Chemical Kinetics Flashcards
Define chemical kinetics.
The study of the time dependence of macroscopic chemical properties.
MTK, Lecture 12, Slide 12
What is the focus of Chemical Dynamics?
To interpret macroscopic properties of chemical reactions in terms of the behavior of atoms and molecules.
MTK, Lecture 12, Slide 12
True or False:
The slope of the curve of product concentration as a function of time at some time, t, is the instantaneous rate of product formation at that time.
True.
MTK, Lecture 12, Slide 13
True or False
The slope of reaction curve (concentration vs. time) approaches a horizontal line as the reaction approaches equilibrium.
True.
MTK, Lecture 12, Slide 5
True or False:
The measurement of the rate of change in the concentration of any one of the reactants or products would suffice to measure the rate of the reaction.
True
MTK, Lecture 12, Slide 13
From the definition of the “Rate Law:”
The rate law for a reaction is defined in terms of the time rate of change in the concentration of ___________________.
One of the reactants or products.
MTK, Lecture 12, Slide 14
True or False:
Generally, the rate coefficient k is not a strong function of temperature.
False.
MTK, Lecture 12, Slide 14
The order of a reaction is _______.
The sum of the exponents that describe the rate of the reaction.
MTK, Lecture 12, Slide 15
True or False:
The order of a reaction can only be determined by experiment.
True.
MTK, Lecture 12, Slide 16
True or False:
Observed rate dependence is not based solely on stoichiometry.
True.
MTK, Lecture 12, Slide 16
What are the four types of microscopic processes observed in nature?
- Zero-molecular
- Unimolecular
- Bimolecular
- Termolecular
MTK, Lecture 12, Slide 17
What are the associated molecularities of the four microscopic processes observed in nature?
- Zero-molecular –> Molecularity = 0
- Unimolecular –> Molecularity = 1
- Bimolecular –> Molecularity = 2
- Termolecular –> Molecularity = 3
MTK, Lecture 12, Slide 17
True or False:
For a zero-molecular reaction the rate is independant of the reactants or products.
True
In other words, Rate = k
MTK, Lecture 12, Slide 17
Describe a zero-molecular microscopic process.
A zero-molecular microscopic process in chemical kinetics refers to a process that involves zero reactant molecules coming together to produce a change. This may sound counterintuitive at first, as chemical reactions are generally thought to involve the interaction between reactant molecules. However, zero-molecular processes are more like spontaneous events that don’t require the collision or interaction of multiple molecules for the process to occur.
An example would be radioactive decay.
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Describe unimolecuar processes.
Unimolecular processes in chemical kinetics refer to reactions that involve the rearrangement or transformation of a single reactant molecule. These reactions are initiated by the internal energy of the molecule itself, often activated by collision with other molecules, even if those other molecules are not directly involved as reactants. Essentially, only one molecule undergoes a change to form the product(s) in a unimolecular process.
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What is the rate of a unimolecular reaction (give the equation)?
Rate = k[A]
In terms of the rate equation, unimolecular processes can often be described as first-order reactions. The rate of a first-order reaction is proportional to the concentration of the single reacting substance.
MTK, Lecture 11, Slide 17
Describe Bimolecular processes.
Bimolecular processes in chemical kinetics refer to reactions in which two reactant molecules collide and interact to form products.
Extra Notes: These kinds of reactions are often represented by second-order rate equations, although there are exceptions.
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For a bimolecular process, how is the rate equation defined?
Rate = k[A][B]
Extra Notes: Usually a second order reaction ([A]^1[B]^1 –> p = 1 + 1 = 2). There can be exceptions though.
MTK, Lecture 11, Slide 18
Describe termolecular reactions.
Termolecular processes in chemical kinetics are reactions in which three reactant molecules simultaneously collide and interact to form products.
ChatGPT.
True or False
Termolecular reactions are common.
False. These types of reactions are relatively rare compared to unimolecular and bimolecular reactions. The rarity is mainly due to the low probability of three molecules simultaneously colliding with the correct orientation and sufficient energy to trigger a reaction.
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Give the rate equation for a termolecular reaction.
Rate = k[A][B][C]
MTK, Lecture 11, Slide 18
True or False:
A fractional exponent in a rate law implies that complex mechanisms are at play.
True.
MTK, Lecture 11, Slide 19
What is the difference between a differential rate law and an integrated rate law?
A differential rate law describes the rate of a reaction.
An integrated rate law describes the time dependence of the concentration of reactants or products.
MTK, Lecture 11, Slide 20
For a zero-molecular reaction, the plots of [A] vs t is a ___________.
Linear function.
MTK, Lecture 11, slide 20
For a zero molecular reaction, the slope of the curve when [A] is plotted vs. time is given as ______.
-k
MTK, Lecture 11, slide 20
The terms, “time constant, decay time, lifetime, and half-life,” are often used in conjunction with __________ order reactions.
First
MTK, Lecture 11, Slide 21
The time constant of a reaction is defined as the time it takes _________________________.
For the concentration to decrease to 1/e of its initial value.
Extra notes: 1/e = 0.3679
MTK, Lecture 11, Slide 21
What is a complex reaction?
A reaction that consists of more than one step.
MTK, Lecture 12, Slide 3
What is the principle of detailed balance?
When a reaction reaches equilibrium, the forward and reverse reactions occur at the same rate.
