Chemical Dynamics: Lecture 1 - Chemical Kinetics!

Lecture Recording:

Lecture Summary/Notes:

OK, so basically, we started this course with chemical kinetics!

Key Questions covered in the lecture:

-What makes a reaction fast or slow?

-How do we measure or define that (the rate of the reaction)?

Kinetics, unlike thermochemistry, is very UNPREDICTABLE!

To explain this - if you look at a curved function, at every point, the tangent is a different slope, right? The energy of a reaction (E) is like a curved function, with a rising hill, and then a drop, like this!

Initially, you have the reactants (left side), then the reactants come together to form something, then they break apart to make the products (right side). At every point in this reaction, the rate is different! The point where the red star is, that's called the transition state! Something is going on there, but we don't know what! But what we DO know, is that if there are 4 reactants, A, B, C, and D, there is NO chance that they are all going to collide one another at the same time to produce the product. This point hints at the mechanism involved in the reaction.

How do we define the rate of the reaction?
**We must define the conditions first (since they are always changing!)

WAYS to define the rate:
Average rate (change in concentration over the change in time).
Initial rate (slope of the tangent at x=0).
Both the average rate and the initial rate are not very accurate ways to measure rate... Initial rate depends on the initial conditions of the reaction!

SO WHAT DO WE USE????
The rate equation (or the definition of a rate)

The overall order of a reaction is the SUM of all the orders.
The order is usually a very small integer or simple fraction!

The Zero Order Reaction (where m = 0 in R= k[A]^m)
Looks like this!

Enzyme catalyzed reactions are usually zero-order in some species!

The rate equation for the zero order equation goes something like this:

R= k[A]^0

R=k

Slope m (from y=mx+b) is -k (since the reactant is disappearing)
[A]t = -kt + [A]

(the concentration of A at any time t = -kt + initial amount of reactant)

At this point, we started talking about Radioactive Decay, which is always a first order reaction! (first order reactions are the most common in nature). That brought us somehow to the topic of the integrated rate law.

The Rate Law was integrated in order to isolate [A], which tells us the concentration of A at any given time :).
We can also use the final result to determine HALF-LIFE, which is just found by setting [A]t = [A]0/2

Reactions Involving Gas