Safety: Goggles should be worn at all times while using chemicals.

Transcription

1 1 Teacher Page Title: Observing Energy in Chemical Reactions Objectives: 1. The student conducts field and laboratory investigations using safe, environmentally appropriate, and ethical practices. 2. Plan and implement investigative procedures including asking questions, formulating testable hypotheses, and selecting and using equipment and technology. 3. Collect data by observing and measuring. 4. Communicate valid conclusions. 5. Construct graphs, tables, maps, and charts using tools including computers to organize, examine, and evaluate data. 6. Collect, analyze, and record information to explain a phenomenon using tools. 7. Identify a design problem and propose a solution. 8. Design and test a model to solve the problem. 9. Evaluate the model and make recommendations for improving the model. 10. Demonstrate that substances may react chemically to form new substances 11. Identify and demonstrate that loss or gain of heat energy occurs during exothermic and endothermic chemical reactions Safety: Goggles should be worn at all times while using chemicals. Background Information: A feature of a chemical reaction is a change in energy, usually light or heat. Every chemical reaction involves energy. As chemical bonds in the reactants break, energy is absorbed. As new bonds form in the products, energy is released. Exothermic Reactions: If the products contain less energy than the reactants, heat is given out (released) to the surrounding environment and the change is called exothermic. The temperature of the system will be observed to increase in an exothermic reaction. 2Na + Cl 2 2NaCl + energy When energy is released it is written as product in chemical equation

2 2 Many times heat is released so slowly we are not aware that it is happening. The rusting of iron is an exothermic reaction, but we do not feel the heat because it is a very slow reaction. Endothermic Reactions: If the products contain more energy than the reactants, heat is taken in (absorbed) from the surroundings and the change is called endothermic. 2H energy 2H When energy is absorbed it is written as reactant in a chemical equation. If the reaction can take place spontaneously, the temperature of the reacting system will decrease but in some cases the reactants must be heated to speed up the reaction and provide the absorbed heat. Cold packs are an example of endothermic reaction. Cold packs contain ammonium nitrate and water separated by some kind of barrier. When the bags are squeezed or shaken, the barrier is broken, and the ammonium nitrate mixes with the water. Heat energy is absorbed from the surrounding air and your skin. Activation Energy: When matter is heated molecules gain kinetic energy and move faster and faster. This increases the chance of collision between reactant molecules. However this is not the main reason for the increased reaction speed because most molecular collisions do not result in chemical change. Before any change takes place because of a collision, the colliding molecules must have a minimum kinetic energy called the Activation Energy. This is not the overall energy change in the reaction. It does not matter whether the reaction is an exothermic or an endothermic energy change. Heated molecules have a greater kinetic energy and more of them have the required activation energy to react.

3 3 The increased chance of higher energy collisions greatly increases the speed of the reaction. The diagram below shows the activation energy diagram for an exothermic reaction. The activation energy is low. The reaction would be fast: The diagram below shows an activation energy diagram for an exothermic reaction. The activation energy is high. The reaction would be slow. This diagram shows the activation energy for an endothermic reaction. The complete reaction has three steps. Most reactions occur through a series of steps called a mechanism. This diagram shows a three-step mechanism. The step with the highest activation energy controls the rate of the reaction. Step three is the slowest step with the highest activation energy

4 4 Materials: LabPro Data Collection Interface Temperature Sensor TI Graphing Calculator or Computer with Logger Pro program Goggles Large Test Tubes or Beakers Spoon Forceps Graduated Cylinder 3% Hydrogen Peroxide Baking Soda Calcium Chloride (rock salt) Vinegar Potato Liver Yeast Epsom Salts Distilled Water Resealable freezer bags Procedure: 1. The following will be used to demonstrate endothermic and exothermic reactions a. Hydrogen Peroxide + Potato b. Calcium Chloride + Baking Soda c. Baking Soda + Vinegar d. Liver + Hydrogen Peroxide e. Yeast + Hydrogen Peroxide f. Epsom Salts + Water 2. To set up the LabPro and computer to collect data: a. Connect the LabPro to the computer and the Temperature sensor to the LabPro. b. Open Logger Pro. This screen will be displayed:

5 5 c. Click Setup Sensors d. Double check for the correct sensor, port, calibration, and details. e. To set parameters for the experiment got to Experiment Sampling f. Set the experiment length for 30 seconds, 1 sample per second:

6 6 g. Click on when you are ready to begin collecting data. 3. Set up a materials station or put supplies in a basket at each lab station for student use 4. Remind students to rinse equipment with distilled water between each use. 5. Have students return clean materials to the supply table or their baskets. Evaluation: Performance Assessment Using information gathered in the investigation, students design a hot pack or cold pack using the materials available and a resealable freezer bag.

7 7 Student Page Name Problem: To observe heat energy changes in chemical reactions. Predict: What kind of energy change will you observe in these reactions? 1. Hydrogen Peroxide + Potato: 2. Hydrogen Peroxide + Yeast: 3. Hydrogen Peroxide + Liver: 4. Baking Soda + Vinegar: 5. Epsom Salts + Water: 6. Rock Salt + Baking Soda + Water: Materials: LabPro & Computer Temperature Sensor Safety Goggles Test Tube or Beaker Test Tube Rack Spoon 50 ml Hydrogen Peroxide 50 ml Vinegar Baking Soda Small Piece of Liver Small Piece of Potato Dry Yeast Rock Salt Distilled Water Graduated Cylinder Forceps Procedure: 1. Set up the computer to collect data: a. Connect the LabPro to the computer and the Temperature sensor to the LabPro. Use the CH1 port for the sensor.

8 8 b. Open Logger Pro on the computer. This screen will be displayed: c. Click Setup Sensors d. Double check for the correct sensor, port, calibration, and details.

9 9 e. To set parameters for the experiment got to Experiment Sampling f. Set the experiment length for 30 seconds, 1 sample per second: 2. Put the temperature sensor in the test tube. 3. Add a small piece of potato. 4. Pour 10 ml of hydrogen peroxide into the test tube and immediately click to begin collecting data. Feel the bottom of the test tube. Record your descriptive observations in the data chart. 5. Rinse the test tube and temperature sensor with distilled water and repeat with ½ spoon of yeast and 10 ml of hydrogen peroxide. 6. Rinse the test tube and temperature sensor with distilled water and repeat with a small piece of liver and 10 ml of hydrogen peroxide.

10 10 7. Rinse the test tube and temperature sensor with distilled water and repeat with ½ spoon of baking soda and 10 ml of vinegar. 8. Rinse the test tube and temperature sensor with distilled water and repeat with ½ spoon of Epsom salts and 10 ml of distilled water. 9. Rinse the test tube and temperature sensor with distilled water and repeat with ½ spoon of rock salt, ½ spoon baking soda, and 10 ml water. Data: Reaction Observations Hydrogen Peroxide + Potato Hydrogen Peroxide + Yeast Hydrogen Peroxide + Liver Baking Soda + Vinegar Epsom Salts + Water Rock Salt + Baking Soda + Water Analysis: 1. Describe the energy change during each reaction. Use data to support what you say.

11 11 2. How do the graphs tell you that the reactions are changing temperature? 3. How are endothermic reactions different from exothermic reactions? Use a diagram to help explain your answer.

12 12 Design an Investigation: Given this information: Most commercial hot packs reach a temperature of around 60º C or 140º F. Optimal cold treatment temperature is 10º C or 50º F; freezing temperatures can damage skin. Use this information plus what you have learned in the previous experiments to design a hot pack OR a cold pack. Record your process below:

13 13 Create a way to display your data here: