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1 Name: Period: Date: UNIT 10: Energy Lesson 1: Entropy vs. Enthalpy By the end of today, you will have an answer to: What conditions are favored during reactions? Definition Enthalpy is another word for heat. A change in enthalpy = ΔH. Examples: If a reaction is exothermic, enthalpy is decreasing because ΔH is negative. If a reaction is endothermic, enthalpy is increasing because ΔH is positive. Fill out the following table: Exothermic Reactions Br + Br Br2 Endothermic Reactions Br2 Br + Br Potential Energy Diagram Energy: released or absorbed Bonds forming or Bonds breaking? Lower enthalpy or higher enthalpy? Products more or less stable?

2 Read the following passage and answer the appropriate annotation questions. Another expression for ΔH: The difference between the Potential Energy of the products and Potential Energy of the reactants is known as the heat of the reaction or ΔH. The heat content (H) is known as the enthalpy of a system. Therefore a change in enthalpy is another way of saying ΔH! Endothermic reactions: If the change in enthalpy is positive, the reaction is endothermic. In an endothermic reaction, heat is listed as a reactant. An example is found below. 2NaHCO 3(s) kj Na 2CO 3(s) + H 2O(g) + CO 2(g) ΔH = +129 kj Exothermic reactions: If the change in enthalpy is negative, the reaction is exothermic. In an exothermic reaction, heat is listed as a product. An example is found below. CaO(s) + H 2O(l) Ca(OH) 2(s) kj ΔH = kj All reactions tend to form bonds to achieve a state of lower energy. Exothermic reactions create products with lower energy. The Universe favors exothermic reactions over endothermic reactions. One more important factor: how random can you be? Exothermic reactions may be favored, but endothermic reactions still take place. There is one more factor that determines if a chemical reaction will occur spontaneously (without adding extra energy). Entropy is a measure of the disorder of a system. It is also known as the randomness of the system. The Universe tends to increase the entropy or randomness of a system. Think about your backpack or bedroom. Over time without effort, your bag or room become more disordered and. Only continual energy can help maintain order. Entropy can be examined through states of matter. Solids have the most structure and least entropy. Liquids have more entropy than solids. Gases have the highest entropy. This picture clearly shows gases are most random; solids are least random. When a solid dissolves in water, its particles are spread out. Therefore aqueous solutions have higher entropy than solid particles. When a gas dissolves in water, its particles become less spread out. Therefore aqueous solutions have lower entropy than gas particles. 1) Describe a new way to say heat of the reaction or ΔH. 2) Describe three differences between a positive change in enthalpy and a negative change in enthalpy. 3) Which type of reaction does the Universe favor? Why? 1) In your own words describe entropy. 2) What naturally happens to entropy in a system? Explain. 3) Describe entropy changes from a solid to liquid to gas. 4) How does entropy change when a solid is dissolved in water? 5) How does entropy change when a gas is dissolved in water?

3 Putting it together: Enthalpy and Entropy and phase changes As you melt a solid and boil a liquid the enthalpy increases. As you melt a solid and boil a liquid the entropy increases. Work hard. Be nice. 1) Why does the enthalpy increase as a substance is heated from a solid to a liquid to a gas? 2) Why does the entropy increase as a substance is heated from a solid to a liquid to a gas? Practice: Enthalpy increase or decreases: Reaction/ Process Melting of ice Freezing of water Evaporation of water Exothermic or Endothermic? Increase or decrease in enthalpy? Practice: Entropy increase or decrease Process/ Reaction Ice melting Organizing your chem notebook Sublimation (when solid turns directly into gas) Deposition (when gas turns directly into solid) Increase or decrease in entropy? Before you answer Regents-Based Questions, use your notes to help you with conceptual understanding. 10 points Part I: Enthalpy 1. Explain, why forming bonds is an exothermic process and breaking bonds in an endothermic process. 2. Explain, in-terms of stability why exothermic reactions are more favorable (likely to occur). Part II: Entropy 3. Explain why reactions that increase entropy are more favorable (likely to occur). Entropy vs. Enthalpy 4. Both of these words start with Ent and both are involved with whether reactions are likely to occur. Explain the difference between these two words. Finally In nature, all systems go toward /10 points

