Energy and Chemical Change

Transcription

1 Energy and Chemical Change Reviewing Vocabulary Match the definition in Column A with the term in Column B. h e d p c f a r m t j i s l u k n q g o Column A 1. The ability to do work or produce heat 2. States that energy cannot be created or destroyed 3. Energy flowing from a warmer to a cooler object 4. The study of heat changes from chemical reactions and phase changes 5. Energy stored in a substance because of its composition 6. required to raise the temperature of one gram of a substance by one degree Celsius 7. An insulated device measuring the heat absorbed or released during a chemical or physical process 8. The system plus the surroundings 9. States that spontaneous processes always proceed in such a way that the entropy of the universe increases 10. Everything in the universe other than the system 11. The change in enthalpy in a chemical reaction 12. A balanced chemical equation that includes the physical states of all reactants and products and the energy change that accompanies the reaction 13. A system s heat content at constant pressure 14. Energy required to vaporize one mole of a liquid 15. Enthalpy change occurring when one mole of a compound in its standard state forms from its constituent elements in their standard states 16. Energy required to melt one mole of a solid 17. A physical or chemical change without outside intervention 18. The enthalpy change for the complete burning of one mole of a substance 19. Energy that is available to do work 20. The SI unit of heat and energy Column B a. calorimeter b. standard enthalpy (heat) of formation c. chemical potential energy d. heat e. law of conservation of energy f. specific heat g. free energy h. energy i. thermochemical equation j. enthalpy (heat) of reaction k. molar enthalpy (heat) of fusion l. molar enthalpy (heat) of vaporization m. law of disorder n. spontaneous process o. joule p. thermochemistry q. enthalpy (heat) of combustion r. universe s. enthalpy t. surroundings Chapter Assessment Chemistry: Matter and Change Chapter 16 91

2 Understanding Main Ideas (Part A) For each statement below, write true or false. true 1. A negative sign for G indicates that, at constant temperature and pressure, the reaction is spontaneous. false 2. For a given substance, the entropy always increases in the following order: gas 0 liquid 0 solid. false 3. For the reaction NH 4 Cl(s) 0 NH 3 (g) HCl(g), the entropy change is negative. true 4. Hess s law states that if two or more thermochemical equations can be added to produce a final equation for a reaction, then the sum of all the enthalpy changes for the individual reactions is the enthalpy change for the final reaction. Use the illustration of three systems to answer the following questions. 5. How do the bottles in the three systems differ? Water vapor Seal 6. What do the arrows in the illustration indicate? 7. Which of the systems will show the greatest change in enthalpy and entropy as time progresses? Which will show the least change? Explain your answers. Water A Water B Water C 8. What are the surroundings in the illustration? 92 Chemistry: Matter and Change Chapter 16 Chapter Assessment

3 Understanding Main Ideas (Part B) Answer the following questions. 1. Consider the following reaction under standard conditions. AgCl(s) 0 Ag (aq) Cl (aq) G 54.1 kj Will this reaction occur spontaneously? Explain. 2. Which of the following species has the highest entropy at 25 C? Explain your answer. a. CH 3 OH(l) b. CO(g) c. MgCO 3 (s) d. H 2 O(l) e. Ni(s) 3. Predict the change in entropy ( S) for the following reactions. Explain your answer. a. H 2 O(g) 0 H 2 O(l) b. 2NO(g) 0 N 2 (g) O 2 (g) c. MgCO 3 (s) 0 MgO(s) CO 2 (g) Use the illustration to answer the following questions. 4. Determine if each process shown in the illustration is endothermic or exothermic. Explain. 5. What happens to the entropy of each system? Explain your answer. 500kJ Iron, 1.0 kg, 0 C 1100 C A 500kJ Ice, 2.0 kg, 0 C Ice, 0.5 kg, 0 C B Chapter Assessment Chemistry: Matter and Change Chapter 16 93

4 Thinking Critically Use the graph to answer the following questions. 1. What does the graph show? Gas Entropy (S) Entropy of fusion Entropy of vaporization Liquid 2. Why does the entropy increase dramatically at the melting and boiling points, but the temperature does not increase at all? Melting point Temperature (T) Boiling point Solid Use the nutrition label to answer the following questions. 3. How much energy is contained in the six-cookie serving size recommended on the label? 4. How was the number of Calories contained in the cookies determined? 5. How much energy in joules is provided by eating six cookies? (1 cal J) 94 Chemistry: Matter and Change Chapter 16 Chapter Assessment

5 Applying Scientific Methods Rachel and Ravi are lab partners in Ms. Santo s chemistry class. After completing a calorimetry experiment to determine the specific heat of lead, they discovered that the results had a percent error of 17.3%. They thought that the large percent error wasn t a good indication of their lab technique. They knew that for each of the three trials they had carefully measured the masses and the temperatures of the dry, lead sinker and the water in the calorimeter before and after they submerged the hot sinker. Perhaps the error wasn t a result of their technique but of other factors, including the design of the experiment, so they decided to investigate further. They recalled that the equation they used to determine c (lead) stemmed from the equation showing that the quantity of heat lost by the lead sinker equals the quantity of heat gained by the water. q lead q water 0 q lead q water Ravi grabbed a pencil and drew a quick sketch of the process represented by the equation. hot sinker cool water System (before) heat transfer from hot sinker to cool water warm sinker and warm water System (after) After a few seconds of thought, he said, We re forgetting that the calorimeter absorbs some heat, and he wrote, q lead (q water q calorimeter ) Sure! agreed Rachel. Here s what the process should look like. 1. Sketch the process represented by the equation that Ravi wrote. hot sinker and cool water and cool calorimeter System (before) heat transfer from hot sinker to cool water and cool calorimeter warm sinker and warm water and warm calorimeter System (after) The lab partners decided to design an experiment to determine the effect of the calorimeter on calorimetry experiments. They determined the amount of heat lost by the sinker from mass and temperature measurements and used the accepted value of the specific heat of lead (c (lead) J/(g C)). They also determine the heat gained by the water from mass and temperature measurements and used the accepted value of the specific heat of water (c (lwater) 4.18 J/(g C)). Chapter Assessment Chemistry: Matter and Change Chapter 16 95

6 Applying Scientific Methods, continued The table shows data taken from a typical trial of the experiment. Initial temperature of sinker ( C) Initial temperature of water ( C) 21.3 Final temperature of sinker and water ( C) 31.4 Mass of sinker (g) Mass of water (g) From the data, determine the value of q calorimeter. Interpret its meaning. T lead C 31.4 C C T water 31.4 C 21.3 C 10.1 C heat gained by calorimeter heat lost by lead heat gained by water q calorimeter q lead q water q lead c (lead) m T lead (0.129 J/(g C))( g)(128.6 C) 2370 J q water c (water) m T water (4.18 J/(g C))(50.0 g)(10.1 C) 2110 J q calorimeter q lead q water 2370 J 2110 J 260 J The calorimeter absorbs 260 J of heat. Ravi assumes that q calorimeter for their calorimeter doesn t change, that is, q calorimeter constant. However, Rachel thinks that q calorimeter might vary with the temperature change of the water, T water. She thinks that for their calorimeter, q calorimeter constant T water. 3. From the results of question 2, what are the value and units of Ravi s constant and of Rachel s constant? 4. Explain if you think Ravi or Rachel is correct. 96 Chemistry: Matter and Change Chapter 16 Chapter Assessment