CHEMLAB Exploring Chemical Changes

Most reactions can be classified into five major types. As you carry

out this experiment, you’ll observe examples of each of these types.

In doing so, you will also learn to recognize many of the physical

changes that accompany reactions.

 

PROBLEM

What are some of the physical changes that indicate that a reaction has occurred?

 

OBJECTIVES

• Observe physical changes that take place during chemical reactions.

• Compare changes that take place during different types of chemical reactions.

 

Hypothesis:

 

MATERIALS

large test tube and one-hole stopper with glass tube and rubber tubing attached

saturated Ca(OH)2 solution, limewater

125-mL flasks (4)                                          balance

hot plate                                                        watch glass

spatula                                                          stirring rod

ring stand                                                      lab burner

test-tube clamp                                             file

0.1M CuSO4                                                                          new penny

granular copper, Cu                                     250-mL flask

powdered sulfur, S                                       ice

CaCO3, finely ground                                   tongs

100-mL graduated cylinder                          6M HCl

0.5M Na2CO3                                                                       0.5M CuCl2

 

SAFETY PRECAUTIONS

Wear an apron and goggles. Use care when handling hot objects. Dispose of the reaction mixture and products as instructed by your teacher.

 

PROCEDURE

For each of the following reactions, record in the data table all changes that you observe.

Synthesis Reaction

1. Place 50 mL 0.1M CuSO4 in a 125-mL flask.

2. Place 1.6 g granular copper and 0.8 g powdered sulfur on a watch glass and mix together thoroughly with a spatula.

3. Heat the flask on a hot plate set at high until the solution begins to boil.

4. Stir the Cu/S mixture into the boiling CuSO4 solution.

5. Continue boiling until a black solid forms.

Decomposition Reaction

1. Place 100 mL of saturated Ca(OH)2 solution (limewater) in the 250-mL flask.

2. Add finely ground CaCO3 to a large test tube until it is one-fourth full. Stopper the tube with the stopper/glass tube/rubber tubing assembly, and clamp the tube to the ring stand.

3. Light a laboratory burner, and begin to heat the test tube. Submerge the end of the rubber tubing into the limewater so that any gas produced in the tube will bubble through the limewater.

4. Continue heating the CaCO3 until you observe a change in the limewater. The presence of CO2 causes limewater to become cloudy.

Single-Displacement Reaction

1. Place 30 mL 6M HCl in a 125-mL flask.

2. Using a file, cut six 0.2-cm notches evenly spaced around the perimeter of a new penny.

3. Place the penny in the flask of acid and leave it in a fume hood overnight.

Double-Displacement Reaction

1. Add 25 mL 0.5M Na2CO3 and 25 mL 0.5M CuCl2 to a 125-mL flask.

2. Swirl the flask gently until you observe the formation of a precipitate.

Combustion Reaction

1. Light a laboratory burner and adjust the air and gas supplies until the flame is blue. Observe what happens.

2. Using tongs, hold a flask or beaker with ice in it about 10 cm over the flame for approximately one minute. Move the flask away from the flame and observe the bottom of the flask.

DATA AND OBSERVATIONS

Reaction

 

 

 

Observations

 

Synthesis

 

 

 

 

Decomposition

 

 

 

 

Single displacement

 

 

 

 

Double displacement

 

 

 

 

Combustion

 

 

 

 

 

ANALYZE AND CONCLUDE

1. Making Inferences Which observations noted during each of the reactions indicated that a reaction had occurred?

 

2. Comparing and Contrasting What did all of the reactions have in common?

 

3. Making Inferences Write the name and formula of the

a) black solid formed in the synthesis reaction.

 

b) gaseous product of the decomposition reaction.

 

c) solid product of the decomposition reaction.

 

d) pale blue precipitate in the double displacement reaction.

 

e) liquid product of the combustion reaction.

 

4. Observing and Inferring Explain how the penny changed during the single- displacement reaction. What would happen if a pre-1983 penny, which is solid copper, were used?

 

5. Relating Concepts Is energy a reactant or product of the combustion

reaction?

 

APPLY AND ASSESS

1. Were there any physical changes that often occur during a reaction that you did not observe while doing this ChemLab? If so, what were they?

 

2. Write balanced chemical equations for all of the reactions carried out.

 

3. Why do you think pennies are no longer made from only copper metal?