Lecture 38 Thermochemistry 1 Read HW BLB 5 3 5 5 BLB 5 4 6 17 29 37 39 41 53 55 Know REVIEW energy enthalpy enthalpy of reactions calorimetry Lectures 2 26 Check out the grad u lator on the Chem110 website FINAL SKILL CHECK TEST DEADLINE MONDAY APRIL 27 Need help Get help TAs in CRC 211 Whitmore and SI hours on Chem 110 website my office hours Mon 12 30 2 Tues 10 30 12 in 324 Chem Bldg or 326 Chem F in al Exa m MONDAY May 4 12 20 pm Concept Final review session with Sheets Thursday 4 30 6pm in 108 Forum Please work through the Concept Exam on lecture note page before the review session bring it with you along with any questions you may have This review is meant to complement to the review sessions that your TAs will be holding in which they will go over the practice exams Sheets Page 1 Lecture 38 Thermochemical equations thermochemistry the study of the relationship between chemical reactions and energy changes a balanced chemical equation that also includes the change H delta H standard standard P 1 bar T usually 25 C NOTE this is NOT the same as STP for gases Recall 1 J 1 kg m2 s2 in chemistry kilojoule 1 kJ 103 J calorie 1 cal 4 184 J Calorie 1 Cal 103 cal 1 kcal dietary Sheets Page 2 Lecture 38 The first law of thermodynamics energy can be converted between various forms but total energy is law of conservation of energy first law of thermodynamics all energy lost by a system under observation must be gained by the surroundings during energy conversion some heat is always produced Sheets Page 3 Lecture 38 State functions state functions are independent independent no memory of previous state and depend only on current state its T P etc E is a state function for elevation path 1 path 2 E for path 1 E for path 2 E Efinal Einitial Sheets Page 4 Lecture 38 Energy enthalpy when changes occur at constant pressure e g those done at bench work w arises from expansion or contraction of system w P V V Vfinal Vinitial E qp wexpansion quantitative definition E qp wp qp P V So qp E P V H H is enthalpy qualitative definition H is also a state function H is the quantity of thermal energy heat absorbed or released by a system at constant pressure for many chemical processes P V is small H E Sheets Page 5 Lecture 38 Examples of enthalpy energy transfers accompany physical changes chemical changes physical changes we ve seen before freezing melting add heat to ice temperature does not change during melting Hfusion heat of fusion vaporizing condensing add heat to water temperature does not change during vaporization Hvaporization heat of vaporization Sheets Page 6 Lecture 38 Enthalpy of reactions chemical changes H Hproducts Hreactants qp Hrxn enthalpy is an extensive property how much stuff you ve got Hrxn is equal in magnitude but opposite in sign for H of reverse reaction depends on states of reactants products e g gas liquid H 0 exothermic reaction qp 0 heat is think of heat as a product will come in handy when we get to equilibrium H 0 endothermic reaction qp 0 heat is think of heat as a reactant will come in handy when we get to equilibrium Sheets Page 7 Lecture 38 Demo great cotton balls of fire Na2O2 s 2H2O l 2Na aq 2 OH aq H2O2 aq sodium peroxide 2 H2O2 aq 2H2O l O2 g what type of rxn reaction produces heat H is have high O2 concentration and cotton has low ignition temperature voila Another reaction demo Ba OH 2 8H2O s 2NH4 SCN s Ba SCN 2 aq 2NH3 g 10H2O l mix two solids highly endothermic reaction H is driving force is increase in disorder entropy go from 2 ordered crystals to lots of ions gases in solution Sheets Page 8 Lecture 38 Example 2H2 g O2 g 2H2O g H 483 6 kJ a Is this reaction exothermic or endothermic b How much heat is given off per mole of O2 c How much heat is given off per mole of H2 d How much heat will be needed to convert 9 0 g of water into hydrogen and oxygen e How much heat will be given off if 10 0 g of H2 is consumed Sheets Page 9 Lecture 38 Example 5 00 g of table sugar C12H22O11 is completely combusted How much heat is released Then why don t you combust when you eat candy H 1348 2 kJ C12H22O11 s 12 O2 g 342 g mol 32 g mol 12 CO2 g 11 H2O g 44 g mol 18 g mol 5 00 g Use Hrxn as you would any other reactant or product That is you need to have a balanced chemical equation use appropriate molar relationships The negative sign indicates that heat is released Sheets Page 10 Lecture 38 Calorimetry review Lecture 26 experimental measure of heat flow used to determine Hrxn review molar heat capacity specific heat capacity q C m T q quantity of heat m mass C specific heat T Tfinal Tinitial m C heat capacity an extensive property for H2O C 4 184 J g C NOTE H2O is usually part of the surroundings qsurroundings Csurroundings m T Sheets Page 11 Lecture 38 Coffee cup calorimeter constant pressure calorimetry heat lost by the reaction qrxn is equal in magnitude to but opposite in sign to heat gained by the solution qsoln and vice versa we can measure temperature and calculate enthalpy heat lost or gained measure qsoln calculate qrxn qsoln qrxn can calculate qrxn which is qp Hrxn Sheets Page 12 Lecture 38 Quantitative calorimetry You mix 50 mL of 0 1 M SO32 aq and 50 mL 0 1 M OCl aq and observe that the temperature increases from 20 0 C to 33 6 C What is H for this reaction Assume all heat capacity is from the water and density of solution is 1 0 g mL SO32 aq OCl aq 50 0 mL 0 10 M 50 0 mL 0 10 M SO42 aq Cl aq H CH2O 4 184 J g C T 33 6 C 20 0 C 13 6 C Is there a limiting reagent problem Always check if you are given amounts of 1 reagent Sheets Page 13 Lecture 38 Example 100 mL of 0 5 M HCl is mixed with 50 mL of 1 0 M NaOH in a coffee cup calorimeter What is the final temperature of this solution The initial temperature is 21 5 C Assume that the total volume of solution is 150 mL that its density is 1 0 g mL and that its specific heat of the solution is the same as that for pure water 4 184 J g K Is this an exothermic or endothermic process HCl NaOH 100 mL 50 mL 0 5 M 1 0 M H2O NaCl Ti 21 5 C Tf H 54 kJ Vsoln …
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