Lab 15: Chemical Reactions II

 

Lab 15: Chemical Reactions II
Introduction
Chemical reactions and laundry: what’s the connec‐
tion?
Laundry sometimes involves different colors and fabrics that re‐
quire special treatment for washing and drying. You might not
want to wash your white dress shirt with a bright and new red
sweater—not unless you want your white shirt to end up not‐so‐
white. To avoid this, it helps to sort your laundry into colors,
darks, and whites. In a similar fashion, chemical reactions can be
sorted into categories based on the characteristics of the reac‐
tions.
There are many types of reaction and many ways to compare and
contrast them. A common way of classifying chemical reactions is
to use the following five categories: combustion, synthesis, single
replacement, double replacement, and decomposition. Most re‐
actions can be placed into these categories, including the reactions we will observe in this lab.
The first reaction you will perform is a combustion reaction. During combustion, a hydrocarbon and oxygen break into two
simple compounds: water vapor and carbon dioxide gas. A hydrocarbon is a molecule that contains only carbon and hydro‐
gen atoms. To illustrate, methane (CH4), sometimes called “natural gas,” will combust with the oxygen in the air. The equa‐
tion for this reaction is:
CH4(g) + O2(g) → CO2(g) + H2O(g)
In the chemical reaction given above, all of the reactants and products are given, but is the equation balanced? From the
law of conservation of mass, we know that atoms are never destroyed or created. In the chemical equation above there is
1 C on both sides, but there is 4 H and 2 O on the left side (reactant side) of the arrow and 2 H and 3 O on the right side
(product side) of the arrow. This means that the equation is not balanced. If you answered no, you are correct!
There are four steps used to balance a chemical equation:
1. Count the number of atoms of each element on both the reactant and the product sides.
2. Determine which atoms are not the same for both sides.
3. Balance one element at a time by changing the coefficients for the molecules in the reaction and not their
chemical formulas.
4. After you think the chemical equation is balanced, check it as in step 1.
Figure 1: Methane, the main component of natural gas,
is a common fuel used for home heating and cooking.
Other gases that combust in a similar way (forming CO2
and H2O) are propane and butane.
Concepts to explore:
 Observe chemical reactions and identify the reactants and products
 Classify types of chemical reactions
 Practice balancing chemical equations158
Now let’s use these four steps to balance the chemical equation.
Step 1:
Step 2: The hydrogen and oxygen are not balanced.
Step 3: Insert a 2 before the O2 on the reactants side and a 2 before the H2O on the products side.
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
Step 4:
Now the chemical equation is balanced! Some of the chemical equations for the reactions that you will observe are given
in this introduction as unbalanced. Try to balance each one as they are given.
The combustion reaction you will observe is the reaction of butane (C4H10) from a lighter with the oxygen in air. The unbal‐
anced reaction equation is
C4H10 + O2 → CO2 + H2O
The next type of reaction you will perform is a synthesis reaction. A synthesis reaction takes two or more substances and
combines them to create a more complex substance. A general reaction equation for this type of reaction is:
A + B → C
The chemical equation for the synthesis reaction that you will peform is:
Hb(s) + O2 HbO2 (s)
Reactants Products
1 C 1 C
4 H 2 H
2 O 3 O
Reactants Products
1 C 1 C
4 H 4 H
4 O 4 O
Hemoglobin Oxygen Oxyhemoglobin
Lab 15: Chemical Reactions II159
In this reaction, oxygen is taken up through the lungs and enters into blood‐
stream. When oxygen levels are high in the lung, oxygen binds to the hemes in
hemoglobin molecules (abbreviated Hb), resulting in the formation of the prod‐
uct, oxyhemoglobin. In the tissues, where the oxygen level is slightly lower, the
reverse reaction occurs, releasing oxygen from the hemoglobin. This reaction
allows the blood to transport and exchange oxygen throughout bodily tissue.
Following the synthesis reaction, you will perform a single replacement reac‐
