Questions tagged [kinetics]
The study of rates and steps in mechanisms of chemical processes.
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Is there a general consensus on the causes of the alpha-effect?
There have been various explanations posited for the α-effect. The α-effect refers to a phenomenon wherein nucleophiles with lone pairs on atoms adjacent (i.e., in the α- position) to the atom bearing ...
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Difference between thermodynamic and kinetic stability
What is the difference between thermodynamic and kinetic stability? I'd like a basic explanation, but not too simple. For example, methane does not burn until lit -- why?
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What is more acidic: D3O+ in D2O or H3O+ in H2O and why?
What is more acidic:
$\ce{D3O+}$ in $\ce{D2O}$
or
$\ce{H3O+}$ in $\ce{H2O}$
and why?
I think it's $\ce{D3O+}$ in $\ce{D2O}$ as I saw somewhere that this property is used in mechanistic studies (...
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Does any known substance ignite on cooling?
As the title says, I'm interested in knowing if there is any substance — or combination of substances — that ignites (or even increases its chance of spontaneous ignition) when cooled.
I've never ...
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Why are solids and liquids not included in the equilibrium constant? What about in a reaction rate calculation?
Take for instance the reaction
$$\ce{H2(g) + I2(s) <=> 2HI(g)}$$
The equilibrium constant would not include the solid $\ce{I2}$, but why is this? I have read that its concentration is a ...
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What's a minimal yet chemically-meaningful kinetic system for an oscillating reaction?
Oscillating reactions are a funny aspect of chemistry. I have tried to find various simplified kinetic models of oscillating reactions such as the Belouzov-Zhabotinsky, the Briggs–Rauscher or the Bray–...
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What are some good examples of rate equations for a math class?
I'm a mathematician who's currently teaching a course on differential equations. Though I don't know much about chemistry, I like to include examples from chemistry in my course, and I prefer for the ...
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Michael reaction or nucleophilic attack at the alpha carbon of an alpha-beta-unsaturated ketone?
What conditions promote a nucleophile to undergo the Michael reaction over the nucleophilic attack at the alpha carbon of the carbonyl group in an alpha-beta-unsaturated ketone? I'm looking for an ...
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How to identify zero order reactions?
I have come across many reactions that are zero order reactions but at one glance I'm unable to tell if they are zero order or not. Is there any criteria that can be used to identify these reactions ...
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Relation between chemical kinetics and chemical equilibrium
In my chemistry book, the law of chemical equilibrium is derived from the law of mass action:
For a reversible chemical reaction $$\ce{aA +bB\rightleftharpoons cC + dD}$$ where $a$, $b$, $c$ and $d$ ...
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How to relate a reaction barrier to the time the reaction needs to proceed?
As I am writing this I am at a conference and one of the participants just asked a question where he linked reaction barriers to durations for the reaction to complete. To paraphrase:
From our ...
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What happens when para-water ice is suddenly melted?
Background (hydrogen)
In the case of recently liquified hydrogen (which is quite cold of course) it must be re-equilibrated before loading on to a rocket as fuel to avoid a sudden exothermic ...
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Why are equilibrium constants unitless?
I haven’t quite reached the point where I can read a full-fledged text on chemical kinetics and thermodynamics yet, so bear with me, please.
I’m wondering why a value like $K_\text{eq} = \frac{[\ce{...
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Can in any case the faster step of the reaction be rate determining?
I found a sentence in book which states the reverse step of this reaction ( I forgot what was it!) has the faster step as rate determining .
Even Rate determining step-Wikipedia states:
In ...
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Can a multi-species system oscillate around equilibrium?
In reading about chemical oscillations such as those that occur in the Belousov–Zhabotinsky reaction (BZ), it's often reported that these reactions were initially not taken seriously, because of a ...
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Is activation energy temperature-independent?
I know that activation energy for a reaction is the extra energy given to the reactants to reach the threshold energy so that they can collide and react. But then, why is it said that the activation ...
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Overall effective activation energy for parallel reactions
Today, I was solving a question from Chemical Kinetics.
