Natural Science #11: Kinetics

In light of the NFL season starting today, let’s consider the action that occurs around a line of scrimmage. A defensive line crashes into an offensive line – hoping to find a gap that allows access to the quarterback, while simultaneously trying to close gaps that allow access for the running back. The offensive line does the same thing but in reverse. Most of the time, the two lines crash in a stalemate – no quarterback is sacked and no rushing yards are gained.

But if we are talking about the Seattle Seahawk’s defensive line, they probably crashed into the offense in just the right way to cause a reaction that would end with a quarterback laying on the ground. If we are talking about the Dallas Cowboy’s offensive line, they probably crashed into the defense in just the right way to allow Ezekiel Elliot a passage into the end zone.

Chemical reactions basically work the same way. Atoms and molecules flow around the environment and occasionally bump into each other. Like with football, these collisions usually result in nothing happening. But if they collide in just the right way – with enough force and at the right angle – a chemical reaction occurs. Today’s post will explore kinetics, the science of how those collisions affect the rate of reactions.

Activation Energy

Every reaction, whether on the football field or in the lab, has a minimum amount of energy necessary to get it started. This threshold is known as the activation energy. Imagine a ball trying to roll up a volcano and fall into the mouth of it. If it only has enough energy to get up half way, it just rolls back down. If it starts at the wrong angle, it will just circle the volcano instead of going to the top. However, if it starts off fast enough and in the right direction, it will reach the top and fall in.

Consider oxygen and carbon, two elements that love to combine. Carbon can be found in wood, and oxygen can be found in the atmosphere. Place a piece of wood outside and watch it. Nothing happens. But if you add energy to the equation, by heating up the molecules in the wood, they might reach the speed nessesary for a carbon atom to climb up the volcano and snap together with an oxygen atom. When they snap together, they vibrate and bump into other atoms, causeing those atoms to speed up and snap together with other atoms. A chain reaction occurs. We have a fire.

In this example, a hydrocarbon (wood, coal, oil, etc) collided with oxygen at just the right speed to cause a reaction which produced carbon dioxide, water, and heat. CH2O + O2 -> CO2 + H2O. A plant will eventually use the sun’s energy to recombine that water and carbon dioxide back into wood and oxygen. We just summarized the carbon cycle.

Rate Laws

After activation energy has been satisfied, the reaction’s rate will vary wildly based on temperature, pressure, concentration, and other factors. The reaction rate can be defined as the rate at which the concentrations of the reactants decrease. In the previous example, it would be the rate at which CH2O + O2 decreased. Since every reaction is different, we have determine the reaction rate experimentally. We use the Rate Law, a mathematical relationship between concentrations of the reactants and the rate of the reaction, to figure this stuff out.

The Rate Law:
rate = k[A]n[B]m

k is some constant
A is the concentration of a reactant
B is the concentration of another reactant
n and m are some power

In our example, rate = k[CH2O]n[O2]m. We can start testing different starting concentrations of CH2O and O2 to see what we get. For example, we can hold CH2O constant and double O2. This might lead to a reaction rate that is twice is fast (n^1). If we hold O2 constant and double the amount of CH2O, we might find that the reaction is eight times as fast (n^3). Using the rate law, we get:

rate = k[CH2O]3[O2]1

Catalysts

When we want a reaction to occur at a faster rate or at a lower temperature than it otherwise would, we use a catalyst. A catalyst is a chemical that speeds up a reaction without actually entering into it. For example, a catalytic stove uses a catalyst to reduce the temperature that smoke catches fire from 1100 degrees to 500 degrees.

In the human body, enzymes catalyze every chemical process that keeps us alive. They are kind of a big deal.

In investing, a catalyst is a corporate event that will unlock value. Without them, you may get the idea right, but the timing can still kill you.

Conclusion

The world would be pretty boring if stuff didn’t bump into other stuff and cause even more stuff to happen. A game of football would be pretty boring if the line of scrimmage never moved.

Motion and reaction keeps the world interesting. The study of kinetics investigates how different experimental conditions will influence the speed of a chemical reaction. Rate laws show the mathematical relationship between concentrations of reactants and the speed of a reaction. Catalysts are used when the natural rate of a reaction isn’t going to cut it.

References:
https://en.wikipedia.org/wiki/Chemical_kinetics
https://en.wikipedia.org/wiki/Combustion
https://youtu.be/7qOFtL3VEBc

 

 

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Author: David Shahrestani

"I have the strength of a bear, that has the strength of TWO bears."

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