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|Unit A—Energy and Matter in Chemical Change |Additional Activities |

| |Line Master |26 |

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Additional Activity 2 — Three Precipitation Reactions

The Question

How can you use chemical equations to identify the precipitate formed in a chemical reaction?

Materials and Equipment

safety goggles, apron, gloves

test tube rack

3 test tubes (centrifuge tubes, if possible)

10-mL graduated cylinder

iron(III) chloride solution

calcium chloride solution

aluminium chloride solution

sodium hydroxide solution

centrifuge (if available)

spatula

Procedure

1. Label three clean, dry test tubes: Fe, Ca, Al.

2. Using a 10-mL graduated cylinder, pour 5 mL of iron(III) chloride solution into the Fe test tube. Into the Ca test tube, pour 5 mL of calcium chloride solution, and into the Al test tube, pour 5 mL of aluminium chloride solution. Examine the solutions and record the colour of each. Note whether each solution is clear or cloudy.

3. To each test tube, add 5 mL of sodium hydroxide solution. Shake each test tube gently to mix. Examine the products, and record your observations.

4. (Optional) Place the test tubes in a centrifuge for 2 minutes. Remove them and record your observations.

5. Dump or use a spatula to scrape the products onto a paper towel, and examine them. Record your observations.

6. Follow your teacher’s instructions for disposing of the chemicals you have used. Some of these chemicals contain metals. They require special proposal procedures.

continued...

|Unit A—Energy and Matter in Chemical Change | |

Analyzing and Interpreting

1. Did a precipitation reaction occur? How do you know?

2. Each reaction in this inquiry has two reactants and two products. The identity of the products can be predicted by swapping the parts of the names of the reactants around. For example, one of the products is iron(III) hydroxide. Write a word equation for each of the three reactions.

3. Translate each word equation into a formula equation, and then balance each equation.

4. Each reaction produces a precipitate involving a hydroxide ion. Reading from your balanced chemical equations, what was the identity of the precipitate in each of the three reactions?

5. Use the solubility table, table C, in Student Reference 12 in Science 10 to confirm the identities of the precipitates.

Forming Conclusions

6. Write a brief summary paragraph that answers the question at the beginning of this lab.

|Unit A—Energy and Matter in Chemical Change | |

TEACHING NOTES

Additional Activity 2 — Three Precipitation Reactions

Groupings: pairs

Time: 45 min

Purpose: To learn to recognize precipitate formation and represent these processes in chemical equations

Activity Notes

• Students produce three precipitates in this activity, involving iron, calcium, and aluminium hydroxides. Each of these reactions also produces sodium chloride, which is soluble. Because sodium chloride is produced in each reaction and each reaction produces a precipitate, it is possible to deduce that the precipitates are not sodium chloride. Thus students can conclude that the metal hydroxides are the precipitates.

• The following amounts per class are needed for each reagent:

- 150 mL of 0.1 mol/L FeCl3(aq) (16.2 g/L FeCl3∙6H2O)

- 150 mL of 0.1 mol/L CaCl2(aq) (23.8 g/L CaCl2∙2H2O)

- 150 mL of 0.1 mol/L AlCl3(aq) (17.0 g/L AlCl3∙6H2O)

- 150 mL of 0.1 mol/L l NaOH(aq) (4.0 g/L)

• Keep in mind that this is not a study of double replacement reactions. (Double replacement reactions are covered in section A3.3 of Science 10, along with other common reaction types.) It may be helpful to remind students that all ionic compounds exist in two parts (cation and anion) and that they all have two parts to their names because of this. One very clever part of this naming system is that when two ionic compounds in solution are mixed, the products can be predicted by swapping parts of the names. For example, when iron(III) chloride and sodium hydroxide are mixed, the sodium can go with the chloride and the iron(III) can go with the hydroxide.

• Safety Notes:

- Students should wear safety goggles, an apron, and gloves during this lab.

- Sodium hydroxide solution is corrosive. Make sure students wash their hands thoroughly before leaving the lab.

- Although the volumes used are small, students should empty the waste solutions into a large beaker before washing the test tubes in the sink.

- Dispose of all chemicals as directed by your school district’s policy.

• Procedure step 1: Use centrifuge tubes if you have a centrifuge.

• Procedure step 2: You may wish to show students the height of 5 mL of solution in the test tubes you are using, or have them measure out 5 mL the first time so that they know what the correct volume looks like. The precise volume used is not critical.

• Procedure step 3: You may wish to demonstrate how to shake a test tube that does not have a stopper at the top. Hold the test tube firmly at the top in one hand, and flick the bottom of the tube gently and quickly with the other. Alternatively, you may wish to put stoppers in the test tubes, which allows them to be inverted while shaking.

• Procedure step 4: Demonstrate safe use of a centrifuge before beginning the lab.

|Unit A—Energy and Matter in Chemical Change | |

Analyzing and Interpreting

1. A precipitation reaction occurred three times. Cloudiness, resembling a gel, formed each time. This was the precipitate. If a centrifuge was used, the solid formed a pellet at the bottom of the test tube.

2. The word equations for the three reactions are:

iron(III) chloride + sodium hydroxide ( iron(III) hydroxide + sodium chloride

calcium chloride + sodium hydroxide ( calcium hydroxide + sodium chloride

aluminium chloride + sodium hydroxide ( aluminium hydroxide + sodium chloride

3. The balanced equations are:

FeCl3(aq) + 3 NaOH(aq) ( Fe(OH)3(s) + 3 NaCl(aq)

CaCl2 (aq) + 2 NaOH(aq) ( Ca(OH)2(s) + 2 NaCl(aq)

AlCl3 (aq) + 2 NaOH(aq) ( Al(OH)3(s) + 2 NaCl(aq)

4. The precipitates were Fe(OH)3(s), Ca(OH)2(s), and Al(OH)3(s).

5. The table shows that NaCl is very soluble, while each of the metal hydroxides has low solubility, demonstrating that they are the precipitates.

Forming Conclusions

6. Precipitates can be recognized by the formation of a cloudy or gel-like substance in the solution. A centrifuging step causes a solid pellet to form at the bottom of the test tube. The metal hydroxides are the precipitates.

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