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1 Return to Lab Menu Stoichiometry Exploring the Reaction between Baking Soda and Vinegar Objectives -to observe and measure mass loss in a gas forming reaction -to calculate CO 2 loss and correlate to a balanced equation -to relate molecular ratios from a balanced equation to mass ratios in an experiment Equipment/Materials balance with accompanying standard weights (e.g. cold water or pennies) extra plastic cups for balance medicine dropper and/or medicine spoon measuring spoon (¼ teaspoon) baking soda vinegar (NOTE: the procedure outline below is derived for 4% vinegar. If you use 5 % vinegar, use 4/5 the amount of vinegar in each part of the procedure.) Introduction Stoichiometry Stoichiometry is a word used by chemists to describe the quantity relationships between reactants and products in a chemical reaction. Let s look at the reaction of hydrogen gas with oxygen gas as an example. Hydrogen gas is a diatomic (2 atom) molecule with the formula, H 2. Oxygen is also a diatomic gas, O 2. When the two gases react with each other, they make water, H 2 O. We can write this as a chemical reaction: H 2 + O 2 H 2 O The problem is that we ve lost an oxygen atom in the process. There are two O atoms on the left side of the equation and only one on the right side. This violates the Law of Conservation of Matter (matter cannot be created or destroyed in a normal chemical process), which is a problem. One possible solution to this problem is to add an oxygen atom to the right side of the equation: H 2 + O 2 H 2 O + O Now the equation is balanced. However, this is not the reaction that occurs in nature. In nature water and only water is formed when hydrogen and oxygen react with each other. Let s try something else: suppose you change the formula of H 2 O to H 2 O 2 : H 2 + O 2 H 2 O 2 AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 1 of 9

2 Again the equation is balanced, but now we are even further away from what actually occurs in nature. The formula for water is H 2 O, not H 2 O 2, which is the formula for hydrogen peroxide. H 2 and O 2 could potentially react to form hydrogen peroxide, but special conditions are required, and we are interested in their reaction to form water. If you were to explore this reaction in a laboratory, you would determine that it takes twice as many hydrogen molecules as oxygen molecules to produce water: 2H 2 + O 2 2H 2 O Now we have four hydrogen atoms on both sides and two oxygen atoms on both sides so we are abiding by the Law of Conservation of Matter. The ratios of hydrogen, oxygen, and water that are represented by this chemical reaction (2:1:2) are the stoichiometric ratios. The problem that chemists have in the laboratory is that we cannot easily measure out individual molecules. We can measure mass and volume quite easily, so we ve devised a way to relate mass to numbers of molecules: molar mass. The mole is a quantity of molecules that represents a particular count. Just as a dozen is 12 or a gross is 144, a mole represents a count of 6.02 x 10 23, albeit a very large count! The mass numbers under the elements in the periodic table represent the mass of a mole of atoms of that particular element. So a mole of carbon atoms (i.e x C atoms) has a mass of g; this is the molar mass of carbon. A mole of zirconium atoms has mass of g; this is the molar mass of zirconium, etc. Let s go back to our hydrogen and oxygen example. Hydrogen, H 2, has two hydrogen atoms per hydrogen molecule. A dozen hydrogen molecules would contain 24 hydrogen atoms. Thus a mole of hydrogen molecules would contain 2 moles of hydrogen atoms. The masses would then look like this: 1 mole hydrogen atoms 1.01 g 2 moles hydrogen atoms: 2.02 g 1 mole of hydrogen molecules (H 2 ): 2.02 g Similarly for oxygen: 1 mole oxygen atoms g 2 moles oxygen atoms: g 1 mole of oxygen molecules (O 2 ): g AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 2 of 9

3 So if we want to react hydrogen (H 2 ) with oxygen (O 2 ) we would need the following kinds of ratios: 2H 2 + O 2 2H 2 O Molar mass of individual atom (from periodic table) 1.01 g g 1.01 (H) g (O) g Molar mass of molecule 2.02 g g g Molar mass of molecule x stoichiometric coefficient from balance equation 4.04 g g g sum to g Note that the masses on the left side of the equation sum to the mass on the right side of the equation right in line with the Law of Conservation of Matter. So, if we were to do this reaction in the laboratory, the ratio of masses of H 2 and O 2 would be 4.04 to We could react any of the following to yield the amount of water shown. 2H 2 + O 2 2H 2 O 4.04 g g g 8.08 g g g 2.02 g g g g + 3,200.0 g g g g g Baking soda and vinegar The reaction between baking soda and vinegar is a classic activity for young scientists. Most children (and adults!) enjoy watching the foamy eruption that occurs upon mixing these two household substances. The reaction involves an acid-base reaction that produces a gas (CO 2 ). Acid-base reactions typically involve the transfer of a hydrogen ion (H + ) from the acid (HA) to the base (B ): HA + B A + B-H acid base The base often (although not always) carries a negative charge. The acid usually (although not always) becomes negatively charged through the course of the reaction. You will notice that the reverse reaction: + B-H HA + B A base acid AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 3 of 9

