Section 16-1

Section 16.1 A Model for Reaction Rates

• Calculate average rates of chemical reactions from experimental data.

• Relate rates of chemical reactions to collisions between reacting particles.

Section 16-1

Section 16.1 A Model for Reaction Rates (cont.)

reaction rate

collision theory

activated complex

activation energy

Collision theory is the key to understanding why some reactions are faster than others.

Section 16-1

Expressing Reaction Rates

• The reaction rate of a chemical reaction is stated as the change in concentration of a reactant or product per unit of time.

• the reaction rate of a chemical reaction is generally stated as the change in concentration of a reactant or product per unit of time, expressed as mol/(L·s). Brackets around the formula for a substance denote its molar concentration. For example, [NO2 ] represents the molar concentration of NO 2 .

Section 16-1

Expressing Reaction Rates (cont.)

• Reaction rates are determined experimentally.

Section 16-1

Collision Theory

• Collision theory states that atoms, ions, and molecules must collide in order to react.

Section 16-1

Collision Theory (cont.)

• An activated complex “ sometimes is called transation state” is a temporary, unstable arrangement of atoms in which old bonds are breaking and new bonds are forming.

• Q: What might prevent the occurrence of a reaction per each collision?

• 1-incorrect orientations

• 2- insufficient energy.

Section 16-1

Collision Theory (cont.)

• The minimum amount of energy that reacting particles must have to form the activated complex and lead to a reaction is called the activation energy ”Ea”.•A high Ea means that relatively few collisions have the required energy to produce the activated complex, and the reaction rate is slow. A low Ea means that more collisions have sufficient energy to react, and the reaction rate is faster.

•Think of this relationship in terms of a person pushing a heavy cart up a hill. If the hill is high, a substantial amount of energy is required to move the cart, and it might take a long time to get it to the top. If the hill is low, less energy is required and the task might be accomplished faster

Section 16-1

Collision Theory (cont.)

Section 16-1

Collision Theory (cont.)

• When an exothermic reaction occurs, molecules collide with enough energy to overcome the activation energy barrier. They form an activated complex, then release energy and form products at a lower energy level.

Section 16-1

Collision Theory (cont.)

• In the reverse reaction, which is endothermic, the reactant molecules are at a lower energy than the products. To react, the reactants must absorb enough energy to overcome the activation energy barrier and form higher-energy products.

• Q1: Relate collision theory to reaction rate.

• Collision theory explains why reactions occur and how reaction rates can be modified.

• Q2: Explain what the reaction rate indicates about a particular chemical reaction.

• The reaction rate indicates the rate of change of the concentration of a reactant or product in mol/(L·s).

• Q3: Compare the concentrations of the reactants and products during the course of a chemical reaction (assuming no additional reactants are added )

• The concentrations of the reactants decrease, and the concentrations of the products increase at the same rate .

• Q5: Explain why the average rate of a reaction depends on the length of the time interval over which the rate is measured.

• The rate of change of a reactant or product in a chemical reaction is not linear in time

• Q6: Describe the relationship between activation energy and the rate of a reaction

• The higher the activation energy, the slower the rate of the reaction.

• Q7: Summarize what happens during the brief existence of an activated complex.

• Bonds in the reactants are in the process of breaking, while new bonds are beginning to form to produce the products.

• Q8: Apply collision theory to explain why collisions between two reacting particles do not always result in the formation of a product.

• The collision must be in a correct orientation and have sufficient energy to form the activated complex.

• Explain the negative sign for the reaction rate of reactants

• Explain why reaction rate for product is positive?

• As concentration of reactant decreases, a negative sign is used to express the rate of reaction in terms of reactants. As the concentration of products increases, a positive sign is used in terms of products.

A. A

B. B

C. C

D. D

Section 16-1

Section 16.1 Assessment

Which of the following is NOT a requirement for a reaction to occur, according to the collision theory?

A. Reacting substances must collide.

B. Reacting substances must be in an exothermic reaction.

C. Reacting substances must collide in the correct orientation.

D. Reacting substances must collide with sufficient energy to form an activated complex.

A. A

B. B

C. C

D. D

Section 16-1

Section 16.1 Assessment

A temporary, unstable arrangement of atoms in which old bonds are breaking and new bonds are forming is called ____.

A. reaction complex

B. reaction substrate

C. activated complex

D. activated molecule

A. A

B. B

C. C

D. D

Chapter Assessment 1

The energy required to initiate a reaction is called ____.

A. initiation energy

B. activation energy

C. complex energy

D. catalyst energy

A. A

B. B

C. C

D. D

STP 1

Which of the following is an acceptable unit for expressing a rate?

A. mol/L ● s

B. L/s

C. M

D. mL/h

End of Section 16.1