Chemical Reactions

Unit Overview

Observing, investigating and inquiring about a wide variety of reactions - rusting automobiles, crumbling concrete, burning paper, electricity from a dry cell, and reactions with chemicals in the laboratory - makes concrete the concept of a chemical reaction.

The importance of chemical reactions in maintaining life, in developing new substances, and in the impact of those substances on the environment make it essential that students understand what chemical reactions are and how pervasive they are.

Topics from this unit can support the development of students' concept of molecules and compounds. By integrating this unit with the stoichiometry unit, students simultaneously can develop their concepts of what chemical reactions are, and how we measure them. To some extent the units Molecules and Compounds, Chemical Reactions, and Mole Concept and Stoichiometry can be treated as one.

Factors of scientific literacy which should be emphasized

• A1 ..........public/private
• A4 ..........replicable
• A5 ..........empirical
• A7 ..........unique

• B1 ..........change
• B2 ..........interaction
• B3 ..........orderliness
• B8 ..........quantification
• B10 ..........cause-effect
• B11 ..........predictability
• B13 ..........energy-matter
• B20 ..........theory
• B21 ..........accuracy
• B23 ..........invariance

• C1 ..........classifying
• C2 ..........communicating
• C3 ..........observing and describing
• C5 ..........measuring
• C8 ..........hypothesizing
• C9 ..........inferring
• C10 ..........predicting
• C11 ..........controlling variables
• C12 ..........interpreting data
• C14 ..........problem solving
• C15 ..........analyzing
• C16 ..........designing experiments
• C20 ..........defining operationally
• C21 ..........synthesizing

• D3 ..........impact of science and technology
• D4 ..........science, technology, and the environment
• D8 ..........limitations of science and technology
• D10 ..........technology controlled by society

• E1 ..........using magnifying instruments
• E3 ..........using equipment safely
• E7 ..........manipulative ability
• E9 ..........measuring volume
• E10 ..........measuring temperature
• E11 ..........measuring mass
• E12 ..........using electronic instruments

• F1 ..........longing to know and understand
• F3 ..........search for data and their meaning
• F4 ..........respect for natural environments
• F7 ..........demand for verification
• F8 ..........consideration of premise

• G1 ..........interest
• G2 ..........confidence
• G5 ..........avocation
• G7 ..........vocation
• G9 ..........valuing contributors

Foundational Objectives for Chemistry and the Common Essential Learnings

Appreciate the importance of chemical reactions.

• Identify changes which indicate that a chemical reaction has taken place .
• Identify chemical reactions that help maintain living organisms.
• Identify chemical reactions that harm living organisms.
• Identify chemical reactions that affect the environment.
• Recognize how chemistry has been involved in product and process development in the last thirty years.

Acquire ability to communicate chemical information through equations for reactions.

• Write an equation representing a chemical reaction using chemical formulas for the species involved.
• Recognize that chemical equations need to be balanced for number of atoms and for charge.
• Balance chemical equations for number of atoms and for charge.
• Apply the Law of Conservation of Mass to writing balanced chemical equations.
• Develop net ionic equations.
• Recognize that an energy term is often shown in a chemical equation.
• Place the energy term on the correct side of an equation, depending on whether a reaction is exothermic or endothermic.
• Develop a balanced chemical equation from a word equation, experimental evidence, or a description of a chemical reaction taking place.

Use a wide range of language experiences to develop understanding about molecules and their reactions. (COM)

• Record, discuss and compare their observations of reactions with others.
• Present findings about reactions by using diagrams, models, analogies or other devices.
• Evaluate readings and videos about reactions in the context of concrete experience with reactions.

Develop an understanding of how knowledge is created, evaluated, refined and changed within chemistry. (CCT)

• Strengthen perceptual abilities through concrete experiences with chemical reactions.
• Reflect upon how the technology available to measure reactions influences the concept of how a reaction occurs.
• Recognize how basic concepts of chemistry change with changing perspective and new information.

Suggested activities and ideas for research projects

• A good series of reactions to illustrate both a variety of chemical reactions and some of the principles of stoichiometry begins with copper(II) chloride. The series of reactions here may be done either qualitatively or quantitatively.

  Since all copper compounds are skin irritants, avoid contact with the skin and wear eye protection at all times. If the compounds do contact the skin, wash with lots of running water.

  Obtain a sample of anhydrous copper(II) chloride. Transfer some of the sample (about the size of a dime) to a dry watch glass. Record a description of the compound, and then put the watch glass aside until the end of the period.

  At the end of the period, look for any changes to the cooper(II) chloride. Label the watch glass and store it until the next class. At the beginning of the next class again examine the copper(II) chloride for evidence of change. Record your observations. Dispose of the compound as instructed.

