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Rate of Reaction

Chemistry Chapter 14
by

Kimberly Cribb

on 25 April 2016

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Transcript of Rate of Reaction

Chemistry Chapter 14: Rates of Reaction
14.1 Kinetics
– the study of rates of reactions and the steps by which they occur.
Thermodynamics answers WILL it react?
Kinetics answers HOW FAST will it react?
If thermodynamically favorable it may still not react automatically. Some reactions need a push to get going.
Thermodynamically favorable conditions are:
Exothermic
reactions ( Heat exits the reaction and feels HOT.) (delta H = -value)
Reactions that increase
Entropy
( Entropy is disorder. Nature tends towards disorder.)
14.3
Collision theory
1. Particles must collide before the can react
2. Particles must be oriented correctly when they hit
3. Particles must have enough energy to form products when they hit

Any factor that increases the number of EFFECTIVE collisions increases the rate of a reaction.
14.4 Activation Energy and Activated Complex
Activation energy
( E_a ) – energy needed to get the reaction started. The initial push. This is the MINIMUM amount of kinetic energy that must be possessed by the colliding molecules before the can react.
Remember this.... reactions are a 2 step process.

1.
BREAKING
of reactant bonds
2.
MAKING
of product bonds
Draw the reaction process.
Time (seconds or minutes or hours)
Activated complex
at the top is
VERY UNSTABLE
so is very short lived. It can break up to form the products OR can reform the reactants. It is the
TRANSITION STATE!
If the activated complex goes on to complete the reaction, a large amount of energy will be released.
In EXOTHERMIC reactions, this energy starts or activates more reactants, and the reaction rate will get faster.
A large activation energy can prevent a favorable reaction from starting.

Rates of Reactions
– tells how fast reactants change into products.

Units like Molarity/Second or moles/L/Hour are common.
(M/Hour)
Reactants
Products
Products
Reactants
Joules
14.6 Five most important factors affecting reaction rates

1.
Nature of reactants
Reactive substances react QUICKLY. Others, like nobles gases with their full octet rarely or never react.



2.
Concentration
HIGH concentrations INCREASE rate of reaction generally, but by how much, if any, depends on the reaction.

3.
Temperature
A rule of thumb is that reaction
rates double
for every
10 degree C
rise in temperature.
HIGHER temperature increase reaction rates by INCREASING the NUMBER of collisions and the FORCE of the collisions.

4.
Surface Area
Smaller particles have greater surface and thus allow the reaction to go faster.
HETEROGENEOUS REACTIONS – when the reactants are in TWO DIFFERENT PHASES like a solid and a liquid.

5.
Presence of a Catalyst
A catalyst is a substance that changes a reaction rate WITHOUT being permanently changed OR consumed by the reaction. It gives the reaction a second path to take, a short cut. It is NOT a reactant OR a product! It affects both the forward and reverse reactions equally. (It is faster to go forward or backwards.) It will not enable a thermodynamically impossible reaction to occur. One theory states that catalysts hold reactants in just the right positions for favorable collisions.
Illustration of a catalyst.

I will draw this for you in class. :)
Homogeneous catalyst
– in the same phase as the reactants or in a solution with a reactant








Heterogeneous catalys
t – in a different phase from the reactants
Enzymes
– a class of catalysts that occurs naturally in biological substances. The presences of an enzyme typically causes a reaction to occur millions of times faster than the reaction without a catalyst. They are usually proteins!
Inhibitor
– used to reduce a catalyst's undesirable effects. They bond to the catalyst to slow the increase in reaction rate, or stop it. This effect is also the result of holding molecules in a certain position, but in this case to prevent collisions.
14B Reaction Mechanisms
The series of steps that make up a reaction is called a reaction mechanism.
The reaction mechanism is often made of two or more possible steps, called
elementary steps
.
The
rate-determining step
is the slowest elementary step.
The elementary steps all together make up a complex reaction.
During a complex reaction there are
intermediates made.
Example from text:
Required for Honors: Rate Law
Honors Tutorial
Favorable!
Review Only
14.2
Thermodynamics relates only to the starting and ending points of a reaction and is NOT DEPENDENT ON THE PATH
KINETICS looks for the PATH, what happens between the start and finish of a reaction

A2 + B2
2AB
Activated complex
Watch in class and rewatch for review.
Video 5
Reaction: _______________________
1
2
3
1
2
3
1
2
3
1
2
3
Beginning and ending is the same
Elementary steps
1
2
Enthalpy or Delta H
Last slide for general chemistry students.
Video 1
Video 2
Video 3
Video 6
Video 4
Video 7
Video 8
Video 9
Video 10
https://quizlet.com/_255drl
Full transcript