tion. This type of reaction takes place when a more reactive element replaces
one component of a compound. Two or more reactants will produce two or
more products. A common example of a single replacement reaction is when
one metal replaces another in a compound. A general reaction equation for this
type of reaction is:
A + BC → AC + B
The unbalanced chemical equation for the single replacement reaction that you
will observe is:
Zn(s) + H3C6H5O7 (aq) → Zn3(C6H5O7)2 (aq) + H2 (g)
This reaction uses a zinc coated (galvanized) washer and citric acid. Zinc is more reactive than the hydrogen it replaces. The
zinc citrate stays in the solution and hydrogen gas is given off. When this reaction is allowed to continue to completion,
the zinc coating almost disappears.
The fourth type of reaction you will perform is a double replacement reaction. It involves two different ionic compounds
that exchange components in the reaction. Most single or double replacement reactions take place in an aqueous solution
where the free ions can float around and react. The general format for the reaction is:
AB + CD → AD + CB
The unbalanced chemical equation for the double replacement reaction that you will observe in this lab is:
Zn(C2H3O2)2 (aq) + Na3PO4 (aq) → NaC2H3O2 (aq) + Zn3(PO4)2 (s)
In this reaction the zinc and sodium change places. The zinc bonds to phosphate ions and sodium bonds to acetate ions.
Zinc phosphate is not soluble in water like the two reactants, and after the two reactants are mixed a precipitate of zinc
phosphate is produced.
A decomposition reaction is the last type of reaction you will do. It is much like a synthesis reaction running in reverse. In
this type of reaction, a more complex compound breaks down into a less complicated compound or elements.
C → A + B
Figure 2: Oxyhemoglobin molecule. Hemo‐
globin molecules transport oxygen in the
bloodstream, and are vital to the circula‐
tory system.
Lab 15: Chemical Reactions II160
Pre‐lab Questions
1. Michelangelo used fresco painting when he painted the Sistine Chapel. Fresco painting involves most of the
types of chemical reactions you just studied. Listed below are some of the reactions used in creating a Fresco
painting. Identify the type of chemical reaction used for each step and balance the chemical equation if
needed.
Initially, some sort of heat must be generated. Propane is an example of a common fuel source used for heat‐
ing.
C3H8 + O2 → CO2 + H2O
This is a reaction.
Next, quicklime (calcium oxide) is made by roasting calcium carbonate (limestone).
CaCO3 → CaO + CO2
This is a reaction.
The quicklime is slaked to form lime plaster.
CaO + H2O → Ca(OH)2
This is a reaction.
The lime plaster is cured or dried.
Ca(OH)2 + CO2 → Ca(OH)(HCO3)
This is a reaction.
Lab 15: Chemical Reactions II
The unbalanced chemical equation for the decomposition reaction that you will observe in this lab is:
(NH4)2CO3 (s) → NH3 (g) + H2O(g) + CO2(g)
As shown, the ammonium carbonate decomposes when heated to form three gases: ammonia, water vapor, and carbon
dioxide. These five categories of reactions will give you a good foundation to understand reaction processes throughout
chemistry.161
This quickly continues to react to form calcium carbonate and water.
Ca(OH)(HCO3) → CaCO3 + H2O
This is a reaction.
Frescos will deteriorate over time when exposed to the damp, acidic environments typical of modern urban
city atmospheres.
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
This is a reaction.
2. Balance each of the chemical equations you will be doing in this laboratory exercise.
Combustion:
C4H10 (g) + O2 (g) → CO2 (g) + H2O(g)
Synthesis:
Hb (s) + O2 → HbO2 (s)
Single Replacement:
Zn (s) + H3C6H5O7 (aq) → Zn3(H3C6H5O7)2 (aq) + H2 (g)
Double Replacement:
Zn(C2H3O2)2 (aq) + Na3PO4 (aq) → NaC2H3O2 (aq) + Zn3(PO4)2 (s)
Decomposition:
(NH4)2CO3 (s) → NH3 + H2O + CO2 (g)
Lab 15: Chemical Reactions II162
Experiment: Getting to Know Your Reactions
Procedure
Part 1: Combustion
C4H10 (g) + O2(g) → CO2 (g) + H2O (g)
1. Light a butane lighter and observe the flame. (The ignition of the flame is a reaction between butane and the
oxygen in the air you breathe.)
2. Record your observations in the data table from when the lighter is turned on until it is turned off.
Part 2: Synthesis (A + B → C)
