Grade Level/Course: Science 8 / Physical Science Lesson/Unit Plan Name: H2O vs. H2O2: Chemical Properties and Formulas Rationale/Lesson Abstract: This lesson will demonstrate some of the differences in chemical properties between water (H2O) and hydrogen peroxide (H2O2), as well as provide an example to begin understanding how chemical formulas are written. Extensions can include designing an experiment to test an effect, chemical bonds, rearrangement of atoms in reactions, as well as balancing equations. Timeframe: 2+ class periods Standard(s): CST standard: REACTIONS
5.
Chemical reactions are processes in which atoms are rearranged into different combinations of molecules.
As a basis for understanding this concept:
a. Students know reactant atoms and molecules interact to form products with different chemical
properties.
b. Students know the idea of atoms explains the conservation of matter: In chemical reactions the
number of atoms stays the same no matter how they are arranged, so their total mass stays the
same.
c. Students know chemical reactions usually liberate heat or absorb heat.
NGSS: MS: CHEMICAL REACTIONS
Disciplinary Core Ideas
PS1.A: Structure and Properties of Matter § Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it. (MS-­‐PS1-­‐2) PS1.B: Chemical Reactions § Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants. (MS-­‐PS1-­‐2),(MS-­‐PS1-­‐5) § The total number of each type of atom is conserved, and thus the mass does not change. (MS-­‐PS1-­‐5) § Some chemical reactions release energy, others store energy. (MS-­‐PS1-­‐6) Page 1 of 12
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Instructional Resources/Materials: Activities may be done as demonstrations for class, or in groups. Food color activity: • Food coloring • Tap water • Hydrogen peroxide • 2 x 250mL beakers Meat activity • small samples of ground beef • 2 x small petri dish or Dixie cup • Tap water • Hydrogen peroxide ‘Elephant toothpaste’ • Empty plastic bottle, 1-­‐2 liter, or large Erlenmeyer flask • Food coloring—blue is nice for this. • Dish soap • Warm tap water • Active dry yeast, (1/2 packet per demo) • 2 x 250 mL beakers • Hydrogen peroxide (6% “20-­‐volume developer”, or 12% “40 volume developer” from a beauty supply store work best for this demo!) • Safety goggles • Metal/plastic tray or newspaper to catch eruption (for easier clean up) Chemical Formula activity • Wooden molecule model sets (or Dots candy and toothpicks) Handouts included below: • Understanding Chemical Formulas • Molecule Models • (extension) Balancing Equations Page 2 of 12
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Activity/Lesson: Background Information The chemical formula for water is H2O. The chemical formula for hydrogen peroxide is H2O2. Though the formulas and physical appearance may seem similar, they are completely different substances, with different chemical properties. The activities and demonstrations in this lesson offer evidence of this fact. For your lab activities and demonstrations, hydrogen peroxide is available in several concentrations. The typical hydrogen peroxide found in grocery and drug stores is 3% H2O2. Beauty supply stores carry “hair developer” called 20-­‐volume (6% H2O2), or 40-­‐volume (12% H2O2). It is even possible to acquire 30% H2O2, but it can present some dangers for untrained presenters. The lab activities and demonstrations provided here demonstrate the decomposition reaction: H2O2 à H2O + O2. Students will observe the release of oxygen—bubbling from meat, and foam in elephant toothpaste. Warm Up / Do Now Fill one beaker with a small amount (≈100mL) of water and the other beaker with an equal amount of hydrogen peroxide. 1. Describe the physical properties of Beaker 1—Water, and Beaker 2—Hydrogen peroxide. Responses may include: both are liquid, clear, colorless, odorless, similar viscosity, etc. 2. Describe what difference you see in the chemical formulas of water (H2O) and hydrogen peroxide (H2O2). Responses may include: Hydrogen peroxide has one more oxygen atom, or an extra ‘O’, or hydrogen peroxide is bigger, or has more atoms, etc. Page 3 of 12
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Activity/Lesson continued: After discussing Warm Up, continue using the same two beakers for Activity 1. The three activities may be demonstration or in groups of students. Activity 1—Food coloring Squeeze one drop of food coloring into each beaker. Do not stir. Students write down observations and differences. (Students should observe that in water, the drop sinks, and spreads slowly. In hydrogen peroxide, the drop will sink and then come up to the surface. The color will spread out mostly on the surface.) Activity 2—Ground beef Place a small bit of ground beef into two small dishes or Dixie cups. Pour water into one cup, covering the meat. Pour hydrogen peroxide into the other cup, covering the meat. Students write down observations and differences. (Students should observe that in water, nothing observable happens. The meat gets wet. However, in hydrogen peroxide, bubbles begin to form around the meat, and soon the solution becomes foamy with bubbles.) Activity 3—‘Elephant toothpaste’ • Quantities that follow are appropriate for a 2-­‐liter bottle container. These can easily be reduced for smaller 500mL water bottles, for example. • Hydrogen peroxide can stain clothes. • Wear safety goggles. • TEST IT ON YOUR OWN BEFORE DOING THIS WITH STUDENTS! Procedures 1. Warm up ≈50mL of water in a beaker to activate the yeast. The water should be ≈100°F, or about what comes out of the hot water tap. 2. Pour in ≈5mL (or ≈1 tsp, or ½ packet) of dry active yeast. Swirl to mix. Let sit a few minutes. 3. Measure out in a beaker ≈125 mL of 12% hydrogen peroxide (‘40-­‐volume hair developer’). 4. Set the 2-­‐liter bottle onto the tray or newspaper. 5. Pour the hydrogen peroxide into the 2-­‐liter bottle. 6. Squeeze in a little dish soap into the 2-­‐liter bottle, say, 10-­‐15mL. 7. Squeeze plenty of food color, several drops, in to the 2-­‐liter bottle. 8. Swirl the 2-­‐liter bottle around to mix the hydrogen peroxide with the soap and color. 9. Ready? Pour the beaker of water and yeast into the 2-­‐liter bottle. 10. Stand back and watch! Students write down observations. (Students should notice rapid eruption of foam, and some heat coming from foam. Bottle will be warm as well.) Page 4 of 12
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Extension Students can use this demonstration, and turn it into an experiment: Ask: “What could you change in this experiment to affect what happened?” Examples might include: changing amount of yeast or water or hydrogen peroxide, different temperatures of water or hydrogen peroxide, different size or shape of container, etc. Students can make a hypothesis, test, measure, record results, make a conclusion, communicate results, etc. Chemical Properties Why do we observe different chemical properties between water and hydrogen peroxide? Although their physical appearance is similar, and the chemical formulas (H2O and H2O2) look similar, they are different substances with different chemical properties. They will behave differently in the same situation. Similarly, even though water and hydrogen peroxide are made of the elements hydrogen and oxygen, when the atoms of those elements are bonded together, they have new, different properties from when the elements are uncombined. What’s happening? The bonds between the oxygen atoms in H2O2 are fairly weak, so they tend to break apart on their own, in what is called a ‘decomposition’ reaction. The bonds that hold H2O2 together break apart and the atoms are rearranged to form water (H2O) and oxygen (O2) molecules. This happens slowly in store-­‐bought hydrogen peroxide, so even in dark bottles, it will ‘expire’ in a year, or less if it’s opened. The yeast in our demonstration acts as a catalyst to speed up this reaction. And higher concentrations of H2O2 produce more oxygen. Why is it warm? Chemical reactions involve energy. Sometimes the energy produced by the reaction is more than what goes into making the reaction happen. When that happens, energy is released, usually as heat, and it is called an “exothermic” reaction. Other examples of exothermic reactions include burning (or combustion) and glow sticks. The opposite can also happen, where energy is absorbed, like in instant cold packs or photosynthesis. These reactions are called “endothermic”. Page 5 of 12
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Chemical Formulas Looking at the difference between water (H2O) and hydrogen peroxide (H2O2) offers a good introduction to how chemical formulas are written. The subscript is the smaller number to the lower right of each symbol. The subscripts show the number of atoms in each molecule of the compound. Where there is no number, one atom is implied. For example, in each molecule of ferric chloride: FeCl3, there is one atom of iron (Fe) and three atoms of chlorine (Cl). You try: Describe which elements and how many atoms are in each molecule of CO2 (carbon dioxide). “There is one carbon atom and two oxygen atoms in every CO2 molecule.” You try: What elements are in Na2S3 (disodium trisulfide), and how many atoms of each element are in each molecule? “There are two sodium atoms and three sulfur atoms in every molecule.” Parentheses are used in chemical formulas for groups of atoms that appear together multiple times in a molecule. For example, in calcium phosphate: Ca3(PO4)2 , there are two groups of phosphate (PO4) in each molecule. A coefficient shows the number of whole molecules in a formula. In a chemical equation, it shows the ratio of molecules present in the reaction. Similar to a coefficient in math, it multiplies all that comes after it. Again, where there is no coefficient, a ‘1’ is implied. For example, 4H2O is 4 separate molecules of water: Building Molecule Models Students can build models of molecules to show their understanding. Wooden or plastic molecule models are great, or alternately, Dots candy and toothpicks will also work. Examples à Hydrogen peroxide (H2O2) Water (H2O) Oxygen (O2) You try: Build models, or draw diagrams, or both. Use different colors for the different elements. Don't worry so much here about arrangement—
just the numbers of atoms in each molecule. 1. ammonia: NH3 2. aluminum oxide: Al2O3 3. sodium sulfate: Na2SO4 4. What would this look like? 2 NaCl: Page 6 of 12
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Balancing Equations Finally, as a high school level extension or prep, this lesson can offer an introduction to balancing equations, also known as “stoichiometry”. The Law of Conservation of Matter states that in a chemical reaction, matter is neither created, nor destroyed. The mass of the reactants will be equal to the mass of the products. All of the atoms in the reactants will be rearranged, and present, in the products. You can easily see, using models, that the Law of Conservation of Matter is not true here in this equation where hydrogen peroxide decomposes into water and oxygen. The reason is that the equation in its general state, H2O2 à H2O + O2, is unbalanced. Balancing equations requires the use of coefficients. In reality, it takes two molecules of hydrogen peroxide for this reaction to happen, and it produces two molecules of water and one molecule of oxygen, shown below: Using coefficients, this is how the balanced equation is written: 2 H2O2 à 2 H2O + O2 , which reflects more realistically how the reaction occurs. Steps to balancing an equation: 1. Write the equation, and draw diagrams of each molecule 2. Count the atoms on each side of the equation: reactants, then products. 3. Use coefficients to balance each side—Do not change subscripts! 4. Check if balanced. (If not, repeat steps.) You try: Balance these equations to show how matter is conserved in a chemical reaction: 1. N2 + O2 à N2O balanced: 2 N2 + O2 à 2 N2O 2. Na + H2O à NaOH + H2 balanced: 2 Na + 2 H2O à 2 NaOH + H2 Assessments: Page 7 of 12
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Example:
The
chemical formula for carbon dioxide is CO2.
