4B5 Chemical Kinetics

Question 1

Define:

rate of a reaction

Answer 1

The change in concentration of reactants or products divided by time.

The rate of reaction quantifies how fast the concentration of reactants or products changes over time.

Equation: Rate = change in concentration / time; expressed in Molarity per second (M/s) or per minute (M/min), depending on the reaction speed.

Question 2

List the four factors that affect the rate of reaction.

Answer 2

1. Temperature
2. Concentration
3. Surface area
4. Catalysts

These factors all influence how often reactant molecules collide and how much energy they have for successful collisions.

Question 3

How does concentration affect the rate of reaction?

Answer 3

Increasing concentration increases the rate of reaction.

Higher concentration means more reactant molecules are present, which leads to more collisions and thus a faster reaction.

Question 4

What happens to the rate of reaction as temperature increases?

Answer 4

It increases.

Higher temperature increases the kinetic energy of molecules, making them move faster and collide more frequently.

Question 5

How does surface area affect the rate of reaction?

Answer 5

More surface area increases the rate of reaction.

More surface area exposes more molecules to collisions, speeding up the reaction.

Example: Crushing a solid into powder increases surface area.

Question 6

Fill in the blank:

In a chemical reaction, rate is proportional to the _________.

Answer 6

number of collisions

The rate of reaction depends on how often molecules collide and how much energy is available for the collision to be effective.

Question 7

Define:

collision theory

Answer 7

Chemical reactions occur when reactant molecules collide with sufficient energy.

More frequent and energetic collisions increase the rate of the reaction.

Question 8

What is the rate law in chemistry?

Answer 8

A quantitative expression linking the concentration of reactants to the rate of a chemical reaction.

The rate law defines how the concentration of reactants affects the speed of a chemical reaction.

Question 9

How can the rate law equation be expressed?

Answer 9

rate = k[A]^m[B]ⁿ [C]^o

Where [A], [B], and [C] are the concentrations and m, n, and o are the partial reaction orders of reactants A, B and C.

Question 10

What is the reaction order?

Answer 10

The sum of the partial reaction orders for all reactants in the rate law equation.

The reaction order provides a way to understand how reactant concentrations affect the reaction rate.

Question 11

Fill in the blank:

The reaction order in a first-order reaction is equal to _____.

Answer 11

1

In first-order reactions, the sum of the exponents (m+n) is 1.

Question 12

In a first-order reaction, how is the rate related to the concentration of reactant A?

Answer 12

The rate is directly proportional to the concentration of A.

If the concentration of A doubles, the rate also doubles in a first-order reaction.

Question 13

How is the rate constant (k) determined?

Answer 13

By experimental data, which involves:

1. Using different concentrations.
2. Measuring reaction rates.

After determining the reaction order, k can be calculated from the experimental data by substituting experimental values into the rate equation.

The rate constant k is specific to each chemical reaction under particular conditions.

Question 14

What does a higher value of the rate constant (k) indicate?

Answer 14

A faster reaction speed.

The rate constant (k) varies with conditions like temperature.

Question 15

What happens when the concentration of reactant A triples in a first-order reaction?

Answer 15

The rate of the reaction triples.

In first-order reactions, the rate is directly proportional to the concentration of the reactant.

Question 16

What is the unit of the rate constant (k) in a first-order reaction?

Answer 16

s⁻¹

In a first-order reaction, the rate constant has units of inverse seconds.

Question 17

What is the unit of the rate constant (k) in a second-order reaction?

Answer 17

L mol⁻¹ s⁻¹

Question 18

In a second-order reaction, how does the rate change when the concentration of A doubles?

Answer 18

The rate quadruples.

In second-order reactions, the rate is proportional to the square of the reactant concentration.

Question 19

What is the overall reaction order in the equation rate = k[A]²[B]⁰?

Answer 19

2

The reaction order is the sum of the exponents in the rate law equation (m+n).

Question 20

True or False:

The reaction order can sometimes be a fraction or negative number.

Answer 20

True

Reaction orders are typically whole numbers but can occasionally be fractions or negative.

Question 21

What happens to the rate of reaction in a zero-order reaction with respect to reactant B?

Answer 21

The rate is independent of the concentration of B.

If the exponent for B is zero, the concentration of B does not affect the rate of the reaction.

Question 22

List the steps involved in determining the rate law for a reaction.

Answer 22

1. Conduct experiments to measure reaction rates.
2. Calculate reaction orders for each reactant.
3. Solve for the rate constant (k).

The rate law is determined through experimentation and calculation based on observed data.

Question 23

What equation is used to calculate the rate constant k?

Answer 23

The Arrhenius equation.

Arrhenius equation: k=Ae^(-Ea/RT).

It expresses the relationship between the rate constant (k), temperature (T), and activation energy (Ea).

Question 24

Which component in the Arrhenius equation represents the frequency factor?

Answer 24

A

The frequency factor is the number of times reactants approach the activation energy per unit time.

It depends on factors like molecular orientation and collision frequency.

Question 25

Which factor **increases** when temperature rises?

Answer 25

The **exponential** factor. ## Footnote e^(-Ea/RT) is the exponential factor.

Question 26

How does the activation energy **affect the exponential** factor?

Answer 26

The **higher** the activation energy, the **smaller the exponential** factor. ## Footnote A higher activation energy means that fewer reactants can overcome the barrier, reducing the exponential factor.

Question 27

What is the value of the exponential factor **if the activation energy is 0**?

