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Chapter 16: Reaction Rates

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Sydney Sturgeon

on 21 March 2016

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Transcript of Chapter 16: Reaction Rates

Chapter 16: Reaction Rates
Average Reaction Rate Example
If the concentration of NO is 0.000M at t = 0.00 s and 0.010M two seconds after the reaction begins, what is the average rate of reaction?
Collision Theory
States that atoms, ions, and molecules must collide in order to react provided that they (1) have enough kinetic energy (KE) to form an activated complex and (2) have the right orientation.
Average Reaction Rate Equation
avg. rxn rate = -
Chemical Reaction Rate
Change in concentration of a reactant or product per unit of time (units usually in mol/L s)
Average Reaction Rate Example
[NO] = 0.000M
[NO] = 0.010M
t = 0.00 s
t = 2 s
Expressing Reaction Rates
Reaction rates depend on the reaction
Lighting a match (fast rxn rate) vs. the formation of coal from dead plants (slow rxn rate)
Rate:
Change in a given quantity during a specific period of time
March 18th - 21st
avg. rate =
______________________________________
quantity
t
The # of atoms, ions, or molecules that react per given unit of time to form products
Often measured as a change in the # of moles/time or as a change in molarity/time
Related to the properties of the atoms, ions, and molecules through the COLLISION THEORY MODEL
___________________________________
[reactant]
t
[reactant] = the change in concentration of a reactant
[product] = the change in concentration of a product
t = the change in time
1
Activated Complex (Transition State)
Temporary unstable arrangement of atoms in which old bonds are breaking and new bonds are forming.
Activation Energy
E
a
Minimum amount of energy that particles must have to form the activated complex and react.
Activation Energy
A HIGH activation energy means that relatively few collisions have the required energy to produce the activated complex, and the reaction rate is slow.
A LOW activation energy means that more collisions have sufficient energy to react, and the reaction rate is faster.
Endothermic vs. Exothermic Reactions
Factors Affecting Reaction Rates
Depend on the Collision Theory which states that particles must collide in order to react provided that they (1) have enough KE to form an activated complex and (2) have the right orientation.
GOAL: To increase the # of collisions
5 Factors:
(1) Nature of Reactants
(2) Concentration
(3) Surface Area
(4) Temperature
(5) Catalysts and Inhibitors
Factors Affecting Reaction Rates
(1) Nature of the reactants
(2) Concentration
The higher the concentration, the greater the # of collisions, and the faster the reaction rate
Factors Affecting Reaction Rates
(3) Surface Area
The greater the surface area, the greater the # of collisions, and the faster the reaction rate
(4) Temperature
The higher the temperature, the greater the # of collisions, and the faster the reaction rate
Factors Affecting Reaction Rates
(5) Catalysts and Inhibitors
Catalyst
Enzyme
Substance that increases the rate of a chemical reaction without being used up in the reaction
Inhibitor
Substance that slows down reaction rates or inhibits a reaction
Factors Affecting Reaction Rates
Chapter 16: Reaction Rates WS
=
________________________________
[product]
t
initial
final
initial
final
avg. rxn rate =
______________
[NO]
t
0.010M - 0.000M
0.010M
= 0.005 M/s or 0.005 mol/L s
2 s - 0.00 s
2 s
=
___________________________________
_________________
(4) Temperature
Activation Energy
Writing Reaction Rate Laws
Rate Law: show the relationship between the rate of a chemical reaction and the concentration of reactants at a given temperature
Specific Rate Constant
- symbol k
- relates the reaction rate and the concentrations of reactants at a given temperature
- unique for every reaction
- changes with temperature, but not with concentration
- having a large k value means that the reactants react quickly to form the products
General Rate Law
aA + bB products
rate = k[A] [B]
m
n
[A] = molar concentration of substance A
[B] = molar concentration of substance B
k = specific rate constant
m = reaction order
n = reaction order
Reaction Order
Shows how the rate is affected by the concentration of a specific reactant.
A reaction order is determined by changing the concentration of that reactant.
Determining Reaction Order
To calculate the overall reaction order add the exponents together.
Example: rate = k[NO] [O ]
2
2
Reaction order for NO = second order
Reaction order for O = first order
Overall reaction order = 2 + 1 = 3 (third order)
2
Reaction Order Example #1
Once you have an answer compare it with someone around you.
Determine the overall reaction order for a reaction between A and B for which the rate law is rate = k[A][B] . Also find the reaction orders for both A and B.
3
Reaction Order Example #1
rate = k[A][B]
3
Reaction order for A = first order
Reaction order for B = third order
Overall reaction order = 1 + 3 = 4 (fourth order)
Reaction Order Example #2
Reaction Order Example #2
Once you have an answer compare it with someone around you.
The rate law for the reaction 2NO(g) + O (g) 2NO (g) is second order in oxygen and third order overall. What is the rate law for the reaction?
2
2
Second order in oxygen and third order overall.
rate = k[O ] [NO]
2
2
Overall reaction order = 2 +1 = 3 (third order)
Instantaneous Reaction Rates
Instantaneous rate: reaction rate at a specific time
To calculate a reaction rate plug values in for the concentrations of the substances as well as the specific rate constant (k) into its rate law.
Calculating a Reaction Rate Example
The following reaction is first order in hydrogen and second order in nitrogen monoxide with a rate constant of 2.90 x 10^2 M /s.
-2
2NO(g) + H (g) N O(g) + H O(g)
2
2
2
Calculate the reaction rate when the reactant concentrations are [NO] = 0.005M and [H ] = 0.002M
2
Calculating a Reaction Rate Example
Reaction Mechanisms
Complex reaction:
reaction that consists of 2 or more elementary steps
Reaction mechanism:
complete sequence of elementary steps that make up a complex reaction
rate = k[NO] [H ]
k = 2.90 x 10^2 M /s
[NO] = 0.005M
[H ] = 0.002M
2
2
-2
2
rate = (2.90 x 10^2 M /s)(0.005M) (o.oo2M)
2
-2
rate = 1.45 x 10^-5 M/s or 1.45 x 10^-5 mol/L s
Is this rxn exothermic or endothermic?
Reaction Mechanism for the formation of oxygen from ozone
Add elementary steps together and cancel out alike terms to get complex reaction.
Elementary step: Cl(g) + O (g) ClO(g) + O (g)
Elementary step: O (g) O(g) + O (g)
Elementary step: ClO(g) + O(g) Cl(g) + O (g)
3
2
2
3
2
_________________________________________
Complex reaction: 2O (g) 3O (g)
3
2
Intermediate:
substance that is produced and consumed in subsequent elementary steps.
(Cl, ClO, and O are intermediates)
Rate-determining Step
Energy of Reaction Graph
Is the overall reaction endothermic or exothermic?
The slowest elementary step
2NO N O (fast)
N O + H N O + H O (slow)
N O + H N + H O (fast)
2
2
2
2
2
2
2
2
2
2
2
Chapter 16: Rate Laws WS
DRAW THIS REACTION PATHWAY INTO YOUR NOTES
DRAW THIS REACTION PATHWAY INTO YOUR NOTES
Example of a reaction mechanism in organic chemistry
Chapter 16 Review KAHOOT & Chapters 14 & 16 TEST REVIEW WS
Before you sit down pick up the following from the green demo table:

(1) Chapter 16: Sec. 1, 2, & 4 Reaction Rates NOTES (salmon)

(2) Chapters 14 & 16 Test Review WS (white)
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