Does the diagram illustrate an exothermic or an endothermic reaction? State one reason, in terms of energy, to support your answer.

Transcription

1 1. Given the reaction: A + B C Does the diagram illustrate an exothermic or an endothermic reaction? State one reason, in terms of energy, to support your answer. The answer is: endothermic. Acceptable reasons include, but are not limited to, these examples: PE of product C is greater than PE of reactants A and B. Product C absorbed energy and is at a higher PE than reactants A and B. 2. Base your answer to the question on the information below. A hot pack contains chemicals that can be activated to produce heat. A cold pack contains chemicals that feel cold when activated. Figure 1 Based on energy flow, state the type of chemical change that occurs in a hot pack. Acceptable responses include, but are not limited to, this example: exothermic

2 3. Explain how a catalyst may increase the rate of a chemical reaction. Acceptable responses include, but are not limited to, these examples: lowers activation energy provides an alternate reaction pathway brings particles together forms a different activated complex at lower activation energy

3 4. Figure 2 Base your answers to the question on the information and potential energy diagram. Chemical cold packs are often used to reduce swelling after an athletic injury. The diagram represents the potential energy changes when a cold pack is activated. Identify a reactant listed in Reference Table I that could be mixed with water for use in a chemical cold pack. Acceptable responses include these examples: KNO 3 NaCl NH 4 Cl NH 4 NO3 Potassium nitrate Sodium chloride Ammonium chloride Ammonium nitrate

4 5. Figure 3 The equation indicates that KNO 3 has formed a saturated solution. Explain, in terms of equilibrium, why the solution is saturated. Acceptable responses include, but are not limited to, these examples: The rate of dissolving KNO 3 is equal to the rate of recrystallizing KNO 3. The KNO 3 is going into the solution at the same rate it precipitates out of the solution.

5 6. Figure 4 Base your answer to the question on the potential energy diagram and the equation. Describe how the potential energy diagram will change if a catalyst is added. Acceptable responses include, but are not limited to, these examples: Arrow 2 gets shorter. The activation energy would be lower. The peak of the curve is lower.

6 7. Base your answer to question on the information below. In a laboratory experiment, grams of an unknown solid is added to milliliters of water and the temperature of the resulting solution is measured over several minutes, as recorded in the table below. Given the statement: Figure 5 The unknown solid is either sodium hydroxide or lithium bromide, and both of these compounds dissolve in water exothermically. a Explain how the experimental data support the statement. b State specific information from Reference Table I to support the statement. a Acceptable responses include, but are not limited to, these examples: The data show an increase in temperature, which indicates an exothermic reaction solution temperature goes up, exothermic b. Acceptable responses include, but are not limited to, these examples: Reference Table I shows that H is negative for NaOH and LiBr, which indicates an exothermic reaction. H is negative for both. NaOH: H = kj/mol LiBr: H = kj/mol

7 8. Figure 6 Base your answer to the questions on the potential energy diagram. Explain, in terms of the function of a catalyst, why the curves on the potential energy diagram for the catalyzed and uncatalyzed reactions are different. A catalyst provides an alternate reaction pathway with a lower activation energy than an uncatalyzed reaction. A catalyst speeds up the reaction. lower activation energy

8 9. Base your answer to the question on the information and balanced equation below. Given the equation for a reaction at equilibrium: Figure 7 Explain, in terms of LeChatelier s principle, why the concentration of SO 2 (g) increases when the temperature is increased. An increase in temperature favors the endothermic reaction, which produces more SO 2 (g). The reaction shifts to the left, increasing the concentration of SO 2 (g). 10. [Refer to figure 7 in question 9] Explain, in terms of collisions between molecules, why increasing the concentration of O 2 (g) produces a decrease in the concentration of SO 2 (g). A higher concentration of O 2 (g) causes more collisions and reactions with SO 2 (g) molecules, decreasing SO 2 (g) concentration. More collisions between reactants shift the reaction to the right.

9 11. Base your answer to the question on the information and table below. A student conducts an experiment to determine how the temperature of water affects the rate at which an antacid tablet dissolves in the water. The student has three antacid tablets of the same size and composition. The student drops one tablet into each of three beakers containing 200. milliliters of water at different temperatures and measures the time it takes for each tablet to completely dissolve. The results are shown in the table below. Figure 8 Explain, in terms of collision theory, how water temperature influences the rate of dissolving. At higher temperatures, the water molecules collide more often and more effectively with the tablet, so the tablet dissolves faster. high temperature more collisions dissolves faster 12. Base your answer to the question on the information below. Given the reaction at equilibrium: Figure 9 Explain, in terms of energy, why the forward reaction is exothermic. Examples: Potential energy of the product is less than potential energy of the reactant. More energy is released than absorbed. Energy appears on the right side of the reaction.

10 13. Base your answer to the question on the information below. An investigation was conducted to study the effect of the concentration of a reactant on the total time needed to complete a chemical reaction. Four trials of the same reaction were performed. In each trial the initial concentration of the reactant was different. The time needed for the chemical reaction to be completed was measured. The data for each of the four trials are shown in the table below. Figure 10 In a different experiment involving the same reaction, it was found that an increase in temperature increased the rate of the reaction. Explain this result in terms of collision theory. The greater the kinetic energy of the reactant particles, the greater the frequency and effectiveness of the collisions. Increasing the temperature causes more collisions. more effective collisions

11 14. Base your answer to the question on the information below. A gasoline engine burns gasoline in the presence of excess oxygen to form carbon dioxide and water. The main components of gasoline are isomers of octane. A structural formula of octane is shown below. One isomer of octane is 2,2,4-trimethylpentane. Figure 11 Explain, in terms of the arrangement of particles, why the entropy of gasoline vapor is greater than the entropy of liquid gasoline. The arrangement of molecules in the vapor state is more random or disordered. Particles in the vapor state are farther apart and move more freely. 15. Base your answer to this question on the information below. Nitrogen gas, hydrogen gas, and ammonia gas are in equilibrium in a closed container at constant temperature and pressure. The equation below represents this equilibrium. The graph below shows the initial concentration of each gas, the changes that occur as a result of adding H 2 (g) to the system, and the final concentrations when equilibrium is reestablished.

12 Figure 12 Explain, in terms of LeChatelier s principle, why the final concentration of NH 3 (g) is greater than the initial concentration of NH 3 (g). The stress of adding H 2 (g) shifts the reaction to the right, producing NH 3 (g). The reaction shifts to the right to relieve the stress.