1. Module 7

1.22. Page 2

Lesson 5

Module 7—Principles of Chemical Equilibrium

Explore

 

In your science and mathematics courses you have come across many “constants.” A constant is a numerical value that describes a relationship between variables within a system. Sometimes constants can have units that help identify the variables they are relating. For example, in previous chemistry courses you were introduced to the universal gas constant, 8.314 L•kPa/(mol•K). You may have even completed an investigation to derive this value from the data you collected.

 

In the next activity you will derive another familiar constant from your observations—pi.

 

Try This
 
Derivation of Pi

 

Materials
  • piece of string 30 cm in length
  • ruler
  • calculator
Procedure

 

Step 1: Download the handout “Try This—Circles.”

 

Step 2: Prepare a data table consisting of four columns and six rows. Label the columns with the following headings: “Circle Number,” “Circumference, C (cm),” “Diameter, d (cm),” “Circumference Divided by Diameter, C/d.”

 

Step 3: Using the piece of string and the ruler, measure the circumference and diameter of each circle on the handout. Record each value in your data table.

 

Step 4: Calculate the circumference divided by the diameter for each circle. Record the value obtained for each circle in the appropriate cell of your data table.

 

Analysis

 

TR 1. Comment on the results of the calculations for each circle.

TR 2. Do the results demonstrate that a constant relationship exists between the variables of the circles being analyzed?

 

Read

 

The constant pi is used in most calculations involving circle geometry. When completing certain calculations involving gases, you may have used the ideal gas constant or the constants for molar volume of a gas at STP or SATP. Do systems at equilibrium each have a constant? What are the variables that describe an equilibrium? How would these variables be used to calculate a constant?

 

In your earlier investigations of equilibrium, you used colour changes to monitor whether or not a system was at equilibrium. You will recall that in an equilibrium, all species in the chemical system are present to some extent. Therefore, the equilibrium you observed was a mixture of colours of the reactant and product species.

 

Depending upon the position of the equilibrium, the colour may have been more like the products or the reactants, but it was always a blending of colours from each group. You also know from your understanding of solutions that colour intensity is directly related to concentration. For example, the higher the concentration of the oxidized form of methylene blue, the more intense the blue colour of the solution in the flask.

 

From what you have learned so far in this module, you know that some relationships regarding equilibrium are important and need to be considered in any calculation for an equilibrium constant.

 

Keep the following three important relationships in an equilibrium in mind when calculating an equilibrium constant:

  • The concentrations of each species in the system are important values.
  • The position of an equilibrium is determined by the relative proportions of products to reactants.
  • A balanced chemical equation describes the proportions of reactants and products involved in the system.