(Ce) Chpater 5: Chemical Kinetics And Equilibrium

Front 1

What will determine whether a reaction will occur on its own or with outside assistance?

Back 1

The change in Gibbs Free Energy will determine whether a reaction is spontaneous or not. 90

Front 2

Mechanism

Back 2

The mechanism is the step by step process by which a reaction occurs. The mechanism of a reaction includes the intermediate steps that are not included in the overall reaction

Front 3

Overall Reaction

Back 3

These are the full reactions that do not include the intermediate steps of the reaction

Front 4

Rate Determining Step

Back 4

The rate determining step is the slowest step of the reaction. The reason why it is the rate determining step is because it is only when this step is complete can the reaction proceed. In every step of the term, the slowest step of the reaction determines how fast a reaction will occur.

Front 5

What does the negative sign in front of the rate mean?

Back 5

When there is a negative sign in front of the rate (over time) it means that that species is disappearing. In other words, if you have a positive sign you know that that species is being created while the species with the negative sign in front of it is disappearing.

Front 6

What are the units of rate?

Back 6

1) (Mol/L/s)
2) (M/s).

Front 7

What does rate tell you?

Back 7

The rate tells you the concentration of a given species in a given reaction. As the reaction proceeds, the concentration of different reactants change as they turn into products.

Front 8

Rate Law

Back 8

The rate law tells you the proportionality concentrations between the different species in the reaction. In other words, the rate law tells you the concentration of one reactant in relation to another

Front 9

Example of Rate Law

Back 9

Rate Law = K {A}^x{B}^y

Front 10

What does K in the rate law mean?

Back 10

It is simply called a constant called the rate constant.

Front 11

How does the rate law measure rate?

Back 11

It measure rate in terms of concentration/time = (mol/sec)

Front 12

Are the exponents of the rate law the same as the stoichiometric coefficients of the equation?

Back 12

No. The stoichiometric coefficients of the reaction cannot and should not necessarily be taken as the exponents of the rate law.

Front 13

Why can't you take the stoichiometric coefficients as the exponents of the rate law?

Back 13

The reason for this is that the rate law give the proportionality concentration between the different species of the reaction. The overall reaction that you deal with does not necessarily include all the intermediate steps so you cannot derive a proportionality between the reactants.

Front 14

In what case can you use the stoichiometric coefficients of the reaction as the order of the rate law?

Back 14

Only in the case where the reaction that you are given represents the mechanism in its entirety.

Front 15

What does Keq tell you?

Back 15

It tells you how far the reaction is from reaching equilibrium. This means that it is telling you how far the reaction is from reaching a point where the rate for product formation is equal to the rate of reactant formation

Front 16

When is the reaction rate taken and why?

Back 16

The reaction rate is taken as the beginning of the reaction to minimize the effects of the the reverse reaction.

Front 17

Regarding the lass of mass action, what is the difference between the kinetic aspect and the equilibrium aspect?

Back 17

The kinetic part only deals with the PRODUCTS while the equilibrium part deals with both the REACTANTS and PRODUCTS.

Front 18

When is equilibrium measured?

Back 18

Equilibrium is measured at the end of the reaction (i.e. when the reaction has reached equilibrium).

Front 19

What is the difference in terms of when equilibrium is measured and when the rate is measured?

Back 19

The rates is measured at the BEGINNING of the reaction and the equilibrium is measured at the END of the reaction.

Front 20

What happens to the temperature in course of determining a rate law?

Back 20

For a given rate law with a rate constant coefficient, k, the temperature will remain constant.

Front 21

How do you determine a rate law from experimental data?

Back 21

1) Find two trials where the concentration remains constant for one reactant and changes for another reactant.
2) Determine the factor by which the concentration changes.
3) Inspect the rate and determine the factor by which it changes
4) Set the factor by which the concentration changes equal to the factors by which the rate changes and raise to the latter to the power X. Your equation should look like (factor by which concentration changes) = (factor by which rate changes) ^ X.
5) Solve for X. This will be the power you raise the species of one element.
6) Re 4: the power to which one of the numbers is raised will make the two number equal. What number should that be?

