Endothermic and Exothermic Process

Things you should know when you leave Discussion today: 1. Endothermic and exothermic process , Mahaffy, 2e section 7.2 2. The First Law of Thermodynamics Mahaffy, 2e section 7.4: ΔU = q + w Where:  ΔU is an internal energy (J)  w is pressure-volume work done by gas (J)  q is heat transferred (J)  qp≡ΔH is heat transferred at constant pressure  qv is heat transferred at constant volume  At constant pressure: ΔU = qp + w  At constant volume (w ≡ 0): ΔU = qv 3. If work is done on the system (by the surroundings), then w > 0 and gas is consumed. 4. If work is done by the system (on the surrounding), then w < 0 and gas is produced. 5. If no work is done w = 0; ΔU = q; qv = qp 6. Enthalpy of vaporization ΔvapH and enthalpy of fusion ΔfusH, Mahaffy, 2e section 7.5 1. Gas is consumed during a reaction. Choose one for each of the following relationships. a. work is done on the system b. w < 0; w > 0; work is done by the system w=0 c. ΔU - q < 0; ΔU - q > 0; ΔU - q = 0 2. For each of the following unbalanced chemical reactions indicate: a) Is work done? Indicate whether work is done on the system or by the system. b) Is the final temperature greater, less than or the same as the initial temperature? c) ΔU >, <, or = qp? d) Draw the energy diagram for each reaction e) Will the surroundings around the reaction get hot or cold? f) Will the final temperature be greater if the reaction is run at constant pressure or constant volume? (Tfinal (constant pressure) vs. Tfinal (constant volume)) I. H2 (g) + O2 (g) → H2O (l) Exothermic Choose one: w < 0 || w > 0 Choose one: ΔU - q < 0 || ΔU - q > 0 Choose one: 1 Tfinal (P-const) > || w = 0 || ΔU - q = 0 < = Tfinal (V-const) II. H2O2 (aq) → H2O (g) + O2 (g) Exothermic Choose one: w < 0 || w > 0 Choose one: ΔU - q < 0 || ΔU - q > 0 Choose one: III. CaCO3 (s) → Tfinal (P-const) > CaO (s) + CO2 (g) Endothermic Choose one: w < 0 || w > 0 IV. H2O (g) + CO2 (g) → Tfinal (P-const) > O2 (g) + CH3OH (l) Endothermic Choose one: w < 0 || w > 0 V. H3O+ (aq) + OH- (aq) → Tfinal (P-const) > H2O (l) Exothermic Choose one: w < 0 || w > 0 Tfinal (P-const) > || w = 0 || ΔU - q = 0 || w = 0 || ΔU - q = 0 < = Tfinal (V-const) Choose one: ΔU - q < 0 || ΔU - q > 0 Choose one: || ΔU - q = 0 < = Tfinal (V-const) Choose one: ΔU - q < 0 || ΔU - q > 0 Choose one: w = 0 < = Tfinal (V-const) Choose one: ΔU - q < 0 || ΔU - q > 0 Choose one: || || w = 0 || ΔU - q = 0 < = Tfinal (V-const) 3. A process is endothermic, circle all that apply. q<0 w<0 ΔU < 0 The surroundings get hot. qv < qp q>0 w>0 ΔU > 0 The surroundings get cold. qv > qp Tfinal(constant pressure) > Tfinal(constant volume) Tfinal(constant pressure) < Tfinal(constant volume) 4. A process is endothermic and an expansion takes place, circle all that apply. q<0 w<0 ΔU < 0 The surroundings get hot. qv < qp q> 0 w>0 ΔU > 0 The surroundings get cold. qv > qp Tfinal (constant pressure > Tfinal (constant volume) Tfinal (constant pressure) < Tfinal (constant volume) HOME: For practice at home redo questions 3 and 4 for an exothermic process, exothermic process and expansion, endothermic process and gas is consumed, exothermic process and gas is consumed. 2 5. If qp = -10.555J and w = -5.000J. What is the value of qv? a. If the heat capacity is 5.000 J/(g∙K) mass is 1.0000g and the initial temperature was 300.000 K, what would the final temperature be for the reaction at (1) constant pressure and (2) constant volume? Tfinal (constant pressure)= Tfinal (constant volume)= 6. A reaction is endothermic. When it is run at constant pressure, it’s found that less cooling occurs in the solution than when it is run at constant volume. Draw the diagram for this reaction. Is work positive, zero, or negative? 