Acid-Base Equilibria
Unit Overview
This core unit provides many opportunities to consider environmental issues. Acid precipitation is only one of the applications which could be considered, through case studies, independent research and laboratory investigations. Others may include the production of acids and bases and their use in industrial processes.
Students should observe both physical and chemical properties of acids and bases to help them understand the operational and conceptual
definitions those classes of substances. In this unit, students apply the principles of quantitative analysis while considering dissociation, conjugate pairs, the common ion effect (especially with water) and neutralization.
Laboratory activities form the foundation upon which analysis and calculations can be done. Activities are essential in this unit.
Factors of scientific literacy which should be emphasized
Foundational Objectives for Chemistry and the Common Essential Learnings
Investigate the nature of acids and bases.
- Identify some acids and some bases which are used in common household products.
- Observe some physical and chemical characteristics of acids and bases.
- Construct an operational definition of an acid and a base, using the characteristic properties of those substances.
- Describe the Brfnsted-Lowry conceptual definition of acids and bases.
- Identify the conjugate bases formed in acid dissociation.
- Associate acid or base strength with magnitudes of Ka and Kb.
- Identify the conjugate acid of any base.
- Recognize substances which are amphiprotic (amphoteric).
- Compare the strengths of the dissociations in the dissociation series for a polyprotic acid.
- Investigate the nature of the production and use of acids and bases in our society.
Consider how the ionization of water interacts with acid and base dissociations.
- Write the equilibrium constant expression for the dissociation of water.
- Show how the common ion effect influences the equilibrium of water's dissociation when H+ ions or OH- ions are added to water.
- Recognize the relationship between the [H+] and [OH-] in an aqueous system.
- Calculate the [H+] in a solution.
- Express the [H+] as a pH value.
- Explain how a logarithmic scale differs from an arithmetic scale.
- Estimate the pH of solutions, using indicator solutions and indicator papers.
Explore the principles of neutralization.
- State the general neutralization equation:
acid + base
salt + water
- Write equations for specific neutralization reactions, identifying the nature of each species.
- Solve mathematical problems involving data from titrations.
- Develop skill in doing titrations.
Strengthen understanding of equilibria through quantitative analysis of acid/base reactions. (NUM)
- Collect and organize data in charts and graphs.
- Interpret collected data.
- Read and interpret the scales on buret tubes.
- Discuss with peers how estimates of values are made.
- Use information from Ka tables to calculate pH values in solutions and check results of calculations with indicators.
Develop an understanding of how knowledge is created, refined and changed within chemistry. (CCT)
- Observe and record carefully during experimental or investigative procedures.
- Develop and conduct investigations and research.
- Understand the meaning of theory in science.
- Compare the nature of scientific knowledge with knowledge in other areas of study.
Understand that technology both shapes and is shaped by society. (TL)
- Appreciate how use of the principles of acid/base reactions has influenced our lives.
- Explore how knowledge about acid/base reactions has both explained existing applications and suggested new applications .
- Value the role of technology in studying acid/base reactions.
Suggested activities and ideas for research projects
- Place some 0.001 M HCl in an Erlenmeyer flask. Add universal indicator until the colour is noticeable. Place the flask against a white background, so that the colour changes is most evident.
Use a magnetic stirrer to distribute NaOH solution dripped into the flask from a buret tube. Open the stopcock to allow the base to drop slowly into the vortex formed in the flask. Observe the colour changes.
The colour changes can be reversed by adding more HCl from another buret above the flask.
- Obtain a 0.1 M HCl solution. Produce a serial dilution of the acid (at one-tenth concentration decrements). After the samples have been prepared, test each one with chemical indicators. This gives an indication of the colour of the indicators at different pH levels.
Repeat as above, starting with a 0.1 M NaOH solution.
Test a variety of solutions with chemical indicators. Determine whether the solutions are acidic, basic, or neutral. Students can determine the approximate pH of each unknown solution with indicators. The serial dilutions prepared previously can be used as standards for comparison.
