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5.1 Defining Rate of Reaction

dylan black

⏱️ April 29, 2020

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🎥 Watch: Introduction to Kinetics

⚛️ What is Kinetics?

Welcome to the first part of unit 5! This unit will cover everything you need to know about kinetics, or the study of the rate of a reaction. Essentially, kinetics studies what makes reactions happen and how quickly reactions occur. You may notice that when doing labs or observing reactions, some reactions go incredibly quickly, whereas others go unbearably slow. For example, if you take a balloon and fill it with methane and put a match to it, the balloon explodes, whereas with something like hydrogen peroxide, it breaks down incredibly slowly. The role of kinetics is to help describe why certain reactions are faster than others. (PS: For those of you wanting to watch that balloon, check this out).

📈 How Do We Measure the Rate of a Reaction?

The rate of a reaction (also shorthanded as "rate of reaction") has a simple definition that when uncovered has a lot of nuance to it. The simple definition of the rate of reaction is how quickly a reaction produces products. However, this definition brings up some issues. How do we measure "how quickly" a reaction occurs? Well, we do this by observing concentrations.

As a reaction progresses, the concentration of the reactants decrease as they are used to create products. A reaction may start with an initial concentration of 0.5M but then after 30 seconds, that concentration may drop to 0.2M. Conversely, the concentration of the products will increase as they are created. Therefore, when dealing with a rate of reaction, it can be written mathematically as: Rate = -Δ[Reactant]/t or as Rate = Δ[Product]/t. The units for rate are mol/Ls, also notated as Ms^-1 or mol L^-1 s^-1. You may also see that seconds will change to hours, etc. You have to be sure to keep track of this!

Showing Rate of Reaction Graphically

Image Courtesy of CK-12

As a reaction progresses, the [Products] increases whereas the [Reactants] decreases until it reaches equilibrium. This is the point at which the rate of the reaction going forwards is the same as the rate of the reaction going backwards. This is covered in depth in the next unit, but it's good to get a taste for it in this graph. Rate graphically is the slope of the line between two points on either curve. This is because the slope of a line represents the change in [] over the change in t, which as we discussed, gives us the rate! We can see this either as an average rate or an instantaneous rate.

The instantaneous rate is the slope of the line tangent to any given point, where the average rate is between two points. To find the instantaneous rate, you need some calculus, so we won't get into it here (for calculus people, you should recognize this as a derivative. [Rate = -d[R]/dt!]).

🧪 Using Stoichiometry with Rate of Reaction

Let's suppose we had the reaction 2A + 3B --> C. This reaction is, of course, imaginary, but using examples like this helps us simplify concepts instead of jumping straight into the chemistry. Let's suppose that in 2 seconds, [A] decreased 0.2M. Therefore, the rate of the reaction in terms of A is -0.2/2 = -0.1 mol A/Ls. Using this information, let's figure out the rate at which B is being used up. We can do this with some simple stoichiometry:

The same can be applied to C to find the rate of production of C.