7. A reaction forms a larger volume of gas and it gives off heat. Draw the diagram for this reaction. Compared to the heat given off at constant pressure, will the heat given off at constant volume be more, the same, or less? 8. A reaction between gases takes place in a balloon. Balloon shrinks and feels colder. If the balloon were rigid, would the cooling be more, the same or less? Hint: Use the diagrams 9. Which process requires more energy: heating the substance or changing the state of the substance? Why? 10. An ice cube, initially at -10°C, with mass 9.0 grams is placed on top of a 2.0 kg iron plate J (cp = 0.50 ). If the temperature of the iron plate drops by 5.0°C, what is the final temperature of the g∙K J J H2O? (cice=2.0g∙K), ΔfusH (water)=6.00kJ/mol, cwater =4.2 g∙K )? 3 11. During a chemical reaction the thermometer in a beaker showed an increase in temperature by 10.°C. Choose all that apply: qsys < = > 0 qsur < = > 0 w < = > 0 ∆Tsys < = > 0 ∆Tsur < = >0 12. Reaction takes place in the balloon. During the reaction balloon expands. Choose all that apply: qsys < = > 0 qsur < = > 0 w < = > 0 ∆Tsys < = > 0 ∆Tsur < = > 0 13. A combustion reaction takes place in the balloon. During the reaction balloon expands. Choose all that apply: qsys < = > 0 qsur < = > 0 w < = > 0 ∆Tsys < = > 0 ∆Tsur < = > 0 14. A 20.0 g hot block of iron at 400. K is placed against a 40.0 g cold block of gold at 200. K. What will the J final temperature (in K) of the blocks be when they reach thermal equilibrium? (Cs[Fe] = 0.400 g∙K), J Cs[Au] = 0.100 g∙K). 15. A 10.0 g object, initially at 75.0°C, is added to a calorimeter containing 100. mL of room temperature water at 25.0°C, which causes the temperature of the water to rise until the calorimeter contents reach g J 45.0°C. (d(H2O) = 1.00 , Cwater = 4.18 ). mL g∙K a. How much heat did the water absorb? b. What is the specific heat capacity of the object? 16. How much energy in kJ is required to fully vaporize (at Tbp=2525°C) a 270 g sample of Al(s), initially at kJ kJ J J 25oC?(ΔvapH=300. , ΔfusH=11 , CAl(s) = 0.9 , CAl(l) = 0.9 , Tmp=625°C) mol mol g∙K g∙K 17. Calculate the energy needed (in kJ) to heat 36.0 g of water from 50.0°C to 100.0°C and then to completely J vaporize it to steam at 100.0°C. The specific heat of water is 4.18g∙K, and its enthalpy change of kJ vaporization is 40.7mol. 18. For following endothermic reaction draw the energy diagram that describe the chemical reaction : Al(OH)3(s)  Al2O3(s) + H2O(g) a. Circle the correct relationship between the final temperature of the surroundings when the reaction takes place at constant pressure versus constant volume. Tfinal (constant pressure) < ≈ > Tfinal (constant volume) 19. If qp = 10.0J and w = 5.00J. What is the value of qv? J a. If the heat capacity is 5.00g∙K, mass is 1.00g and the initial temperature was 300. K. What would the final temperature be for the reaction? Tfinal (at constant pressure) = Tfinal (at constant volume) = 5 20. When magnesium metal reacts, both at constant pressure and constant volume, with a strong acid, like HCl, hydrogen gas is generated and the temperature of the solution increases. For each relationship below, circle the correct symbol (<, =, or >); if more information is required, circle nothing. ΔH < |qv| < = = > 0 > |qp| w < = > 0 ΔU < = > 0 Tfinal (constant pressure) < = > Tfinal (constant volume) 21. We are going to cool 28 g of water from 10.