JHS Regents Chemistry Department Rates of Chemical Reactions Inquiry Investigation

Transcription

1 Name: Date: Partner: Period: JHS Regents Chemistry Department Rates of Chemical Reactions Inquiry Investigation Introduction: According to the collision theory, the rate of a reaction depends on the frequency of collisions between reacting particles. The more frequent the collisions, the faster the rate of the reaction. However, in order for the collisions to be effective, the particles must collide with enough energy (activation energy). Furthermore, the particles must collide with proper orientation. Alka-Seltzer is a common, over the counter medication designed to relieve symptoms stomach pain and discomfort while neutralizing excess stomach acid. When the tablets are dissolved in water, they produce an effervescent reaction releasing gas and bubbles. Alka-Seltzer s ingredients include: aspirin, sodium bicarbonate and citric acid. As the tablets dissolve, the base (sodium bicarbonate) and the acid (citric acid) react vigorously to produce carbon dioxide gas. This reaction also produces enough agitation to allow the active ingredients to dissolve slowly. The patient then ingests the resulting solution. Purpose: Chemical reactions occur at different rates. In this experiment, you will consider some of the key factors that influence the rate of a reaction. These factors include nature of reactants (ionic or covalent bonding); surface area/particle size; temperature; concentration; catalysts. Problem: What factors increase the rate of chemical reactions? Pre-Lab Demonstration & Questions: 1. What are the three requirements for a chemical reaction to occur, according to the collision theory? 2. Teacher will drop ¼ tablet of Alka Seltzer into 5 ml water in a film canister, cover the canister and record the time to pop. Time = seconds 3. When the Alka-Seltzer was added to the water in the canister a [physical / chemical] reaction occurred. As gas was released, [pressure / volume / temperature] increased inside the container Because the [pressure / volume / temperature] was constant, the pressure inside the canister forced the lid to pop off. The amount of time it takes for a reaction to occur is called the. **INFER**: What do you think can be done to INCREASE the rate of reaction? List two things.

2 Objectives: To hypothesize and examine the factors that influence reaction rates Investigate a hypotheses of what can be done to change the rate of the Alka- Seltzer/water reaction Provide relevant data to support inferences and conclusions. Select the appropriate instrument; measure, calculate, and record in metric units, length, volume, temperature and mass, to the accuracy of instruments used. CAUTION: Goggles must be worn over eyes. Exercise caution when working with acids and bases. Wash your hands after completing the lab. Exercise caution when handling hot glassware and hot plates. Do not point exploding film cannisters toward yourself or others. Materials (Per Table): Alka-Seltzer tablets Water Film canister with lid Stopwatch Graduated cylinder Metal scoopula Bunsen burner Paper towels Mortar & pestle Pipette Weighing boats Scales Hot plates Ice 1 M HCl (aq) Procedure: 1. Choose two of the variables that you and your partner listed during the pre-lab demonstration to investigate. 2. Develop a hypothesis for each variable to be tested. 3. Design a controlled experiment to test your hypothesis. Identify your independent variable, dependent variable, and controlled variables. 4. Each partner will be responsible for writing one lab report summary. Your write-up must include a step-wise procedure, data table, graph, and conclusion statement based on your findings. Be prepared to share your findings with the class. Discussion Questions: 1. Based on the experiments conducted by the class, what factors can affect the rate of a chemical reaction? 2. In terms of collision theory, explain why each of the three factors identified in question 1 increases the rate of chemical reactions.

3 3. Make a sketch of each of the relationships below. Label the x-axes. 4. Other factors that can affect the rate of a chemical reaction are addition of a catalyst, nature of reactants, and pressure (gases, only!). a. What types of substances, ionic or covalent, do you think react faster? (Hint: Which substances require less rearrangement of electrons?) b. Increasing the amount of pressure on a gaseous system effectively increases the concentration of the reactants, because the same amount of particles are present in less volume. Does increasing the pressure on a gaseous system increase or decrease the rate of a chemical reaction? Make a sketch of this relationship below. 5. Which combination of all factors tested today do you think would result in the greatest rate of reaction? 6. When Alka-Seltzer dissolves in water, the following reaction occurs: The citric acid and sodium bicarbonate in the Alka-Seltzer tablet react with water to produce carbon dioxide gas and sodium citrate. a. Word Equation: b. Chemical Equation: (look up chemical formulas using the internet!)

4 Practice Regents Questions 1. Given the balanced equation representing a reaction: Zn(s) + 2HCl(aq) H 2 (g) + ZnCl 2 (aq) Which set of reaction conditions produces H 2 (g) at the fastest rate? a 1.0-g lump of Zn(s) in 50. ml of 0.5 M HCl(aq) at 20. C b. 1.0-g lump of Zn(s) in 50. ml of 0.5 M HCl(aq) at 30. C c. 1.0 g of powdered Zn(s) in 50. ml of 1.0 M HCl(aq) at 20. C d. 1.0 g of powdered Zn(s) in 50. ml of 1.0 M HCl(aq) at 30. C 2. Ionic substances react faster than covalent substances because less rearrangement of electrons is required. Based on the nature of the reactants (type of bonding) in each of the equations below, which reaction at 25 C will occur at the fastest rate? a. C(s) + O 2 (g) CO 2 (g) b. NaOH(aq) + HCl(aq) NaCl(aq) + H 2 O() c. CH 3 OH(l) + CH 3 COOH(l) CH 3 COOCH 3 (aq) + H 2 O(l) d. CaCO 3 (s) CaO(s) + CO 2 (g) 3. Given the reaction: A + B C + D, the reaction will most likely occur at the greatest rate if A and B represent a. nonpolar molecular compounds in the solid phase b. ionic compounds in the solid phase c. solutions of nonpolar molecular compounds d. solutions of ionic compounds 4. A 1-cubic-centimeter cube of sodium reacts more rapidly in water at 25 C than does a 1- cubic-centimeter cube of calcium at 25 C. This difference in rate of reaction is most closely associated with the different a. surface area of the metal cubes b. nature of the metals c. density of the metals d. concentration of the metals 5. Given the reaction below, at which temperature will the reaction occur at the greatest rate? Mg + 2H 2 O Mg(OH) 2 + H 2 a. 25ºC b. 50ºC c. 75ºC d. 100ºC

5 6. Base your answer on the table below, which represents the production of 50 milliliters of CO 2 in the reaction of HCl with NaHCO 3. Five trials were performed under different conditions as shown. (The same mass of NaHCO 3 was used in each trial.) Which two trials could be used to measure the effect of surface area? a. trials A and B b. trials A and C c. trials A and D d. trials B and D 7. Which statement explains why increasing the temperature increases the rate of a chemical reaction, while other conditions remain the same? a. The reacting particles have less energy and collide less frequently. b. The reacting particles have less energy and collide more frequently. c. The reacting particles have more energy and collide less frequently. d. The reacting particles have more energy and collide more frequently. 8. In each of the four beakers shown below, a 2.0-centimeter strip of magnesium ribbon reacts with 100 milliliters of HCl(aq) under the conditions shown. In which beaker will the reaction occur at the fastest rate? 9. At STP, which 4.0-gram zinc sample will react fastest with dilute hydrochloric acid? a. lump b. bar c. powdered d. sheet metal

6 Controlled Experiment Write-Up (You may write or type & to 1. Problem: 2. Hypothesis: 3. Materials: 4. Independent Variable: 5. Dependent Variable: 6. Controlled Variables: 7. Stepwise Procedure: 8. Data Table: 9. Graph(s): (attach) 10. Conclusion: