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 WCCUSD 11/07/14 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 WCCUSD 11/07/14 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 WCCUSD 11/07/14 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 WCCUSD 11/07/14 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 WCCUSD 11/07/14 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.