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UGA CHEM 1211 - Ch. 5 Textbook Notes

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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 heat5.1 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 increaseKinetic Energy and Potential Energy:- Kinetic energy: The energy of motiono Magnitude depends on object’s mass and speedoo Increases as speed or mass increases- Potential energy: Energy dependent on the object’s position relative to other objectso “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 particleso Energy is proportional to electric charges on the two interacting objects, Q1 and Q2, and inversely proportional to distance, d, separating themoo 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 energyo Zero of electrostatic potential energy is infinite separation of charged particleso Same charge sign of objects creates repulsion, decreasing potential energy, and opp. signcauses 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 energyUnits of Energy:- Joule: SI unit for energyo James Joule (1818-1889), British scientist found 1 J= 1 kg-m2/ 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° Co Now, 1 cal= 4.184 J- Nutritional Calorie (Cal): 1 Cal= 1000 cal= 1 kcalSystem and Surroundings:- System: Portion of universe singled out for study of energy changeso Open system: Matter and energy can be exchanged with surroundingso Closed system: Energy, but not matter exchanged with surroundings Energy exchanged as work and heato Isolated system: Neither energy nor matter exchanged- Surroundings: Everything else in universe besides systemTransferring 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 objecto w= F x d- Force: Any push or pull exerted on an object- Heat: Energy transferred from hotter object to colder one5.2 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. ΔE= system gained energy from surroundings; neg. ΔE= system lost energy to surroundings- In chemical rxn, initial state of system is reactants and final state is products.Relating ΔE 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, ΔE increases.-Endothermic and Exothermic Processes:- Endothermic: Process in which system absorbs heato Heat flows into system from surroundings.- Exothermic: Process in which system loses heatState 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 (interms 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 ΔE 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 flowo H= E+ PV E= internal energy, P= pressure, V= volume All state functions, meaning H is tooPressure-Volume Work:- Typically, only kind of work produced by chemical or physical changes open to atmosphere is mechanical work associated with a Δ in V.- Pressure-volume work (P-V work): Work involved in the expansion or compression of gaseso When pressure is constant, w= -PΔVo Pressure always pos. or zeroo 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, ΔV is neg., so w is pos.Enthalpy Change:- At constant pressure, ΔH= ΔE+ PΔV- Remember, ΔE= q+ w and w= -PΔV- Change in enthalpy equals heat qp gained or lost at constant pressure.- Pos. ΔH means endothermic process, neg. means exothermic. - ΔH depends only on initial and final states of system.5.4 Enthalpies of Reaction- Enthalpy change for a chemical rxno ΔH= Hproducts- Hreactantso Called enthalpy of rxn or heat of rxn- Coefficients of balanced equations represent # of moles of reactants and products producing ΔH- Thermochemical equations: Balanced chemical equations that show associated ΔH- Guidelines to thermochemical equations and enthalpy diagrams:1. Enthalpy is an extensive prop.a. Magnitude of ΔH proportional to amount of reactant2. The enthalpy change for a rxn is equal in magnitude, but opp. in sign, to ΔH for thereverse rxn.3. The enthalpy change for a rxn depends on states of reactants and products.5.5 Calorimetry- ΔH can be found by heat flow of rxn, and heat flow found by magnitude of temp. Δ produced by heat flow- Calorimetry: Measurement of heat flow- Calorimeter: Device used to measure heat flowHeat Capacity and


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