Try to react each of the unknown samples with a small strip of magnesium ribbon. Record the results and look for a correlation between the pH and the reactivity with magnesium for each substance.
Once the standard and unknown solutions have been tested, a variety of household materials can be tested. (Some of these samples, such as oven cleaners, may be corrosive. Caution the students to observe normal safety precautions when handling household chemicals.)
- Place a few drops of phenolphthalein indicator into a beaker of water. Put a small piece of sodium metal on the water. Caution: Use only a very small piece of sodium, and view from a distance.
- What is aqua regia? For what purpose was it used?
- What acid is commonly called muriatic acid? What is the origin of that name?
- Place equal amounts of calcium carbonate suspension in two cylinders. Buffer one cylinder with a sodium acetate solution before putting equal amounts of 2 M acetic acid into each. Discuss how the common ion effect alters the rate of the reactions. (This activity was adapted from CHEM13 NEWS, #81, November 1976, based on an idea contributed by C. McNeill, Savannah, GA. He attributed the idea to Denman Evans.)
- One acid-rain simulation is to burn a small sample of sulfur in a gas jar which contains a small amount of water. Cover the jar and shake. Test the water with chemical indicators. Where does the sulfur dioxide in the atmospere come from. Is the burning of sulfur used commercially to prepare acids from sulfur?
Another involves collecting automobile exhaust samples and proceeding in the same way. (Caution: Collect the samples carefully to avoid the toxic effects of the exhaust and avoid burns from the tail pipe.) Is the automobile exhaust toxic due to an acid effect of carbon monoxide? Is the "acid rain" produced here a result of a reaction of carbon monoxide with water?
- Burn a strip of magnesium ribbon in air. Place the white, powdery magnesium oxide that results into water. The magnesium oxide reacts with the water to form magnesium hydroxide in water. Test the solution with chemical indicators.
- Are acid-wash jeans really washed in acid? If so, what effect does it have on the fabric? What acid is used in the process?
- Some shampoos advertise that they are low pH. Others advertise that they are pH-balanced. Why do they make these claims? What do the claims mean? Look at advertising to find other claims made about the acidic or basic character of products. Test some shampoos and detergents to determine their pH values.
- Tomatoes can be canned in a hot water bath without fear of botulism developing in the tomatoes. Corn must be canned in a pressure canner at high pressures in order to eliminate the possibiltiy of botulism. Why the difference?
- In July 1991 at St. Lazare, Manitoba, a train carrying acetic anhydride derailed. The chemical spilled and the town was evacuated. What is acetic anhydride? What effect does it have on the environment? How was the spill cleaned up? How would a similar spill be handled if it occurred in your area? Where would the people to organize and carry out the evacuation and clean-up come from?
- How is acid used in uranium milling and refining? What other industrial processes in Saskatchewan use acids?
- Caustic soda is a an important industrial chemical. What is its chemical name? Where and how is it produced? What is its industrial use? What is its domestic use?
- Prepare a "simulated stomach" by placing 5 mL of 2 M HCl into 50 mL of water in a 250 mL Erlenmeyer flask. Add a tablet of an antacid, such as Tums, Rolaids, milk of magnesia tablets, etc. Allow the neutralization reaction to proceed.
When the fizzing has stopped, complete a titration using 0.2 M NaOH with a phenolphthalein indicator. Calculate the number of moles of acid to begin with, the number of moles of NaOH needed to neutralize the acid, and the number of moles of HCl neutralized by the antacid.
Compare the results from a variety of brands of antacid tablets. Any claims made by the makers of the antacid tablets can then be subjected to scrutiny, based on experimental evidence. How do these products relieve `acid-indigestion'? What are the causes and symptoms of acid-indigestion? Discuss the use of models to simulate processes or events. The flask represents the human stomach. How are they similar? How are they different? Are there factors involved in the human stomach which haven't been taken into account by the flask-stomach model?
