Chemistry Notes Chapter 6 Thermochemistry http faculty sdmiramar edu nsinkaset studyguides Chap6SG pdf http faculty sdmiramar edu nsinkaset chem200MW studyguides Forms of Energy and Their Introconversion Potential Energy PE is from position chem bonds Kinetic Energy KE due to motion PE is converted to KE as falls Some transferred KE Heat PE in chemical bonds of food or fuel converted to KE ability to do work give heat Whenever energy transferred from one obj to another its work W and or heat q Delta E Energy flows to or from a system System is part of universe whose change is to be measured Sum of KE and PE of all particles in the system is the internal energy E or U Delta E E final E initial E products E reactants Energy Change Delta E q w Chem system can change its internal energy in two ways losing energy to surroundings OR gaining energy from surroundings E enters or leaves a system as heat and or work No work heat is exchanged W 0 so Delta E q Energy transferred only as work q 0 Delta E w Work when Volume change occurs against external pressure is PV work W P deltaV LxAtm If external P raised system V decreases Inc E by doing work delta E 0 W 0 SI unit is joule J 1 Joule 1 kg x m 2 S 2 Both heat and work in joules Calorie cal 1 cal 4 184 J 1 Kilojoule kJ 1000 J 2390 kcal Law of Conservation of Energy If system gains E surroundings lose an equivalent amount Energy can be transferred but NOT created or destroyed Delta E universe Delta E system Delta E surroundings 0 Determining Energy Change of a system determine system remember delta E q w Heat released q is negative gained positive Work done w neg work done on sys Add values given convert if necessary Delta E surroundings Opposite or delta E of system Delta E is a state rxn only depends on initial and final states Way Delta E changes doesn t matter Internal Energy E is a state fxn matters only on current state Delta E D P and D V depend only on initial and final states Q and P are not state fxns they depend on path system takes Enthalpy physical and chemical changes usually occur under constant pressure PV Work P atm x D V L or P Vfinal Vinitial xL atm 1 L atm 101 J Delta E q w q PD V Exothermic and Endothermic rxns Rxn enthalpy change heat of rxn or delta h Delta H H final H initial H products H reactants SIGN of delta H shows heat is absorbed or released Find sign by viewing heat as reactant or product Heat released show on right exothermic Heat absorbed show on left endothermic Endothermic increase in heat melting ice cube Combustion is decrease or exothermic Calorimeters Heat released or absorbed by a system is related to its temperature change and heat capacity More heat obj absorbs higher its T Q Constant x DeltaT Constant depends on material we re looking at Heat capacity is proportionality constant Heat capacity is to q DT In units of J K Specific heat capacity c is amount of heat needed to change the T of one gram of a substance by one k Specific heat Capacity q mass x DT J gXK Specific Heat Capacity of H2O 4 184 J g K From Specific heat capacity and mass measure temp change to calc heat change q q j c x mass x Delta T K Object becomes hotter q 0 Calorimetry AT CONSTANT PRESSURE Measures heat released or absorbed by a process whenever have a calorimetry problem 1st write down q loss q gained System loses system q gained by surroundings water Cup calorimeter q sys q water Measure Delta T to calc q water Qh2o C h2o x mass h2o x Delta T h2o Determine specific heat capacity of a solid weigh solid system heat to known T boiling point of water add to H2O surroundings of known T and mass in calorimeter q lost q gained q solid q water c solid x mass solid x deltat solid C h2o x Mass h2o x deltat h2o C solid C h2o x Mass h2o x deltat h2o mass solid x deltat solid Determining C of the solid use above equation Identify each variable Finding q of a rxn Remember that water 1 ml 1 g acid base rxns Add ml of bases and acid and take as grams Be aware of units and keep in mind looking for qrxn or qh2o Delta H Q rxn Moles rxn Kilojoules per mole mass x C x DeltaT Finding Delta H in acid base rxns set up balanced chemical equations With amount of product find the limiting reactant Acid Base rxns OH H H2O Find Q Find moles produced use formula above pay attention or Bomb Calorimeter Heat of Combustion q rxn q calorimeter q h2o Heat change measured by a bomb calorimeter is at constant volume qv q calorimeter c calorimeter x delta t Which equals deltaE NOT deltaH but often approx deltaH Q specific heat x delta t x mass remember units Bond energies in fuels and foods Common fuels are hydrocarbons and coal organisms fats and carbohydrates Molecs have c c and c h bonds that break in combustion CO and OH bonds CO2 H2O DeltaH combustion values for carbon cmpds Ethane C2H6 1560 kJ mol and 51 88 kJ g Ethanol C2H5OH 1367 kJ mol and 29 67 kJ g Higher heat of comb more heat released Question less than X Calories confirm given heat capacity and temp inc calculate heat gained by the calorimeter q rxn q calorimeter Q calorimeter heat capacity x deltaT 10 Calories 10kcal 41 84kJ Stoichiometry of Thermochemical Equations common equation accompanied by deltaH of that particular rxn rep ed by eq delta h endothermic exothermic Magnitude of delta h is proportional to amount of substance of the rxn FRACTIONS OK bc express what delta h is for one mole of reactant Delta H Just another quantitative variable Make into conversion factor Use delta H to convert between Mol of substance and equivalent heat change Hess s Law of Summation finding unknown H given other thermochemical equations Pay attention to coefficients from given equations but we are allowed to do anything mathematically to the original equations multiply divide flip etc Whatever you do to make the eqyation match must do to the delta h as well Things on both sides of the 2 equations will cancel when merged together Standard Heats of Rxn standard states to compare thermodynamic data gas standard 1atm Aqueous 1M concentration 1 mol liter Element or compound is at 1 atm and 25 degrees celcius or 298k Delta H of Rxn is measured with all substances in their standard states standard heat of rxn DeltaH degree symbol rxn delta H not Delta H not delta h of the formation rxn with substances in standard states Table 6 3 has Delta H not Any element in its elemental …