°C down to 0.0°C. Circle all that apply? Hint: Specific heat of 𝐽 water is 4.18 𝑔∙𝐾. a. b. c. d. e. Heat is absorbed by the water or qsys < 0 qsys > 0 qsurr < 0 qsurr > 0 What is qsys in Joules? What is qsurr Joules? Heat is released by the water qsys = 0 qsurr = 0 22. Define enthalpy of fusion ∆𝐻fus: kJ 23. We are going to freeze 28 g of water at 0.0°C down to 0.0°C ice (∆𝐻fus = 6.00 mol; melting). Circle all that apply: a. Net gain in the number of H-bondsor Net loss in the number of H-bonds b. Heat is absorbed by the water or Heat is released by the water c. Process is exothermic or Process is endothermic d. qsys < 0 qsys > 0 qsys = 0 e. qsurr < 0 qsurr > 0 qsurr = 0 f. What is a temperature change of the system? g. Calculate qsys in Joules? h. What is qsurr in Joules? 24. Calculate the heat (q) in joules that is involved in cooling 28g of liquid water starting at 10.°C to ice at 0.°C? 25. Answer the following questions related to the plot below of the final temperature of 1g object vs. the amount of heat the object absorbed. During the heating the object changed phase from solid to liquid. a) What is ΔHfusion for the object? Please give your answer in J. Temperature (K) 335 330 325 320 b) Which of the following is true about the object (circle one)? 315 Csolid > Cliquid ; Csolid < Cliquid; Csolid = Cliquid 310 c) Calculate the heat capacity in J/K of the object in the liquid phase. 305 50 100 150 Heat (J) 200 250 6 Numerical Answers: 1. a. b. c. 3. 4. Work is done on system w>0 ∆𝑈 − 𝑞 > 0 2. 5. I. 2 H2(g) + O2(g) → 2 H2O(l) a. Yes, work is done on the system. b. Tfinal > Tinitial c. ∆𝑈 > 𝑞𝑝 d. II. III. IV. V. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. e. Hot f. Constant pressure 2 H2O2(aq) → 2 H2O(g) + O2(g) a. Yes, work is done by the system b. Tfinal > Tinitial c. ∆𝑈 < 𝑞𝑝 d. e. Hot f. Constant volume CaCO3(s) → CaO(s) + CO2(g) a. Yes, work is done by the system b. Tfinal < Tinitial c. ∆𝑈 < 𝑞𝑝 d. e. Cold f. Constant volume 4 H2O(g) + 2 CO2(g) → 3 O2(g) + 2 CH3OH(l) a. Yes, work is done on the system b. Tfinal < Tinitial c. ∆𝑈 > 𝑞𝑝 d. e. Cold f. Constant pressure H3O+(aq) + OH–(aq) → 2 H2O(l) a. No b. Tfinal > Tinitial c. ∆𝑈 = 𝑞𝑝 d. e. Hot f. Neither, they’re the same. 16. 17. 18. 19. 20. q > 0; ΔU > 0; The surroundings get cold w < 0; qv < qp; q > 0; ΔU > 0; The surroundings get cold; Tfinal (const. pressure) < Tfinal (const. volume) qv= -15.555 J; Tfinal (constant pressure) = 302.111 K; Tfinal (constant volume) = 303.111 K w>0 More heat given off at constant volume. More cooling if rigid. Changing the state of the substance requires more energy; more of the intermolecular forces has to be broken. 48°C qsys < 0; qsurr > 0; ∆𝑇sur > 0 w<0 qsys < 0; qsurr > 0; w < 0; ∆𝑇sur > 0 333 K a. 8.38 kJ b. 27.9 J/g·K 3700 kJ 88.9 kJ Tfinal (const. pressure) < Tfinal (const. volume) 15.0J ; Tfinal (at constant pressure)= 298 K; Tfinal (at constant volume)= 297K ΔH < 0; w < 0; ∆𝑈 < 0; |qv| > |qp|; Tfinal (const. pressure) < Tfinal (const. volume) 21. a. b. c. d. e. Heat is released by water qsys < 0 qsurr > 0 -1.2 kJ 1.2 kJ 22. 23. a. Net gain b. Heat is released c. Exothermic d. qsys < 0 e. qsurr > 0 f. No change in temperature g. -9.3 kJ h. 9.3 kJ 24. -11 kJ 25. a. 50 J b. Csolid < Cliquid c. 10 J/K 7 Answers: 1. a. 5.38mol b. 5.38mol c. 50.4% 2. 0.582mol 3. 565kJ 4. a. b. c. 0 mols 5. 5.0× 10−4 𝑚𝑜𝑙 6. a. D b. C c. Trigonal planer d. Bent e. 75g 7. Cu2+(𝑎𝑞) + Al(𝑠) → Cu(𝑠) + Al3+(𝑎𝑞) a. Al(𝑠) → Al3+(𝑎𝑞) + 3e− b. Cu2+(aq) c. 1.50 mol 8. a. 0.09 b. LDF c. NH2Cl(aq) + H2O(l)  NH3Cl+(aq) + OH−(aq) 9. a. Pb2+ (𝑎𝑞); NO− 3 (𝑎𝑞) 2+ b. Pb (𝑎𝑞) + 𝑆𝑂42− (𝑎𝑞)  PbSO4(𝑠) c. 79.4 d. 7.00× 10−2 𝑚𝑜𝑙 e. 0.233M 10. Acid: Cu2+(𝑎𝑞); Base: NH3(𝑎𝑞); H2O(𝑙) 11. a. 50.% b. CO2(𝑎𝑞) + H2O(𝑙)  HCO2+(𝑎𝑞)+ OH− (aq) c. Close to 0.114M 8