Test 3 Study Review Chapters 6 and 7 Energy Changes There are many types of energy physical chemical thermal etc They can be converted to another type but the energy is never destroyed Thermodynamics studies the interconversion of various kinds of energy which is the first law of thermodynamics energy can be The Law of Conservation of Energy neither created nor destroyed in any physical or chemical processes This means that the total amount of energy in the universe is constant System a specific part of the universe chosen by us for investigation volume etc State functions how the state was achieved State 3 types of systems a set of values of measurable properties of the system ex temperature properties that are determined by the state of the system regardless of 1 Open System 2 Closed System Isolated System 3 can exchange mass and energy with it s surroundings can exchange energy but not mass with it s surroundings can t exchange either The Internal energy U is a state function U the change in system internal energy represents the energy transferred to or from the system at constant volume Usystem Ufinal Uinitial so for a chemical reaction Ureaction Uproducts Ureactants All of the energy lost by the system is gained by it s surroundings and vice versa So Usystem Usurroundings Surroundings the rest of the universe outside our chosen system SI unit of energy Joule J 1 J 1 Kgm2 s2 1 calorie cal 4 184 J The energy change of the system that includes heat and work is calculated by this formula U system q w where q represents heat and w represents work When q is negative heat is lost by the system exothermic When q is positive heat is gained by the system endothermic When w is negative work is done by the system onto the surroundings expansion When w is positive work is done by the surroundings on the system contraction Expansion Work the amount of work of a system when it expands or contracts against a constant pressure W Pexternal V where w work P pressure V Volume Expansion happens when work is negative which means V has to be positive Compression happens when work is positive so V needs to be negative Thermochemistry the study of heat changes in chemical reactions Exothermic reactions give off heat to it s surroundings Endothermic reactions absorb heat from surroundings Specific Heat Capacity Cs is the amount of heat required to raise the temperature on gram of the substance by one degree oC q mCs T where q heat J m mass g Cs specific heat J goC T change in temp oC You get the specific heat capacity from this table If it s not given on the test you might want to have this on your cheat sheet Calorimetry Measure of heat changes in physical and chemical processes A calorimeter represents an isolated system and is carried out under either constant pressure or constant volume In a constant volume calorimetry where V 0 the change of the internal energy can be calculated through this equation Usystem q w q p V and since V 0 you are just left with q In other words the change of internal energy is equal to the heat gained or lossed by the system Enthalpy can is calculated by E H PV a thermodynamic function of a system can be expressed as H U pV where H heat changes U internal energy p pressure and V volume The heat change at constant pressure qp H where H is the change in system enthalpy represents the energy transferred to or from the system at constant pressure H Hfinal Hinitial H Hproducts Hreactants When H 0 the process is exothermic When H 0 the process is endothermic Helpful hints when solving Enthalpy questions 1 Always specify the physical states of all reactants and products g l aq 2 If both sides of an equation are multiplied by some number then H must change by the same factor 3 When an equation is reversed H changes signs Since we can t measure the absolute energy content of a substance assume the standard enthalpy of formation of any element in its most stable form is 0 The Standard Enthalpies of Formation the heat change of the reaction where 1 mole of the compound is formed form its constituent elements at 1 atm The Standard Enthalpy of Reaction the heat change of the reaction carried out at 1 atm reaction m Hf HO stoichiometric coefficients o products n Hf o reactants where means the sum and m and n are the This table will most likely be given but make sure you can read it Hess s Law the standard enthalpy change of a reaction is the sum of the standard enthalpies of reactions into which the net reaction may be divided It depends only on the reactants and products of the overall reaction and does not depend on the reaction pathway or number of steps Applying H to quantify chemical reactions 1 The direct method is used to determine the standard reaction enthalpy rHO rHO vHm o products vHm o reactants where v stoichiometric coefficient and Hm o molar enthalpy Only use this method if you begin with pure separate reactants at standard condition because then you should end with pure separate products 2 The indirect method Need to come up with a balanced thermochemical equation Intro to Quantum Mechanics Albert Einstein explained the photoelectric effect He said Light consisted of photons that each had a particular amount of energy or a quantum of energy When they collided each photon can transfer its energy to a single electron So the more photons that strike the surface of a metal the more electrons are liberated and the higher the current is Max Planck described electromagnetic radiation in terms of particles called photons Each photon is a particular amount of energy carried by a wave Plank s equation h 6 662x10 34Js relates the energy of the photon to the frequency of radiation E h where When electric current passes through a sample of gas at very low pressure light is emitted Every element has a unique emission or absorption spectrum They appear as a collection of very sharp lines corresponding to well defined energies Balmer Rydberg Equation an empirical equation that relates the wavelengths of the lines in the hydrogen spectrum This equation suggests that atoms have more complex underlying structure 1 R 1 n1 2 where n1 n2 and R 1 097x107 m 1 2 1 n2 Neils Bohr applied Planck s quantum theory to explain the hydrogen spectrum He determined that an atom has a number of orbits in which an electron may revolve without emitting or absorbing electromagnetic radiation As the orbital radius increases so does the energy of the electron Aka An electron may move from one energy orbit
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