Unit 7: Acids and Bases Station #1
Analysis of Commercial Antacid Tablets
Instructions: Work in a group of 2 or 3 people. Every person must answer all the questions on their own sheet of paper. Your group will share one set of investigation materials.
Hydrochloric acid (HCl) is one of the substances found in gastric juices secreted by the lining of the stomach. HCl is needed by the enzyme pepsin to catalyze the digestion of proteins in the food we eat. Heartburn is a symptom that results when the stomach produces too much acid (hyperacidity).
Antacids are bases used to neutralize the acid that causes heartburn. Despite the many commercial brands, almost all antacids act on excess stomach acid by neutralizing it with weak bases. The most common of these bases are hydroxides, carbonates, or bicarbonates. The following table contains a list of the active ingredients found in several common commercial antacids, and the reactions by which these antacids neutralize the HCl in stomach acid.
Compound / Chemical Formula / Chemical ReactionAluminum hydroxide / Al(OH)3 / Al(OH)3(s) + 3 HCl(aq) -----> AlCl3(aq) + 3 H2O(l)
Calcium carbonate / CaCO3 / CaCO3(s) + 2 HCl(aq) -----> CaCl2(aq) + H2O(l) + CO2(g)
Magnesium carbonate / MgCO3 / MgCO3(s) + 2 HCl(aq) -----> MgCl2(aq) + H2O(l) + CO2(g)
Magnesium hydroxide / Mg(OH)2 / Mg(OH)2(s) + 2 HCl(aq) -----> MgCl2(aq) + 2 H2O(l)
Sodium bicarbonate / NaHCO3 / NaHCO3(aq) + HCl(aq) -----> NaCl(aq) + H2O(l) + CO2(g)
Read the information above and answer the following questions on your own sheet of paper.
(1) Look at the ingredients of Tums. What is the active ingredient in Tums?
(2) What is the name and chemical formula for the acid found in the stomach?
(3) Find the reaction in the chart that occurs with stomach acid and Tums and write it on your paper.
(a) Label the reactants as an acid and a base.
(b) Draw a box around the salt and a triangle around the gas that is produced when Tums reacts with stomach acid.
Investigation: Perform the investigation as a group. Use one Tums tablet per group.
Procedure: Pour 40 mL Vinegar in a small beaker. Test the pH with pH paper. Record the result. Place one tums tablet inside. Observe for at least two minutes.
· (Question 4) Describe what you observed (see, hear, smell) in at least 5 sentences.
· Test the pH with pH paper again. Record the result.
· (Question 5) Did the pH change? Did the solution become more basic or more acidic? Why?
Unit 7: Acids and Bases Station #3
Chemistry and Cooking
Instructions: Work in a group of 2 or 3 people. Every person must answer all the questions on their own sheet of paper. Your group will share one set of investigation materials.
Part 1: Introduction
What makes breads, muffins, cakes, and cookies rise? The gas CO2 is released by various processes, and it is this gas that makes the bubbles you see in bread.
Observe: Examine the piece of bread at this station.
(1) Draw a portion of bread. Show the ‘bubbles’ that were formed in the bread.
(2) What gas formed those bubbles?
Part 2: Learn more. Read the following excerpt to learn more about where that CO2 came from . . .
Many recipes use no yeast. Things like muffins, biscuits, cakes and cookies usually use baking powder instead [to produce CO2]. Baking powder is normally made of three different parts:
· An acid
· A base
· A filler of some sort
All three need to be dry powders that can be mixed together. For example, baking soda (a base), cream of tartar (an acid) and corn starch (the filler) are three common ingredients.
Baking soda, also known as sodium bicarbonate, has the chemical formula NaHCO3. Cream of tartar, also known as tartrate salt, has the formula KHC4H4O6.
The reaction is:
NaHCO3 + KHC4H4O6 ----> KNaC4H4O6 + H2O + CO2
(Question 3) Copy the chemical reaction equation. Label the reactants as an acid and a base. For a clue, look to the paragraph above. Label the products by drawing a box around the salt that is produced and a triangle around the gas that is produced.
