Sulfuric acid is hazardous: Safety glasses are REQUIRED during this experiment.

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1 CHEMICAL COMPOSITION Life exists on Earth because of the abundant presence of liquid water. While other planets have water, it may be primarily found as either a gas, as on Venus, or as a solid, such as on Mars. However, recent explorations of the surface of Mars have revealed the presence of water that existed millions of years ago. Thus the chemical nature of water is an important chemical to us, all living organisms, and to the survival of our planet. THE CHEMICAL COMPOSITION OF WATER Compounds can be separated into their elements by chemical means, such as heating them (to very high temperatures) or passing electricity through them. In any chemical reaction, atoms are conserved. This means that if a given number of atoms react in a chemical change, the same kind and number of atoms will be present in the products. In other words, chemical reactions produce new arrangements of atoms, not new atoms. For the decomposition of water, we can see that atoms are conserved by comparing the number of atoms (not molecules) present in the reactants and products. In your supplies you will find magnetic spheres of different colors and sizes. The larger spheres represent oxygen atoms and the smaller spheres represent hydrogen atoms. Construct 6 models of water molecules. How many hydrogen ATOMS in total do the models contain? 12 How many oxygen ATOMS? 6 What is the ratio of the hydrogen atoms to oxygen atoms? 2 : 1 Decompose the water molecule models to form molecules of hydrogen gas (H 2 ) and oxygen gas (O 2 ). Hydrogen and oxygen gas are examples of diatomic molecules, that is, they only exist free in nature as a combination of two atoms. Nitrogen gas (N 2 ) and chlorine gas (Cl 2 ) are also diatomic gases. How many oxygen MOLECULES are formed by the DECOMPOSITION? 3 How many hydrogen MOLECULES are formed? 6 What is the ratio of hydrogen to oxygen molecules? 2 : 1 Write a balanced chemical equation for the DECOMPOSITION of water. 2 H2O 2 H2 + O2 PART I: ELECTROLYSIS OF WATER Materials: DC electrical supply, electrodes, 2-20 ml test tubes, beaker, wood splints, matches, wire connectors, sulfuric acid solution. Sulfuric acid is hazardous: Safety glasses are REQUIRED during this experiment. The decomposition of a substance by passing electricity through it is called ELECTROLYSIS. Set up the apparatus as shown in the diagram to the right. Make sure that all connections are tight. Notice that the end of the electrode should not go all the way up into the graduated cylinder. Also, note the color of wires that are connected to the terminals of the power source: Positive Terminal (red) Negative Terminal (white) Plug in the power source. Chemical Composition 1

2 Since this reaction is very slow with pure water it is necessary to add a substance to aid in the conductance of electricity. Add approximately 20 ml of sulfuric acid to the water and stir. Make certain that the electrodes are under the collecting cylinders and that all the gas produced is collected into the cylinders. When one of the graduated cylinders is full, unplug the power source. Estimate the volume of gas in the other cylinder. Positive Terminal (red) Gas Volume: Half Full Negative Terminal (white) Gas Volume: Full Remove the electrode from the cylinder full of gas, but keep the electrode in the beaker. Place your finger over the end of the cylinder and remove the cylinder from the beaker. Keep the cylinder upsidedown once it has been removed. This cylinder contains hydrogen gas. To test for this gas, ignite a wood splint and place the BLAZING splint quickly inside of the cylinder. What are your observations? Small pop. This is the hydrogen gas reacting with the oxygen gas in the air. This reaction is similar to the balloon in lecture except on a smaller scale. Reconnect the power supply and continue collecting oxygen gas in the other cylinder until it is full. To test for oxygen, ignite a wood splint and blow it out so that an orange GLOWING ember is present. Place the glowing splint into the cylinder and record your observations. The splint re-ignited. Oxygen is required for combustion (burning). PART II: COLLECTING OXYGEN & HYDROGEN IN THE SAME TUBE Repeat the experiment, this time collecting BOTH gases together in one cylinder. Be sure that the two electrodes do not touch while in the cylinder. Ignite the mixture of gases with a BLAZING SPLINT. What are your observations? A much larger pop occurs since there is hydrogen and oxygen in the tube. This reaction is more vigorous since the amount of oxygen present in the test tube is greater than the concentration in the air. In the space shuttle s main engines, liquefied hydrogen and oxygen (which are stored in the central external fuel tank) react to form water. The three main components of the external tank are an oxygen compartment containing approximately 550,000 liters of liquefied oxygen at -300 o F and a hydrogen compartment that holds 1.5 million liters at a temperature of -423 o F. The hydrogen tank is 2.5 times larger than the oxygen tank but weighs only one-third as much when filled to capacity. The reason for the difference in weight is that liquid oxygen is 16 times heavier than liquid hydrogen. Disassemble the apparatus and rinse all the components with water. Wipe up any spills which occurred during the experiment. Chemical Composition 2

3 PART III: COMBINING OXYGEN & HYDROGEN In the last reaction where a mixture of gases was ignited, hydrogen and oxygen gas reacted to form water vapor. As in the decomposition process, 2 parts of hydrogen will combine with one part of oxygen. If more hydrogen or oxygen is used than in a 2:1 ratio, some of the remaining gas will remain unreacted. The test tube to the right contains a mixture of 10.0 ml of hydrogen and 10.0 ml of oxygen gas which is suspended in a beaker of water. To react the gases, an electrical spark can be used, the reaction occurs with a bang (as you saw in the last experiment and the hydrogen and oxygen gas react at a 2:1 ratio to form water. Since 10.0 ml of hydrogen gas will react with 5.0 ml of oxygen gas, there was 5.0 ml excess of oxygen gas which remains in the test tube after ignition. The water in the beaker then rises to fill the volume where the reacted gases once occupied. Before Ignition gas water 10 ml Hydrogen 10 ml Oxygen After Ignition water 5 ml Oxygen For the following sets of reactions of hydrogen and oxygen gas determine the identity and quantity of unreacted gas (if any). Sketch a line indicating the volume of unreacted gas that remains (if any) at the top of the cylinder and water below it. Before Ignition After Ignition Before Ignition After Ignition Before Ignition After Ignition 5 ml Hydrogen 15 ml Oxygen 12.5 ml O2 20 ml Hydrogen 20 ml Oxygen 10 ml O2 20 ml Hydrogen 10 ml Oxygen No gas remains. PART IV: CHEMICAL COMPOSITION OF HYDROGEN PEROXIDE O H While hydrogen peroxide (H 2 O 2 ) resembles the chemical composition of water, it has H O significantly different chemical properties. The bond between the two oxygen atoms is weak, so that H 2 O 2 readily decomposes. H 2 O 2, when pure, is extremely unstable, therefore it is commonly sold as a 3% solution in water, making it much safer to handle, but it is still quite reactive. The decomposition of H 2 O 2 is also accelerated by the presence of light, so H 2 O 2 is normally sold and stored in brown bottles. In this experiment yeast is added to act as a catalyst. A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently changed. In the previous experiment sulfuric acid was also used as a catalyst. This does not mean that it is not involved in the reaction. It is "attached" or bonded to the reactants temporarily in the reaction, but the catalyst then returns to its former composition by the time the reaction is complete. Chemical Composition 3

