Thermochem: It s DYNAMIC!

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1 Thermochem: It s DYNAMIC! Presented by: Hiral Mathur Pre-AP, AP, and Advanced Placement are registered trademarks of the College Board, which was not involved in the production of, and does not endorse, this product. The presenter(s) are solely responsible for the required permissions and content of the lessons. LTF and Laying the Foundation are registered trademarks of Laying the Foundation, Inc. Visit: for more information.

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3 Laws of Thermodynamics Zeroth Law: Heat flows from hot to cold First Law: Energy and matter are conserved Second Law: Matter tends towards chaos Third Law: Entropy of a pure crystal at 0K is zero What does each term tell us? Enthalpy (ΔH) Energy content + endothermic - exothermic Entropy (ΔS) Disorder + increase in disorder - decrease in disorder Free energy (ΔG) Spontaneity + non-spontaneous - spontaneous Equilibrium (K ) Extent of reaction >1 favors products <1 favors reactants Enthalpy Heat content H Endothermic (+) or Exothermic (-) Calculate: Table of standard values Hess s Law Calorimetry Stoichiometry Bond energies ΔH f Enthalpy of formation. Production of ONE mole of compound FROM ITS ELEMENTS in their standard states ( ) 0 for ELEMENTS in standard states: 25 C (298 K), 1 atm, 1M Big Mamma Equation: ΔH rxn = Σ ΔH f (products) - Σ ΔH f (reactants) Page 3

4 Example 1) The enthalpy for the formation of Na2O is shown below, 4Na(s) + O2(g) 2Na2O(s) H rxn = kj mol 1 Calculate the amount of heat released when 1 mol of Na2O is formed? 2) Calculate the enthalpy for the reaction below and indicate if the reaction is endothermic or exothermic. 3 Al(s) + 3 NH 4 ClO 4 (s) Al 2 O 3 (s) + AlCl 3 (s) + 3 NO(g) + 6 H 2 O(g) Given the following values: Substance ΔH f (kj/mol) NH 4 ClO 4 (s) -295 Al 2 O 3 (s) AlCl 3 (s) -704 NO(g) 90.0 H 2 O(g) ) Find the ΔH f of C 6 H 12 O 6 (s) from the following information: C 6 H 12 O 6 (s) + 6 O 2 (g) 6 CO 2 (g) + 6 H 2 O( ) kj mol 1 Substance ΔH f (kj/mol) CO 2 (g) H 2 O(l) Hess s Law Enthalpy is not dependent on the reaction pathway. If you can find a combination of chemical equations that add up to give you the desired overall equation, you can also sum up the ΔH s for the individual reactions to get the overall ΔH rxn. Remember this: First decide how to rearrange equations so reactants and products are on appropriate sides of the arrows. If equations had to be reversed, reverse the sign of ΔH If equations had be multiplied to get a correct coefficient, multiply the ΔH by this coefficient since ΔH s are in kj/mole (division applies similarly) Check to ensure that everything cancels out to give you the exact equation you want. Hint** It is often helpful to begin your work backwards from the answer that you want! Page 4

5 Example 4) Given the enthalpy change for the two reactions below, C(s) O 2(g) CO(g) C(s) + O2(g) CO2(g) H = 111 kj H = 394 kj Calculate the enthalpy change for the reaction, CO(g) O 2(g) CO2(g) 5) Using the following standard enthalpy of reaction data, 2C(s) + 3H2(g) C2H6(g) C(s) + O2(g) CO2(g) H2(g) O 2(g) H2O(l) H = kj H = 394 kj H = 286 kj Using the reactions above, calculate the heat of reaction for the combustion of 1 mol of ethane (C2H6). Calorimetry The process of measuring heat based on observing the temperature change when a body absorbs or discharges energy as heat. Types of calorimetry: (1) Coffee-cup calorimetry in the lab this is how we experiment to find energy of a particular system. We use a Styrofoam cup, reactants that begin at the same temperature and look for change in temperature. After all data is collected (mass or volume; initial and final temperatures) we can use the specific formula to find the energy released or absorbed. This process is referred to as constant pressure calorimetry. Energy (q) released or gained q= mcp Δ T q = quantity of heat ( Joules or calories) ΔT = T f - T i (final initial) C p = specific heat capacity ( J/g C) m = mass in grams * q = - ΔH and the specific heat of water (liquid state) = J/g C (2) Bomb calorimetry weighed reactants are placed inside a steel container and ignited. Often referred to as constant volume. This is used by industry to determine number of food calories that we consume! Page 5

6 Example 6) When g of NaOH is dissolved in g of water initially at C in a coffee-cup calorimeter, the final temperature is found to be C. Calculate the heat liberated when NaOH dissolves in water. (Assume the specific heat of the solution is the same as that of water and no heat is absorbed by the calorimeter.) Note: Heat lost by substance = heat gained by water 7) A 28.4 g sample of an unknown metal was heated to C and plunged into a 100 g sample of water initially at a temperature of C. The final temperature of the mixture was C. Calculate the specific heat of the metal. Identify the metal. 8) The overall reaction in commercial heat packs can be represented by 4Fe(s) + 3O 2 2Fe 2 O 3 ΔH rxn = kj/mol rxn How much heat is released when 1.00 g iron is reacted with excess O 2? 9) Calculate the ΔH rxn using only the information provided below. H 2(g) + F 2(g) 2 HF (g) Bond Type H-H F-F H-F Bond Energy 432 kj/mol 154 kj/mol 565 kj/mol Entropy Disorder Less Disorder (-) or More disorder (+) Calculate: Table of standard values Hess s Law Page 6