  Add the remainder of the sample to about 150 mL distilled water in a 250 mL beaker. Describe what occurs when the compound is added to the water. Stir the mixture gently until a solution is formed.

  Over a low heat, raise the temperature of the solution to 70°C or so. Add a piece of aluminum metal and describe the reaction which ensues. How can you tell when the reaction is complete?

  Separate the copper metal from the aluminum metal. Which component was the limiting factor in this reaction? Wash and dry the copper.

  Put the copper metal in a labelled 500 mL beaker. Add 50 mL of dilute nitric acid. Put a watch glass over the top of the beaker and watch the formation of nitrogen dioxide gas until the beaker is half full of the gas. Then move the beaker and watch glass to the fume hood until the reaction is complete and the nitrogen dioxide has diffused from the beaker.

  Test a sample of sodium hydroxide with pH paper. Also test the blue copper(II) nitrate solution. Add 25 mL samples of the sodium hydroxide solution to the copper(II) nitrate solution, stirring gently as the addition is made, until the mixture has the same pH as the sodium hydroxide.

  Let the bluish-white copper(II) hydroxide precipitate settle and decant as much of the sodium nitrate solution from it as possible without losing any of the solid.

  Add enough water to bring the volume to 200 mL and heat until the copper(II) hydroxide has been converted to black copper(II) oxide. Let the mixture settle and decant the solution, which still contains sodium nitrate. Add a 50 mL portion of distilled water and swirl gently. Decant and repeat the process twice more, with 50 mL distilled water each time.

  Finally, to the copper(II) oxide, add dilute sulphuric acid drop by drop, swirling after each addition, until all the copper(II) oxide is dissolved.

  Allow the solution to evaporate, producing crystals of copper(II) sulphate. After the crystals have been examined and described, transfer them to test tube and heat them gently. What change is happening to the crystals? Dump the dried crystals from the test tube onto a piece of paper. Add a drop of water to the pile. Record the results.

  From the paper transfer the copper(II) sulphate back to a 250 mL beaker. Add enough water to dissolve the solid. Add some zinc metal and heat gently. How can you tell when this reaction is complete? Pour the solution which remains into an evaporating dish and evaporate at room temperature. How do the crystals formed on this step compare with the copper(II) sulphate crystals? What happens if you heat these crystals? What is the name of these crystals?

  Write word or formula equations for each of the reactions in the activity. Use molecular models (ball and spring, space-filling, etc.) to illustrate each reaction. Perhaps each lab group could be responsible for creating a poster with an equation of one of the reactions, a description of what was observed during the reaction and an example of a similar reaction which is used in some mining, manufacturing or industrial process. If there are more than six lab groups, perhaps some of them could be merged for the purpose of this activity since there are only six reactions.

  Each group could make an oral presentation to the class, using the poster for illustration. The posters could be posted on a bulletin board in the order the reactions occur in the activity.

• Mix 1.7 g iron filings and 1.0 g powdered sulphur in a crucible. Heat the crucible with a low flame for three minutes and then for five minutes with a hot flame. Compare the characteristics of the product (after it has cooled) to the characteristics of the reactants.

• Get a Material Safety Data Sheet (MSDS) for one of the chemicals which is used in your laboratory. Study the type of information that is presented on the sheet. From a list of chemicals provided by your teacher, select one chemical and prepare an MSDS for it. (This activity was adapted from CHEM13 NEWS, #195, May 1990, page 12, based on an idea from H.E. Pence of Oneonta, NY)

• Here are three quick ways to generate gases for illustrating tests. They are demonstrations which Michael Faraday did in annual Christmas chemistry lectures for children.

  • A half glass of soda water (or vinegar with baking soda) will produce enough CO₂ gas in the top part of the glass to extinguish a candle flame or burning splint thrust into that space.

  • Oxygen gas can be generated in another glass by adding a pinch of CoCl₂ (cobalt(II) chloride) to household bleach. Alternatively, 3% H₂O₂ (hydrogen peroxide) and a pinch of MnO₂ (manganese dioxide) can be used. A glowing splint will burst into flame when inserted into the top part of the glass.

  • Generate H₂ gas (hydrogen gas) by adding a small piece of aluminum foil to about 2 mL of 3.0 M NaOH solution (sodium hydroxide, or lye) in a 13 x 100 mm test tube. A burning splint held at the mouth of the test tube will produce a 'pop'. A glowing splint quickly inserted into the test tube will go out.

  What are the equations for the reactions in each case? (This activity was adapted from CHEM13 NEWS, #205, September 1991, page 9, from an article contributed by John Fortman, Dayton, OH)

• A microscale procedure for the collection of O₂ gas (oxygen gas) by the displacement of water is described in CHEM13 NEWS #183, February 1989, page 13.