Consider the following parallel, first order reactions:
$$\begin{align}
\ce{A -> P} \tag{1} \\
\ce{A -> Q} \tag{2}
\end{align}$$
with ...
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Why is proton transfer so fast?
Why is proton transfer always kinetically favored? In other words, why are Brønsted acid-base reactions so quick?
Is it because protons are generally unhindered, sterically? This seems plausible; ...
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Do coefficients matter in rate law?
For reaction $\ce{2A→B}$ (elementary step), according to the rate law, rate $= k [A]^2$.
In some calculations, we use $k[A]^2$ as the production rate of B. Why isn't it ${1 \over 2} k[A]^2$?
In this ...
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Is there a difference between equilibrium and steady state?
The term equilibrium is used in the context of reversible reactions that reach a point where concentrations no longer change. The term steady-state is used in enzyme kinetics when the concentration of ...
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Analytical solution for kinetics of bimolecular reaction
Consider two chemicals, $\ce{A}$ and $\ce{B}$ that react with each other to make $\ce{C}$ with a reaction rate $k$. The reaction can be expressed as $$\ce{A + B->C}$$ The equation expressing the ...
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Definitions of autocatalysis
Is there a formal definition or algorithm that can take a list of reactions and tell me whether it contains a set of species that can produce itself autocatalytically?
It is clear that the following ...
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Is the rate determining step the step with the largest Ea?
I've seen some controversy on this question while doing a brief search. For example, this SE answer quotes Wikipedia and says that the RDS is the step with the largest $E_a$. However, this UC Davis ...
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Does the pre-exponential factor (Arrhenius constant) depend on temperature?
I was taught that the the pre-exponential factor $A$ in the Arrhenius equation \eqref{arrhenius} does not increase with temperature.
$$k = A\cdot\mathrm \exp\left(-\frac{E_\mathrm a}{RT}\right)\tag1\...
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Can a zeroth order reaction be reversible?
After recently studying about chemical equilibrium, I was convinced that the forward and backward reaction rates meet each other at equilibrium.
However thinking about zeroth order reaction annoyed ...
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Kinetic Isotopic Effect in EAS
It is observed that rate of reaction of two isotopologues of benzene, $\ce{C6D6}$ and $\ce{C6H6}$, is the same for electrophilic aromatic reactions, as the deprotonation is not the rate limiting step. ...
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Rate Constant Units and Eyring Equation
Rearranging the Eyring equation leads to the following:
$$\Delta^\ddagger S^\circ = R \ln{\frac{k \times h}{{k_\text{B}}{T}}}+\frac{\Delta^\ddagger H^\circ}{T}$$
where $k$ is the rate constant, $h$ ...
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What is the molecularity of a reversible reaction?
I know that the definition of molecularity of a reaction is number of species reacting in an elementary step. But considering the theory of microscopic reversibility for elementary reactions, each ...
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Reaction molecularity and order
Question:
A reaction involving two different reactants can never be a
unimolecular reaction
bimolecular reaction
second order reaction
first order reaction
The answer as per my ...
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How is it that the equilibrium constant does not depend on the mechanism?
For a reaction of the form
$$\ce{aA + bB <=> cC + dD}$$
the equilibrium constant is
$$K_c=\frac{[\ce{C}]^c[\ce{D}]^d}{[\ce{A}]^a[\ce{B}]^b}$$
regardless of the mechanism of the reaction. ...
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Is it possible to make an anticatalyst?
I'm wondering if it is possible, theoretically, to create compounds which perform the opposite function of a catalyst (thus an anticatalyst). That is to say, could a compound be made which raises the ...
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What is the difference between Molecularity and Order Of Reaction?
In the field of Chemical Kinetics, terms like Molecularity and Order Of Reaction are often used. I am a bit confused between these two terms.
Can someone explain to me about these two terms and their ...
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Has IUPAC been inaccurate in their 1994 definition of kinetic stationary state?
Summary of IUPAC definition
$\def\d{\mathrm{d}}$In IUPAC recommendations from 1994[1, 1166–1167], the authors discuss the process
$$\ce{A <-->[$k_1$][$k_{-1}$] X\\ X + C ->[$k_2$] D}$$
A ...
