Chemical Changes and Equations

Objectives

In this experiment, you will

  • perform four types of chemical reactions and identify macroscopic evidence of chemical change
  • identify the reactants and the products of the reactions
  • write and balance equations for the reactions observed

Data Table

Reaction / Reactants / Observations / Products
Synthesis
Decomposition
Single Displacement
Double Displacement
Example: Synthesis / Na + Cl2 / Light, heat given off; white crystals form / NaCl

Hypothesis ~

Read through the procedures carefully. Based on the reaction type, the description and the reactants given, make a prediction about the products you will form in each reaction.

Procedure

  1. Synthesis
  1. Obtain a piece of magnesium about 13 cm long and roll it into a loose ball. Place the magnesium in a clean, dry crucible and measure the mass of the crucible and its contents carefully to the nearest 0.01 g. Record the mass.
  2. Place the crucible in a clay triangle, and place the clay triangle on a ring attached to the ring stand. Begin heating, slowly at first. Gradually increase the intensity of heat to the hottest flame of your laboratory burner.
  3. When the magnesium begins to glow brightly, stop heating. IF THE MAGNESIUM DOES BURN, DO NOT LOOK DIRECTLY INTO THE FLAME.After the crucible has cooled slightly, remove it from the clay triangle with forceps. Place the crucible on the lab countertop and cool completely. (There are 2 synthesis reactions at play here! What two gases in the atmosphere might the magnesium metal be reacting with?)
  4. Measure the mass when the crucible has completely cooled.Record and compare the new mass with the original mass. Empty the crucible’s contents into an evaporating dish.
  5. Examine the contents for a change in composition. Add a few drops of water to the residue and try to detect the odor of ammonia gas, NH3.
  6. Discard the solid contents into the waste container designated by your teacher.
  1. Decomposition
  1. CAUTION: Avoid skin contact with copper (II) carbonate and limewater.
  2. Obtain two small spatulas full of copper (II) carbonateand place them in a large, clean, dry test tube.
  3. Clamp the larger test tube onto the ring stand, and then insert the one-hole rubber stopper with bent glass tubing into the test tube. Angle the bent glass tubing downward.
  4. Place approximately 3 cm3(3 droppers full) of limewater, Ca(OH)2(aq), into a second, smaller test tube. Use the test tube holder to secure this test tube.
  5. Heat the tube containing the copper (II) carbonate while the end of the bent glass tubing is in the limewater solution. Observe changes that occur to both copper (II) carbonate and the limewater solution.
  6. Discard the solid left over from heating the CuCO3 into the waste container designated by your teacher. The limewater solution may be rinsed down the drain using plenty of water.

  1. Single Displacement
  1. CAUTION: Hydrochloric acid causes burns; avoid skin contact.
  2. Place a small piece of zinc in a large, clean, dry test tube. Clamp the test tub in place and then insert the one-hole rubber stopper with bent glass tubing into the test tube. Angle the bent tube so that it is pointing up (perpendicular to the floor).
  3. Holding an additional test tube with your test tube holder, place it over the opening of the glass tubing.
  4. Remove the stopper from the apparatus, and add 5 cm3 of 6Mhydrochloric acid. Immediately reinsert the stopper with tubing and additional test tube.
  5. A reaction should occur and a gas should escape from the tubing. With the glass tubing turned upward, the gas being liberated will be collected into the inverted test tube. (This collection method is called air displacement.)
  6. Keeping the test tube with the gas inverted, remove the test tube from the glass tubing. Keep it inverted and bring a burning splint near its mouth. A “pop” or “bark” indicates the presence of hydrogen gas.
  7. Remove the rubber stopper from the test tube. Carefully, feel the base of the test tube containing the zinc and hydrochloric acid. Carefully add tap water to the contents of the test tube and decant the liquid part into the sink with plenty of water running down the drain.
  8. The solid waste may be discarded into a waste container or returned to the reagent table as directed by your teacher.
  1. Double Displacement
  1. CAUTION: Nitrates can be corrosive. Avoid skin contact.
  2. Add 5 cm3 of sodium sulfate solution to a clean, dry test tube. Add about 10 drops of barium nitrate solution, to the sodium sulfate solution.
  3. After observing the formation of a precipitate, empty the liquid and solid contents into the waste container designated by your teacher.

ANALYSIS/CONCLUSION QUESTIONS

  1. Write balanced chemical equations for each of your five (there are 2 for the synthesis) reactions. Indicate the type of reaction for each.
  2. Which of the reactions in this experiment were exothermic and which were endothermic? Provide evidence to support your decision.
  3. How can you tell if a chemical reaction has occurred? What are some distinctive macroscopic changes that can be observed?
  4. For the synthesis reaction:
  5. Describe the change in mass from your reactant to your product(s). Explain the reason for this change.
  6. When water was added to your product, were you able to detect the smell of ammonia (NH3)? Why?
  7. Write balanced chemical equations for the following reactions including symbols for states of matter, and identify the type of reaction that is being described for each:
  8. Aqueous solutions of lead (II) nitrate and phosphoric acid (H3PO4) react to form a precipitate of lead (II) phosphate and an aqueous nitric acid (HNO3) solution.
  9. Sulfur trioxide gas is passed over solid magnesium oxide to form solid magnesium sulfate.
  10. Solid ammonium nitrate is heated and yields dinitrogen monoxide and water vapor.
  11. Aqueous chromium (III) nitrite and calcium metal react to produce chromium metal and calcium nitrite.