Investigation 10B Chemical Reactions

2 Investigation 10B: Chemical Reactions When does a chemical reaction occur? How do we know if a chemical reaction has occurred? Mixing sand with water? Mixing water and paint? Adding bleach to colored water? There is a color change. There is a temperature change. A new substance such as a gas or a precipitate is formed.

3 Use caution when handling HCl (a strong acid) and KOH (a strong base). These substances are caustic and can burn your skin. If you do come in contact with them, rinse your hands with running tap water for 3-5 min. Always wear goggles. Never breathe chemical fumes. If you spill any chemicals, inform your instructor immediately. Investigation 10B: Chemical Reactions Part 1: Chemical safety

4 Metal and air Metal and water Metal oxide and water Metal and acid Heat a small strip of copper metal in the light of a candle for 2–3 min. Do not hold the strip with your bare hands. Make observations about how the metal looks before and after heating. Place the piece of paper on a watch glass and scrape off some of the coating. Make some observations. Investigation 10B: Chemical Reactions Part 2: Reactions of metals 1.Metal and air

5 Metal and air Metal and water Metal oxide and water Metal and acid When the reaction appears to be over, gently touch the test tube, and record how it feels. Add 2 drops of phenolphthalein, and record your observations. Rinse into the sink with tap water. Investigation 10B: Chemical Reactions Part 2: Reactions of metals 2.Metal and water Fill a test tube about half way with distilled water. Add a small piece of calcium metal to the test tube.

6 Metal and air Metal and water Metal oxide and water Metal and acid Fill a clean test tube with about 2 in of distilled water. Add a small spatula (pea-sized amount) of calcium oxide to the test tube, and mix the contents with a stirring rod. Add 2 drops of phenolphthalein. Record your observations. Rinse the contents of the test tube into the sink with tap water. Investigation 10B: Chemical Reactions Part 2: Reactions of metals 3.Metal oxide and water

7 Metal and air Metal and water Metal oxide and water Metal and acid Add a small strip of Mg ribbon to about 1.5 in of 1.0 M HCl. Immediately cover with another test tube. When the Mg ribbon is done reacting, have your lab partner light a wooden splint. Record your observations. Rinse the contents into the sink. Investigation 10B: Chemical Reactions Part 2: Reactions of metals 4.Metal and acid

8 a.What gases in air could react with copper? Which one do you think reacted the most? b.What do you think the coating on the copper in Reaction 1 was? Explain. c.What does the phenolphthalein tell you about the solution in Reaction 3? d.What did the flaming splint do when it was introduced to the test tube in Reaction 4? Why do you think this happened? Metal and air Metal and water Metal oxide and water Metal and acid Investigation 10B: Chemical Reactions Part 3: What happened?

9 Sodium bicarbonate Ammonium carbonate Metal carbonate Add 10.0 g of baking soda (sodium bicarbonate) to a dry test tube. Place it in the Lab-Master heater. Attach a one-hole stopper and a rubber tubing assembly. Place the other end of the tubing in a second test tube containing 2 in of lime water. Investigation 10B: Chemical Reactions Part 4: Decomposition reactions 1.Sodium bicarbonate

10 Set the heater to 120 o C. If no bubbles appear within a minute or two, increase the heater temperature to 150 o C. Watch the lime water. Remove the rubber stopper and hold a piece of cobalt chloride paper into the mouth of the test tube. Allow the test tube to cool for 5 min. Discard the solid NaHCO 3 and rinse the test tube in the sink. Investigation 10B: Chemical Reactions Part 4: Decomposition reactions 1.Sodium bicarbonate (continued)

11 Sodium bicarbonate Ammonium carbonate Metal carbonate Add about 5.0 g of ammonium carbonate, (NH 4 ) 2 CO 3, to a dry test tube. Use the Lab-Master to heat it up to 70 o C. Watch for condensation and the smell of ammonia. Hold a piece of cobalt chloride paper down inside the test tube and note any color change. Do not touch the sides of the tube with the paper. Investigation 10B: Chemical Reactions Part 4: Decomposition reactions 2.Ammonium carbonate

12 Continue heating and place a flaming splint into the mouth of the test tube. What happens to the splint? Is there any solid left after heating? Let the test tube cool before cleaning it with water. Investigation 10B: Chemical Reactions Part 4: Decomposition reactions 2.Ammonium carbonate (continued)

13 Sodium bicarbonate Ammonium carbonate Metal carbonate Add about 1.0 g of copper(II) carbonate (CuCO 3 ) to a desiccator. Describe the appearance of the powder. Set the heater to 150 o C and heat for a few minutes. Do you notice a color change? If the reaction does not occur in your desiccator, what could be the main reason for it? CuCO 3 (s) CuO(s) + CO 2 (g) heat What would you do differently to make sure that this reaction takes place? Investigation 10B: Chemical Reactions Part 4: Decomposition reactions 3.Metal carbonate

14 a.What happened to the flaming splint when you placed it into the mouth of the test tube in Reaction 1? b.What gas do you think the flaming splint was testing for? c.What color did the cobalt chloride paper turn? Cobalt chloride paper turns pink in the presence of moisture or water vapor. d.What color was the cobalt chloride paper initially? e.Was there any other evidence that water vapor may have been produced? f.Each of the three experiments produced the same gas. Can you guess the identity of that gas? Explain. g.Can you guess which of the three experiments produced ammonia? Explain your reasoning. Sodium bicarbonate Ammonium carbonate Metal carbonate Investigation 10B: Chemical Reactions Part 5: Thinking about it

15 Add about 2.0 mL of 1.0 M HCl to a small test tube. Add 2–3 drops of phenolphthalein to the acid and swirl to mix. Add 1.0 M KOH drop by drop from a dropper bottle. Swirl after each additional drop. Record the total number of drops you have added when a pink color persists after swirling. Investigation 10B: Chemical Reactions Part 6: Acid-base reactions Neutralization

16 a.What happens to the acid solution color when KOH is added? b.How many drops did the solution require to change color? c.Compare your results with those of other groups. Was yours the same? Explain in a couple of sentences why they may or may not have required the same number of drops to change color. Neutralization Investigation 10B: Chemical Reactions Part 7: Thinking about it

17 a.List the reactants for each of the 4 reactions, and label the physical states with (s), (l), (g), or (aq). c. Compare your results with another group. For Part 2: Reactions of Metals Investigation 10B: Chemical Reactions Part 8: Writing chemical equations b.Assess your observations and try to predict the products for each reaction. Look at your reactants to see whether all the elements have been used to form products. Example:

18 For Part 4: Decomposition Reactions Investigation 10B: Chemical Reactions Part 8: Writing chemical equations d.List the reactants for each of the 4 reactions, and label the physical states with (s), (l), (g), or (aq). e.Assess your observations and try to predict the products for each reaction. Look at your reactants to see whether all the elements have been used to form products. General approach:

19 f.Compare your results with those of another group. For Part 4: Decomposition Reactions Investigation 10B: Chemical Reactions Part 8: Writing chemical equations General approach: g.Identify each of the gases you tested for. How were you able to determine which gas was which? Explain.

20 What did we learn? Carbonates decompose with heat to produce carbon dioxide gas. Acids and bases can neutralize each other. Heat causes metal to oxidize in air. A popping sound is produced when a flame is exposed to hydrogen gas. Investigation 10B: Chemical Reactions