reaction kinetics honors chemistry. what is reaction kinetics / reaction kinetics is the study of...
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Reaction KineticsReaction Kinetics
Honors ChemistryHonors Chemistry
What is Reaction Kinetics
What is Reaction Kinetics
Reaction kinetics is the study of rates of chemical processes. We will look into:
how rxns happen how to speed rxns up Rxn mechanisms Predicting the rate of a rxn.
Reaction kinetics is the study of rates of chemical processes. We will look into:
how rxns happen how to speed rxns up Rxn mechanisms Predicting the rate of a rxn.
Collision TheoryCollision Theory For a reaction to occur between two substances, the molecules of the 2 substances must collide at the right angles and with enough energy to break and/or remake bonds.
For a reaction to occur between two substances, the molecules of the 2 substances must collide at the right angles and with enough energy to break and/or remake bonds.
Consider this:Consider this:
Which reaction should occur the fastest? Which is slowest?
CH4 (g) + 2 O2 (g) --> CO2 (g) + 2 H2O (g)
H2 (g) + Cl2 (g) --> 2HCl (g)
3 Fe (s) + 2 I2 (s) --> 2 FeI3 (s) Let’s look at them one by one.
Which reaction should occur the fastest? Which is slowest?
CH4 (g) + 2 O2 (g) --> CO2 (g) + 2 H2O (g)
H2 (g) + Cl2 (g) --> 2HCl (g)
3 Fe (s) + 2 I2 (s) --> 2 FeI3 (s) Let’s look at them one by one.
CH4 (g) + 2 O2 (g) --> CO2 (g) + 2 H2O (g)
CH4 (g) + 2 O2 (g) --> CO2 (g) + 2 H2O (g)
For this rxn to occur quickly, one methane molecule must collide with two oxygen molecules simultaneously. The fact that the reactants are gases is good, because this means that they have more energy to break/make bonds.
For this rxn to occur quickly, one methane molecule must collide with two oxygen molecules simultaneously. The fact that the reactants are gases is good, because this means that they have more energy to break/make bonds.
H2 (g) + Cl2 (g) --> 2HCl (g)
H2 (g) + Cl2 (g) --> 2HCl (g)
For this reaction to occur quickly, one gaseous hydrogen molecule must collide one gaseous chlorine molecule. This seems pretty simple - and it is. This is the fastest of the 3 rxns.
For this reaction to occur quickly, one gaseous hydrogen molecule must collide one gaseous chlorine molecule. This seems pretty simple - and it is. This is the fastest of the 3 rxns.
2 Fe (s) + 3 I2 (s) --> 2 FeI3 (s)
2 Fe (s) + 3 I2 (s) --> 2 FeI3 (s)
For this reaction to occur quickly, 3 solid iron atoms must somehow come in contact with 2 solids iodine atoms. This is extremely unklikely, so this reaction is the slowest by far.
For this reaction to occur quickly, 3 solid iron atoms must somehow come in contact with 2 solids iodine atoms. This is extremely unklikely, so this reaction is the slowest by far.
Reaction MechanismReaction Mechanism
In the reaction: 2 NO2 + F2 --> 2NO2F 3 molecules must collide simultaneously. This is virtually impossible. What is more likely to happen is that two collide, make some product(s) which then collide with the third molecule. The possible steps for each collision is called a reaction mechanism.
In the reaction: 2 NO2 + F2 --> 2NO2F 3 molecules must collide simultaneously. This is virtually impossible. What is more likely to happen is that two collide, make some product(s) which then collide with the third molecule. The possible steps for each collision is called a reaction mechanism.
More on Reaction Mechanisms
More on Reaction Mechanisms
2 NO2 + F2 --> 2NO2F Step 1: NO2 + F2 --> NO2F + F Step 2: NO2 + F --> NO2F If we add these two steps we get the same rxn as above. The F atom seen in both of the two steps cancels out, so it is not part of the overall rxn; F would be called an intermediate.
2 NO2 + F2 --> 2NO2F Step 1: NO2 + F2 --> NO2F + F Step 2: NO2 + F --> NO2F If we add these two steps we get the same rxn as above. The F atom seen in both of the two steps cancels out, so it is not part of the overall rxn; F would be called an intermediate.
Rate Determining StepRate Determining Step
The Rate Determining Step is the slowest step in the reaction.
Why? Consider a funnel: the rate at which water moves through the funnel depends most on the stem and little on the rest, because it slows the water down the most.
