chemical kinetics Flashcards
How can you measure the concentration of reacants during a reaction?
you can take in-situ measurements (inside the reaction vessel) or ex-situ measurements (outside the reaction vessel.
For ex-situ measurements, we either need to stop the whole reaction, then measure the concentration, or take a sample from the vessel and stop the reaction in that sample.
What 4 methods could be used to take in-situ measurements?
Many methods of in-situ measurement of concentration exist, including: Observation, Spectroscopy, Chemical sensor, Gas pressure.
What are the units for rate constant?
for 0th, 1st, 2nd, and 3rd order reactions
Overall Reaction Order Units of k
0 = mol dm–3 s–1
1 = s–1
2 = mol–1 dm3 s–1
3 = mol–2 dm6 s–1
What is an Sn1 reaction, and whats the reaction order?
Substitution Nucleophilic
Unimolecular. It only involves one molecule, therefore is first order. It is a two-step reaction.
What is an Sn2 reaction?
Substitution Nucleophilic
Bimolecular. This step involves two molecules, so the reaction is overall
second order. It is a one-step reaction.
What is integrated rate laws?
These tell us the shape of the curves for different orders in reactions with a single reactant. We plot the data in different ways to test for each order.
If the rate depends upon only one reactant, we can solve
this equation for the rate to get an equation for the timedependence of the concentration of this single reactant.
What is the initial rates method?
For slow reactions, we measure the initial rate at several different starting concentrations. A single plot of the data gives us the order.
ln(R0) = ln(k) + q ln([A]0)
A plot of ln(R0) against ln([A]0) will give a straight line with gradient q and intercept ln(k).
What is the isolation method?
This allows us to determine the orders for reactions with multiple reactants.
We increase the concentration of all reactants except one sothey are in large excess. We then measure the concentration of the remaining reactant during the reaction. to get the rate law: k’ = k [B]0^y.
With this new rate law, we can use the methods already discussed to find the value of x, the order with respect to A.
What does a plot of a first order reaction give?
If we plot ln([A]) against time, a first-order reaction will give a straight line with gradient –k and intercept ln([A]0).
What does a plot of a second order?
If we plot 1/[A] against time, a second-order reaction will give a straight line with gradient k and intercept 1/[A]0.
Describe what a half life is and what the formula is.
The half life of a reaction is the time taken for the reactant concentration to be halved.
The concept is only useful for first-order reactions because the half life is constant.
t1/2= ln(2)/k
What is pseudo reaction order?
If we find that this is first order then, under conditions where all the other reactants are in excess, we can say that the reaction is pseudo-first order.
What is the activation energy? Does temperature affect it?
This is the energy barrier that must be overcome when molecules collide for the reaction to occur. Typical values of Ea are in the tens to hundreds of kJ/mol.
At this higher temperature, more molecules have energy greater than Ea.
In the Arrhenius equation what is the pre exponential factor?
It depends on two factors.
The first considers how frequently collisions occur between molecules.
The second is the steric factor.
The collision frequency term will typically give a very large number while the steric factor can vary between a very small
value and 1. A small value for the steric factor means a very specific geometry is required. A steric factor of 1 means any collision can lead to the reaction (assuming enough energy is present). The pre-exponential factor can have typical
values of between 10^7 and 10^15.
What does an Arrhenius plot look like?
ln(k) = ln(A) – Ea/R T
If we plot ln(k) against 1/T we should get a straight line with intercept ln(A) and gradient –Ea/R. From these, we can calculate A and Ea
