CHM 123Chapter 12-Chemical Kinetic
Factors Affecting Reaction Rates
The Nature of Reactants
- The reactive nature of reactants can determine the rate of reactions
- Some substances react more readily than others such as the alkali metals.
Concentration
- Reactions speed up when the concentrations of reacting particles are increased
- The reaction speeds up because the amount of collisions between to reacting particles increases.
Surface Area
- When the surface area increase the reaction rate increases
- Increasing the surface area allows more particles to collide with one another, thus increasing the reaction rate.
Temperature
- Usually increasing the temperature of a reaction generally increases the reaction rate.
- This occurs because when temperature is increased, the particles move more quickly thus more collisions occur.
- Also, when temperature is increased the energy of the collisions is greater.
Catalysts
- A catalyst is a substance that increases the rate of a chemical reaction without being consumed itself.
- A catalyst lowers the activation energy in reactions.
- Enzymes that are present in living cells are examples of catalyst.
Inhibitors (Use an Inhibitor to Slow Down the Reaction)
- Inhibitors slow down reaction rates and some stop reactions from occurring all together.
- The food industry uses inhibitors to keep foods fresher, longer
12.11 – Reaction rates and Temperature: The Arrhenius Equation
The change in rate constant with temperature with temperature varies considerably from one reaction to another. To understand why reaction rates depend on temperature, we need a picture of how reaction takes place
Collision Theory of reaction rates is a theory that assumes that, for reaction to occur, reactant molecules must collide with an energy greater than some minimum value and with the proper orientation.
As temperature increases, the molecules move faster and the collision frequency increases, the greater the number of collisions the faster the rate.
As temperature increases, the molecules move faster and the collision frequency increases.
Thus reaction rate should increase with an increase in temperature
Also, the more molecules present, the greater the probability of collision and the faster the rate.
Thus reaction rate should increase with an increase in the concentration of reactant molecules
The minimum energy of collision required for two molecules to react is called activation energy, Ea. The value of Ea depends on the particular reaction.
(1) In order to form products, bonds must be broken in the reactants.
(2) Bond breakage requires energy.
(3) Molecules moving too slowly, with too little kinetic energy, don’t react when they collide.
Effect of Temperate on the rate of reaction
The fraction of colision that lead to products is further reduced by an orientation requirement. Even if the reactant collide with sufficient energy, the won’t react unless the orientation of the reaction parnters is correct for the formation of the transition state.
Which colision is more effective?
12.12 – Using The Arrhenius equation
Arrhenius discovered that most reaction-rate data obeyed an equation based on three factors:
(1) The number of collisions per unit time.
(2) The fraction of collisions that occur with the correct orientation.
(3) The fraction of the colliding molecules that have an energy greater than or equal to Ea.
Rate constant for most chemical reactions closely follow an Arrhenius equation
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Determining the Activation Energy
One can determine the activation energy of a reaction by measuring the rate constant at two temperatures:
Writing the Arrhenius equation for each temperature:
Take the natural log of the ratio k2 over k1 and substituting the value of Ea, we find
Example: The rate constant for the formation of hydrogen iodide from the elements
H2(g) + I2(g) 2 HI(g)
is 2.7 x 10-4 L/(mol ·s) at 600K and 3.5 x 10-3 L/(mol· s) at 650 K.
a)Find the activation energy Ea
b)Calculate the rate constant at 700K
Example: The activation energy of a first order reaction is 50.2 kJ/mol at 25oC. At what temperature will the rate constant double?
12.9 – Reaction Mechanism
A balanced chemical equation defines the identities of reactants and the products as well as the stoichiometry (mole-mole ratios) between them but says nothing about the process by which reactants are actually transformed into the products.A mechanism for a reaction is a collection of elementary processes (also called elementary steps or elementary reactions) that explains how the overall reaction proceeds.
A mechanism is a proposal from which you can work out a rate law that agrees with the observed rate laws. The fact that a mechanism explains the experimental results is not a proof that the mechanism is correct. A mechanism is our rationalization of a chemical reaction, and devising mechanism is an excellent academic exercise. For that reason, although chemists can definitely rule out some propose mechanism, they can never definitely “prove” that a proposed mechanism is the correct set of step that a reaction is actually using.
The number of particle involved in an elementary step is called the molecularity, and in general, we consider only the molecularity of 1, 2, and 3. Types of elementary steps are summarized below. In the table, A, B, and C represent reactants, intermediates, or products in the elementary process.
Molecularity / Elementary step / Rate law1 / A -> products / rate = k [A]
2 / A + A -> products
A + B -> products / rate = k [A]2
rate = k [A] [B]
3 / A + A + A -> products
A + 2 B -> products
A + B + C -> products / rate = k [A]3
rate = k [A] [B]2
rate = k [A] [B] [C]
Facts about a reaction mechanism:
1. The sum of the fundamental steps in the mechanism mustequal the correct overall reaction.
2. The rate of the reaction, and therefore the rate law for the reactions are determined by the rate of the slow step.
Example:Experimental evidence suggests that the reaction between NO2 and CO takes place by a two-step mechanism:
Do the steps of the proposed mechanism add up to the overall reaction? Are there any reaction intermediates?