The Rate Determining Step is the slowest step in the reaction.
Why? Consider a funnel: the rate at which water moves through the funnel depends most on the stem and little on the rest, because it slows the water down the most.
Which is the R.D.S?Which is the R.D.S?
So which step do you think is the RDS in:
2 NO2 + F2 --> 2NO2FStep 1: NO2 + F2 --> NO2F + FStep 2: NO2 + F --> NO2F
Step 1 requires a little more energy because the F2 must be broken so it would be slower.
So which step do you think is the RDS in:
2 NO2 + F2 --> 2NO2FStep 1: NO2 + F2 --> NO2F + FStep 2: NO2 + F --> NO2F
Step 1 requires a little more energy because the F2 must be broken so it would be slower.
So how can we speed up a rxn?
So how can we speed up a rxn?
Let’s say that I have a ball of aluminum foil which I am trying to react with some hydrochloric acid.
The rxn is: 2Al (s) + 6HCl (aq) --> 3H2 (g) + 2AlCl3 (s).
How can I make this reaction happen faster?
Let’s say that I have a ball of aluminum foil which I am trying to react with some hydrochloric acid.
The rxn is: 2Al (s) + 6HCl (aq) --> 3H2 (g) + 2AlCl3 (s).
How can I make this reaction happen faster?
1. Use more of one reactant
1. Use more of one reactant
If I add more HCl, then there is a much greater chance of HCl molecules colliding with Al atoms.
If I add more HCl, then there is a much greater chance of HCl molecules colliding with Al atoms.
2. Increase surface area2. Increase surface area
If I spread the aluminum out, or shred it, the HCl molecules will collide easier with Al atoms.
If I spread the aluminum out, or shred it, the HCl molecules will collide easier with Al atoms.
3. Increase the concentrations3. Increase the concentrations
If I use a stronger concentration of acid (for example 10.0 M HCl instead of .100 M HCl) there are more HCl molecules, so collisions are more likely to happen. (For a gas this might mean increasing pressure.)
If I use a stronger concentration of acid (for example 10.0 M HCl instead of .100 M HCl) there are more HCl molecules, so collisions are more likely to happen. (For a gas this might mean increasing pressure.)
4. Add energy4. Add energy When energy is added, the reactants have more energy, and thus collide more often and stronger. Of course heating the Al or the HCl would be dangerous, but effective.
When energy is added, the reactants have more energy, and thus collide more often and stronger. Of course heating the Al or the HCl would be dangerous, but effective.
5. Add a catalyst5. Add a catalyst A catalyst is a chemical that speeds up a rxn without being consumed in the rxn. This will be explained more later on.
In the case of our rxn water is a catalyst.
A catalyst is a chemical that speeds up a rxn without being consumed in the rxn. This will be explained more later on.
In the case of our rxn water is a catalyst.
Enthalpy DiagramEnthalpy Diagram
An enthalpy diagram is an energy graph.
This enthalpy diagram is of an exothermic reaction. How do we know this?
An enthalpy diagram is an energy graph.
This enthalpy diagram is of an exothermic reaction. How do we know this?
Activated ComplexActivated Complex
A complex is an unstable molecule formed as an intermediate.
The activated complex is the complex requiring the most energy to form.
A complex is an unstable molecule formed as an intermediate.
The activated complex is the complex requiring the most energy to form.
Activation EnergyActivation Energy One of the stipulations of the collision theory is that the collision must have enough energy. This energy, called activation energy, is the energy needed to convert the reactants into the activated complex.
What is the activation energy in this rxn?
One of the stipulations of the collision theory is that the collision must have enough energy. This energy, called activation energy, is the energy needed to convert the reactants into the activated complex.
What is the activation energy in this rxn?
Some questions to think about
Some questions to think about
What is the activation energy?
What is H? Is the reaction endothermic or exothermic?
If the reaction were reversed, how would these answers change?
What is the activation energy?
What is H? Is the reaction endothermic or exothermic?
If the reaction were reversed, how would these answers change?
Effect of a catalystEffect of a catalyst A catalyst speeds up a rxn by lowering the activation energy. This often occurs by offering a different mechanism.
A catalyst speeds up a rxn by lowering the activation energy. This often occurs by offering a different mechanism.
Rate Law ExpressionRate Law Expression We have said that the rate of a reaction depends on a number of factors, but the most important is the the amount of reactants.
