Ch 14 Chemical Kinetics

Question 1

Define Chemical Kinetics

Answer 1

The study of the factors that affect the rates of chemical reactions, such as temperature.

Question 2

Define Reaction Rate

Answer 2

The speed of a chemical reaction; the measure of how fast the reaction makes products or uses reactants.

Question 3

How is The rate of chemical reaction measured?

Answer 3

The rate of chemical reaction is generally measured in terms of how much the concentration of a reactant decreases (or product concentration increases) in a given period of time.

Question 4

Define Instantaneous Rate

Answer 4

The change in concentration at any one particular time.

The slope at one point of a curve.

Question 5

What are the two ways to calculate the rate of a chemical change?

Answer 5

1) Rate = - (1🔺[A]/a🔺t)

Or -({1🔺[A]t2 - [A]t1} / {t2-t1})

2) determine the slope at a particular time. Slope = 🔺y/🔺x

Question 6

As time goes on, what usually happens to the rate of chemical reaction? Why?

Answer 6

The rate of the reaction generally slows down because the concentration of the reactants decreases.

Question 7

Define Average Rate. How is it calculated?

Answer 7

The average rate is the change in measured concentrations in any particular time period.
Average Rate = Concentration change / 🔺time

Question 8

What effect does the Time Interval have on the average rate?

Answer 8

The larger the time interval, the more the average rate deviates from the instantaneous rate.

Question 9

Define the Rate Law

Answer 9

The Rate Law is an equation (only using REACTANTS) relating concentration of reactants to rate when the reverse reaction is negligible.

Rate = k[A] ^n

n is called the order, usually it’s an integer that determines rate dependence on reactant concentration.
K is the rate constant

Question 10

Define Order of the the Reaction.

Answer 10

The sum of the exponents on the REACTANTS.

Question 11

Give the order of Reaction for each of the following reactants and the entire reaction: k[NO]^2[O2]

Answer 11

1) [NO]^2 = 2
2) [O2] = 1
3) k[NO]^2[O2] = 3

Question 12

Give the characteristic that describes Rate orders zero, first and second.

Answer 12

1) Zero Order - The Rate of the Reaction is always the same. Doubling [A] will have no effect on the reaction.
2) First Order - The Rate is directly proportional to the reactant concentration. Doubling [A] will double the rate of the reaction.
3) Second Order - The Rate is directly proportional to the square of the reactant. Doubling [A] will quadruple the rate of the reaction.

Question 13

What’s the difference between the Rate Law and the Integrated Rate Law?

Answer 13

The Rate Law shows the relationship between rate and concentration.

The Integrated Rate Law shows the relationship between the concentration of A and the time of the reaction.

Question 14

What are the Rate Laws for Zero, First and Second Order Reactions?

Answer 14

1) Zero-Order: k[A]^0 or just k
2) First-Order: k[A]^1 or k*[A]
3) Second-Order: k[A]^2

Question 15

What are the Integrated Rate Laws for Zero, First and Second-Orders?

Answer 15

1) Zero-Order: [A]t = -kt + [A]0
2) First-Order: ln[A]t = -kt + ln[A]0 or ln ([A]t/[A]0) = -kt
3) Second-Order: 1/[A]t = kt + 1/[A]0

Question 16

What are the units of k for Zero, First and Second Order Rate Laws?

Answer 16

1) Zero-Order: M * s^-1
2) First-Order: s^-1
3) Second-Order: M^-1 * s^-1

Question 17

How do you calculate Half Life for Zero, First and Second Order Rate Laws?

Answer 17

1) Zero-Order: t1/2 = [A]0/2k
2) First-Order: t1/2 = 0.693/k
3) Second-Order: t1/2 = 1/k[A]0

Question 18

Explain the equation of y = mx + b

Answer 18

The equation of any straight line can be written as y = mx + b; where m is the slope of the line and b is the y intercept, which represents the value of y at the point where the line crosses the y axis.

Question 19

Given a balanced equation:
aA + bB —> cC + dD
How is rate calculated?

Answer 19

-1/a * (🔺[A]/🔺[t])

Question 20

What is the significance of the Arrhenius Equation? Explain the components of the equation.

Answer 20

Arrhenius ‘ Equation =
k = Ae ^-Ea/RT

Arrhenius ‘ Equation shows that the rate constant (k) of the Rate Law is dependent on temperature.

T is the temp in kelvins
R is the gas constant 8.314
A is the frequency factor, which represents the frequency of collisions between reactant molecules.
Ea is the activation energy or the minimum needed to start the reaction.

Question 21

Define A Activation Energy

Answer 21

Activation Energy Aka the “Energy Barrier” is the amount of energy needed to convert reactants into the activated complex (also known as the transition state).

Question 22

Define Activated Complex

Answer 22

A chemical species with partially broken and partially formed bonds.
Bonds of reactants are broken but product is not yet formed.

Question 23

What is the significance of the Frequency Factor in Arrhenius’ Equation?

Answer 23

The Frequency Factor represents the number of approaches to the activation barrier time. In other words, it’s the amount of time the molecule wags or vibrates side to side.

Question 24

How does Activation Energy and Temperature affect the reaction rate?

Answer 24

A higher Activation Energy slows down or increase the reaction rate time, while a higher Temperature will speed up or decrease the reaction rate time.

