Chemistry 11 Lab #19

 

Reaction Equilibrium Revisited

 

 

Theory:

 

The general reversible reaction can be written:

 

aA + bB ↔ cC + dD

 

where the capital letters stand for the chemical substance (e.g. A could be H₂O and B could be CO₂) and the lower case letters represent the coefficients of the balanced equation. 

 

The expression for the equilibrium constant, K_eq, for the reaction is:

 

                                                                        K_eq = [C]^c[D]^d/[A]^a [B]^b

 

where you ignore the concentration term (the thing with square brackets) for liquids and solids AND the value of the constant is different at different temperatures.

 

We have done lots of problems using this for solutions where the concentration is commonly used, but I have also mentioned that you can define (and change) the concentration of a gas in a similar way:

 

                                                                        [gas] = moles gas/volume gas

 

But we used these concepts (moles of gas and volume of gas) in our discussion of the ideal gas law:

 

                                                                        PV = nRT

 

Where n represents moles and V volume (P pressure, T temperature and R the gas constant).  Using this notation we see that the concentration of gas can be rewritten:

 

                                                                        [gas] = moles gas/volume gas = n/V = P/RT                 

 

BUT……..

 

Since R is a constant and we define equilibrium constants at a particular temperature (T is constant) which means when we deal with gases in the equilibrium constant expression we can use the pressure, P IN PLACE OF THE CONCENTRATION.  You will see this done in the online labs below.

 

Procedure

Go to the Virtual Chemistry Experiments and Exercises web site and run through the simulations available there on chemical equilibrium.

1. Run the first experiment from the Chemical Equilibrium section (Chemical Equilibria: Basic Concepts)  and use the graph and what you know about equilibrium to:
1. write an expression for the equilibrium constant.
2. evaluate the equilibrium concentrations of the reactants and products
3. calculate the equilibrium constant for the reaction.
2. Run the second experiment (Chemical Equilibria: Equilibrium Constant) and follow the procedure there.
3. Run the rest of the experiments in the section (for a total of six experiments) following the procedures there.