In a Rate Law Expression, we show that the rate of a reaction depends on the concentrations of reactants. Specifically, the rate law is a constant times the concentrations of the reactants.
Since solids and liquids are not compressible, we cannot change their concentrations or densities like we can with gases or solutions. So, solids and liquids are not used in Rate Law Expressions.
We have said that the rate of a reaction depends on a number of factors, but the most important is the the amount of reactants.
In a Rate Law Expression, we show that the rate of a reaction depends on the concentrations of reactants. Specifically, the rate law is a constant times the concentrations of the reactants.
Since solids and liquids are not compressible, we cannot change their concentrations or densities like we can with gases or solutions. So, solids and liquids are not used in Rate Law Expressions.
Theoretical Rate LawTheoretical Rate Law Let’s start simple with a simple rxn.
H2 (g) + Cl2 (g) --> 2 HCl The rate of this reaction depends largely on the amount of hydrogen and the amount of chlorine gases present.
The Theoretical rate law expression would be: rate = k*[H2]*[Cl2]
K is a constant based on the temperature and the nature of the chemicals. It is a different constant for any rxn. Let’s not worry about that now.
Let’s start simple with a simple rxn. H2 (g) + Cl2 (g) --> 2 HCl The rate of this reaction depends largely on the amount of hydrogen and the amount of chlorine gases present.
The Theoretical rate law expression would be: rate = k*[H2]*[Cl2]
K is a constant based on the temperature and the nature of the chemicals. It is a different constant for any rxn. Let’s not worry about that now.
Why is it “Theoretical”Why is it “Theoretical”
We already said that a reaction mechanism depends most on the Rate Determining Step. Without seeing the mechanism or experimental data, we can’t know for sure to what extent each reactant actually contributes.
We’ll move on to actual rate laws later.
We already said that a reaction mechanism depends most on the Rate Determining Step. Without seeing the mechanism or experimental data, we can’t know for sure to what extent each reactant actually contributes.
We’ll move on to actual rate laws later.
Try these:Try these:
Write the Theoretical Rate Law Expressions for: H2 (g) + I2 (s) --> 2 HI (s) 2 H2 (g) + O2 (g) --> 2 H2O 2C2H6 (g) + 7O2 (g) --> 4CO2 (g) + 6H2O (g)
Write the Theoretical Rate Law Expressions for: H2 (g) + I2 (s) --> 2 HI (s) 2 H2 (g) + O2 (g) --> 2 H2O 2C2H6 (g) + 7O2 (g) --> 4CO2 (g) + 6H2O (g)
The answers are:The answers are:
H2 (g) + I2 (s) --> 2 HI (s) Rate = k*[H2]
2 H2 (g) + O2 (g) --> 2 H2O Rate = k*[H2]2*[O2]
2C2H6 (g) + 7O2 (g) --> 4CO2 (g) + 6H2O (g) Rate = k*[C2H6]2*[O2]7
H2 (g) + I2 (s) --> 2 HI (s) Rate = k*[H2]
2 H2 (g) + O2 (g) --> 2 H2O Rate = k*[H2]2*[O2]
2C2H6 (g) + 7O2 (g) --> 4CO2 (g) + 6H2O (g) Rate = k*[C2H6]2*[O2]7
Actual Rate LawActual Rate Law
In order to determine the actual (nontheoretical rate law) we need experimental data of the concentrations of reactants and the rate of the reaction.
In order to determine the actual (nontheoretical rate law) we need experimental data of the concentrations of reactants and the rate of the reaction.
Determining the Rate LawDetermining the Rate Law 2 H2 + O2 --> 2H2O The data on the right depicts how the rate of water production varies with differing concentration of H2 and O2.
2 H2 + O2 --> 2H2O The data on the right depicts how the rate of water production varies with differing concentration of H2 and O2.
[H2] [O2] rate
.25 M
.25 M
1 M/s
.50 .25 M
2 M/s
.50 M
.50 M
4 M/s
Isolate your variablesIsolate your variables
Let’s look at the 1st two rows.
The [H2] changes, but the [O2] doesn’t. That means we can compare the [H2] directly to the rate.
Let’s look at the 1st two rows.
The [H2] changes, but the [O2] doesn’t. That means we can compare the [H2] directly to the rate.
[H2] [O2] rate
.25 M
.25 M
1 M/s
.50 .25 M
2 M/s
.50 M
.50 M
4 M/s
Isolate your variablesIsolate your variables
Compare the H2 and the rate.
