Questions tagged [kinetics]
The study of rates and steps in mechanisms of chemical processes.
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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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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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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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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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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 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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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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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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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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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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Are there any reactions with no activation energies?
Are there any reactions with no activation energies? Our professor just told us there are no chemical cliffs. Is this true?
I read something about nuclear decay as being a reaction with no ...
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Prove that a 10-Degree Temperature Increase Doubles the Rate Constant (k), when the Activation Energy is Approximately 50 kJ/mol
I read that increasing the temperature by $10~^\circ\mathrm C$ will double the rate constant ($k$), when the activation energy for the reaction is relatively close to $50~\mathrm{kJ/mol}$.
However, ...
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Y-axis of the reaction co-ordinate graph
In the graph of the reaction co-ordinate, is the $y$-axis that is used $∆H,$ $∆U$ or $∆G$?
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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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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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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 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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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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Is negative activation energy possible?
Is negative activation possible? And, in coupled reactions is there any difference?
Because I saw in the paper - Chemical Engineering Science 1996, 51 (11), 2995–2999 - the following conclusion:
...
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Belousov-Zhabotinsky reaction: questions about rate determining step, k and activation energy
In one of our lab courses, we performed a variation of the BZ reaction, the experimental procedure is outlined below:
Experimental Procedure
10 mL of 3.0 M $\ce{H2SO4}$ was added to test tube T1.
...
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Law of mass action
If you have the forward reaction
$$\ce{2X ->[K] P}$$
which of the following systems of differential equations would model the reaction's kinetics?
$$\begin{array}{rl} \dfrac{\mathrm{d}[\ce{X}]}{...
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Why is the rate of a reaction proportional to the concentrations of reactants raised to their stoichiometric coefficients?
Consider a gaseous state elementary reaction $$\ce{aA(g) + bB(g)} \overset{k_\mathrm{f}}{\underset{k_{\mathrm{b}}}{\ce{<=>}}}\ce{ cC(g) + dD(g)}$$
I know that for this reaction, $$\Delta G = \...
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Why are the stoichiometric coefficients the powers in the rate law?
Generally, for a reaction of the form
$$\ce{$n$_A A + $n$_B B + \dots -> products},$$
the rate law is given by the following:
$$\text{rate} = k [\ce{A}]^a [\ce{B}]^b.$$
Do the stoichiometric ...
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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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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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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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Unit consistency in rate equations
I suppose that my problem is not one of great profundity, but it is an annoying one. The problem is related to the measurement units involved in rate equations of different order. Not being a chemist ...
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How exactly is activation energy defined?
In a common interpretation of the Arrhenius rate equation
$$k = A\exp\left(-\frac{E_\mathrm a}{RT}\right),$$
the activation energy $E_\mathrm a$ is understood to represent the difference in the ...
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Dependence of rates of neighbouring group participation on length of alkyl chain
On the topic on neigbouring group participation, it is mentioned in Carey & Sundberg (2007)[1] that the effectiveness of the participation is dependent on on the ease with which the molecular ...
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Understanding and usage of Arrhenius equation
A widely used rule-of-thumb for the temperature dependence of a reaction rate is that a ten degree rise in the temperature approximately doubles the rate. This is not generally true, especially when a ...
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What is the correct definition of the Gibbs free energy of activation?
Is the following correct?
Gibbs free energy of activation is used in energy profiles where the stabilities of the species are expressed as changes in Gibbs energy, while the activation energy Ea is ...
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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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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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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 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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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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Why is the equilibrium constant defined that way intuitively?
Suppose we have chemical equation: $$\ce{ aA + bB <=> cC + dD }$$
then equilibrium constant is defined: $$K=\frac{[C]^c[D]^d}{[A]^a[B]^b}$$
but why don't we define it as: $$K=\frac{cd[C][D]...
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How can a catalyst not be included in a rate equation if, by definition, it speeds up a chemical reaction?
I thought that anything not in a rate equation was automatically zeroth order and therefore did not affect the reaction. However, I have heard that catalysts can be involved in a reaction while not ...
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Does a first order reaction really get completed only at infinity?
For a $1^{\text{st}}$ order reaction $ A \rightarrow B$, the corresponding rate law is: $$\text{rate} = k[A]^{1}$$ and the integrated rate law for this is $$ \ln \left(\frac{A}{A-x}\right) = kt$$
...
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How to derive the rate laws for three consecutive reactions?
Given the following three consecutive unimolecular reactions $$\ce{A ->[$k_1$] I_1 ->[$k_2$] I_2 ->[$k_3$] P}$$ write the rate of change for each.
I have gotten the following:$\newcommand{\...
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How does the inductive effect, explain the reactivity of tertiary alcohols with halogen acids?
The reactions of primary and secondary alcohols with halogen acids ($\ce{HX}$, where $\ce{X}$=halogen) require the presence of a catalyst, $\ce{ZnCl_2}$. With tertiary alcohols, the reaction is ...
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Why can't the half-life be determined in a reaction with more than one reactant?
We have a second order irreversible reaction with 2 reactants (A and B, order 1 for both) so that the initial concentrations are different. I've read that the half-life doesn't make sense in this ...
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Representing woody biomass concentration for kinetic reactions
Woody biomass is composed of cellulose, hemicellulose, and lignin components. The following diagram provides representative formulas for each component:
Biomass can be represented by it's density, ...
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Half-life equation for 2nd order kinetics
My friends and I were doing some problems from this year's IChO Preparatory Problems (PDF from the 49th International Chemistry Olympiad (2017)) when we stumbled upon a question which we had some ...
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Which step below is rate determining
What is the rate determining step in the following energy profile? To clarify, the reaction is:
A -> B -> C
The energy of A is greater than B which in turn is also greater than C. The intermediate A-...
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How to find the average half life of radioactive nuclide which undergoes two different decays?
Find the average life of a radio nuclide which decays by parallel paths,
\begin{align}
A &\rightarrow B\\
2A &\rightarrow B,
\end{align}
where the decay constants are $\lambda_1 = \pu{0....
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Is the kinetic energy turning into activation energy when the reaction takes place? [closed]
So I want maybe a simple question that I had on a test today. For a reaction to take place it needs a minimum energy called activation energy. At school I heard that this energy is coming from the ...
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Conservation equation in an enzyme-catalysed reaction
Consider the reaction scheme:
$$\ce{S + E ->[k_1] C1} \qquad \ce{C1 ->[k_2] E + P} \qquad \ce{S + C1 <=>[k_3][k_4] C2}$$
where $\ce{S}$ is the substrate, $\ce{E}$ is the enzyme, $\ce{P}$ ...
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Formation of hydrogen bromide: Does the concept of "order" apply to this reaction?
Started reading Levine's Physical Chemistry [1] yesterday, and I hit upon (another) wall.
On page 517, Chapter 15, she includes a reaction resulting in the formation of $\ce{HBr}$ from $\ce{H2}$ and $\...
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Equilibrium constant for heterogeneous equilibria having aqueous as well as gaseous reactants
Suppose we have a heterogenous equilibrium :
$$\ce{A(aq) +B(aq) <=> C(g) +D(aq)}$$
Which equilibrium constant is used here?
Both pressure and concentration terms are there. So, according to me, ...
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