Unimolecular reaction: is an elementary reaction that involves a single reactant molecule
The asterisk on O3 indicates that the ozone molecule is an energetically excited state. It has absorbed ultraviolet light from the sun, causing the bond to break without any collision.
Bimolecular reaction: is an elementary reaction that results from an energetic collision between reactants atoms or molecules.
Termolecular reactions: which involves three atoms or molecule and not common in mechanism. Three particles collide simultaneously only infrequently. When a reaction mechanism includes a termolecular step, it is generally a “slow” reaction since three-body collision occurs only infrequently.
12.10 – 12.11 – Rate Laws for Elementary Reactions
The rate law for an elementary reaction follows directly from its molecularity because an elementary reaction is an individual molecular event.
Example: Two students proposed a mechanism for the given reaction below. Who proposed the correct one? A + B C + D
Rate law predicted by “student 1” proposed mechanism: Rate = k [A][B]
Rate law predicted by “student 2” proposed mechanism: Rate = k [B]2
Rate law from experiment: Rate = k [B]2
For mechanism in which there are two or more elementary steps, one of the step is often much slower than any of the others. This step is called the “rate-determining step”.
Rate of overall reaction is equal to the rate-determining step
The orders of the reactants observed in the rate law are related to the mechanism of the reaction, including the identity of the rate-determining step.
Identify the rate determining step from the question above
Example:
Consider the following reaction: 2 NO + O2 2 NO2
This reaction actually is the sum of two steps:
Step 1: 2 NON2O2 (fast step)
Step 2: N2O2 + O2 2 NO2 (slow step)
Experimental rate law = k[NO]2[O2]
Determine the rate law
Example: For the following reaction
2H2(g) + 2NO(g) N2(g) + 2H2O(g)
the experimentally determined rate law is
rate = k [H2][NO]2
For each of the following three mechanisms, state whether it is possible or not possible for it to describe the overall reaction. Show all of your work.
Mechanism I
H2(g) + NO(g) H2O(g) + N(g) (slow)
N(g) + NO(g) N2(g) + O(g) (fast)
O(g) + H2(g) H2O(g) (fast)
Mechanism II
H2 (g) + 2NO(g) H2O(g) + N2O(g) (slow)
N2O(g) + H2 (g) N2 (g) + H2O(g) (fast)
Example: Consider the following elementary steps
2 A + 2 B C + D
Step 1: A + A X (fast)
Step 2: X + B C + Y (slow)
Step 3: Y + B D (fast)
Is the mechanism consistent with the rate law? Rate = k[A]2[B]
12.13 – Catalyst
Reaction rates are affected not only by reactant concentrations and temperature but also by the presence of catalysts.
Catalyst: A substance that increases the rate of a reaction without itself being consumed in the reaction. A catalyst is used in one step and regenerated in a later step
How does it work? A catalyst accelerates the rate of a reaction by making available different, more efficient mechanism for the conversion of reactants to products.
Consider the following reaction
The catalyst does not appear in the overall reaction BUT since the catalyst is involved in the rate determining step, it often appears in the rate law.
12.14 – Homogeneous and Heterogeneous Catalysts
Homogeneous Catalyst: A catalyst that exists in the same phase as the reactants.
Heterogeneous Catalyst: A catalyst that exists in a different phase from that of the reactants
Mechanism complex and not well understood
2. Important steps
a. Adsorption of reactants onto the surface of the catalyst
b. Conversion of reactants to products on the surface
c. Desorption of products from the surface
3. Adsorption steps.
a. Chemical bonding of reactants to the highly reactive metal atoms on the surface
b. Breaking or weakening of bonds in the reactants
4. Industrial chemical processes use mostly heterogeneous catalysts due to the ease of separation of the catalyst from the reaction products.
5. Used in automobile catalytic converters
Enzymes
Enzymes are protein catalysts that, like all catalysts, speed up the rate of a chemical reaction without being used up in the process.
How Enzymes Work
• Enzyme-catalyzed reactions are characterized by the formation of a complex
between the enzyme and its substrate (the ES complex)
• Substrate binding occurs in a pocket on the enzyme called the active site
• Enzymes accelerate reactions by lowering the free energy of activation Ea
The equilibrium of the reaction remains unaffected by the enzyme
• Enzymes do this by binding the transition state of the reaction better than the
Substrate
Mechanism:
The “lock and key” model – in this model the substrate has a shape matching the enzyme’s active site
The “induced fit model” – the active site has a shape complementary to that of the substrate after the substrate is bound
Dang 1