Student Notes. Chemical Reactions LINK

Transcription

1 LCPS Core Experience Chemical Reactions Student Notes OBJECTIVES Students will: investigate the relationship between reactants and products. investigate an exothermic reaction. investigate an endothermic reaction. LINK 1. How can you tell if an energy change has taken place in a chemical reaction? Background Chemical reactions usually have two or more chemicals called reactants that are different from one another. The reactants interact during the chemical reaction to form one or more products. These are substances that are different from the reactants and from each other. The reactants and products in a chemical reaction can be written as chemical formulas using letters to represent elements. All chemical reactions can be written as equations that are like sentences. The reactant and product formulas are like the words in a sentence. In an equation, the reactants and products are separated by an arrow ( ). When reading a chemical equation, the word yield is used for the arrow symbol ( ). The chemical equation is then read by saying that the interaction of the reactants yields the products. In a chemical reaction, the chemical bonds that hold the atoms of the reactants together are broken and reformed in a different arrangement to form the products. Breaking and reforming these bonds occurs when the reactants react with each other. The different arrangement of atoms can be seen by the different formulas that represent the reactants and products. When all of the reactants are consumed in a chemical reaction, they no longer exist in their original form. All the atoms originally found in the reactants can now be found in the products. Atoms and molecules react with each other in specified ratios in order for a chemical reaction to take place. The balanced chemical equation that represents each chemical reaction stipulates the relative numbers of reactants and products. In a chemical reaction every atom that is part of the reactants is part of the products. Page 1

2 LCPS Core Experience Background (continued) Chemical reactions are classified into two broad types: ones in which energy is released (exothermic) and ones in which energy is absorbed (endothermic). A clue that a chemical reaction has taken place is a transfer of energy. EXPERIMENT In this investigation, you will be investigating an endothermic reaction and an exothermic reaction. Lesson One: Measuring Energy Change in a Chemical Reaction Materials Per Lab Group for Lesson One 30 ml of hydrogen peroxide 1 weigh dish (3% solution) 1 triple beam balance 1 50 ml graduated cylinder 1 stopwatch 1 thermometer 1 pair of goggles ml Erlenmeyer flask 1 lab apron 2 g of yeast 1. Obtain the materials for Lesson One. You will be investigating the following chemical reaction. Yeast 2H2O2 2H2O + O2 Hydrogen Water Oxygen peroxide 2. Measure 30 ml of hydrogen peroxide using a 50 ml graduated cylinder. Pour the hydrogen peroxide into a 250 ml Erlenmeyer flask. 3. Put a thermometer into the flask and wait two minutes to allow the thermometer to reach the temperature of the hydrogen peroxide. During the time you are waiting, measure 2 g of yeast with the balance. 4. Record the starting temperature. One person should keep track of the time. Another person should keep track of the temperature. You will be measuring the temperature every 30 seconds. 5. Add the yeast to the beaker and immediately record the temperature in Table A, this is time zero, while gently stirring the contents of the flask. Continue to record the temperature in Table A every 30 seconds for 3 minutes. 6. Did the temperature increase or decrease? Page 2

3 LCPS Core Experience 7. Was energy taken in or released in the reaction? 8. Pour the mixture down the drain and rinse out the flask with water. Rinse the thermometer off. Lesson Two: Measuring Energy Change in a Chemical Reaction Materials Per Lab Group for Lesson Two 30 ml of vinegar (acetic acid) 1 triple beam balance 1 50 ml graduated cylinder 2 g of baking soda 1 125mL Erlenmeyer flask 1 stopwatch 1 thermometer 1 pair of goggles 1 weigh dish 1 lab apron 1. Obtain the materials for Lesson Two. You will be investigating the following chemical reaction. heat NaHCO3 + CH3COOH CH3COONa + H2O + CO2 Baking Soda Acetic Acid 2. Put a thermometer into the flask and wait two minutes to allow the thermometer to reach the temperature of the vinegar. During the time you are waiting, measure 2 g of baking soda with the balance. Refer to the procedural toolbox Using a Weigh Dish. 3. Record the starting temperature. Sodium Acetate Water Carbon Dioxide 4. Add the baking soda to the vinegar and immediately record the temperature, as time zero in Table A as you swirl the contents of the flask. Continue to record the temperature every 30 seconds as you observe the reaction for 3 minutes. 5. Did the temperature increase or decrease? 6. Was energy taken in or released in the reaction? Page 3

4 LCPS Core Experience 7. Pour the mixture down the drain and rinse out the flask with water. Rinse the thermometer. Table A: The Effect of a Chemical Reaction on the Temperature of a Solution Lesson One: Hydrogen Peroxide and Yeast Reaction Lesson Two: Vinegar and Baking Soda Reaction Time (min) Temperature ( 0 C) Temperature ( 0 C) Graph: Use the data from Table A to plot a graph of time vs. temperature for both solutions on the same graph paper. a. Use a different color for each set of data.. b. Determine which variable is the independent variable and which is the dependent variable. STOP AND DISCUSS Page 4

5 LCPS Core Experience LEARNING REVIEW List three things you learned about energy, endothermic and exothermic reactions. a. b. c. STOP AND DISCUSS EVALUATION 1. Which reaction was endothermic? Write the chemical equation. 2. Why did you select this reaction as the endothermic reaction? Page 5

6 LCPS Core Experience 3. Which reaction was exothermic? Write the chemical equation. 4. Why did you select this reaction as the exothermic reaction? Page 6