15. Chemical Kinetics

Collision Theory

14.4 Collision Theory and the Arrhenius Equation

Why don’t all collisions between reactant molecules lead to 
a chemical reaction?

What fraction of the molecules in a gas at 300 K collide with an 
energy equal to or greater than Ea when Ea equals 50 kJ/mol? 
What is the value of this fraction when Ea is 100 kJ/mol?

Collision theory | Kinetics | AP Chemistry | Khan Academy

Collision Theory Concept 2

(a) What factors determine whether a collision between
two molecules will lead to a chemical reaction?

Collision Theory Concept 1

A chemical reaction is successful if reactant molecules can combine together to form a new product.&#160;

Based on their activation energies and energy changes 
and assuming that all collision factors are the same, rank 
the following reactions from slowest to fastest. 
(a) Ea = 45 kJ>mol; E = -25 kJ>mol 
(b) Ea = 35 kJ>mol; E = -10 kJ>mol 
(c) Ea = 55 kJ>mol; E = 10 kJ>mol

Indicate whether each statement is true or false.
(a) If you measure the rate constant for a reaction at different
temperatures, you can calculate the overall enthalpy
change for the reaction.

Consider the two reactions:
O + N2¡NO + N Ea = 315 kJ>mol
Cl + H2¡HCl + H Ea = 23 kJ>mol
a. Why is the activation barrier for the first reaction so much
higher than that for the second?

Frequency Factor is made up of 2 variables:&nbsp;1. Orientation Factor (p)2. Collision Frequency (z)

The gas-phase reaction of NO with F2 to form NOF and F
has an activation energy of Ea = 6.3 kJ>mol. and a frequency
factor of A = 6.0 * 108 M-1 s-1. The reaction is
believed to be bimolecular:
NO1g2 + F21g2 ¡ NOF1g2 + F1g2
(e) Suggest a reason for the low activation
energy for the reaction.

Indicate whether each statement is true or false.
(c) Increasing the reaction temperature increases the
fraction of successful collisions between reactants.

Consider the two reactions:
O + N2¡NO + N Ea = 315 kJ>mol
Cl + H2¡HCl + H Ea = 23 kJ>mol
b. The frequency factors for these two reactions are very close
to each other in value. Assuming that they are the same, calculate
the ratio of the reaction rate constants for these two
reactions at 25 °C.

Collision Theory Example 2

Two reactions have the same activation energy, but their 
rates at the same temperature differ by a factor of 10. 
Explain.

Introduction to Collision Theory

(b) Does
the rate constant for a reaction generally increase or decrease
with an increase in reaction temperature?

When the temperature of a gas is raised by 10 °C, the collision 
frequency increases by only about 2%, but the reaction 
rate increases by 100% (a factor of 2) or more. Explain.

Indicate whether each statement is true or false.
(c) If you double the temperature for a reaction, you cut
the activation energy in half.

This reaction has an activation energy of zero in the gas phase:
CH3 + CH3¡C2H6
a. Would you expect the rate of this reaction to change very
much with temperature?

Energy Diagrams, Catalysts, and Reaction Mechanisms

Collision Theory Example 1

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Intro to Chemical Kinetics

Chemical Kinetics is the study of factors affecting reaction rates, which examines the speed by which reactants are consumed to make products.

Average Rate of Reaction

Average (General) Rate is the change in the concentration of a compound over a period of time.

Arrhenius Equation

The Arrhenius Equation&nbsp;illustrates the temperature dependence of the rate constant k.&nbsp;

Rate Law

Rate Law represents an equation for a chemical reaction that connects the reaction rate with the concentrations or pressures of the reactants and the rate constant.&nbsp;

Factors Influencing Rates

There are 4 major factors that directly influence how fast or slow a reaction proceeds.

Stoichiometric Rate Calculations

When the rate of one compound is know, the rate of another compound is calculated through rate-to-rate comparison.

According to Collision Theory:chemical reaction is successful when 2 energetic reactants successfully collide.

Integrated Rate Law

When we include the variable of time to our Rate Law then we obtain the Integrated Rate Laws.&nbsp;

Catalyst

Catalyst is a substance that increases the rate of reaction by lowering the Activation Energy (Ea) of a reaction.

Instantaneous Rate

<div title="Page 1">&nbsp;Instantaneous Rate is the rate a reaction at any particular point in time.</div>

Reaction Mechanism

<div title="Page 1">A Reaction Mechanism is a step-by-step sequence of elementary steps by which an overall chemical change occurs.</div>

Half-Life

Half-Life is the time it takes for half&nbsp;of a reactant to decay in a certain time period.

Energy Diagrams

Energy Diagram represents a curved plot illustrating the energies of reactants, products and transition states of a reaction.

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