.50/.25 = 2 2/1 = 2 As the hydrogen doubled, the rate doubled. This means that the hydrogen is 1st order.
Compare the H2 and the rate.
.50/.25 = 2 2/1 = 2 As the hydrogen doubled, the rate doubled. This means that the hydrogen is 1st order.
[H2] [O2] rate
.25 M
.25 M
1 M/s
.50 .25 M
2 M/s
.50 M
.50 M
4 M/s
Now isolate the other reactant
Now isolate the other reactant
In the last two rows, the [H2] doesn’t change, but the [O2] does.
The [O2] doubles (.50/.25=2) and the rate doubles (4/2=2).
This means that [O2] is 1st order.
In the last two rows, the [H2] doesn’t change, but the [O2] does.
The [O2] doubles (.50/.25=2) and the rate doubles (4/2=2).
This means that [O2] is 1st order.
[H2] [O2] rate
.25 M
.25 M
1 M/s
.50 M
.25 M
2 M/s
.50 M
.50 M
4 M/s
So….So….
Since the [H2] is first order and the [O2] is first order, the rate law is now:
Rate = k*[H2]*[O2] In the theoretical rate law, the hydrogen was second order, yet we see in the actual rate law that it is really first order.
Since each of the 2 reactants are first order, the reaction is second order.
Since the [H2] is first order and the [O2] is first order, the rate law is now:
Rate = k*[H2]*[O2] In the theoretical rate law, the hydrogen was second order, yet we see in the actual rate law that it is really first order.
Since each of the 2 reactants are first order, the reaction is second order.
Try this: N2 + 3H2 --> 2 NH3
Try this: N2 + 3H2 --> 2 NH3
[N2] [H2] Rate
.125 atm .0333 atm 1.75 atm/hr
.125 atm .1332 atm 28.0 atm/hr
.375 atm .0333 atm 1.75 atm/hr
Rate = k*[H2]2Rate = k*[H2]2
In the 1st two rows, the [N2] is constant. The [H2] went up by 4 times, the rate went up by 16 times. 4x=16, so x=2.
Hydrogen is second order. In the 1st and 3rd rows, the [H2] is constant. The [N2] went up by 2 times, the rate didn’t change. 2x=1, so x=0.
nitrogen is zero order, meaning it does not affect the rate.
In the 1st two rows, the [N2] is constant. The [H2] went up by 4 times, the rate went up by 16 times. 4x=16, so x=2.
Hydrogen is second order. In the 1st and 3rd rows, the [H2] is constant. The [N2] went up by 2 times, the rate didn’t change. 2x=1, so x=0.
nitrogen is zero order, meaning it does not affect the rate.
Determining the value of K
Determining the value of K
In the last example, we found a rate law of rate=k*[H2]2 for the reaction N2 + 3H2 --> 2NH3.
So let’s substitute in any row of data:
In the last example, we found a rate law of rate=k*[H2]2 for the reaction N2 + 3H2 --> 2NH3.
So let’s substitute in any row of data:
[N2] [H2] Rate
.125 atm
.0333 atm
1.75 atm/hr
.125 atm
.1332 atm
28.0 atm/hr
.375 atm
.0333 atm
1.75 atm/hr
First RowFirst Row
Rate=k*[H2]2 1.75 atm/hr = k*[.0333 atm]2
1.75 atm/hr = k*.00111 atm2
1578 1/atm*hr = k Since the data for H2 and the rate are the same in the first and 3rd row, k must be the same for both rows.
Rate=k*[H2]2 1.75 atm/hr = k*[.0333 atm]2
1.75 atm/hr = k*.00111 atm2
1578 1/atm*hr = k Since the data for H2 and the rate are the same in the first and 3rd row, k must be the same for both rows.
Second RowSecond Row
Rate=k*[H2]2 28.0 atm/hr = k*[.1332 atm]2
28.0 atm/hr = k*.01774 atm2
1578 1/atm*hr = k
Rate=k*[H2]2 28.0 atm/hr = k*[.1332 atm]2
28.0 atm/hr = k*.01774 atm2
1578 1/atm*hr = k
Rate StoichiometryRate Stoichiometry
Rates can be compared stoichiometrically, because they are based on concentrations, not on masses.
Given a rxn: xA + yB zC Raterxn = Rate A = Rate B = Rate C x b c
Rates can be compared stoichiometrically, because they are based on concentrations, not on masses.