MTK, Lecture 12, Slide 9
True or False:
The principle of detailed balance can be applied when a reaction is not near equilibirum.
False. It is only valid near equilibrium.
MTK, Lecture 12, Slide 10
Explain the basic idea behind the Lindemann Mechanism.
The Lindemann mechanism proposes that two molecules must collide to activate one of them. The activated molecule can either react to form products or deactivate back to the original molecule.
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True or False:
Arrhenius posed that most collisions between molecules are ineffective at leading to reaction because the energy of the collision is insufficient or perhaps the collision geometry is unfavorable for a reaction to occur.
True.
MTK, Lecture 14, Slide 9
The pre-exponential factor in the Arrhenius equation is somtimes called the ___________.
Frequency factor.
MTK, Lecture 14, Slide 13
Why is the pre-exponential factor in the Arrhenius equation sometimes called the frequency factor?
Because it is proportional to the rate at which collisions that have enough energy to react occur with the right geometry for reaction.
MTK, Lecture 14, Slide 13
A negative activation energy implies that reducing the temperature will result in a ________ reaction.
Faster
MTK, Lecture 14, Slide 17
What are the three steps of a chain reaction?
- Initiation
- Propagation
- Termination
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What is a catlysist?
A chemical substance that increases the rate of a chemical reaction without itself being consumed.
MTK, Lecture 16, Slide 2
True or False:
A catlysist can change the equilibrium state of a reaction.
False. It only changes the rate at which equilibrium is reached.
MTK, Lecture 16, Slide 2
A catalyst _________ the activation energy.
Reduces.
MTK, Lecture 16, Slide 3
True or False:
The net enthalpy change for a reaction involving a catalyst is no different than the reaction which excludes the catalyst.
True.
MTK, Lecture 16, Slide 3
True or False:
An Arrhenius plot may be constructed by plotting the natural log of the rate constant k vs. 1/T; the slope of the curve is equal to the activation energy E_A.
True.
Molecular Physical Chemistry for Engineers, Pg. 353
When two molecules come together to form an activated complex, the entropy of the system will _________.
Decrease.
Molecular Physical Chemistry for Engineers, Pg. 363
Define “transition complex.”
In the context of chemical kinetics, a “transition complex” refers to an unstable species that is formed when reactant species collide with both proper orientation and adequate energy.
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Define “free radical.”
A “free radical” is an especially reactive atom or group of atoms that has one or more unpaired electrons.
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Define “adsorption.”
The collision and subsequent bonding of molecules/atoms with surfaces.
Molecular Physical Chemistry for Engineers, Pg. 392
What is the difference between “adsorption” and “absorption?”
Adsorption describes atoms of molecules adhering to the surface of a material through chemical bonds.
Absorption describes the incorporation of atoms or molecules into a material.
Molecular Physical Chemistry for Engineers, Pg. 392
What are the two main types of adsorption phenomena at surfaces?
- Physisorption
- Chemisorption
Molecular Physical Chemistry for Engineers, Pg. 392
True or False:
The weaker form of adsorption is physisorption.
True.
Molecular Physical Chemistry for Engineers, Pg. 392
Physisorption is caused by __________ between the molecule and the surface.
Weak van der Waals forces
Molecular Physical Chemistry for Engineers, Pg. 392
Weak van der Waals forces are caused by what three mechanisms?
- Instantaneous dipole changes
- Permanent dipole (unsymmetric molecules)
- Overlapping electron densities causing a repulsive force
Molecular Physical Chemistry for Engineers, Pg. 392-393
Describe chemisorption.
When a molecule is chemisorbed, there is an exchange of electrons between the molecule and the surface, and a type of covalent surface chemical bond is produced.
Molecular Physical Chemistry for Engineers, Pg. 394
True or False:
When a molecule is physisorbed, its chemical structure is largely unmodified.
True.
Molecular Physical Chemistry for Engineers, Pg. 395
True or False:
The entropy of the system will always increase during adsorption.
False. The entropy of the system will always decrease during adsorption since the translational freedom of the molecule decreases during adsorption.
Molecular Physical Chemistry for Engineers, Pg. 396
What is the difference between the binding strength of Physisorption and Chemisorption?
Physisorption: 20 kJ/mol or less
Chemisorption: 300-500 kJ/mol
Lecture 17, Slide 2
What does the symbol θ mean with regard to the Langmuir model of adsorption?
Fraction of surface sites that are occupied by adsorbate molecules.
θ = number of sites ocupied/total number of sites available
MTK, Lecture 17, Slide 3
True or False:
The pressure of surface adsorbed molecules is equal to the pressure lost from the gas phase when the molecules adsorb on the surface.
True.
MTK, Lecture 17, Slide 5
True or False:
When θ = 1, a volume of gas equivalent to one monolayer (Vm) is adsorbed onto a given surface.
True.
MTK, Lecture 17, Slide 9
When surface coverage is constant (θ = constant), conditions are said to be ________.
Isosteric.
MTK, Lecture 17, Slide 10
What are the two general mechanisms for bimolecular gas-phase reactions?
- Langmuir-Hinshelwood
- Eley Rideal mechanism
MTK, Lecture 18, Slide 2
O_2 –> 2O is an example of _______.
Dissociation.
MTK, Lecture 18, Slide 2