Hb(s) + O2 → HbO2 (s)
1. Take a deep breath, hold it as long as possible and then exhale. Visualize the reaction occurring.
2. Record your observations before inhaling and after you exhale.
3. Construct an oxyhemoglobin molecule with modeling clay and toothpicks. HINT: See figure of molecule in intro‐
duction as a guide.
Part 3: Single Replacement (A + BC → AC + B)
Zn (s) + H3C6H5O7 (aq) → Zn3(C6H5O7)2 (aq) + H2 (g)
1. Place a test tube in a test tube rack or small beaker.
Lab 15: Chemical Reactions II
(unbalanced)
(unbalanced)
(unbalanced)
Materials
Safety Equipment: Safety goggles, gloves Spatula
2 mL 0.1M Zinc acetate (Zn(C2H3O2)2) Tea light candle
2 mL 0.1M Sodium phosphate tribasic (Na3PO4) 3 test tubes
Ammonium carbonate powder, (NH4)2CO3 Test tube holder
2 mL Saturated citric acid solution Test tube rack
Zinc‐coated (galvanized) washer Modeling clay
Baking soda Toothpicks
Butane lighter163
2. Slightly tilt a test tube and slide a small zinc‐coated (galvanized) washer down the side.
3. Use a 10 mL graduated cylinder to measure out approximately 2 mL of saturated citric acid and carefully
pour it into the test tube containing the zinc washer. CAUTION: Citric acid is irritating to the eyes and skin.
4. Observe the reaction for several minutes, and record your observations in the data table.
5. To clean up, separate the acid solution from the washer by pouring it into a small beaker while leaving the
washer in the test tube. This is called decanting. Rinse the test tube containing the washer several times with
water and add each rinse to the beaker—CAUTION: Do not pour the acid directly down the drain. To neu‐
tralize the acid, add small amounts of baking soda to the solution in the beaker and stir with a stirring rod.
6. Continue stirring and adding small amounts of baking soda until gas no longer forms. Pour the liquid down
the drain and throw the washer in the trash.
Part 4: Double Replacement (AB + CD → CB + AD)
Zn(C2H3O2)2 (aq) + Na3PO4 (aq) → NaC2H3O2 (aq) + Zn3(PO4)2 (s)
1. Pour approximately 2 mL of 0.1 M zinc acetate (Zn(C2H3O2)2) into a clean test tube.
2. Add approximately 2 mL of 0.1 M sodium phosphate tribasic (Na3PO4) into the test tube.
3. Record your observations before and after the addition of Na3PO4 in the data table.
4. To clean up, pour the contents of the test tube down the drain.
Part 5: Decomposition (AB → A + B)
(NH4)2CO3 (s) → NH3 + H2O + CO2 (g)
1. Place a spatula tip full (approximately 0.02 g) of ammonium carbonate (NH4)2CO3, powder into a test tube.
CAUTION: Do not inhale the strong ammonia odor. Try to work in well ventilated area.
2. Light the candle using the butane lighter. CAUTION: Long hair should be tied up and loose clothing re‐
strained when around an open flame to prevent fire and burns. Be sure you are wearing your safety gog‐
gles.
3. Use a test tube holder to hold the test tube containing the ammonium carbonate at a slight angle in the can‐
dle flame. Keep the open end of the tube pointed away from you and other students. Continue to heat the
sample until the reaction is finished. Hint: Remember the products of this reaction are all gases.
4. Record your observations in the Data section.
5. Allow the test tube to cool to room temperature before touching it. CAUTION: The test tube will be very
hot and can burn your skin if touched before it cools. Hint: After the test tube has cooled for a few sec‐
onds, place it in a small beaker or test tube rack to finish cooling.
6. Extinguish the candle. Wash out the test tube with soap and water.
Lab 15: Chemical Reactions II
(unbalanced)
(unbalanced)164
Data
Reaction Before the Reaction After the Reaction
Combustion
Synthesis
Single
Replacement
Double
Replacement
Decomposition
Table 1: Reaction Observations, Procedures 1‐5
Lab 15: Chemical Reactions II165
Post‐lab Questions
1. Write the combustion reaction that occurs when you cook out on a propane gas grill. Propane has the chemi‐
cal formula C3H8. Make sure to balance the reaction equation.
Balance the following equations and identify the type of reaction.
BaCl2 (s) + K2SO4 (aq) → BaSO4 (s) + KCl (aq)
KClO3 (s) → KCl (s) + O2(g)
H2 (g) + O2 (g) → H2O (l)
F2 (g) + LiCl (aq) → LiF (aq) + Cl2 (l)
a.
b.
c.
d.
Lab 15: Chemical Reactions II

 

 

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