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Chemistry PS1 6 Equilibrium

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ANSWERS to SMOG Activity
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Investigating SMOG (NO2)                       

 

Preparing the NO2 /N2O4 equilibrium mixture: 

Wear goggles and gloves + do experiment in a FUME HOOD Check out all Safety Data Sheets

 

Lab Preparation: 

REACTANTS   

mix copper + concentrated nitric acid (CARE)

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PRODUCTS are  NO2  +  Cu(NO3)2 +  H2O

 

Write the equation for the preparation of the NO2 /N2O4 mixture

 

Word equation

 

Copper   +    nitric acid   -->   copper nitrate   +   water   +   nitrogen dioxide

 

 

Add Formulas

   Cu        +     HNO3    -->      Cu(NO3)2        +     H2O   +         NO2

 

 

 

BALANCE the equation

   Cu      +     4HNO3    -->      Cu(NO3)2        +   2H2O   +      2NO2

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The nitrogen dioxide, NO2, establishes an equilibrium with dinitrogen tetroxide, N2O4

 

                         2NO2 (g)           N2O4 (g)

 

So you are seeing a mixture of the two oxides of nitrogen

 

A syringe is placed in a large beaker of hot water. 

 

Note the color of the NO2 /N2O4 mixture

-->

<--

In HOT water

COLOUR DARKENS – more NO2

in equilibrium mixture

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The exercise was repeated in ice water below.

Again note the color of the NO2 /N2O4 mixture. 

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COLOUR LIGHTENS

 

  1. In the equation:  N2O4 (g)      =       2NO2  (g)       ΔH =  +                                                                                                                        pale yellow       dark brown    

 

What can you say about the relative rates of the forward and reverse reactions?   

Dynamic equilibrium: at equilibrium, forward and reverse rates equal

 

   2. How did cooling affect the reaction?

In which direction did the reaction shift?  Explain what is happening . . .

     Colour lightens

     More N2O4 in equilibrium mixture

     Shifts to the LEFT

 

    3.  Is this in agreement with Le Chatelier’s Principle?

Hint:  think about whether heat is a reactant or a product of the reaction

   Drawing the potential energy diagram of the reaction may help you get your answer!

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N2O4 (g)      =       2NO2  (g)       ΔH =  +

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From the energy level diagram for the reaction, the energy of the NO2 product is greater than the energy in the N2O4 reactant

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Energy has to be TAKEN IN for the reaction to occur in this direction   ΔH =  + 58 kJ

This is ENDOTHERMIC in the forward direction

Heat is effectively a reactant of the forward reaction

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ADD heat and the reaction goes to the right (towards the NO2  - the darker material and away from the heat and the lighter N2O4) reinforcing Le Chatelier's Principle

Predicting:  Effect of Pressure on the Position of Equilibrium

 

  4.  What would happen to the color of the gases if the pressure was increased?

   The coefficients (balancing numbers) in the equation will help.

 

Increase pressure and favour the production of the gas with the smaller coefficien† in the equation (the smaller number of moles)—the N2O4

 

 Explain your prediction in terms of Le Chatelier’s principle? 

 

Increasing the pressure of a gas is the equivalent of increasing the concentration—increasing the concentration of one chemical (here NO2 with 2 moles for every one mole of N2O4) and the equilibrium will shift away from the increased pressure/concentration

 

 

 

EXTRA CREDIT  Briefly summarize and evaluate ONE of the web sites below.  Requirement: at least one page (bullet points acceptable) of good quality writing:-  what is appealing about the site/useful/made an impact on you/etc.

 

http://www.smogcity.com/welcome.htm

Smog and health effects at http://www.aqmd.gov/smog/inhealth.html#source

http://www.epa.gov/airnow/health/

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