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Is the Arrhenius Equation only valid for reactants in the gas state?
I couldn't find a source that explicitly say this, but given the gas constant R is used, is the Arrhenius equation only valid when all reactants are gases? Do they have to be ideal gases?
If the ...
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Does deuterated water slow down the overall metabolism of a cell?
Would deuterated water, being heavier, slow down the metabolic rate of the cell and subsequently the aging process?
edit: lets say I wanted to observe a cellular event, like the formation of the ...
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Difference between Reaction Rate and Rate Law?
Lets say you have the reaction
$$\ce{$a$A + $b$B -> $c$C + $d$D}$$
The rate of the reaction is given by the change in initial concentration over the change in time.
$$-\frac{\Delta \left[\ce A \...
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Noyori hydrogenation and the Curtin–Hammett principle
My question is relating to how the Curtin–Hammett principle applies to homogeneous hydrogenation using chiral ligands.
Figure 0: An example of a quadrant diagram used as a mnemonic to predict the ...
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Steady-state approximation for the destruction of ozone
As a personal problem, I am trying to find a rate law for the reaction $\ce{2 O3 -> 3 O2}$ using the steady-state approximation.
The elementary reactions are, \eqref{eq:equilib} (forward rate ...
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Are pseudo-zeroth order reactions possible?
Are pseudo zeroth-order reactions possible?
I have been taught that pseudo first-order reactions take place and they can be considered pseudo zeroth-order reactions because one reactant is in excess ...
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Will heating diatomic oxygen enough break the O=O bonds?
The bond enthalpy associated with a $\ce{O=O}$ double bond is equal to 495 kJ/mol. Does that mean that adding enough kinetic energy in the form of heat will eventually cause the bonds to break and ...
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Does a reaction have to have a rate determining step?
I am a bit confused about the concept of the rate determining step. From what I understand, a step in a reaction is the RDS if it meets the following requirements:
It is the slowest step in a ...
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Chemical kinetics of a reversible reaction
I am having trouble with the following question:
Consider the following reversible reaction in which the reaction is first order in both
directions: $$\ce{[A] <=> [B]}$$ $k_\mathrm a$ is ...
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True or false: "If a reaction has a large negative value of ∆G, then it will be a fast reaction."
I recently took a Gen. Chem. 2 exam that contained this question. I answered false, as I knew that thermodynamics and kinetics are separate matters. My professor said the answer is true. I pressed him ...
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What is the significance of the kink in the [A] vs T graph for a 2nd order reaction?
The 2nd order graph shows a bit of a kink, whereas the 1st order graph is smooth.
What is the significance of this kink, and why does it appear in only the 2nd order graph.
What is the significance ...
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Why is a third body needed in the recombination of two hydrogen atoms?
In the article Discuss. Faraday Soc. 1962, 33, 205, the authors say that in order to form molecular
hydrogen from collision to atom we need a third body to remove the excess energy. That is we have ...
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In the elementary steps of a reaction, why do we assume that the coefficients of a reactant correspond to its order?
Recently, I was taught that the coefficients of the reactants in an elementary step of a reaction indicates that reactant's order. Why is this the case? I thought that the order of a reactant was not ...
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How does an oscillating reaction work?
I watched a video showing an orange solution that goes to clear and then back to orange (and so forth.)
The reaction goes through a number of cycles before it will no longer oscillate. This tells me ...
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Determining the rate of stratospheric ozone destruction
I am a sophomore. A friend and I ran across a kinetic chemistry exercise. We solved it in a slightly different way and we can't come to an agreement on who is right. We would appreciate some help:
...
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What does "correct orientation" in a chemical reaction mean conceptually?
My chemistry teacher was talking about how there are two requirements for a reaction to occur:
The reactants must have enough energy; and
they must also have the correct orientation for a reaction ...
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Relationship between magnitudes of forward and reverse kinetic rate constants
Consider the reversible unimolecular reaction:
$$\ce{A <=>[k_1][k_2] B}$$
We know that the forward reaction is often considerably more thermodynamically favourable than the reverse reaction, ...
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