Front 22

Math Note: Number raised to the power of 0

Back 22

Any number raised to the power of 0 will be one

Front 23

Only factor that can change the rate of a 0 order reaction.

Back 23

The only factor that can change the rate of a 0 order reaction is the temperature of the reaction.

Front 24

What does a catalyst do?

Back 24

It lowers the activation energy of the reaction.

Front 25

Zero Order Reaction

Back 25

In this reaction the exponents of the reactants are equal to 0.

Front 26

For a zero order reaction, how will changing the concentration of one reactant affect the formation of the product?

Back 26

There will be no impact on the rate of formation of the product. For zero order reaction, the rate of formation of the product is independent of the concentration of the reactants.

Front 27

First Order Reactions

Back 27

In these reactions, the sum of the exponents will be equal to one. This means that one of the exponents will be 0 and other one.

Front 28

For a first order reaction, how will changing the concentration of one reactant affect the rate of formation of the product?

Back 28

The rate of formation of the product will changed such that the doubling the concentration of one the reactants (the reactant with the exponent of 1) will result in a doubling of the rate of formation of the product.

Front 29

Second Order Reactions

Back 29

In these reactions, the overall rate order will be the sum of the exponents equaling two. This can take a variety of forms in that one exponent can be 1 the other 1, one exponent 0 the other 2.

Front 30

Mixed Order Reaction

Back 30

A mixed order reaction is one where the rates is a FRACTION. It also means that the rate of the reaction is one that will CHANGE OVER TIME

Front 31

Collision Theory of Chemical Kinetics

Back 31

Rate of a reaction is proportional to the number of collisions per second between reacting molecules.

Front 32

According to the Collision Theory of Chemical Kinetics what are the two necessary factors for a reaction to occur?

Back 32

The molecules need to collide with the right energy and at the right orientation so that the chemical bonds holding the molecules together break.

Front 33

What is the minimum energy needed for a reaction to take place?

Back 33

Activation Energy

Front 34

Transition State Theory

Back 34

This theory states that when molecule collide, they form a transition state where old bonds are weakened and new bonds are being strengthened. As the transition state dissociates, new bonds are formed and the old one break completely

Front 35

Transition State / Activates Complex

Back 35

This is a theoretical structure (!) in which the older bonds are being broken and the newer bonds are being formed. This non-real structure has the highest energy in the reaction, higher than both the products and the reactants. The energy equal to the activation energy is necessary to bring the reactants to this this state.

Front 36

Enthalpy Change

Back 36

Enthalpy is the difference in potential energy of the reactants and the product of the reaction. This is a very simple and important thing to know. Again, to find the enthalpy change just subtract the potential energy of the reactants from the potential energy of the reactants..

Front 37

Negative Enthalpy change

Back 37

A negative enthalpy change is an exothermic reaction in which heat is given off

Front 38

Positive enthalpy change

Back 38

A positive enthalpy change is an endothermic reaction in which heat is absorbed by the reaction.

Front 39

Difference between the energy of the reactants the energy of the activated complex.

Back 39

1) The energy difference between the reactants and the products is the rate of the reaction. The reactants can be either end of the reaction.
2) This energy difference is also the activation energy, the energy you need to start the reaction.

Front 40

Two types of catalysts

Back 40

Homogeneous = where the catalyst is in the same phase as the reactants
Heterogeneous = where the catalyst is in a different phase as the reactants

Front 41

Four Factors that affect the rate of a reaction

Back 41

1) Concentration. Increasing the concentration will increase the rate of reaction since there will be more collisions per unit time (with the exception of zero order reactions).
2) Temperature. Very generally, increasing the temperature by 10 C will double the rate of the reaction. This is usually true for biological systems.
3) Medium. Polar solvents are usually preferred because they polarize the bonds by lengthening them which makes them weaker. Also, some reactions take place better in aqueous mediums while other reactions takes place in non aqueous solutions.
4) Catalysts. They lower the activation energy.

Front 42

Will a catalyst make a non spontaneous reaction occur spontaneously?

