Chapter 14 Thermochemistry

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## Chapter 14 Thermochemistry

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**Chapter 14**Thermochemistry**OBJECTIVES: students will be able to understand…**Concept of energy and its various forms Relationship btwn energy, work, and heat The key features of a state function How to use Hess’s law & thermochemical cycles to calculate enthalpy D’s of chem rxtns How to use calorimetric data to calculate enthalpy D’s**Thermochemistry**Energy and Work: Describing energy D’s during a rxtn. Energy D’s are often of prime importance:**Thermite rxtn**Gives off heat even under water**Chemical Hand Warmers**Most hand warmers heat released from slow oxidation of Fe 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)**Energy**– Takes many forms – Some can be seen or felt – Defined by its effect on matter – How many can you name?**Chemical:**particular arrangement of atoms in a chem compound. Heat & light produced in this rxtn due to E released during**Isopropyl alcohol and oxygen**Chemical Energy: C3H7OH(g) + 3O2(g) 2CO2(g) + 3H2O(g) + E C3H7OH(g) E (kJ/mol) CO2(g)**Nature of Energy**Chemistry studies matter energy affects matter Energy = anything w/ capacity to do work Work= ___________ x ______________ Heat = flow of E difference in temp E exchanged btwn objects thru contact = collisions**Energy, Heat, and Work**Think of energy as a quantity an object(s) can possess Think of heat & work as 2 diff ways that an object can exchange E with other objects:**Classification of Energy**• Kinetic energy (KE) = • E of motion or • E being transferred • Thermal energy = E associated with temp • thermal E = a form of KE**Classification of Energy**Potential energy (PE) = stored E in an object, or energy associated with**Surroundings**Surroundings System System System and Surroundings • system material or process being studied • Surroundings:everything else • system can exchange energy w/ surroundings**System + Surroundings =Universe**forms of E that flow: qsys = and wsys = Work can be expressed as: w = “PV work” is either expansion or compression**When is q positive? When is it negative?**When is w positive? When is it negative?**q is pos when heat flows**q isneg when heat flows w is pos when work is done w is neg when work is done __**Comparing Amount of E in System & Surroundings During**Transfer Conservation of E: amount of E gained or lost by system must = amount of E lost or gained by surroundings**Law of Conservation of Energy**E cannot be created nor destroyed in a chem rxtn When E is transferred btwn objects, or converted from one form to another, total amount of**Units of Energy**• KE of an object directly a its mass & velocity KE = ½mv2 When mass is kg & velocity is m/s, then unit for**Units of Energy**1 Joule of E = Energy needed to move 1 kg mass at speed = to 1 m/s 1J=**Units of Energy**• joule (J) = amount of E needed to move a 1-kg mass a distance of 1 meter • 1 J =**Units of Energy**• calorie (cal) = energy needed to raise temp of one gram of water 1°C • kcal = • food Calories =**The First Law of ThermodynamicsLaw of Conservation of Energy**Thermodynamics: study of E & its _______________________ Total amount of E in universe is constant No system can be designed that will continue to produce E without some source of E**Energy Flow and Conservation of Energy**• sum of ED’s in system & surroundings must be • DE universe = DE system + DE surroundings • DE universe =**Internal Energy**• internal energy= sum of KE & PE of all particles that compose the system • D in internal Esystem only depends on amount of E in system at beginning & end**Internal Energyis a state function:**• depends only on initial & final conditions, • noton the process used; • E = Efinal – Einitial • DErxtn = Eproducts − Ereactants**State Functions:**a thermodynamic property that depends only on the state of the system, noton the pathway used to get there the opposite is an energy transfer function**Either of 2 trails to reach top of the mountain:**1st = long & winding, 2nd = short but steep. State Function**Regardless of which trail you take, when you**reach the top you will be 10,000 ft above base. State Function Dist from base to peak is a state function. Depends only on diff in elevation btwn base & peak, not on how you arrive there!**A lake’s water level**is a state function: a property of the system Outflow/inflow & evaporation/precipitation Are processes that D the water level = path functions. We may not know which process causes a D, but we do know the sum of the processes (water level)**Internal E (DU),**a state function, is affected by q & w (path functions). They are processes that alter the system. Once a D occurs in U, we can’t say which process caused the D. We can see the sum of these processes reflected in the D in internal energy**final**energy added DE = + Internal Energy initial initial energy removed DE = ─ Internal Energy final Energy Diagrams E diagrams show the direction of E flow during a process**final**energy added DE = + Internal Energy initial initial energy removed DE = ─ Internal Energy final Energy Diagrams If final condition has larger amount of internal E than initial condition, D in internal E will be (+)**initial**energy removed DE = ─ Internal Energy final Energy Diagrams If final condition has smaller amount of internal E than initial condition, D in internal E will be (─)**energy**released DErxn = ─ E Flow in a ChemRxtn • Total amount of Einternal in 1mol of C(s) & 1 mole of O2(g) is > Einternal in 1 mole of CO2(g) • In rxtn C(s) + O2(g) → CO2(g), there’s a net release of E into the surroundings C(s), O2(g) Internal Energy CO2(g) Surroundings System C + O2→ CO2**E Flow in a ChemRxtn**energy absorbed DErxn = + C(s), O2(g) Internal Energy CO2(g) Surroundings System CO2 →C + O2 System C + O2→ CO2 • Total amount of Einternal in 1mol of C(s) & 1 mole of O2(g) is > Einternal in 1 mole of CO2(g) (at same T and P) • In rxtn CO2(g) → C(s) + O2(g), E is absorbedfrom surroundings into the system**Surroundings**DE + System DE ─ Energy Flow • When E flows out of system, DEsystem is (─) • When energy flows into the surroundings, DEsurroundings is (+) • Therefore:**Surroundings**DE ─ System DE + Energy Flow • When E flows into a system, it must come from surroundings • When E flows into a system, DEsystem is**Surroundings**DE ─ System DE + Energy Flow • When E flows out of the surroundings, DEsurroundingsis • Therefore: DEsystem= ─DEsurroundings**Energy Exchange**• Energy is exchanged btwn system & surroundings thru heat&work • q = heat (thermal) energy • w = work energy • q and w are NOT state functions, their value depends on the process DE = q + w**Energy Exchange**Energy exchanged btwn system & surroundings by either heat exchangeorwork being done**Heat & Work**White ball = initial amount of 5.0 J of KE As it rolls some E converted to heat by friction Rest of KE transferred to purple ball by collision**Heat & Work**On a smooth table, most KEwhite ball is transferred from white to purple ball. Small amount lost thru friction as heat DE for white ball: DE= KEfinal − KEinitial = 0 J − 5.0 J = −5.0 J KE transferred to purple ball, w = −4.5 J KE lost as heat, q = −0.5 J q + w = (−0.5 J) + (−4.5 J)**Heat & Work**DE for white ball: DE = KEfinal − KEinitial = 0 J − 5.0 J = −5.0 J KE transferred to purple ball, w = −2.0 J KE lost as heat, q = −3.0 J q + w = (−3.0 J) + (−2.0 J) On a rough table, most KEwhiteball is lost thru friction: < 1/2 is transferred to purple ball**Heat, Work, and Internal Energy**DE of white ball is = for both cases, but q and w are not On rougher table, heat loss, q, is greater (q = more neg #)**Heat, Work, and Internal Energy**But on rougher table, less KE transferred to purple ball, so work,w, done by white ball, is less (less neg #)**Heat, Work, and Internal Energy**DE is a state function depends only on velocity of white ball before and after the collision.