Answer 27

1 ## Footnote If the activation energy is 0, the exponential factor is 1, meaning all reactants can overcome the barrier.

Question 28

What does the Arrhenius equation indicate about the relationship between **rate constant and temperature**?

Answer 28

The rate constant **increases as temperature increases**. ## Footnote According to the Arrhenius equation, k increases as the temperature T increases, leading to a faster reaction rate.

Question 29

# Define: activation energy

Answer 29

The minimum amount of **energy required** for a chemical **reaction to occur**. ## Footnote A higher activation energy means a slower reaction rate at a given temperature since more energy is required to overcome the barrier.

Question 30

List *two* factors that **contribute to the activation energy** of a reaction.

Answer 30

1. Molecular orientation 1. Bond breaking ## Footnote Reactant molecules must collide with proper orientation, and bonds must break before new bonds can form.

Question 31

What is the effect of **thermal energy on the activation energy**?

Answer 31

It **increases the kinetic energy** of molecules, helping them overcome the activation energy. ## Footnote This energy is usually provided by heat sources like Bunsen burners or hot plates in laboratory settings.

Question 32

# Define: transition state

Answer 32

A **high-energy state** in which reactants rearrange to form products. ## Footnote It is a temporary unstable state in which bonds are breaking and forming, and the system has reached its maximum potential energy.

Question 33

What is the effect of a **catalyst on the activation energy** of a reaction?

Answer 33

It **lowers the activation energy** by providing an alternative reaction mechanism. ## Footnote Catalysts speed up reactions without being consumed by reducing the energy barrier needed for the reaction to proceed.

Question 34

List *three* **examples of catalysts**.

Answer 34

1. Zinc oxide (ZnO) 1. Chromium oxide (Cr₂O₃) 1. Nickel (Ni) ## Footnote These catalysts are used in commercial reactions like methanol synthesis, steam reforming, and the water-gas shift reaction.

Question 35

# Define: activation complex

Answer 35

The **unstable intermediate** formed during the transition state when bonds are breaking and forming. ## Footnote It is the state where the reactants are in the process of converting into products but have not yet completed the reaction.

Question 36

# True or False: Catalysts **change the products** of a reaction.

Answer 36

False ## Footnote Catalysts *do not alter the products of a reaction;* they only affect how quickly the reaction reaches the products.

Question 37

What type of catalysts are found **in living organisms**?

Answer 37

Enzymes ## Footnote These are biological catalysts, typically proteins or RNA. They have a unique active site that specifically interacts with a particular substrate, allowing for more efficient catalysis.

Question 38

How do **enzymes work as catalysts** in biological systems?

Answer 38

They **lower activation energy** by binding to substrates and aligning them in the correct orientation for the reaction to occur. ## Footnote Enzymes differ from catalysts since they are highly specific to their substrates and use the induced fit mechanism to bind and catalyze reactions.

Question 39

What is the purpose of a catalyst in a chemical **manufacturing laboratory**?

Answer 39

1. To increase the **efficiency** of reactions. 1. To reduce the **time** required to complete reactions. ## Footnote Catalysts are used to speed up reactions in industrial settings where time and resource efficiency are critical.

Question 40

What is a **reaction mechanism?**

Answer 40

The step-by-step **sequence of elementary steps** by which reactants become products. ## Footnote A reaction mechanism describes how molecules interact and transform during a chemical reaction.

Question 41

Which step in a reaction mechanism is considered the **rate-determining step**?

Answer 41

The **slowest elementary step** in the reaction mechanism. ## Footnote This step has the highest energy barrier and controls the overall reaction rate.

Question 42

# Define: intermediate

Answer 42

A molecule produced in one elementary step and **consumed in a subsequent step**. ## Footnote Intermediates are crucial to understanding reaction mechanisms but do not appear in the overall reaction equation.

Question 43

# Fill in the blank: In a reaction mechanism, a reaction with multiple steps has a \_\_\_\_\_\_\_ **transition state for each step**.

Answer 43

unique ## Footnote Each elementary step in a multi-step reaction has its own transition state, with a corresponding activation energy.

Question 44

# Define: homogeneous reactions

Answer 44

Reactions that involve reactants and products in the **same physical state**. ## Footnote This makes the rate laws for homogeneous reactions simpler compared to heterogeneous reactions.

Question 45

List the **steps to find the rate law** for a reaction.

Answer 45

1. Write out the elementary steps, including intermediates. 1. Identify the rate-determining step. 1. Find its rate law. 1. If necessary, substitute intermediates with initial reactants. ## Footnote These steps help determine the rate law by focusing on the slowest step and any intermediates involved.

Question 46

What happens when there are **intermediates in the rate law equation**?

Answer 46

Intermediates **must be substituted** with the concentrations of initial reactants. ## Footnote This substitution is done using the rate laws of prior elementary steps.

Question 47

# True or False: The rate law for the rate-determining step **is always** the rate law for the overall reaction.

Answer 47

True ## Footnote Since the rate-determining step controls the overall reaction rate, its rate law is used for the entire reaction.

Question 48

# Define: molecularity

Answer 48

It is the **number of molecules that must collide** to undergo a reaction in an elementary step. ## Footnote The molecularity is often reflected in the exponents of the reactant concentrations in the rate law.

Question 49

What is a rate law for a reaction **dependent on**?

Answer 49

* Concentrations of reactants in the rate-determining step. * The rate constant (k). ## Footnote The rate law is independent of the concentration of products.