Quantitative Information from Chemical Equations

CH104  Introductory Chemistry I

INTRODUCTION

Chemical equations provide various types of information about chemical reactions we might observe in the laboratory.  Typically this includes the identities of reactants and products. Sometimes information about the conditions necessary for the reaction or energy changes during a reaction may also be included. 

Not all information in a balanced equation is qualitative, however.  Chemical equations also provide us with quantitative information.  For instance, we can figure out from an equation how much of each reactant is necessary to produce a certain amount of the product.  For example, consider the following reaction that describes the combustion of methane gas (natural gas, chemical formula CH₄):

CH₄  +  2O₂  à  CO₂  +  2H₂O

This balanced equation tells us that it takes two oxygen molecules to react with each methane molecule in this reaction. 

Of course, we are not able to measure single molecules in the laboratory. The proportion of methane to oxygen gas in the reaction remains the same, though, regardless of scale.  Just as one methane reacts with 2 oxygen molecules to produce one carbon dioxide and 2 waters, 2 methanes would react with 4 oxygens, 5 methanes with 10 oxygens, etc..  6.0 x 10²³ molecules of methane would react with twice as many oxygens.

We can weigh out about 6.0 x 10²³ molecules of these substances.  This number of molecules is an amount we call one mole.  The mass of a mole of anything equals its formula mass in grams.  (The formula mass of a substance is the sum of the atomic masses for all atoms in the formula.)  One mole of methane therefore is 16 grams, and one mole of oxygen gas weighs 32 grams.  If one mole of methane reacts with two moles of oxygen, this means that every 16 grams of methane reacts with 64 grams of oxygen to produce 44 grams (1 mole) of carbon dioxide and 36 grams (2 moles) of water.

In this experiment, we will look at the reaction between sodium bicarbonate (NaHCO₃) and hydrochloric acid (HCl in water) to produce sodium chloride, carbon dioxide and water.  The reaction we are performing is described below:

NaHCO₃  + HCl   à  NaCl  +  CO₂  +  H₂O

We will carefully measure a sample of sodium bicarbonate, perform the reaction, and then measure the quantity of sodium chloride produced.  The amount obtained experimentally will be compared to what is expected based on the chemical equation.

PROCEDURE: 

1.  Put a test tube into a 250 ml beaker, and add a boiling chip or glass bead.  Weigh and record the mass to the nearest 0.001 gram.  Tare the balance, and add a small sample of sodium bicarbonate.  Use a sample of approximately 1 gram of sodium bicarbonate; weigh the sample to the nearest 0.001 gram and record this mass.  Be careful not to spill:  if you do, carefully clean the area around the balance with a dry brush.  Take the sample back to the lab bench.

2.  Add 10% (or 3M) HCl from a dropper bottle, one drop at a time, noting any evidence of a reaction.  Add the acid slowly.  Try to get the liquid down into the bicarbonate completely.  If it sticks to the side of the test tube, tap the test tube gently to coax it down.  When there is no more evidence of reaction, stop adding acid. 

3.  Heat the test tube gently in a Bunsen burner, pointing the end away from your face (or anyone else’s!) and heating slowly.  Heat until the liquid has evaporated away.  Be thorough:  water which remains will add weight to your final measurement and throw off your result.  When completely dry, there should be no splattering, and no water vapor should be condensing at the neck of the test tube.

4.  Let the test tube cool to room temperature (at least 5 minutes).  Weigh the test tube and contents and find the mass of sodium chloride by subtracting the mass of the test tube, boiling chip and beaker from the total mass (all these items plus product). 

5.  Repeat with a new sample of sodium bicarbonate, recording all appropriate data.

ANALYSIS/CONCLUSION:

For each of the two trials, calculate:

1.)  the number of moles of sodium bicarbonate reacted

2.)  the number of moles of sodium chloride produced

3.)  the ratio of moles sodium chloride to moles sodium bicarbonate from experimental data

4.)  the percent yield, calculating this according to the following equation:

% yield = (experimental yield/theoretical yield) x 100.

Compare this experimental result with that you would expect, from looking at the reaction.  Write a short conclusion paragraph restating your result and explaining any deviation from the theoretical ratio.