Carbon
dioxide is a compound, because it is made of multiple elements: carbon, oxygen.
• Use the symbols on the periodic table to find which elements are present in a compound.
Each
molecule of carbon dioxide (CO2) has one atom of carbon and two atoms of oxygen.
• The subscript tells you how many atoms of that element are present in each molecule.
• The coefficient tells you the number of molecules present.
• So a diagram for 3 CO2 might look like this:
Answer the following. You will need a Periodic Table.
How many different elements are in boric acid: H3BO3? ________
1.
Which elements are in ferric chloride: FeCl3? ________________________________________
2.
3.
Which elements are in sodium silicate: Na2SiO3? _____________________________________
4.
Which elements are in acetaminophen: C8H9NO2? ____________________________________
5.
How many boron atoms are in each molecule of boric acid: H3BO3? ________
6. What does the ‘5’ mean in vanadium pentoxide: V2O5? ________________________________
7. How many atoms, in total, are in each molecule of Vitamin C: C6H8O6? ________
How many atoms of oxygen are in each molecule of magnesium phosphate: Mg (PO ) ? ______
8.
3
4 2
How many whole molecules are in 3 (NH4)2SO4? ________
9.
10.
Which of these represents 2 H2O2?
H
H
H
H
H H H
O
H
a.
b.
c.
d. O
O
O
O O
H
H
O
H
H
H
O
O O
O
11.
Which of these could be a model of sodium silicate, Na2SiO3?
O
Na Si O
SiO
O
i
a
O
Na
Na
S
SiO
a. Si
b. N
c.
d. Na Si O
Na
Na
a
O
O
O
O
SiO
O
O
12.
Sketch a diagram of 3 NH3 :
Understanding+Chemical+Formulas+
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H
GROUP MEMBERS: __________________
__________________
__________________
PLEASE
READ THE INSTRUCTIONS:
• Draw a diagram of each of the following molecules, using letters to represent atoms.
• Then build models of the molecules.
o Each element should be represented by one color. You can change the colors from one model to
the next. If you need more of a color, trade with another group.
• Raise your hand when you are finished with a model and I will check it, and initial it for
credit. (If I am busy, just move on to the next one.) After you have received credit for a
model, you may dismantle it.
2
3
Draw
your diagram of H2O in this box.
Then
build your model.
I will come around and check it off.
3
4
MOLECULE MODELS
_____ 1. H O
_____ 2. NH
_____ 3. KClO
_____ 4. CaSO
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_____ 5. Ca(OH)2
_____ 6. Pb(NO3)2
_____ 7. 2 H2O2
_____ 8. 3 KCl
IF YOU HAVE TIME…
DRAW DIAGRAMS AND BUILD MOLECULE MODELS
FOR EACH MOLECULE IN THE FOLLOWING REACTIONS.
_____ 9.
N2
+
H2
→
NH3
_____ 10.
N2
+
3 H2
→
2 NH3
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Name ________________________
Date _______________
Period _____
Balancing Equations
Balance the following equations. Remember that you may only use coefficients to show more
whole
molecules. DO NOT change the subscripts!
Example:
CH4
+
O2
→
CO2
+
CH4
+ 2 O2
→
CO2
+ 2 H2O
H
H–C–H
H
1.
Na
+
I2
→
O–O → O–C–O
O–O
H2O
(unbalanced)
(balanced)
H–O–H
H–O–H
NaI
4.
KClO3
5.
S
6.
Al
→
KCl
+
O2
2.
N2
+
O2
→
N2O
+
O2
→
SO3
3.
N2
+
H2
→
NH3
+
Cl2
→
AlCl3
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more on the back →
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7.
Fe
+
H2O
→
Fe3O4
+
H2
8.
K3PO4
+
HCl
KCl
→
+
H3PO4
9.
HCl
+
Ca(OH)2
CaCl2
→
+
H2O
10.
Fe
+
O2
Fe2O3
→
These are a little harder…
11.
Mg(NO3)2
+
K3PO4
12.
C2H6
O2
→
+
Mg3(PO4)2
→
CO2
+
+
KNO3
H2O
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