Chemical Reactions and Their Consequences

CHEMICAL REACTIONS AND THEIR CONSEQUENCES

How do we know if a chemical reaction has occurred? Many people confuse physical and chemical changes, but the early alchemists made many observations over hundreds of years to enable future scientists to distinguish between the two phenomena. In this lab you will be investigating what are called 'indicators of chemical reactions.' You will observe four of those indicators: formation of a precipitate, formation of a gas, change of odor, and change of color. In addition to these four, a fifth indicator is often observed by chemists, that of a change in temperature.

Procedure

I. Formation of a precipitate:

1.Place 2.0 mL of 0.1 M CaCl2 in a test tube. Add a few drops of 0.1 M sodium oxalate, Na2C2O4 and record your observations.

2.Place 2.0 mL of 0.1 M CaCl2 in a test tube. Add a few drops of 0.1 M NaCl. Record your observations.

3.Label a clean, dry vial and weigh. Add 2.0 mL of 0.1 M Pb(NO3)2, lead nitrate. Reweigh and determine the weight of the solution by subtraction. Weigh another vial and add 1.0 mL of K2CrO4, potassium chromate. Again reweigh to determine the weight of the K2CrO4 by subtraction. Add the potassium chromate to the vial containing the lead nitrate. Make observations, then reweigh. Record these data on the data sheet and complete the table.

II. Formation of a gas:

1.Place 2.0 mL of 2.0 M NaHCO3 in a test tube. Add 1.0 mL of 1.0 M HCl. Watch the test and record observations.

2.Place 2.0 mL of HCl in a test tube and add 1.0 mL of 0.1 M CaCl2. Watch the test tube and record your observations.

III. Change of odor:

1.Add 3.0 mL of 8 M NaOH to a test tube. Wave your hand over the test tube and note the odor. To another test tube add 3.0 mL of 1.0 M NH4Cl. Again note the odor. Pour the NH4Cl into the NaOH. Mix well, and again cautiously note the odor. Record your observations.

IV. Change of color:

1.Place 5.0 mL of CuSO4 solution in a large 20 x 150 mm test tube and note the color of this solution. Add 6 mL of 3.0 M NH4OH, stir, and set the tube in the block for 5 minutes. Compare the color of the resulting solution with the color of the original CuSO4.

2.Place 5.0 mL of the CuSO4 solution in a large 20 x 150 mm test tube and note the color. Add 2.0 mL of saturated KBr. Stir and note the color by comparing with the original color of the CuSO4.

V. Testing unknowns:

Place 2 mL of each solution noted on the data sheet together and note the results. Make a conclusion as to whether or not a reaction has taken place by noting 'yes' or 'no' under the column marked REACTION. Give your reason, that is, list one or more of the above indicators you have observed.

Report Format

Date ______Name ______

______

I. Formation of a precipitate

Observations / Do you believe a chemical reaction took place? / Why or why not?
CaCl2 + Na2C2O4
CaCl2 + NaCl
Pb(NO3)2 + K2CrO4
Wt vial A + Pb(NO3)2 / Wt Vial B + K2CrO4
Wt. Vial A / Wt Vial B
Wt Pb(NO3)2 / Wt K2CrO4

Wt vial A + Pb(NO3)2 + K2CrO4 ______

The Law of Conservation of Mass states that in a chemical reaction, the total weight of the reactants equal the total weight of the products. Use the above numbers to confirm that statement.

II. Formation of a gas

Observations / Do you believe a chemical reaction took place? / Why or why not?
NaHCO3 + HCl
CaCl2 + HCl

III. Change of odor

Observations / Do you believe a chemical reaction took place? / Why or why not?
NaOH + NH4Cl

IV. Change of color

Observations / Do you believe a chemical reaction took place? / Why or why not?
CuSO4 + NH4OH
CuSO4 + KBr

V. Testing Unknowns

Observations / Do you believe a chemical reaction took place? / Why or why not?
A + B
B + E
A + E
D + B
D + E

Revised Ali/Fall 2006