As a further extension of this activity, place a universal indicator into the flask containing the simulated stomach acid. Place an antacid tablet into the container. Use a magnetic stirrer if one is available. Observe the changes that occur in the flask as the tablet dissolves and neutralizes some of the acid.
- Prepare a cabbage juice solution by boiling red cabbage in water. Allow the juice to cool. Strain the solution. Add vinegar to the cabbage juice. Note any colour change. Tea also serves as a good chemical indicator. What happens to tea when lemon juice is added to it? Carrot, beet, and blueberry juices also act as indicators. Determine over what range each operates.
- Add sufficient sodium hydroxide solution and universal indicator to water in a 1 000 mL beaker to produce a deep violet colour. Place several pieces of dry ice in another beaker. Pour the sodium hydroxide solution into the dry ice, then continue pouring it back and forth between beakers. The solution undergoes continuous colour changes while this is taking place.
The dry ice dissolves in the water and forms carbonic acid, which then begins to neutralize the sodium hydroxide. If the solution has turned orange and some dry ice still remains, more sodium hydroxide can be added to repeat the entire process.
- Have students check the pH of their own saliva using Hydrion paper. Make sure that they dispose of the paper immediately after recording the pH. Collect the class data and prepare a graph showing the class results. (This activity was adapted from CHEM13 NEWS, #81, November 1976, page 17, based on an idea contributed by C. McNeill, Savannah, GA.)
- Perform a titration to neutralize a standard solution of a strong base with a strong acid. If a pH meter is available, collect data as the titration proceeds. Use the data to plot a titration curve.
- The susceptibility of the soils and lakes of Saskatchewan to the effects of acid rain can be demonstrated with a microscale activity. Each lab group will need a 12 by 8 well micro-spotplate, a one litre ziplock bag, some chalk fragments, a supply of universal indicator, a micropipet and a 15 mm piece of plastic drinking straw. The plate can be trimmed to produce a trapezoidal plate which is 6 wells wide at one end and eight wide at the other to more closely resemble the shape of Saskatchewan.
Assuming that the rows of the spotplate are labelled from the top with the letters A through L and the columns are labelled from the left with numbers 1 through 8, place a small fragment of chalk (CaCO3(s)) in the wells E1, F2, F3, F4, G5, F6, G7, H8 and in all the wells below that line. Half-fill each well but F1 with universal indicator solution.
In well F1 place a 15 mm length of plastic drinking straw as a smoke stack. Add two drops of 6.0 M HNO3(aq) to well F1 and carefully place the plate into a one litre ziplock bag. Drop a 1 mm piece of #14 copper wire (ordinary house wiring gauge) into the stack in well F1. Immediately seal the bag.
What effects are noticed? What does the spotplate represent (if you haven't told them)? What is the purpose of the chalk in some of the wells? Why were those wells selected to have chalk in them?
Add 300 mL of water to the bag to sufficiently dilute any remaining nitric acid. The plate, bag, straw and copper wire can all be reused after rinsing and drying. The chalk fragments can be discarded.
This activity could also be demonstrated on an overhead projector. (This activity was adapted from CHEM13 NEWS, #207, November 1991, page 7, based on an idea contributed by Denise Gordon of Fort Worth, TX. She adapted the idea from an activity designed by Donna Bogner.)
- How many foods can you identify which contain acids? For each food, list the name(s) of the acid(s).
- Find a recipe for sauerkraut. Use the bottom two-thirds of a clear plastic 2 L pop bottle to produce the sauerkraut. A plastic petri dish cover will fit into the bottle as a press for the top of the mixture. The clear plastic will let you monitor the production. Measure the pH of the liquid each day until the sauerkraut is finished. What acid is produced? What gas is produced? Where does the liquid come from? What chemical reactions are occurring? What causes these reactions to occur?
- The activity "Deadly Skies" from the Project WILD Activity Guide (page 319) could be used during this unit. It involves a simulation and discussion of the effects of acid rain.
Sample ideas for evaluation and for encouraging thinking