4 Hydrogen peroxide decomposes when applied to a cut. The catalyst that speeds up the decomposition in this case, is an enzyme that is found in the blood. Enzymes are complex proteins that catalyze many reactions in the body. When the hydrogen peroxide decomposes, oxygen is released which kills anaerobic bacteria that often cause infections. Using the magnetic spheres, construct 2 models of hydrogen peroxide molecules. How many hydrogen ATOMS in total do the models contain? 4 How many oxygen ATOMS? 4 What is the ratio of the hydrogen atoms to oxygen atoms? 1 : 1 Decompose the hydrogen peroxide molecule models to form molecules of oxygen gas (O 2 ) and water (H 2 O). How many oxygen MOLECULES are formed by the decomposition? 1 How many water MOLECULES are formed? 2 Construct a balanced chemical equation for the decomposition of hydrogen peroxide. 2 H2O2 O2 + 2 H2O DECOMPOSITION OF HYDROGEN PEROXIDE Materials: 50 ml beaker, spatula, wood splints, matches, 3% hydrogen peroxide solution, packaged yeast. Pour 20 ml of hydrogen peroxide into the beaker. Add a small sample of yeast to the hydrogen peroxide and observe the chemical reaction. After 30 seconds or so, you will notice that the reaction becomes more vigorous. Light a wooden splint and gently blow it out to form a glowing splint. Place the glowing splint into the beaker directly above the solution. What evidence did you observe indicating the production of oxygen gas? The splint re-ignites due to the presence of oxygen gas. Dispose the mixture in the sink and rinse out the beaker. PART V: BAGGIE CHEMISTRY There are many types of chemical reactions. When chemicals are mixed several possible observations indicate that a chemical change has taken place: In some chemical reactions heat is generated, thus we can feel the reaction to see if the products of the reaction are getting hotter. Other chemical reactions absorb heat, thus the products become cooler. In some chemical reactions, one of the products is a gas. Evidence of this type of reaction is that gas bubbles form when the reaction takes place. In some chemical reactions one of the reactants may change color. In some chemical reactions, one of the products may not be soluble in water, and a precipitate will form. Evidence of this type of reaction is the formation of an insoluble solid. Chemical Composition 4

5 In this experiment, two chemicals that can be found around the house will be mixed, and several chemical changes will be observed. CALCIUM CHLORIDE is the ingredient in some types of products that are sold in the winter to melt ice on sidewalks. BAKING SODA or SODIUM BICARBONATE is found in most kitchens and is used in cooking. The third chemical that we will use is UNIVERSAL INDICATOR. Universal indicator changes color when the reaction involves an acid or base. The reaction that occurs in the experiment is: CaCl 2 (aq) + 2 NaHCO 3 (aq) 2 NaCl (aq) + 2 CO 2 (g) + Ca(OH) 2 (aq) 1. Pinch the zip-loc bag with your thumb and forefinger at the bottom to form two partitioned areas. 2. Place one spoon of calcium chloride into one corner of a zip-loc plastic baggie. 3. Place one spoonful of baking soda into the other corner of the bag. Do not mix the two chemicals at this point. 4. Place approximately 30 ml of water and 5 drops of universal indicator into the bag WITH THE CALCIUM CHLORIDE ONLY. Make sure that the water does not come in contact with the baking soda. Feel the temperature as the calcium chloride reacts with water. 5. Record your observations that indicate a chemical reaction is taking place. The calcium chloride and water became warm. This change in temperature indicates that a chemical reaction is taking place. 6. Seal the bag. Un-pinch the baggie and mix the calcium chloride solution with the baking soda. Again, feel the temperature of the bag. Record your observations that indicate a chemical reaction is taking place. The baggie and contents became cool to the touch. A gas is being produced, and the ph indicator changed color due to a change in ph. Ignite a wooden splint. Carefully open the baggie and place the blazing splint into the gas that was generated during the chemical reaction. Observations: The flame is extinguished. What gas was produced during the chemical reaction? Carbon dioxide ( CO2 ) 7. If you were to measure the mass of the baggie and chemicals prior sealing the baggie, and again measured the mass after the chemical reaction was completed, would the mass of the baggie and its contents changed in mass? (Increased, decreased, or remained constant?) Defend your answer. No change in mass since the baggie was not opened and nothing could enter or escape the bag. 8. Rinse the baggie in the sink with plenty of water and dispose of the baggie in the trash. Chemical Composition 5