4 is also an acid-base reaction with A acting as the base and B-H acting as the acid. Which direction the reaction will go depends on the relative strengths of the acids and bases involved. The reaction always proceeds from the stronger acid and base to the weaker acid and base. Stronger Weaker The chemical name for baking soda is sodium bicarbonate. It has the formula NaHCO 3. It dissociates in water to form sodium ion (Na + ) and bicarbonate ion (HCO 3 ). NaHCO 3 dissolves in water Na + + HCO 3 sodium bicarbonate sodium bicarbonate ion ion The bicarbonate anion (HCO 3 ) can act as a base. It accepts a hydrogen ion from the acetic acid (CH 3 CO 2 H), which is the acidic component of vinegar: HCO 3 + HC 2 H 3 O 2 H 2 CO 3 + C 2 H 3 O 2 bicarbonate acetic carbonic acetate ion acid acid ion The carbonic acid that is formed (H 2 CO 3 ) decomposes to form water and carbon dioxide: H 2 CO 3 H 2 O + CO 2 (g) carbonic water carbon acid dioxide The latter reaction (production of carbon dioxide) accounts for the bubbles and the foaming that is observed upon mixing vinegar and baking soda. So the overall, combined reaction is: HCO 3 + HC 2 H 3 O 2 H 2 O + CO 2 (g) + C 2 H 3 O 2 bicarbonate acetic water carbon acetate ion acid dioxide ion In this experiment, you will actually do this reaction in one of the cups on your balance. As the reaction occurs, you will monitor the change in mass as a result of the reaction. Using changes in mass to monitor a process is called a gravimetric analysis. Why might we expect the mass to change? Would you expect the mass to increase or decrease? AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 4 of 9

6 Download Data Sheet Data Sheet Name 1. Determine the molar mass of sodium bicarbonate (from periodic table). molar mass of Na x 1 mol Na = molar mass of H x 1 mol H = molar mass of C x 1 mol C = molar mass of O x 3 mol O = Sum for total molar mass 2. Determine the number of moles of sodium bicarbonate in your sample of baking soda: Run I grams of sodium bicarbonate (from experiment): molar mass of sodium bicarbonate (from #1 above) number of moles of sodium bicarbonate Run II grams of sodium bicarbonate (from experiment): molar mass of sodium bicarbonate (from #1 above) number of moles of sodium bicarbonate Run III (which was optional) grams of sodium bicarbonate (from experiment): molar mass of sodium bicarbonate (from #1 above) number of moles of sodium bicarbonate AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 6 of 9

7 3. Determine the mass of acetic used in the experiment. If your vinegar is 4 %, this means that every 100 g of vinegar contains 4 g of acetic acid. (If it is 5 %, then 100 g contains 5 g of acetic acid). Run I: 4 grams acetic acid 100 grams of vinegar =? grams acetic acid grams of vinegar This is the mass of vinegar you measured in the 1 st experiment. Run II: 4 grams acetic acid 100 grams of vinegar =? grams acetic acid grams of vinegar This is the mass of vinegar you measured in the 2 nd experiment. Run III: 4 grams acetic acid 100 grams of vinegar =? grams acetic acid grams of vinegar 4. Determine the molar mass of acetic acid, C 3 H 4 O 2. molar mass of C x 3 mol C = molar mass of H x 4 mol H = molar mass of O x 2 mol O = Sum for total molar mass 5. Determine the number of moles of acetic acid in each sample of vinegar. Run I grams of acetic acid (from #3 above): molar mass of acetic acid (from #4 above) number of moles of acetic acid AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 7 of 9

8 Run II grams of acetic acid (from #3 above): molar mass of acetic acid (from #4 above) number of moles of acetic acid Run III (which was optional) grams of acetic acid (from #3 above): molar mass of acetic acid (from #4 above) number of moles of acetic acid 6. Determine the total mass gain or loss for the reaction by comparing your initial mass (combined mass of baking soda and vinegar) to your final mass. Run I: Run II: Run III: Initial total mass Initial total mass Initial total mass Final total mass Final total mass Final total mass Difference Difference Difference 7. Calculate the molar mass of carbon dioxide. molar mass of C x 1 mol C = molar mass of O x 2 mol O = Sum for total molar mass 8. Determine the number of grams of carbon dioxide that the reaction should theoretically produce, remembering that one mole of acetic acid or sodium bicarbonate should produce one mole of carbon dioxide Run I: moles of carbon dioxide* molar mass of carbon dioxide (from #7) X grams of carbon dioxide (the product) Run II: moles of carbon dioxide* molar mass of carbon dioxide (from #7) grams of carbon dioxide (the product) X Run III: moles of carbon dioxide* molar mass of carbon dioxide (from #7) X grams of carbon dioxide (the product) *moles of carbon dioxide = moles of acetic acid (from #5 above) AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 8 of 9

9 Questions (attach answers) 1. Figure out why the mass increased or decreased. Correlate this increase or decrease to the products yielded by the reaction. Does the stoichiometry of the reaction (coupled with the results of the calculations above) account for all of the mass difference? Discuss possible reasons for any discrepancies. 2. In this particular example the change in mass through the course of the reaction provides evidence that a reaction is taking place. Is it necessary to have a change in mass in order to have a reaction? Can you provide an example of a reaction where no mass change would be observed? (Extra credit) Explain how this particular reaction might be supportive of the phlogiston theory that was popular among scientists prior to the 18 th century. AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 9 of 9

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