Week 12

This past week in AP chemistry we were introduced to the concept of equilibrium in chemical reactions. We were first told that most of the information we had learned about chemical reactions going to completion was not accurate in reality, instead, every reaction has a proportion that it will stay at once it is reached under certain conditions. This is the basis of equilibrium. Using information of temperature, volume, concentration or pressure, and constants, you can often calculate more useful information. After becoming familiar with the topic, you are also able find how a change in any characteristic of a system will affect the particles contained, as well as how you may make such a change.

Reactions under identical conditions will approach the same reaction quotient of equilibrium. A reaction quotient is equal to the product of the concentrations of the products, each to their stoichiometric coefficients, divided by the product of the concentrations of the reactants, each to their stoichiometric coefficients. This is called the Kp of a reaction under these conditions. This value is always constant for a particular reaction under constant temperature. Temperature is the only value that can affect the Kp of a reaction, while many others still affect the system as a whole. In the same way, you are able to find the Qp of a system. This value tells you the current place on the equilibrium scale and with it you can find which way the reaction will shift.

Even in equilibrium, reactions never stop. Reactants and products are continuously being converted between each other. Only in equilibrium are the rates of each direction equal, resulting in a constant concentration for each group. This is why adding or removing any reactant or product to a system results in a significant change in the proportions of each. Adding reactant will make a reaction shift to the right, toward the products, while adding product will make a reaction shift back to the left, toward the products. Removing either of these two does just the opposite. In the case of adding one of the reactants, you are able to push the reaction further, decreasing the remaining amount of the other reactants, while increasing the amount of the one you are adding. Chemical engineers can use this technique to save money, pushing more of an expensive reactant through with an inexpensive one (see link below for practice).

An  illustration of manipulating conditions to obtain desirable products

In class we were given a challenge, requiring us to use our prior knowledge in combination with the topics we have just learned. We were given a reaction and a yield and told to find the reactant quotient of the reaction and draw a diagram. I found this exercise very useful in improving my ability to draw together larger concepts to find useful and more realistic values in the real world.

Practice with manipulating reactions through conditions
http://www.files.chem.vt.edu/chem-ed/courses/equil/intro/lechatel.html