Given a rxn: xA + yB zC Raterxn = Rate A = Rate B = Rate C x b c
An exampleAn example
Given the reaction: H2 + 3 O2 + N2 --> 2 HNO3
If the rate of RXN is 1.5 atm/hr, what is the rate of consumption of Oxygen?
1.5 atm/hr * -3 = -4.5 atm/hr What is the rate of production of ammonia?
1.5 atm/hr * +2 = 3.0 atm/hr
Given the reaction: H2 + 3 O2 + N2 --> 2 HNO3
If the rate of RXN is 1.5 atm/hr, what is the rate of consumption of Oxygen?
1.5 atm/hr * -3 = -4.5 atm/hr What is the rate of production of ammonia?
1.5 atm/hr * +2 = 3.0 atm/hr
I.C.E, I.C.E. BabyI.C.E, I.C.E. Baby
When we are comparing starting values to final values, we have a three step process, called I.C.E.
Let’s do an example to show this concept:
When we are comparing starting values to final values, we have a three step process, called I.C.E.
Let’s do an example to show this concept:
C3H8 + 5 O2 --> 3 CO2 + 4H2O
C3H8 + 5 O2 --> 3 CO2 + 4H2O
If we begin the reaction with 10 atm of propane and 10 atm of oxygen, we find that carbon dioxide is made at a rate of .04 atm/ min. If the rate stays reasonably constant for one hour, what is the partial pressure of each gas?
If we begin the reaction with 10 atm of propane and 10 atm of oxygen, we find that carbon dioxide is made at a rate of .04 atm/ min. If the rate stays reasonably constant for one hour, what is the partial pressure of each gas?
I stands for initialI stands for initial
C3H8 + 5 O2 --> 3 CO2 + 4H2O I:10 atm 10 atm 0 atm 0 atm
We know we started with 10 atm of each reactant, and we can safely infer that we do not start with products.
C3H8 + 5 O2 --> 3 CO2 + 4H2O I:10 atm 10 atm 0 atm 0 atm
We know we started with 10 atm of each reactant, and we can safely infer that we do not start with products.
C. Stands for ChangeC. Stands for Change
C3H8 + 5 O2 --> 3 CO2 + 4H2O I:10 atm 10 atm 0 atm 0 atm
C:-.8 atm -4 atm +2.4 atm +3.2atm
We know that carbon dioxide is made at .04 atm/min, so in 60 min 2.4 atm will be made. Now we do stoichiometry to compare CO2 to the other chemicals. 2.4 atm CO2 * (-1 C3H8/3 CO2) = -.8 atm C3H8 2.4 atm CO2 * (-5 O2/3 CO2) = -4 atm O2 2.4 atm CO2 * (+4 H2O /3 CO2) = +3.2 atm H2O
C3H8 + 5 O2 --> 3 CO2 + 4H2O I:10 atm 10 atm 0 atm 0 atm
C:-.8 atm -4 atm +2.4 atm +3.2atm
We know that carbon dioxide is made at .04 atm/min, so in 60 min 2.4 atm will be made. Now we do stoichiometry to compare CO2 to the other chemicals. 2.4 atm CO2 * (-1 C3H8/3 CO2) = -.8 atm C3H8 2.4 atm CO2 * (-5 O2/3 CO2) = -4 atm O2 2.4 atm CO2 * (+4 H2O /3 CO2) = +3.2 atm H2O
E. Stands for EndE. Stands for End
C3H8 + 5 O2 --> 3 CO2 + 4H2O I:10 atm 10 atm 0 atm 0 atm C:-.8 atm -4 atm +2.4 atm +3.2atm
E:9.2 atm 6 atm 2.4 atm 3.2 atm
To determine the end results, we just add up our initial with our change. So the answers are:
[C3H8] = 9.2 atm [O2] = 6.0 atm [CO2] = 2.4 atm [H2O] = 3.2 atm
C3H8 + 5 O2 --> 3 CO2 + 4H2O I:10 atm 10 atm 0 atm 0 atm C:-.8 atm -4 atm +2.4 atm +3.2atm
E:9.2 atm 6 atm 2.4 atm 3.2 atm
To determine the end results, we just add up our initial with our change. So the answers are:
[C3H8] = 9.2 atm [O2] = 6.0 atm [CO2] = 2.4 atm [H2O] = 3.2 atm