Some baking powders contain sodium aluminum sulfate: NaAl(SO4)2. The reaction there is:
NaAl(SO4)2 + 3 NaHCO3 ----> Al(OH)3 + 2 Na2SO4 + 3 CO2
(Question 4) Copy the chemical reaction equation. Label the reactants as an acid and a base. For a clue, look to the paragraph above. Label the products by drawing a box around the salt that is produced and a triangle around the gas that is produced.
Part 3: Investigation. In chemistry class, we performed the experiment where you mixed baking soda (a base) and vinegar (an acid) and got a bubbling reaction. Baking powder works the same way. When you add water to baking powder, the dry acid and base go into solution and start reacting to produce carbon dioxide bubbles.
· Single-acting baking powder produces all of its bubbles when it gets wet.
· Double-acting baking powder produces bubbles again when it gets hot.
Procedure: Add a small scoop of soda to the beaker. Add 100mL warm water. (As long as the baking powder is fresh, you will definitely see the reaction!) Observe for at least two minutes. Pour down the drain to clean up.
(Question 5) Describe what you observed (see, hear, smell) in at least 5 sentences.
Conclusion: Many recipes call simply for baking soda rather than baking powder. Usually these recipes use some kind of liquid acid like buttermilk or yogurt to react with the baking soda to produce the bubbles.
The reason why people often prefer baking powder to yeast is because yeast takes so long -- usually two to three hours -- to produce its bubbles. Baking powder is instant, so you can mix up a batch of biscuits and eat them 15 minutes later.
Now that you understand how baking powder works, you can understand two things you often see in recipes:
· Many recipes instruct you to mix all of the dry ingredients together and then add the liquid. That keeps the baking powder from reacting until the end of the mixing process.
· Many recipes tell you to mix only briefly -- just until the ingredients are moistened. That minimizes the escape of the gas from the batter. If you were to stir for a long time, the reaction would end and the stirring would have allowed the bubbles to escape.
(Question 6) Why add liquids at the last minute when making muffins or cakes?
Unit 7: Acids and Bases Station #2
Acid/Base Calculations
Instructions: Work in a group of 2 or 3 people. Read the following introduction to pH calculations and answer the reading questions. Then, choose at least 6 problems to solve on your own sheet of paper. Show all your work and label your answers.
Introduction:
In 1909, Danish biochemist S.P. L Sorensen introduced a system in which acidity was expressed as the negative logarithm of the Hydrogen (H+) ion or Hydronium (H3O+) ion concentration. In this way, the acidity of a solution having [H3O+] of 10-4 M would have a value of 4. Today we know this system as the pH scale. pH stands for “power of hydrogen” because the power of 10 was part of the number. The formula we use to calculate the pH from the [H3O+] is:
pH = -log[H3O+] (The opposite equation is: [H3O+] = 10-pH)
Similarly, the value of pOH (power of hydroxide) equals the negative logarithm of the [OH-]. Bases dissociate to form hydroxide ions, so this equation is helpful when working with bases.
pOH = -log[OH-]
Water molecules interact with each other and ionize. At the same time, the ions in solution reform molecules of water. This is demonstrated by the following reversible reaction:
H2O(l) + H2O(l) ↔ H3O+(aq) + OH-(aq)
In pure water, the concentration of hydronium ions and hydroxide ions are always equal. At 25 degrees Celsius, both [H3O+] and [OH-] are 10-7. This information is used to derive the following formulas:
[H3O+]*[OH-] = 1.0 x 10-14 and pH + pOH = 14
Reading Questions
(a) What equation can you use to convert from pH to pOH or pOH to pH?
(b) What equation can you use to calculate the pH of a solution if given the hydronium ion concentration?
(c) What equation can you use to calculate the hydronium ion concentration of a solution if give the pH?
(d) What equation can you use to calculate the hydronium ion concentration if given the hydroxide ion concentration?
