and "climb" the energy barrier, kinetic energy is converted to potential energy. Provided they have enough kinetic energy in the first place, they will surmount the barrier. At the top of the energy profile, the potential energy of the system will be at a maximum and the kinetic energy will be at a minimum. At the end of the reaction, the system will have less kinetic energy and more potential energy than it had originally.

Page 531 Section 12.1 Question 4

They differ in that the enthalpy change is the net difference in potential energy between reactants and products whereas the activation energy is the height of the energy barrier that must be surmounted to achieve a successful collision. They are similar in that each represents a difference in potential energy. In the case of the activation energy, it is the difference in potential energy between the reactants and the activated complex rather than between reactants and products.


Page 531 Section 12.1 Question 5
  1. There must be an adequate supply of oxygen to burn the hydrogen, and the reaction needs to be initiated by a spark or flame to overcome the activation energy.







Page 534 Section 12.2 Question 1

Bond energy is the energy required to break a chemical bond and it is the energy released when a chemical bond is formed.


Page 534 Section 12.2 Question 2

In the reactant molecules, if the bonds that are being broken are strong, the activation energy is likely to be high. If the bonds being broken are weak, the activation energy likely will be relatively low.


Page 534 Section 12.2 Question 3

By definition, a chemical reaction involves a rearrangement of bonds to produce new substances in which atoms are conserved but the bonding is different. This can happen only if at least some bonds are broken and new ones are formed. For example, in the decomposition of water, O-H bonds are broken and H-H and O=O bonds are formed.



Page 542 Section 12.3 Question 1

Same: the reactants, products, potential energy (position of reactants and products on a potential energy diagram), overall enthalpy change

Different: activation energy, rate (speed) of reaction, reaction intermediates, activated complex that occurs during the reaction.


Page 542 Section 12.3 Question 2
  1. C and D
  2. A, B and E
  3. F
  4. 2A + B + E → F

Page 542 Section 12.3 Question 3
  1. The activation energy for the forward reaction is 60 kJ.
  2. The activation energy for the reverse reaction is 95 kJ.
  3. The enthalpy change for the forward reaction is -35 kJ.
  4. The enthalpy change for the reverse reaction is +35 kJ.
  5. The forward reaction is exothermic.

Page 542 Section 12.3 Question 4

If only 55 kJ of kinetic energy were available, the reactant molecules would rebound without reacting successfully.


Page 542 Section 12.3 Question 5

In the reverse direction, a collision with kinetic energy equivalent to 55 kJ would also result in rebound - and an unsuccessful reaction.