Determining a Chemical Equation

 

                As we have already discussed, chemical reactions occur with the breaking of chemical bonds, the rearrangement of atoms, and the formation of new chemical bonds.  So, what do chemists do to determine how the atoms rearrange, and what products are formed from the reaction?  As always, we look for observations to help us deduce the behavior of matter.  This week, we are presented with the problem of determining the products of a common kitchen chemical reaction; the decomposition of baking soda with heat.  (Very important when baking chocolate chip cookies!)  Baking soda is sodium bicarbonate, and could undergo one of the following decomposition reactions: 

 

NaHCO₃(s)    à     NaOH(s)     +     CO₂(g)

 

2 NaHCO₃(s)   à      2 Na(s)   +   H₂(g)    +   2 C(s)   +   3 O₂(g)

 

2 NaHCO₃(s)    à      Na₂CO₃(s)    +     CO₂(g)    +     H₂O(g)

 

2 NaHCO₃(s)    à       Na₂O(s)       +     2 CO₂(g)     +    H₂O(g)

 

where (s) indicates a solid and (g) indicates a gas.  The coefficient in front of the molecular formula indicates the number of that type of molecule involved in the reaction.  No number means one molecule of that type is involved.

                Come to your laboratory session with at least two methods to determine which of these reactions takes place. Your lab instructor must approve your methods before you may begin your experiments.  Here are a few hints to help you get started: 

1.       Consider the products that are formed and the properties of these products.

2.       Mass may be a molecular property you may want to consider.  Can atomic mass be used to determine a molecular mass?  Could molecular mass be helpful in differentiating between products? 

 

                In the second part of this experiment, we will re-visit the chemical reaction we performed a couple of weeks ago; the combination reaction between the elements magnesium and oxygen.  (Your laboratory instructor will show you the balanced chemical equation representing this reaction.)  Predict the mass of magnesium oxide formed from the mass of magnesium you used in that experiment week.  Compare the predicted (calculated mass) to your observed (experimental) mass.  Determine the per cent difference by taking the difference between your predicted value and the experimental value.  Divide this difference by the predicted value.  Multiply by 100.  Put your data and results into a neat table.  You will not be repeating this experiment, just performing the calculation and comparing the predicted value to the value you found last week.  Don’t forget to bring that data with you to lab. 

               

                Answer the following questions in your conclusion: 

1.       Can the mass of the products of any chemical reaction be predicted?  If so, how; if not, why not? 

2.       How many grams of aluminum are formed from 10.0 grams of aluminum oxide given the following chemical equation? 

 

2 Al₂O₃ (s)     à     4 Al(s)     +     3 O₂(g)

 

Using circles (o) for aluminum atoms, dots (·) for oxygen atoms, and lines for chemical bonds, draw a molecular picture of this reaction.  What type of chemical reaction is this?