Modifying Reactions

Reaction Speed

Chemical reactions happen because of molecules colliding into other molecules with enough energy to break these bonds. The speed which reactants form products is known as the reaction rate and it increases when either more collisions happen or when substances have increased kinetic energy to more easily break the initial bonds.

Once a reaction can occur, there are a few factors that can speed a reaction up:

Increasing reactant concentration (more reactants mean more possible collisions)
Increasing temperature (faster particles means more collisions)
Increasing movement of the particles (stirring or mixing means the particles are moving more quickly as well)
Increasing pressure (closer together particles means more collisions)
Increasing surface area (Solids only, more surface means more locations for reactions to happen)
Using a catalyst, a substance that isn't part of the reaction itself but makes it easier to react (Lower required energy means the extra energy can be used to quicken the reaction)

We can calculate reaction rates in terms of concentration and time from the following relationship:

Reaction Rate = Change in concentration of one reactant or product / Change in time

Equilibrium

Many reactions are reversible, having the ability to go forwards or backwards. This happens because collisions between the products and reactants take place during the reaction, making some of the products revert back to reactants. When the forward and backward reactions are happening at the same rate a system is said to be in equilibrium.

We can change where equilibrium falls by changing parts of our system. When we change these systems, they shift equilibrium to fix what changed as much as possible. This is known as Le Chatelier’s Principle.

Changing Concentration: The reaction tries to make less of what’s added or more of what’s removed.
Changing Temperature: Adding temperature means the reaction will shift towards the side without an energy term, trying to either use up more energy or make less energy.
Changing Volume/Pressure: Volume is space, so increasing it makes the shift go towards the side with more moles since it can spread out more. Pressure will do the opposite, trying to relieve the pressure by spreading out less.

Equilibrium has its own constant as well that relates the different concentrations of solutions in a reversible reaction and where their point of equilibrium lies. To calculate the equilibrium constant we use the following:

Reactions in the Body (PSA)

Biochemical reactions are reactions that involve living things, such as ourselves. These biochemical reactions happen within our bodies all the time, continuously taking in, storing, using, and releasing chemicals and energy we take into ourselves. Most of these can only happen thanks to enzymes (the catalysts of our bodies). The presence of enzymes help position reactants at the correct locations to react, help bind molecules at different active sites in the body, and can even make new pathways for a reaction to occur in the body.

When looking at all chemical reactions in the body we get a body's metabolism, or how the body changes food into energy. Catabolic reactions in this process are exothermic, taking large molecules, breaking them down, and releasing energy. Anabolic reactions are the opposite, being endothermic and using this energy and these broken down molecules to make new molecules, such as proteins, ATP, and lipids.

Release Limits (PSA)

When doing chemical processes businesses have release limits they are required to follow when making products. These products are usually for different materials that can cause health effects or environmental pollution and are enforced by the EPA and other environmental regulatory agencies. When getting rid of these pollutants a reaction can either be slowed down, the pollutant can be captured or further reacted, the pollutant can be diluted in some way, or the business can use less of the reaction making the waste product. Many of these techniques are still emerging technologies however, such as the conversion of carbon dioxide into useful materials (and other forms of carbon capture).

Additional Resources

Crash Course: Equilibrium

Interesting Links

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