Chemical Changes and Equations
There are many ways to detect or “observe” a chemical change. Some highly precise
instruments enable us to measure not only the degree of change in composition but also
the change in energy which takes place when substances react chemically.
Endothermic reactions: uses energy as a reactant.
Exothermic reactions: Releases energy as a product.
A chemical equation represents a chemical reaction and indicates the substances that
interact, the reactants and the new substances, or products that are formed. For the
equation to be completely correct it must be balanced. The balanced equation also
indicates the ratio by which the reactants combine and the relative amount of product
formed.
In this experiment you will examine four classes of chemical reactions: synthesis,
decomposition, single replacement and double displacement. You will be asked to
identify some of the products of the reactions and write balanced equations for the
reactions observed.
Equipment
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2 large test tubes
clay triangle
small test tubes
clamp
stopper with tubing
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crucible
forceps
ring stand and ring
wooden splint
evaporating dish
Procedure
Part A: Synthesis
1. Obtain a piece of magnesium ribbon about 13cm long and roll it into a loose ball.
Place the Mg in a clean dry crucible and measure the mass of the crucible and its
contents to the nearest 0.01g. Record the mass.
2. Place the crucible in a clay triangle and place the clay triangle on a ring stand with
a Bunsen burner under the clay triangle.
3. Begin heating, slowly at first and gradually increase the intensity of the heat by
increasing the flame.
4. The Mg will begin to burn and give off a bright light.
Caution: Do not look directly at the light.
5. When the reaction is complete turn off the flame. Allow the crucible to cool
slightly and remove it from the clay triangle with the forceps.
6. Measure the mass to the nearest 0.01g.
7. Examine for any observable changes.
8. Empty crucible contents into waste beaker.
Part B: Decomposition
1. Obtain two small spatulas full of copper (II) carbonate (CuCO3) and place them in
a large dry test tube.
Caution: CuCO3 is poisonous. Avoid contact with skin and eyes.
2. Insert the stopper with the glass tubing in the test tube containing CuCO3.
3. Attach the stoppered test tube to a test tube clamp on a ring stand as shown in the
diagram:
4. Pour about 5mL of limewater into a small test tube.
Caution: Limewater causes burns, avoid contact with skin.
5. Place the end of the glass tube from the stopper into the limewater in the second
test tube. Be careful that the glass tube is not touching the bottom of the test tube
filled with limewater.
6. Heat the tube containing the CuCO3 slowly while holding the end of the glass
tube in the limewater solution.
7. Continue heating until the limewater turns a cloudy or “milky” colour.
8. A cloudy appearance indicates the presence of CO2. Record your observations.
9. Let the test tube cool completely and empty the contents of both tubes into the
waste beaker.
Part C: Single Displacement
1. Using the same apparatus as in Part B, place a small piece of zinc in a test tube
and add 5mL of 6M HCl. Caution: Keep away from open flame. HCl causes
burns; avoid contact with skin and eyes.
2. Insert a rubber stopper containing the glass delivery tube with the glass tube
pointing up.
3. A reaction should occur and a gas should escape from the tubing. Collect some of
the gas produced by holding a test tube (with a test tube holder) inverted over the
end of the glass tubing.
4. Remove the test tube containing the gaseous product from the glass tubing but
keep it inverted. Bring a burning splint near the mouth of the test tube. A “pop”
indicates the presence of H2 gas,
5. Once the reaction has stopped, pour the contents of the test tube into the waste
beaker and rinse all test tubes well with water.
Part D: Double Displacement
1. Add 5mL of potassium iodide solution to a large test tube. Add about 10 drops of
lead (II) nitrate solution to the KI in the test tube.
Caution: Pb(NO3)2 is poisonous. Avoid contact with skin and eyes.
2. Observe the reaction and record these observations.
3. Pour contents of test tube into the labelled waste beaker. Do not pour down the
sink; do not rinse test tubes in the sink.
Results:
Prepare a data table for your experiments. Here is an example:
Reactants
Sodium and water
Observations
Products
Sodium moves across the Sodium hydroxide
water as it reacts, a gas is hydrogen gas
released
and
Analysis:
1. What evidence can be seen in your results to show that the reaction that occurred
in Part A is in fact a synthesis reaction?
2. What evidence is present in Part A and Part B to indicate a chemical change?
3. Why must the gas tube point down for Part B and up for Part C?
4. Why does a “pop” indicate the presence of hydrogen gas?
5. Was a precipitate, a gas or water formed in part D? If so, indicate which product
was the precipitate, the gas or if there was water.
6. Which of the reactions in this lab were endothermic and which were exothermic.
How can you tell?