Ch 5 Thermochemistry Thermodynamics The study of energy and its transformations Thermochemistry The portion of thermodynamics that examines the relationships between chemical reactions and energy changes that involve heat 5 1 Energy Energy The capacity to do work or transfer heat Work The energy used to cause an object to move against a force Heat The energy used to cause the temperature of an object to increase Kinetic Energy and Potential Energy Kinetic energy The energy of motion o Magnitude depends on object s mass and speed o o Increases as speed or mass increases Potential energy Energy dependent on the object s position relative to other objects o Stored energy arising from the attractions and repulsions an object experiences relative to other objects Kinetic and potential energy are interconvertible Forces from electrical charges are more important than those from gravity Electrostatic potential energy Eel Energy that arises from interactions btwn charged particles o Energy is proportional to electric charges on the two interacting objects Q 1 and Q2 and inversely proportional to distance d separating them o o k is proportionality constant 8 99 x 109 J m C2 relating units for energy to units for charges and distance of separation C is coulomb unit of electrical charge J is joule unit of energy o Zero of electrostatic potential energy is infinite separation of charged particles o Same charge sign of objects creates repulsion decreasing potential energy and opp sign causes attraction increasing potential energy Chemical energy of fuel is due to potential energy stored in arrangements of atoms When fuel burns chemical energy converted to thermal energy Units of Energy Joule SI unit for energy o James Joule 1818 1889 British scientist found 1 J 1 kg m 2 s2 Kilojoules kJ typically used bc joules are small Energy changes in chemical rxns often use calories as units Calorie cal Originally amount of energy to raise temp of 1 g of water from 14 5 to 15 5 C o Now 1 cal 4 184 J Nutritional Calorie Cal 1 Cal 1000 cal 1 kcal System and Surroundings System Portion of universe singled out for study of energy changes o Open system Matter and energy can be exchanged with surroundings o Closed system Energy but not matter exchanged with surroundings Energy exchanged as work and heat o Isolated system Neither energy nor matter exchanged Surroundings Everything else in universe besides system Transferring Energy Work and Heat Generally energy transferred into or out of a system by causing motion of an object against a force and causing a temp change Work Energy transferred when a force moves an object o w F x d Force Any push or pull exerted on an object Heat Energy transferred from hotter object to colder one 5 2 The First Law of Thermodynamics Energy conversions and transfers follow that energy can be neither created nor destroyed First law of thermodynamics Energy is conserved Internal Energy Internal energy E The sum of all the kinetic and potential energies of the components of the system Numerical value of system s internal energy generally unknown Concerned with change in E accompanying change in the system Thermodynamics have three parts number and unit magnitude and sign direction Pos E system gained energy from surroundings neg E system lost energy to surroundings In chemical rxn initial state of system is reactants and final state is products Relating E to Heat and Work Internal energy of system changes in magnitude when heat is added or removed or work is done on or by the system o q is sum of heat added to or liberated from the system and w is work done on or by system When heat is added to a system or work is done on a system E increases Endothermic and Exothermic Processes Endothermic Process in which system absorbs heat o Heat flows into system from surroundings Exothermic Process in which system loses heat State Functions Internal energy has fixed value for a given set of conditions o Conditions include temp and pressure E is proportional to total quantity of matter bc energy is extensive prop State function Property of a system determined by specifying the system s condition or state in terms of temp pressure etc o Value of state function depends only on present state of system not path taken to state E is state function so E depends only on initial and final states of system q and w are not state functions If path increases q it also decreases w 5 3 Enthalpy Chemical and physical changes can result in change in heat or work Enthalpy H A thermodynamic fxn that is a state fxn and relates mainly to heat flow o H E PV E internal energy P pressure V volume All state functions meaning H is too Pressure Volume Work Typically only kind of work produced by chemical or physical changes open to atmosphere is mechanical work associated with a in V Pressure volume work P V work Work involved in the expansion or compression of gases o When pressure is constant w P V o Pressure always pos or zero o Units expressed as L atm to convert 1 L atm 101 3 J If gas expands V is pos system does work on surroundings which is why w is neg If gas is compressed V is neg so w is pos Enthalpy Change At constant pressure H E P V Remember E q w and w P V Change in enthalpy equals heat qp gained or lost at constant pressure Pos H means endothermic process neg means exothermic H depends only on initial and final states of system 5 4 Enthalpies of Reaction Enthalpy change for a chemical rxn o H Hproducts Hreactants o Called enthalpy of rxn or heat of rxn Coefficients of balanced equations represent of moles of reactants and products producing H Thermochemical equations Balanced chemical equations that show associated H Guidelines to thermochemical equations and enthalpy diagrams 1 Enthalpy is an extensive prop a Magnitude of H proportional to amount of reactant 2 The enthalpy change for a rxn is equal in magnitude but opp in sign to H for the reverse rxn 3 The enthalpy change for a rxn depends on states of reactants and products 5 5 Calorimetry H can be found by heat flow of rxn and heat flow found by magnitude of temp produced by heat flow Calorimetry Measurement of heat flow Calorimeter Device used to measure heat flow Heat Capacity and Specific Heat Heat capacity C Amount of heat to raise temp of object by 1 K or 1 C o Greater heat capacity great heat needed to raise temp Molar heat capacity Cm Heat capacity of one mole of a substance Specific heat Cs Heat capacity of one gram of a substance o o o Unit