Back 42

No. Catalysts only work on reactions that occur spontaneously

Front 43

You have a reaction between A and B. What is the concentration of the two at equilibrium?

Back 43

The concentration of the two is constant but not necessarily the same. The rate of the formation of product and the formation of reactants (in the reverse reaction) is equal but it does not mean that the AMOUNT of reactants and products are equal.

Front 44

Irreversible Reactions vs Reversible Reactions

Back 44

Irreversible reactions are generic reactions in which a reactant A will fully go to product B. In Reversible reactions, there is an equilibrium between the formation of A and B. As A turns to B, B will also turn into A. The equilibrium point (Keq) is the point where the rate of formation of A is equal to the rate of formation of B.

Front 45

Dynamic Equilibrium vs Static Equilibrium

Back 45

Dynamic = The concentrations of both A and B are constant. But A is turning into B and B is turning into A
Static = The concentration of both A and B are constant. There is no change of A and B.

Front 46

What is the relationship between Kc and Keq?

Back 46

They are the same things when the reaction is in equilibrium. You find the Kc by multiplying the products together and divining that by the product of the reactants.

Front 47

When a reaction is at equilibrium and you are given a reaction, how do you find the Keq or Kc?

Back 47

Simply take the products and place them in the numerator and in the denominator place the reactants. In this case, the stoichiometric coefficients are the exponents of the Keq expression. This is very different from the rate law in which the exponents can only be found from data. In the case of equilibrium problems, you can simply take the values that you are given in the reaction and use them as the exponents

Front 48

Law of Mass Action

Back 48

This law states that for a system in equilibrium at a given temperature, the following ratio is constant:

Keq = {C)^c{D}^d/{A}^a{B}^b

Front 49

Qc (reaction quotient) and how you measure how close a reaction is to equilibrium.

Back 49

Qc can be used as a measure to determine how close a reaction is to equilibrium. We determine this with the Qc = {C)^c{D}^d/{A}^a{B}^b. This value is only of use when it is compared to the value that you have for the Keq or equilibrium value. It is only when compared to this other value that you know where a reaction is relative to the point of equilibrium

Front 50

Qc < Keq

Back 50

When Keq is greater than the reaction quotient, the reaction has not reach equilibrium

Front 51

Qc > Keq

Back 51

When the Keq is less than the reaction quotient that reaction has exceeded the equilibrium

Front 52

Qc = Keq

Back 52

When the two values are equal then the reaction has reached a dynamic equilibrium

Front 53

Do the concentrations of constants or solids appear in the equilibrium expression?

Back 53

No. Since they are constant they do not appear in the expression and can be ignored.

Front 54

Equilibrium Constant and temperature

Back 54

The equilibrium constant is the temperature dependent.

Front 55

What does it mean if we have a large Keq?

Back 55

It means that the reaction is more to the right and more complete.

Front 56

How do you write the equilibrium constant (Keq) in the reverse direction?

Back 56

You simply write the Keq that you have as 1/Keq.

Front 57

Le Chatelier's Principle

Back 57

This principle state that if you place a stress on a system, then that system will act to relieve that stress.

Front 58

How does Le Chateliers Principle apply to the addition or removal of reactants and products?

Back 58

Simply put, if you add or remove, then the reaction will move in the direction opposite the addition or removal. In other words, if you add product, the reaction will remove in the reverse direction and vice versa.

Front 59

In an endothermic reaction, how will the addition of heat affect the reaction?

Back 59

In an endothermic reaction, look at heat as a reactant. So if you add heat, then you are adding reactant and the reaction will move in the forward direction.

Front 60

In an exothermic reaction, how will the addition of heat affect the rate of reaction?

Back 60

In an exothermic reaction, look as heat as a product. So if you add hear then you are adding a product and the reaction will move in the reverse direction.

Front 61

If you add pressure to a system, how will the system react?

Back 61

The system will move to the side where there are less moles of gas to as to decrease the pressure. The reason for this is that since there is an addition of pressure the system will want to move in a direction where there is less pressure and since gasses exert partial pressure the system will move to the side with less pressure.