5.9: Reaction Rate
Reaction Rates (Kinetics)
When a reaction releases energy, the chemical change is exothermic. Release of energy does not guarantee that the reaction will take place within a reasonable amount of time. A measure of how quickly products form, is called the reaction rate.
Reaction Energy Diagrams
Reaction energy diagrams summarize the important information about a reaction in a single graph. The y-axis represents the energy and the x-axis represents the reaction progress (time). The relative energies of the reactants and products are determined by the potential energy stored within the molecules as chemical bonds. For an exothermic reaction A, the products are lower in energy than the reactants resulting in a release of heat. For an endothermic reaction B, the products are higher in energy than the reactants requiring the constant addition of energy to keep the reaction going. The reactants must possess a minimum amount of energy for a reaction to occur. In a reaction energy diagram, the minimum energy is described as the “activation energy” (\(E_a'\)). Because the activation energy can be viewed as a barrier to a reaction, a larger activation energy indicates a slower the reaction rate and a smaller activation energy indicates a faster the reaction rate.
Reaction A Reaction B
Figure \(\PageIndex{1}\): Reaction A is exothermic, Reaction B is endothermic.
Catalysts are substances that speed up a reaction without being consumed or changed by the reaction. Because of these properties, catalysts can be added in small quantities to increase reaction rates. Catalysts increase reaction rates by decreasing the activation energy of a reaction. The transition state represents the reactants in the correct orientation with the minimum energy for a reaction to occur. The energy difference between the reactants and the transition state is the activation energy. The addition of a catalyst to Reaction C increases the reaction rate by decreasing the activation energy as shown below. Catalysts do not affect the ΔH of the reaction. Most chemical reactions that take place in living things are catalyzed by proteins called enzymes .
Reaction C
No Catalyst Reaction With Catalyst Reaction
Figure \(\PageIndex{2}\): Addition of a catalyst lowers the activation energy of the reaction and speeds up a chemical reaction.
Another way to alter the rate of a reaction is to change the temperature . The higher the temperature the faster the reaction. At higher temperatures the kinetic energy of the of the reactants are greater, collisions between reactants take place more frequently and with greater energy, so reactants are able to find their way over the activation barrier to form products.
The rate of the reaction can also be varied by changing the concentration of one or more reactants. Concentration refers to the amount of substance that is present in a mixture. A higher concentration of reactants can increase the rate of a reaction. The more crowded the reaction mixture is with reactants, the more likely is the collisions between reactant molecules.
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Use the reaction diagrams from Figure 5.9.1 to answer the following questions.
- Label the ΔH and indicate whether it is positive or negative.
- What is activation energy?
- Overlay a second reaction energy curve for Reaction B when a catalyst is added.
- Predict which reaction occurs at a faster rate in figure 5.9.2.