Section 16.2 Factors Affecting Reaction Rates Identify factors that affect the rates of chemical reactions. Explain the role of a catalyst. Section 16-2

The Nature of Reactants Some substances react more readily than others. Section 16-2

Concentration Chemists change reaction rates by changing concentrations of reactants. When concentrations are increased, more molecules are available to collide, and therefore collisions occur more frequently. Thus the rate of reaction will increase. Section 16-2

Reaction rate increases with increasing surface area. Greater surface area allows particles to collide with many more particles per unit of time. For the same mass, many small particles have more surface area than one large particle. Reaction rate increases with increasing surface area. Section 16-2

Increasing temperature generally increases reaction rate. For example, you know that the reactions that cause foods to spoil occur faster at room temperature than when the foods are refrigerated. Increasing temperature increases the kinetic energy of the particles. Reacting particles collide more frequently at higher temperatures. Section 16-2

High-energy collisions are more frequent at a higher temperature. Temperature (cont.) High-energy collisions are more frequent at a higher temperature. As temperature increases, reaction rate increases. Section 16-2

Temperature (cont.) This graph compares the numbers of particles that have sufficient energy to react at temperatures T1 and T2 , where T2 is greater than T1 .The number of high-energy collisions at the higher temperature, T2 , is greater than the number at the lower temperature, T1 . Therefore, as the temperature increases, more collisions result in a reaction. Section 16-2

Catalysts and Inhibitors A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the reaction. Catalysts are used extensively in manufacturing because producing more of a product quickly reduces its cost. A catalyst does not yield more product and is not included in either the reactants or the products of the reaction. Thus, catalysts are not included in chemical equations. An inhibitor is a substance that slows or prevents a reaction. Section 16-2

How catalysts and inhibitors work :A catalyst lowers the activation energy required for a reaction to take place at a given temperature. Recall that a low activation energy means that more of the collisions between particles will have sufficient energy to overcome the activation energy barrier and bring about a reaction. By lowering the activation energy, a catalyst increases the average reaction rate .

Inhibitors can act in a variety of ways Inhibitors can act in a variety of ways. Some block lower energy pathways and thus raise the activation energy of a reaction. Others react with the catalyst and destroy it or prevent it from performing its function. In biological reactions, an inhibitor might bind the enzyme that catalyzes a reaction and prevent the reaction from occurring. In the food industry, inhibitors are called preservatives or antioxidants

Catalysts and Inhibitors (cont.) Catalysts lower the activation energy. Low activation energy means more collisions between particles have sufficient energy to react. Section 16-2

Catalysts and Inhibitors (cont.) A heterogeneous catalyst exists in a physical state different than that of the reaction it catalyzes. A homogeneous catalyst exists in the same physical state as the reaction it catalyzes. Section 16-2

Which of the following generally does not increase the rate of a chemical reaction? A. increasing concentration B. adding a catalyst C. adding an inhibitor D. increasing temperature A B C D Section 16-2

A B C D High-energy particle collisions are more frequent: A. when an inhibitor is present B. when temperature is decreased C. when activation energy is higher D. when temperature is increased A B C D Section 16-2

Q1:Explain how collision theory accounts for the effect of concentration on reaction rate. Increasing reactant concentration increases collision frequency between reactant particles. Q2: Explain the difference between a catalyst and an inhibitor. A catalyst speeds up the rate of a reaction by lowering the activation energy. An inhibitor slows or even stops a reaction by interfering with the reactants or with the catalyst .

Q3: Describe the effect on the rate of a reaction if one of the reactants is ground to a powder rather than used as a single chunk. The rate of the reaction increases because more surface area is available for reaction. Q4: Infer If increasing the temperature of a reaction by 10 K approximately doubles the reaction rate, what would be the effect of increasing the temperature by 20 K? The rate would quadruple.

Increasing the temperature of a reaction increases the rate of reaction by: A. increasing the collision frequency B. increasing the number of high-energy collisions C. both a and b D. none of the above A B C D Chapter Assessment 5

End of section 16.2