4 Part II: Regents-based questions (10 points): 1. Given the balanced equation: I 2(s) + energy I 2(g) As a sample of I 2(s) sublimes to I 2(g), the entropy of the sample (1) increases because the particles are less (2) increases because the particles are more (3) decreases because the particles are less (4) decreases because the particles are more Work hard. Be nice. 4. Which list of the phases of H 2O is arranged in order of increasing entropy? (1) ice, steam, and liquid water (2) ice, liquid water, and steam (3) steam, liquid water, and ice (4) steam, ice, and liquid water 5. As carbon dioxide sublimes, its entropy (1) decreases (2) increases (3) remains the same (4) decreases then increases 2. Given the balanced equation: Which statement best describes this process? (1) It is endothermic and entropy increases. (2) It is endothermic and entropy decreases. (3) It is exothermic and entropy increases. (4) It is exothermic and entropy decreases. 3. Systems in nature tend to undergo changes toward (1) lower energy and lower entropy (2) lower energy and higher entropy (3) higher energy and lower entropy (4) higher energy and higher entropy 6. Given the balanced equation representing a reaction: What occurs during this change? (1) Energy is absorbed and a bond is broken. (2) Energy is absorbed and a bond is formed. (3) Energy is released and a bond is broken. (4) Energy is released and a bond is formed. 7. The entropy of a sample of CO2 increases as the CO2 changes from (1) gas to liquid (3) liquid to solid (2) gas to solid (4) solid to gas The Solvay process is a multistep industrial process used to produce washing soda, Na2CO3(s). In the last step of the Solvay process, NaHCO3(s) is heated to 300 C, producing washing soda, water, and carbon dioxide. This reaction is represented by the balanced equation below. 2NaHCO3(s) heat Na2CO3(s) H2O(g) CO2(g) 8. State evidence that indicates the entropy of the products is greater than the entropy of the reactant. [1] 9. Explain, in terms of the arrangement of particles, why the entropy of gasoline vapor is greater than the entropy of liquid gasoline. 10. Base your answers to questions a) through c) on the reaction represented by the balanced equation below. a) Identify the information in this equation that indicates the reaction is exothermic. b) On the axes give below, draw a potential energy diagram for the reaction represented by this equation. c) Explain why the entropy of the system decreases as the reaction proceeds. Name: Pd: Date:

5 Exit Ticket Quiz 9.1 Enthalpy vs. Entropy 1. Given the balanced equation representing a phase change: C6H4Cl2(s) energy C6H4Cl2(g) Which statement describes this change? (1) It is endothermic, and entropy decreases. (2) It is endothermic, and entropy increases. (3) It is exothermic, and entropy decreases. (4) It is exothermic, and entropy increases. 2. Which process is exothermic? (1) boiling of water (2) melting of copper (3) condensation of ethanol vapor (4) sublimation of iodine 3 points The balanced equation below represents the decomposition of potassium chlorate. 2KClO3(s) 2KCl(s) 3O2(g) 3. State why the entropy of the reactant is less than the entropy of the products. [1] Name: Pd: Date: Exit Ticket Quiz 9.1 Enthalpy vs. Entropy 3 points 1. Given the balanced equation representing a phase change: C6H4Cl2(s) energy C6H4Cl2(g) Which statement describes this change? (1) It is endothermic, and entropy decreases. (2) It is endothermic, and entropy increases. (3) It is exothermic, and entropy decreases. (4) It is exothermic, and entropy increases. 2. Which process is exothermic? (1) boiling of water (2) melting of copper (3) condensation of ethanol vapor (4) sublimation of iodine The balanced equation below represents the decomposition of potassium chlorate. 2KClO3(s) 2KCl(s) 3O2(g) 3. State why the entropy of the reactant is less than the entropy of the products. [1]