Thermochemistry. Thermochem and Heat transfer notes notebook. October 18, Crash Course Chemistry Intro: Energy and Chemistry.

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1 Thermochemistry Crash Course Chemistry Intro: Nearly every chemical and physical change is accompanied by a change in energy S L G This E is exchanged btw a system and its surroundings Energy and Chemistry Enthalpy Calorimetry rdqk&list=pl8dpuualjxtphzzyuwy6fyeax9mqq8ogr Entropy Oct 20 9:58 AM Energy = the ability to do work KE - E of motion, depends on mass & velocity of particles PE - stored E/chemical E, depends on bonds, positions, attraction, repulsion, etc Potential Energy Diagram (aka Energy Diagram) R Enthalpy Diagram Temp = avg molecular KE of particles in sample, random motions Heat = total sum of KE transferred btw objects in collisions Joules (kg m 2 / s 2 ) Internal Energy - total of all molecular KE in all particles and PE stored Is E being released from the system or absorbed from the surroundings? P Oct 20 9:53 AM Heat: (aka Enthalpy H) > Exothermic heat leaves the system (heat of the system decreases) «Sign ( ) > Endothermic heat enters the system (heat of the system increases) Work: «Sign (+) Is E being released from the system or absorbed from the surroundings? > If system does work on the surroundings (i.e. Energy flows out of the system) W= ( ) > If work is done on the system (i.e. E flows into the system) W= (+) Oct 16 9:00 PM 1

2 Energy: Is a state funcon > Independent of the pathway or mechanism (net change maers) Heat (q) and Work (w) are NOT state funcons Work usually applies to the expansion and compression of gases ( expansion does work on surroundings and compression is work on the system) w = P ( V) 1st Law of Thermodynamics: C of E > Energy of the universe is constant > Heat flow from a system = to heat flow to the surroundings ΔE system = ΔE surroundings Internal Energy the sum of the KE and PE of the system When a gas expands, work is ; During compression of gases, work is. Example problems 1) Calculate the change in internal Energy for a system undergoing an endothermic process in which 15.6 kj of heat flows and where 1.4 kj of work is done on the system 2) A system released 125 kj of heat while 104 kj of work is done on it. Calculate the change in internal energy. 3) A balloon contains 313g of He at a pressure of 1.00 atm. The volume of the balloon is 1910L. The temperature decreased by 15.0 C as the volume decreases to L while the pressure remained constant. Calculate q, w, and E for the helium in the balloon ( the molar heat capacity of helium gas is 20.8 J/ C mol. ) Energy and Work Problem Set then a few more notes on enthalpy so more work time tomorrow! 1 cal = 4.18 J What is a calorie in food? Cal = 1000 cal so we also call Cal a kcal! Oct 23 6:36 PM 2

3 Heat Transfer and Calorimetry Internal heat (when not transferred) > q = mc T Heat transfer > Heat lost by the system = heat gained by surroundings > q syst = q surr (one is neg) Other forms of Calorimetry > Heat absorbed by objects can be calculated with heat capacity of the object Heat capacity (C) is amount of heat absorbed by an object for every C increase J/ C (this is an extensive property of maer) Bomb Calorimeters (constant V, so the w from the equaon is eliminated making E = q only) Heat of Rxn... Calorimetry uses the 1st L of TD Oct 16 9:04 PM > Heat absorbed or released by substances Coffee cup calorimeter! ( constant P, but w is done! Calc with enthalpy = E + P V ; then q= mc T) c = Specific heat capacity J/g C (this is an intensive property of maer) c is a constant for each substance for each state of maer «ex: H 2O (s) = 2.1 J/ g C «H 2O (l) = 4.18 J/ g C «H 2O (g) = 1.99 J/ g C Oct 23 6:06 PM Oct 16 9:36 PM Calorimetry Don't write! the science of measuring the amount of heat change in a process heat generated (exothermic reaction) heat consumed (endothermic reaction) heat dissipated by a sample (physical transfer from hotter to cooler object) Oct 16 9:39 PM Oct 16 9:14 PM 3

4 A calorimeter is a device used to measure heat of reaction. It can be sophisticated and expensive or simple and cheap. Any process that results in heat being generated and exchanged with the environment can be studied using calorimetry: measuring Calories in foods drug design in the pharmaceutical industry improving manufacturing processes, determining thermal waste the study of metabolic rates in biological systems Oct 16 9:15 PM Oct 16 9:16 PM Calorimetry Calcs: 1) What is the specific heat of 50.00g of a metal that raises 100.0mL of water in a calorimeter from 34.0C to 36.0C. The initial temp of the metal was 95C. The final temp of the water and the metal is 36.0C. > Molar Heats Molar heat capacity J/mol C Molar heat of reacon ΔHrxn (from a thermochemical equaon) «ΔH rxn «ΔH comb «ΔH neut > Molar Heats for non chemical processes (calc heat when temp is not changing during a phase change) ΔH vap ΔH fus 2) A block of rhenium metal (c= cal/g C) is heated to 88.2 C and then dropped into 100.0g water initially at 26.4C. The final temp of the water and the metal is 32.4C. What is the mass of the block of rhenium? ΔH soln «ΔH fus H 2O = 6.02 kj/mol «ΔH vap H 2O = 40.7 kj/ mol Oct 21 9:28 AM Oct 16 9:04 PM a graph of temperature versus time for processes where energy is added at a constant rate shows the physical changes that occur as heat is added (changes in temperature AND changes of state) Oct 21 9:37 AM 4

5 3) Example Problems 28.0g of a substance with a molecular weight of 52.0 g/mol is placed in a bomb calorimeter with a heat capacity of 2.60 J/ C and combusted. What is the molar heat of combuson for the substance if the temperature of the calorimeter went from 25.0 C to C? Convert to J and Cal. 4) The temperature of a 95.4 g piece of copper increases from 25.0 C to 48.0 C when the metal absorbs 849 J of heat. What is the specific heat of copper? Oct 21 9:32 AM 5) The heat of fusion of water is 40.1kJ/mol. How much heat will be transferred when 50.10g of water melts. What is the sign for q in this process? 5) Oct 21 9:29 AM 6) How much heat is required to convert 55.0g of liquid water at 22.0 C to steam at 105 C? Oct 20 3:33 PM 5