7 Example 10) Predict whether the entropy of the system increases, remains constant or decreases when the following processes occur. Explain your reasoning. a) Ice melts at 0 C. b) A precipitate forms in aqueous solution. c) A solid dissolves in water. d) A gas condenses to a liquid. 11) Determine the sign of the entropy change, ΔS, in each reaction below. a) 2NO 2 (g) N 2 O 4 (g) b) H 2 O( ) H 2 O(g) c) CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) 12) Calculate the entropy change at 25 C, in J/K for the reaction Given the following data 2 SO 2 (g) + O 2 (g) 2 SO 3 (g) Substance SO 2(g) O 2(g) SO 3(g) ΔS J/KΧ mol J/KΧmol J/KΧ mol Free Energy Spontaneity Spontaneous(-) or non-spontaneous(+) Calculate: Table of standard values Hess s Law Calculations ΔH, ΔS, and ΔG may all be calculated from tables of standard values, from Hess Law or from the Gibb s equation ΔGº = ΔHº - TΔSº Caution on units: ΔH and ΔG are typically given on kj mol 1 whereas ΔS typically given as J K 1 mol 1 Page 7

8 Other important relationships: ΔG = - RT lnk ΔG = ΔGº + RT log Q ΔG º = - nfeº Conditions of ΔG ΔH ΔS Result negative positive spontaneous at all temp positive positive spontaneous at high temp negative negative spontaneous at low temp positive negative not spontaneous, EVER Relationship to K ΔG K E 0 at equilibrium 0 negative >1, products favored + positive <1, reactants favored Example 13) Consider the first ionization of sulfurous acid: H 2 SO 3 (aq) H + (aq) + HSO 3 (aq) Certain related thermodynamic data are provided below: H 2 SO 3 (aq) H + (aq) HSO 3 (aq) H f kcal/mole S cal/mole K (a) Calculate the value of ΔG at 25 C for the ionization reaction. (b) Calculate the value of K at 25 C for the ionization reaction. Page 8

9 14) Given Kb for ammonia is 1.8 x 10 5 at 298 K, calculate ΔG for the reaction NH3(aq) + H2O(l) NH4 + (aq) + OH - (aq) 15) For each of the reactions listed determine the effect on ΔG rxn when the temperature is increased. a) CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + heat b) heat + H2O(l) H2O(g) c) heat + 2NO2(g) N2O4(g) Page 9

10 Problems Mg(s) + 2 HCl(aq) MgCl 2 (aq) + H 2 (g) H rxn = kj mol 1 1. The reaction above was carried out in an open polystyrene cup calorimeter. The magnesium was added to 50.0 ml of a 1.00 M HCl solution. Assume the specific heat of the solution to be 4.18 J/g C and the total mass of the solution in the calorimeter to be g. (a) If 0.65 g of the magnesium were reacted, determine the amount of heat released into the calorimeter. (b) Determine the temperature change in the calorimeter. Answer the following questions about the oxidation of magnesium. Mg(s) + ½ O 2 (g) MgO(s) (c) Determine the value of the standard enthalpy change for the reaction in the box above. MgO(s) + 2 HCl(aq) MgCl 2 (aq) + H 2 O(g) H 2 (g) + ½ O 2 (g) H 2 O( ) ΔH rxn = 45.7 kj mol 1 ΔH rxn = 286 kj mol 1 (d) Determine the value of the standard entropy change, ΔS rxn, for the oxidation of using the information in the following table. Substance ΔS (J mol 1 K 1 ) Mg 33 O MgO 27 (e) Calculate ΔG for the oxidation of magnesium at 25 C (f) Indicate whether the reaction is spontaneous or non-spontaneous at 25 C. Justify your answer Page 10

11 2) Which of the following reactions has the negative entropy change? a. 2IF(g) --> I2(g) + F2(g) b. Hg(s) --> Hg(l) c. C6H6(l) + 15/2O2(g) --> 6CO2(g) + 3H2O(g) d. H2O(l) -->H2O(s) 3) For the following general reaction, what can be said about spontaneity at different temperatures? A + B --> C + D H o is positive, S o is positive a. spontaneous at all temperatures b. spontaneous only at high temperatures c. spontaneous only at low temperatures d. not spontaneous at any temperature 4) In the reaction of two Cl atoms to give a Cl2 molecule [2Cl(g) --> Cl2(g)], the sign of the enthalpy change is and the sign of the entropy change is. a. negative, positive b. positive, negative c. positive, positive d. negative, negative 5) Which of the following processes would ΔS have a positive value? I. Ca OH -1 Ca(OH) 2 (s) II. 2Fe 2 O 3 (s) 4Fe(s) + 3 O 2 (g) III. H 2 (g) + C 2 H 4 (g) C 2 H 6 (g) (A) I only (B) II only (C) I and III only (D) II and III only (E) I, II, and III 6) Calculate the heat of formation of ethyl alcohol, C 2 H 5 OH, from the following heats of combustion: C(s) + O 2 (g) ---> CO 2 (g) ΔH rxn = kj H 2 (g) + 1/2 O 2 (g) ---> H 2 O(g) ΔH rxn = kj C 2 H 5 OH(l) + 3O 2 (g) ---> 2CO 2 (g) + 3H 2 O(g) ΔH rxn = kj (A) kj/mol (B) kj/mol (C) kj/mol (D) kj/mol Page 11

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