Kinetics and Reaction Rates
Kinetics and Reaction Rates
Chemical Kinetics — the area of chemistry concerning reaction rates or, the “rate of a reaction.”
In the first semester of General Chemistry we learned how to write and balance chemical equations, and using stoichiometry we learned how to determine how much reactants were consumed and how much products were formed.
We learned nothing about how fast those reactions were occurring.
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Why is chemical kinetics so important?
The “spontaneity” of a reaction has nothing to do with how fast a reaction occurs.
While spontaneity refers to a reaction’s tendency to occur, a reaction must occur at a reasonable rate to be useful.
25 million tons of ammonia are produced annually.
So how come ammonia (NH₃) is not produced by the following inexpensive method, where the reactants (N₂ and H₂) are cheap and readily available?
As stated above, thermodynamically this reaction is favorable at 25°C, but the process ( reaction rate) is so slow that the 2 reactants essentially coexist indefinitely.
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Reaction Rates
Consider the reaction presented below.
Table 12.1 shows the concentrations of reactant and products over time, and the data is plotted below in Figure 12.1.
Reaction Rate — the change in concentration of reactant(s) or products per unit time.
In the reaction above, we have:
2NO₂(g) ➝ 2NO(g) + O₂(g)
We can write the reaction rate, or “rate” in terms of any of the reactants or in terms of any of the products.
To be consistent however, it’s customary to represent the reaction rate in terms of the reactant(s).
That’s why you see I’ve boxed up and triple-starred the last equation above.
Here it is again:
rate = -Δ[NO2] / Δt
An Examination of Figure 12–1 Above.
At any single point on the NO₂ curve, we can calculate the instantaneous rate of the reaction.
The instantaneous rate is calculated by finding the slope (m) of the line tangent to the curve at the specific time (t).
ex: Calculate the instantaneous rate of the reaction after 100. seconds has elapsed since the reaction started.
Here are the results of that calculation:
Now calculate the average reaction rate at which the [NO₂] reactant changes over the first 50. seconds of the reaction.
For the average reaction rate — we’ll need to use Table 12.1 above to look at the (x,y) data points at x = 0 and x = 50. seconds.
Like this:
**NOTE: Reaction Rate is not constant but decreases over time, as the reactant(s) concentration goes down. In Table 12–2 below, we see that the lesser the amount of reactant remaining, the slower the reaction rate:
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