1.7 Energy changes in chemical reactions  (Page 5/5)

Activation energy

Activation energy or 'threshold energy' is the energy that must be overcome in order for a chemical reaction to occur.

It is possible to draw an energy diagram to show the energy changes that take place during a particular reaction. Let's consider an example:

The reaction between $H_2\left(g\right)$ and $F_2\left(g\right)$ ( [link] ) needs energy in order to proceed, and this is the activation energy. Once the reaction has started, an in-between, temporary state is reached where the two reactants combine to give $H_2{F}_2$ . This state is sometimes called a transition state and the energy that is needed to reach this state is equal to the activation energy for the reaction. The compound that is formed in this transition state is called the activated complex . The transition state lasts for only a very short time, after which either the original bonds reform, or the bonds are broken and a new product forms. In this example, the final product is HF and it has a lower energy than the reactants. The reaction is exothermic and $\Delta$ H is negative.

Activated complex

The activated complex is a transitional structure in a chemical reaction that results from the effective collisions between reactant molecules, and which remains while old bonds break and new bonds form.

In endothermic reactions, the final products have a higher energy than the reactants. An energy diagram is shown below ( [link] ) for the endothermic reaction $XY+Z\to X+YZ$ . In this example, the activated complex has the formula XYZ. Notice that the activation energy for the endothermic reaction is much greater than for the exothermic reaction.

Energy and reactions

1. Carbon reacts with water according to the following equation: $\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{CO}+{\mathrm{H}}_2$ $\Delta$ H $>$ 0
   1. Is this reaction endothermic or exothermic?
   2. Give a reason for your answer.
2. Refer to the graph below and then answer the questions that follow:

   

   1. What is the energy of the reactants?
   2. What is the energy of the products?
   3. Calculate $\Delta$ H.
   4. What is the activation energy for this reaction?

Summary

• When a reaction occurs, some bonds break and new bonds form. These changes involve energy .
• When bonds break, energy is absorbed and when new bonds form, energy is released .
• The bond energy is the amount of energy that is needed to break the chemical bond between two atoms.
• If the energy that is needed to break the bonds is greater than the energy that is released when new bonds form , then the reaction is endothermic . The energy of the product is greater than the energy of the reactants.
• If the energy that is needed to break the bonds is less than the energy that is released when new bonds form , then the reaction is exothermic . The energy of the product is less than the energy of the reactants.
• An endothermic reaction is one that absorbs energy in the form of heat, while an exothermic reaction is one that releases energy in the form of heat and light.
• The difference in energy between the reactants and the product is called the heat of reaction and has the symbol $\Delta$ H.
• In an endothermic reaction, $\Delta$ H is a positive number, and in an exothermic reaction, $\Delta$ H will be negative.
• Photosynthesis, evaporation and the thermal decomposition of limestone, are all examples of endothermic reactions.
• Combustion reactions and respiration are both examples of exothermic reactions.
• A reaction which proceeds without additional energy being added, is called a spontaneous reaction .
• Reactions where energy must be continuosly supplied for the reaction to contiune, are called non-spontaneous reactions.
• In any reaction, some minimum energy must be overcome before the reaction will proceed. This is called the activation energy of the reaction.
• The activated complex is the transitional product that is formed during a chemical reaction while old bonds break and new bonds form.

Summary exercise

1. For each of the following, say whether the statement is true or false . If it is false, give a reason for your answer.
   1. Energy is released in all chemical reactions.
   2. The condensation of water vapour is an example of an endothermic reaction.
   3. In an exothermic reaction $\Delta$ H is less than zero.
   4. All non-spontaneous reactions are endothermic.
2. For each of the following, choose the one correct answer.
   1. For the following reaction: $\mathrm{A}+\mathrm{B}\to \mathrm{AB}$ $\Delta$ H = -129 kJ.mol ${}^{-1}$
      1. The energy of the reactants is less than the energy of the product.
      2. The energy of the product is less than the energy of the reactants.
      3. The reaction is non-spontaneous.
      4. The overall energy of the system increases during the reaction.
   2. Consider the following chemical equilibrium: $2{\mathrm{NO}}_2\rightleftharpoons {\mathrm{N}}_2{\mathrm{O}}_4$ $\Delta$ H<0 Which one of the following graphs best represents the changes in potential energy that take place during the production of N ${}_2$ O ${}_4$ ?

      

3. The cellular respiration reaction is catalysed by enzymes. The equation for the reaction is: ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2\to 6{\mathrm{CO}}_2+6{\mathrm{H}}_2\mathrm{O}$ The change in potential energy during this reaction is shown below:

   

   1. Will the value of $\Delta$ H be positive or negative? Give a reason for your answer.
   2. Explain what is meant by 'activation energy'.
   3. What role do enzymes play in this reaction?
   4. Glucose is one of the reactants in cellular respiration. What important chemical reaction produces glucose?
   5. Is the reaction in your answer above an endothermic or an exothermic one? Explain your answer.
   6. Explain why proper nutrition and regular exercise are important in maintaining a healthy body.