Practice Problems
The hydroxide ion concentration of an aqueous solution is 1.0 x 10-5. What is the hydronium ion concentration? What is the pH? What is the pOH?
A HCl solution has a concentration of 0.00010 M. Calculate [OH-] and [H3O+] for this solution. HCl is a strong acid, so assume it is 100% ionized.
Calculate the [OH-] and [H3O+] in a 0.001 M solution of HNO3, a strong acid.
Calculate the pH and pOH of a 1.0 x 10-7 M solution of acetic acid. Acetic acid is a weak acid. Assume 10% dissociation.
Determine the pH and pOH of a 0.001 M solution of NaOH, a strong base.
What is the pH of a solution that has a hydronium ion concentration of 1.0 M? What is the pOH?
What is the pH of a 10.0 M solution of HCl, assuming the acid remains 100% ionized? What is the pOH?
A solution of citric acid has a pH of 4.0. Calculate the pOH, [OH-] and [H3O+].
A solution of citric acid has a [H3O+] of 5.0 x 10-4 M. What is the [OH-]?
A solution of acetic acid has a [H3O+] of 3.0 x 10-3 M. What is the [OH-]?
A solution of sodium hydroxide has a [OH-] of 7.0 x 10-3 M. What is [H3O+]?
Challenge Questions
What is the [OH-] and [H3O+] of a solution formed by mixing 80.0 g of NaOH (strong base) in 40.0 L of water?
What is the [OH-] and [H3O+] of a solution formed by mixing 180.0 g of HCl (strong acid) in 10.0 L of water?
Acids and Bases Station #4
What Happens When the Ocean Absorbs Carbon Dioxide From the Atmosphere?
Instructions: Work in a group of 2 or 3 people. Every person must answer all the questions on their own sheet of paper. Your group will share one set of investigation materials.
Introduction: The world’s oceans play a vital role in keeping the Earth's carbon cycle in balance. As people add more greenhouse gases to the atmosphere by burning fossil fuels, the oceans respond by absorbing more CO2. This experiment mimics CO2 absorption by oceans by blowing air into a jar of water and indicator. As you exhale CO2 into the cabbage juice water, some of it will be absorbed, become dissolved, and change the pH of the water. Your task is to determine whether adding carbon dioxide to the ocean makes it more acidic or more basic.
Procedure:
1. Observe the acid, base, and control solutions on the counter. Record the color of the acid solution, the basic solution, and the control solution. These solutions were made by adding an acid or a base to a indicator solution. The indicator used was cabbage juice. An indicator is a substance that changes color based on its pH.
2. Copy the following chemical reaction equations for the absorption of carbon dioxide in water.
CO2 (aq) + H2O ↔ H2CO3
H2CO3 ↔ HCO3- + H+ ↔ CO32- + 2H+
3. Based on the equations shown above, draft your hypothesis. Will the increased carbon dioxide from my breath make the cabbage juice more acidic (pinkish) or more basic (greenish)?
4. Obtain a beaker of cabbage juice water, a straw, and a plastic bag or piece of foil.
5. Use the foil or plastic bag as a lid on your beaker to prevent spilling.
6. Choose one person from your group to use the straw to blow air bubbles into the cabbage juice water. This mimics putting carbon dioxide into the ocean.
7. Blow bubbles for about five minutes or until the liquid changes color. Record the color change.
8. Clean up -- Pour the cabbage juice down the sink. Place the beaker on the counter. Throw away the straw.
Analysis:
1. Did the pH of the solution change when you blew carbon dioxide into it?
2. Did the solution become more acidic or more basic? How do you know?
3. Use the chemical reaction equations shown above to explain your results. In your response, use as many of the words as you can: acid, base, conjugate acid, conjugate base, hydrogen ion concentration, pH, dissociation, proton donor.
4. Write the chemical reaction equation for the dissociation of carbonic acid (H2CO3) and water. Label the acid, base, conjugate acid, and conjugate base. Draw an arrow showing the transfer of a hydrogen ion.