• A colour video camera equipped with lenses and adapters designed to fit student microscopes gives students a chance to view a variety of microscale reactions in great detail. If you can borrow some binocular dissecting microscopes from the biology department, this activity also works well when the reactions are observed under those instruments. Porcelain or plastic spot plates are used as the reaction vessels.

  Place a small pinch of iron filings in a depression of the spot plate. Observe and describe the filings. Add 1 or 2 drops of 0.1 M CuSO₄ (copper(II) sulphate) solution. Record your observations. Write an equation for the reaction which is occurring.

  Repeat the procedure twice more, substituting zinc metal and lead metal for the iron filings.

  Place a few strands of copper turnings or a few copper shot in a depression. Observe the copper metal. Then add 1 or 2 drops of 0.1 M AgNO₃ (silver nitrate) solution. Write an equation for the reaction which is occurring.

  Repeat this procedure twice more, substituting iron filings and lead metal for the copper.

  To some granular zinc in one of the depressions, add 1 or 2 drops of 1.0 M HCl (hydrochloric acid) solution. Record observations. What do the bubbles indicate? Now add one drop of 6.0 M HCl solution. Observe. Write an equation for the reaction.

  Repeat the procedure, using a short piece of magnesium ribbon instead of zinc. (This activity was adapted from CHEM13 NEWS, #199, December 1990, page 14, based on an idea from Norm Hoekstra of Holland, MI as reported in Periodic Reports and Retorts (March 1990).)

• Compare the textures of Plaster of Paris and PolyFilla^TM. How does each behave when water is added? How does the proportion of water to powder to produce a usable mixture compare? How long does each take to set? What are the chemicals in each? What chemical reactions occur when the powder is mixed with water? Are chemical reactions with the air involved in the curing process? (This activity was adapted from CHEM13 NEWS, #197, October 1990, page 1, based on an idea contributed by David Banks, Gloucester, ON)

• In a 1.0 litre ziplock bag, put 5.0 g of solid CaCl₂×2H₂O and 5.7 g of solid NaHCO₃. Squeeze the air out of the bag and then seal it so that there is a minimum of air in the bag. Record your observations of the system.

  Open the corner of the bag and add 10 mL of phenol red indicator solution. Reseal the bag quickly and record your observations.

  What are the chemical reactions involved in this system? What causes the heat effects, the colour changes and the evolution of gas? (This activity was adapted from CHEM13 NEWS, #192, February 1990, page 2, based on an idea from Linda Woodward of University of Southwestern Louisiana. She attributes the idea to Mickey and Jerry Sarquis of Oxford, OH.)

• Remove the grease from some steel wool. (Dipping in acetone or washing in warm soapy water will do this. Why does most steel wool which you buy have a thin coating of grease?) Dry the steel wool completely before use. Record all observations during this activity. (What do you see? Is there anything you can measure?)

  Put the steel wool in a test tube or small jar. Add enough warm vinegar to cover the steel wool. If a microwave oven is available, warm the mixture in it to just about boiling temperature. Alternatively, warm the container in a warm water bath for about 30 minutes. Then let the mixture sit for a day. Make up a strong solution of tea. Put it aside until the next day.

  Decant the liquid from the vinegar/steel wool mixture. Mix equal volumes of the tea and vinegar solutions in another container. Stir briefly and the use the mixture to write your name on a sheet of paper. Use a small paint brush or a wooden splinter to do this. Set the paper and the container aside where they will not be disturbed. Observe once an hour for the rest of the day and then again on the next day.

  Interpret your observations. Write word equations and formula equations for any reactions which you believe have occurred.

Sample ideas for evaluation and for encouraging thinking

• "Water is the solvent, the medium, the participant and the catalyst in most of the chemical reactions occuring in our environment." Comment on this quotation, taken from Fact Sheet #1: Water - Nature's Magician (Environment Canada, 1990).

• Crushed limestone is used in the blast furnaces which reduce iron ore to iron. What is the purpose of the limestone? What chemical reactions is it involved in?

• Identify the chemical processes which are used in drinking water treatment and in waste water treatment. Chlorine gas is commonly added to the water. Is the chlorine involved in any chemical reactions?

• Why do rockets carry tanks of liquid oxygen?

• Why do the alkali metals all react similarly with water?

• A metal can made of iron left half buried in the ground will corrode to leave only a crumbled shell in a few years. A can made of aluminum half buried will last indefinitely. Why is there such a difference? What metals would corrode more quickly than iron?

• As copper weathers, it turns a dark brown and then various shades of green. What reactions are responsible for these colour changes?

• In their solid phases, aluminum conducts electricity and sodium chloride does not. As liquids, both conduct electricity. What is the reason for this difference in behaviour? Are there some substances which conduct in their solid phase but not in their liquid phase?