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Topic for collage is Chemical Kinetics-----Basic Cocepts
Middle of Collage:
Thermodynamics tells if a reaction is product- or reactant-favored.
But this gives us no info on HOW FAST reaction goes from reactants to products.
There’s still one important feature of chemical reactions we haven’t talked about yet:
This is the study of chemical kinetics-The study of Reaction Rates and their relation to the way the reaction proceed ie Mechanism.
Question is what is reaction rate
Reaction rate = change in concentration of a reactant or product with time.
Factors Affecting Rates
1.Concentrations
2.and physical state of reactants and products
3.Temperature
4.Catalysts
Left Side Of Collage
Concentrations & Rates
Left Side Of Collage
A---à Product
Rate = - ∆ A/ ∆t =k[A]
Integrating
ln[A]/A0 = -kt ,where A/A0 =Fraction remaining after time t has elapsed
Above equation is rate law
The rate law expresses the relationship of the rate of a reaction to the specific rate constant k and the concentrations of the reactants raised to some powers.
The specific rate constant k for a reaction is a proportionality constant relating the concentration of reactants to the rate of reaction.
The value of the rate constant k is large if the product form quickly
The value of rate constant k is small if the products form slowly.
The order of reaction is the power to which the concentration of reactant must be raised to give the experimentally observed relationship between concentaration and rate
For the equation :Aa +Bb ---àCc +d D
Rate =k[A]a[B]b
where ‘a’ is order of reaction with respect to A
Where ‘b’ is order of reaction with respect to B
What we really want to do with rate laws is figure out how long we’ll have to wait for a reaction to finish.
Suppose, for example, I need the concentration of products to be 1.00 M. How long will I have to wait .It turns out that we can figure this out if we know the reaction order of our reaction.
For the following reaction
2N2O5(g)--à 4NO2 (g) + O2(g)
Rate =k[N2O5]
ln [N2O5] vs time is a straight line
All 1st order reactions have straight line plot for ln [A] vs. time.
(2nd order gives straight line for plot of 1/[A] vs. time)
One very helpful property of a reaction is its half-life. This is the amount of time it takes to use up half the reactants.
Half-Life is the time it takes for 1/2 a sample to disappear.
For 1st order reactions, the concept of Half-Life is especially useful.
• Reaction after 1 half-life.
• 1/2 of the reactant has been consumed and 1/2 remains.
• After 2 half-lives 1/4 of the reactant remains.
• A 3 half-lives 1/8 of the reactant remains.
• After 4 half-lives 1/16 of the reactant remains.
• Figure below shows the plot of Conc. off Reactant vs Half life
• ln[1/2] =-k.t 1/2
• t1/2 =0.693/k
Right side of Collage:
Why Do Reactions Happen
. One way to think of this is that there’s an energy barrier (like a hill) that the molecules must overcome.
. This barrier is called the activation energy, Ea.
. Activation Energy = Enough Energy Molecules need a minimum amount of energy to react. Visualized as an energy barrier - activation energy, Ea.
. Below is shown an analogy to chemical activation energy, for the volleyball to go over the net ,the player must give it sufficient energy.
Figure above shows activation energy for non catalyst reaction.chemical catalyst reaction and enzyme catalyst reaction
The activation energy determines how difficult it is to perform a chemical reaction, so it’s something we often want to know.Rate constant k of reaction is related to Ea by the following equation,known as Arraneius equation.
Where A is frequency factor which is difficult to measure
It would be helpful if we could calculate Ea without having to know A.
Fortunately, we can! If we measure the rate constant at two different temperatures, we can rewrite this equation as:
Notice that this equation doesn’t contain A! This makes it much easier to find the activation energy.
Molecules need a minimum amount of energy to react.
Visualized as an energy barrier - activation energy, Ea.
• Reaction passes through a Transition State where there is an activated complex that has sufficient energy to become a product.
• Activation Energy, Ea = energy req’d to form activated complex.
• Why is trans-butene <--> cis-butene reaction observed to be 1st order?
• As [trans] doubles, number of molecules with enough Ea also doubles.
• Why is the trans <--> cis reaction faster at higher temperature?
• \
• Reacting molecules must collide with one another ,they must collide with sufficient energy to brake bonds
• Fraction of molecules with sufficient activation energy increases with T. As it is shown in figure above.
• As you increase the temperature the rate of reaction increases. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10°C rise in temperature.
• Increasing the temperature increases reaction rates because of the disproportionately large increase in the number of high energy collisions. It is only these collisions (possessing at least the activation energy for the reaction) which result in a reaction..
• Catalyst decreases the activation energy of reaction.
• Above figure shows plot of Reaction progress vs Potential Energy
• Graph shows when no catalyst was used Ea was much higher
• Therefore particle Crossing it are much less in number is shown in figure below.
• Ea catalyst< Ea without catalyst, so number of particles crossing Ea catalyst has increased
• Main points covered in present collage are-reaction rates,reaction energy, activation energy, rate law, order of reaction. half life of reaction
•
• Take Home Message:Kinetics and Thermodynamics are shown as two little cats in collage which are important branches of chemistry, without knowing their basic concepts the knowledge of chemistry is in complete.
What we really want to do with rate laws is figure out how long we’ll have to wait for a reaction to finish.
Suppose, for example, I need the concentration of products to be 1.00 M. How long will I have to wait
It turns out that we can figure this out if we know the reaction order of our reaction.
All 1st order reactions have straight line plot for ln [A] vs. time.
(2nd order gives straight line for plot of 1/[A] vs. time)
One very helpful property of a reaction is its half-life. This is the amount of time it takes to use up half the reactants.
HALF-LIFE is the time it takes for 1/2 a sample is disappear.
For 1st order reactions, the concept of HALF-LIFE is especially useful.
• Reaction after 1 half-life.
• 1/2 of the reactant has been consumed and 1/2 remains.
• After 2 half-lives 1/4 of the reactant remains.
• A 3 half-lives 1/8 of the reactant remains.
• After 4 half-lives 1/16 of the reactant remains.
• Sugar is fermented in a 1st order process (using an enzyme as a catalyst).
• sugar + enzyme --> products
• Rate of disappear of sugar = k[sugar]
t1/2 = 0.693 / k
Why Do Reactions Happen
One way to think of this is that there’s an energy barrier (like a hill) that the molecules must overcome.
This barrier is called the activation energy, Ea.
The activation energy determines how difficult it is to perform a chemical reaction, so it’s something we often want to know.
A is frequency factor which is difficult to measure
It would be helpful if we could calculate Ea without having to know A.
Fortunately, we can! If we measure the rate constant at two different temperatures, we can rewrite this equation as:
Notice that this equation doesn’t contain A! This makes it much easier to find the activation energy.
Molecules need a minimum amount of energy to react.
Visualized as an energy barrier - activation energy, Ea.
• Reaction passes thru a TRANSITION STATE where there is an activated complex that has sufficient energy to become a product.
• ACTIVATION ENERGY, Ea
= energy req’d to form activated complex.
• Why is trans-butene <--> cis-butene reaction observed to be 1st order?
• As [trans] doubles, number of molecules with enough E also doubles.
• 2. Why is the trans <--> cis reaction faster at higher temperature?
• Fraction of molecules with sufficient activation energy increases with T.
• As you increase the temperature the rate of reaction increases. As a rough approximation, for many reactions happening at around room temperature, the rate of reaction doubles for every 10°C rise in temperature.
• Increasing the temperature increases reaction rates because of the disproportionately large increase in the number of high energy collisions. It is only these collisions (possessing at least the activation energy for the reaction) which result in a reaction..
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