Last Sections Before Final 1 System Surroundings a System i The specific part of the universe that is of interest in the study ii State of a system 1 Values of a set of measurable properties sufficient to determine all properties of a system iii State functions 1 Properties that are determined by the state of the system regardless of how that condition was achieved 2 Examples a Temperature pressure volume density altitude enthalpy Gibbs free energy entropy iv The potential energy of 2 hikers is the SAME even though they took different paths to get there b Surroundings i Everything in the universe EXCEPT the system 1 Very very difficult basically impossible to measure 2 First Law of Thermodynamics destroyed a Energy can be converted from one form to another but CANNOT be created or i The combined amount of energy in the universe is constant ii Known as the Law of Conservation of Energy 1 Energy is neither created nor destroyed in chemical reactions and physical changes 3 Enthalpy H constant pressure a Used to quantify the heat flow into or out of a system in a process that occurs at i ii Delta H reaction is the heat given off or absorbed during a reaction at constant pressure 1 Exothermic a Heat given off by system to the surroundings b Hproducts Hreactants i Delta H 0 c Enthalpically favorable 2 Endothermic a Heat absorbed by the system from the surroundings b Hproducts Hreactants i Delta H 0 c Enthalpically unfavorable 4 Standard Enthalpy of Formation a Is the heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm b When calculating remember i An element in its standard state is given a delta H formation of zero 1 It is by itself or connected to copies of itself ii Example 1 Na s P4 s Cl2 g S8 g iii Most compounds have a negative delta h formation iv To find the delta H of the reaction use the following formula 1 5 Standard Enthalpy of Reaction a Enthalpy of a reaction carried out at 1 atm i b Hess s Law i The enthalpy change delta H of an overall process is the sum of the enthalpy change of its individual steps ii When reactants are converted to products the change in enthalpy is the same whether the reaction takes place in 1 step or in a series of steps iii Example 1 For the following example calculate the unknown delta H from the given delta H values for the other equations Calculate the delta H for a S s 3 2 O2 g SO3 g 2 Given the following set of reactions a S s O2 g SO2 g b 2 SO3 g 2 SO2 g O2 g 3 First start with S s where do I see S s delta H1 296 8 kJ delta H2 198 4 kJ a Only in the first equation b Does not match the moles are different c We need to multiply it by 2 the whole equation d They all become 1 mole i S s O2 g SO2 g delta H1 593 6 kJ 4 Next look at O2 where do I see O2 a Twice so I have to skip it 5 Now look at SO3 a We need it to be a product like up above b Reverse equation c 2SO2 g O2 2SO3 i Reverses sign of delta H d Divide by 2 to get rid of it e SO2 g O2 g SO3 g 6 Get rid of intermediates delta H2 198 kJ delta H2 99 2 kJ a Reactant in one equation product in another b c Cancel each other out It is SO2 7 Bring down what didn t get canceled out a Same as above equation b S s 3 2 O2 g SO3 g i 593 6 kJ 99 2 kJ 692 8 kJ delta H3 combine H1 and H2 c Enthalpy is a state function it doesn t matter how you get there only where you start and end 6 Spontaneity a Tendency for a process to advance to equilibrium without external influence b Something that happens naturally is spontaneous c Any process will be spontaneous is one direction d The reverse is nonspontaneous e Spontaneous natural i Process that DOES occur under a specific set of conditions ii Waterfall runs downhill iii Sugar dissolves in coffee iv Smell diffusing in a room v Ball rolling down a hill f Nonspontaneous unnatural i Process that DOES NOT occur under a specific set of conditions ii Opposite reverse direction of all those things g Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures h Speed i Above 0 degrees C it is spontaneous for ice to melt natural i Speed of a reaction is NOT an indicator of its spontaneity ii Length of time doesn t matter iii Spontaneity is determined by the relative positions of the initial and final states iv Spontaneity does NOT tell you how fast a reaction is v Spontaneity is thermodynamics NOT kinetics speed i Often times spontaneous processes are exothermic BUT not always i Exo endothermic 7 Entropy S a A measure of randomness or disorder of a system b INCREASE order DECREASE entropy i Increase disorder increase entropy c The second law of thermodynamics i Molecular systems tend to move spontaneously to a state of maximum randomness or disorder 1 Universe doesn t want to be in a neat package 2 The universe is going to shit This is the natural state 3 Entropy of the universe is always increasing and must always be greater than 0 4 a Using this if we know delta S surroundings we can calculate the delta S of the universe i And delta S system is products reactants d At 0 Kelvin entropy or chaos will stop e In general as we move from solid liquid gas entropy will INCREASE and its sign will be POSITIVE i Opposite way switches f Entropy and Reactions system 1 Delta S S final S initial i All chemical and physical changes involve a change in entropy delta S for the ii If the change from initial to final results in an increase in randomness and number of states 1 Delta S 0 2 1 iii Structure iv Temperature Less structure more states more entropy 1 Higher temp more energy states more entropy v Number of particles atoms 1 More particles more states more entropy g Example i Which should have the highest molar entropy at 25 degrees C 1 Ga l 2 Ga s 3 Ga g 4 All of them have the same entropy ii Which substance has greater molar entropy 1 CH4 g or CCl4 l 2 Ne g or Xe g 3 CH3OH l or C6H5OH l a First look at the PHASE b Next look at COMPLEXITY of the compounds i The more elements in the compound the more entropy c Next look at the MASS of the compound i Greater mass greater entropy h Heat and Entropy i …
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