1. Chapter 15: Kinetics
The speed with which the reactants disappear and the products form is called the rate of the reaction
A study of the rate of reaction can give detailed information about how reactants change into products
The series of individual steps that add up to the overall observed reaction is called the reaction mechanism

2. The progress of the reaction A  B
The progress of the reaction A  B. The number of A molecules (in red) decreases with time while the number of B molecules (in blue) increases. The steeper the concentration versus time curve, the faster the reaction rate. The film strip represents the relative number of A and B molecules at each time.

3. There are five principle factors that influence reaction rates:
Chemical nature of the reactants
Ability of the reactants to come in contact with each other
Concentration of the reactants
Temperature
Availability of of rate-accelerating agents called catalysts

4. Chemical nature of the reactants
Bonds break and form during reactions
The most fundamental difference in reaction rates lie in the reactants themselves
Some reactions are fast by nature and others slow
Ability of the reactants to meet
Most reactions require that particles (atoms, molecules, or ions) collide before the reaction can occur
This depends on the phase of the reactants

5. In a homogeneous reaction the reactants are in the same phase
For example both reactants in the gas (vapor) phase
In a heterogeneous reaction the reactants are in different phases
For example one reactant in the liquid and the second in the solid phase
In heterogeneous reactions the reactants meet only at the intersection between the phases
Thus the area of contact between the phases determines the rate of the reaction

6. Effect of crushing a solid
Effect of crushing a solid. When a single solid is subdivided into much smaller pieces, the total surface area on all of the pieces becomes very large.

7. Concentration of the reactants
Both homogeneous and heterogeneous reaction rates are affected by reactant concentration
For example, red hot steel wool bursts into flames in the presence of pure oxygen
Temperature of the system
The rates for almost all chemical reactions increase as the temperature is increased
Cold-blooded creatures, such as insects and reptiles, become sluggish at lower temperatures as their metabolism slows down

8. A rate is always expressed as a ratio
Presence of a catalyst
A catalysts is a substance that increases the rate of a chemical reaction without being consumed
Enzymes are biological catalysts that direct our body chemistry
A rate is always expressed as a ratio
One way to describe a reaction rate is to select one component of the reaction and describe the change in concentration per unit of time

9. Molarity (mol/L) is normally the concentration unit and the second (s) is the most often used unit of time
Typically, the reaction rate has the units

10. A reaction rate is generally not constant throughout the reaction
Since most reactions depend on the concentration of reactants, the rate changes as they are used up
The rate at any particular moment is called the instantaneous rate
It can be calculated from a concentration versus time plot

11. A plot of the concentration of HI versus time for the reaction:
2HI(g)  H2(g) + I2(g). The slope is negative because we are measuring the disappearance of HI. When used to express the rate it is used as a positive number.

12. The rate of a homogeneous reaction at any instant is proportional to the product of the molar concentrations of the reactants raised to a power determined from experiment

13. Consider the following reaction:
From experiment, the rate law (determined from initial rates) is
At 0oC, k equals 5.0 x 105 L5 mol-5 s-1
Thus, at 0oC

14. The exponents in the rate law are generally unrelated to the chemical equation’s coefficients
Never simply assume the exponents and coefficients are the same
The exponents must be determined from the results of experiments
The exponent in a rate law is called the order of reaction with respect to the corresponding reactant

15. Exponents in a rate law can be fractional, negative, and even zero
For the rate law:
We can say
The reaction is first order with respect to H2SeO3
The reaction is third order with respect to I-
The reaction is second order with respect to H+
The reaction order is sixth order overall
Exponents in a rate law can be fractional, negative, and even zero

16. Looking for patterns in experimental data provide way to determine the exponents in a rate law
One of the easiest ways to reveal patterns in data is to form ratios of results using different sets of conditions
This technique is generally applicable
Again consider the hypothetical reaction

17. Suppose the experimental concentration-rate data for five experiments is:

18. For experiments 1, 2, and 3 [B] is held constant, so any change in rate must be due to changes in [A]
The rate law says that at constant [B] the rate is proportional to [A]m
Thus m=1

19. This means that the reactions is first order with respect to reactant A
For experiments 3, 4, and 5 [A] is held constant, so any change must be due to changes in [B]
The rate law says that at constant [A] the rate is proportional to [B]n
Using the results from experiment 3 and 4:

20. The reaction is second order in B and
rate=k[A][B]2
Thus n=2

21. The rate constant (k) can be determined using data from any experiment
Using experiment 1:
Using data from a different experiment might give a slightly different value