Question 25

What are the characteristics of the Exponential Factor in Arrhenius’ Equation?

Answer 25

1) It is a number between 0 and 1. e^Ea/RT = a number between 0 and 1 2) It represents the fraction of reactant molecules with sufficient energy that can make it over the energy barrier. The higher the energy barrier (larger activation energy) the fewer molecules that have sufficient energy to overcome it. 3) Increasing temperature will increase the number of molecules with sufficient energy, thus increase the reaction rate.

Question 26

What is the linear equation version of Arrhenius’ Equation?

Answer 26

ln k = -(Ea/R) *(1/T) + ln A Y = mx + b

Question 27

What is the Two-Point Form versions of the Arrhenius Equation? When is it used?

Answer 27

ln (k2/k1) = Ea/r (1/T1 - 1/T2) Can be used when you only have two (T,k) data points.

Question 28

Define Reaction Mechanism

Answer 28

The series of individual steps by which an overall chemical reaction occurs. Knowing the Rate Law of the Reaction helps us to understand the sequence of reactions in the mechanism.

Question 29

Define Reaction Intermediates

Answer 29

Materials that are products in an early mechanism step, but then reactants in a later step.

Question 30

Explain the term Elementary Steps as it applies to a chemical reaction.

Answer 30

Elementary steps represents a written chemical reaction that cannot be broken down into simpler steps and that the molecules actually interact directly.

Question 31

Define Molecularity

Answer 31

The number of reactant particles in an elementary step.

Question 32

What is the molecularity of the following reactions and how is the rate calculated? 1) A —> products 2) A + A —> products 3) A + B —> products 4) A + A + A —> products 5) A + A + B —> products 6) A + B + C —> products

Answer 32

1) A —> products = 1 (Rate = k[A]) 2) A + A —> products = 2 (Rate = k[A]^2) 3) A + B —> products = 2 (Rate = k[A][B]) 4) A + A + A —> products = 3 (Rate = k[A]^3) 5) A + A + B —> products = 3 (Rate = k[A]^2[B]) 6) A + B + C —> products = 3 (Rate = k[A][B] [C])

Question 33

What’s the difference between determining the Rate Law for an overall reaction vs the Rate Law for for an Elementary Step?

Answer 33

The Rate Law for an overall reaction must be determined experimentally while the Rate Law for an elementary step can be deduced from the equation of the step. Each step in the mechanism is like its own little reaction with its own activation energy and its own Rate Law.

Question 34

What is the Rate Determining Step? What are it’s characteristics?

Answer 34

The slowest step in a mechanism is called the Rate Determining Step (RDS). 1) The RDS has the largest activation energy 2) The Rate Law of the RDS determines the Rate Law for the overall reaction

Question 35

Which two conditions have to be met in order to validate a mechanism?

Answer 35

1) The elementary steps must sum to the overall reaction. | 2) The Rate Law predicted by the mechanism must be consistent with the experimentally observed Rate Law.

Question 36

What are the characteristics of a mechanism with a fast initial step?

Answer 36

1) when a mechanism contains a fast initial step, the RDS may contain intermediates. 2) When a previous step is rapid and reaches equilibrium, the forward and reverse reaction rates are equal, so the concentrations of the reactants and products of the step are related, and the product is an intermediate. 3) Substituting into the Rate Law of the RDS will produce a Rate Law in terms of just reactants.

Question 37

What is a Catalyst? What are its characteristics?

Answer 37

Catalysts are substances that affect the rate of a reaction without being consumed. It works by providing an alternative mechanism for the reaction with a lower activation energy which in turn speeds up the reaction rate. Catalysts are consumed in an early step and then made in a later step. Catalyst speeds up the reaction rate

Question 38

What is one real life practical example, given in lecture, of a Catalyst?

Answer 38

A catalytic converter in the exhaust system of a car. It helps eliminate pollutants in the exhaust.

Question 39

What are the types of Catalysis?

Answer 39

1) Homogeneous Catalysis - Catalyst in the same phase as reactants. 2) Heterogeneous Catalysis - Catalyst In different phase than reactants. i.e. catalytic hydrogenation of ethane.

Question 40

What are the four steps in the catalytic hydrogenation of ethane?

Answer 40

1) Adsorption 2) Diffusion 3) Reaction 4) Desorption

Question 41

What are Enzymes? How do they work?

Answer 41

Enzymes are protein molecules that catalyze biological reactions. Enzymes work by adsorbing the substrate reactant onto an active site that orients the substrate for reaction.

Question 42

Why do most biological reactions require a Catalyst?

Answer 42

Because Catalysts speed up reaction rates, most biological reactions use catalysts because many biological molecules are large and complex and they require a Catalyst to proceed at a reasonable rate.

Question 43

ln (x) = ?

Answer 43

Ln (x) = e^x

Question 44

How do you solve 2 = 2^x

Answer 44

Take the “log” of both sides. Log of 2 = .3 Log of 2^x = (log of 2) * y

Question 45

How do you solve for t 1/2 when you have t = 2.08k and t 1/2 = .693/k?

Answer 45

Multiply t by the reciprocal of t1/2. (2.08/k) * (k/.693) = 2.08/.693 = 3

Question 46

How do you calculate Kelvin?

Answer 46

Kelvin = Degrees in Felicia + 273