Name Thermo Unit Entropy of Physical and Chemical Changes PHYSICAL CHANGE Let s look at a process with which we are all familiar Ice melting in a glass of water Imagine you start with a glass that has 36 g of solid ice The initial temperature of the ice is 0 C If you leave this glass in a room that has a constant temperature of 25 C the ice will spontaneously melt and you will end up with all liquid water at 25 C Here we will calculate the entropy change for the process of the ice melting and increasing in temperature to 25 C 1 First identify what the system and surroundings are what are they composed of what is their temperature etc Often it is helpful to make a sketch or diagram Surroundings 25 C System 36 g block of ice at 0 C 2 Now identify what the initial and final states of the system are Again a diagram equation or sketch can be helpful You should break this down into two steps ice melting then water warming up Initial 0 C Ice Middle 0 C Water Final 25 C Water 3 Now look at the energy change for this process Does energy flow into or out of the system If so as heat or work Energy flows in as heat Revised SH 7 17 13 LaBrake Vanden Bout 2013 Name 4 Given that fusH 6 02 kJ mol 1 and that the heat capacity for water is 4 184 J g 1 C 1 How much heat flows into the system for this process in KJ There are two steps in this process melting the ice and warming the water For the first step we have to convert grams into moles 36 g H2O 1 mole H2O 18 g H2O 2 moles H2O Melting the Ice q1 H fus x moles kJ x 2moles mol q1 12 04kJ q1 6 02 Warming the Water q2 Cm Tm J q2 4 184 25 C 0 C 36g g C q2 3765 6J 3 77kJ Total Heat qtotal q1 q2 12 04 kJ 3 77 kJ 15 81 kJ 5 What is the change in entropy for the melting of the ice What is the change in entropy for the melting of the water What is the change in entropy for the system Work qrev 12 04kJ kJ J 0 0441 44 1 ice melting T 273K K K T J 298K J S mCs ln f 36g 4 184 ln 13 12 water war min g Ti g C 273K K J J J Ssys 44 1 13 12 57 22 K K K S 6 Did heat flow into or out of the surroundings during this change Out of surroundings Revised SH 7 17 13 LaBrake Vanden Bout 2013 Name 7 What is the entropy change for the surroundings Work Ssur qrev 15 81kJ kJ J 0 053 53 T 298K K K 8 What is the entropy change for the universe for this process Work Suniverse Ssys Ssur 57 2 J J J 53 4 2 K K K 9 The answer to part 8 is extensive this for the 36 g of solid ice that melts in this process What is the entropy change per mole of solid ice that melts under these conditions We already know from working question 4 that 36 g of ice is 2 moles of H2O J 4 2 Suniverse K 2 1 J moles 2moles molK CHEMICAL CHANGE The equation S qrev T holds for reversible heat flow However during a chemical change the heat flow is not reversible Therefore we need another way to determine the change in entropy for the system such that we can predict the spontaneity of chemical change The Boltzman definition allows a method for determining the standard molar entropy a compound because S kBln When there is only one microstate S 0 This occurs at 0 K in a perfect crystal thus there is an absolute value for entropy The standard molar entropies for elements and compounds have been computed and are available in tables One can use these values to determine the change in entropy for chemical change Remember that entropy is a state function and the change in the state function only depends on the final condition minus the initial condition Revised SH 7 17 13 LaBrake Vanden Bout 2013 Name 1 Use the following tabulated data S H2 g 131 J K mol S O2 g 205 J K mol S H2O l 70 J K mol to determine the change in entropy for the combustion of 1 mole of hydrogen First we need to write out a chemical equation for the combustion of hydrogen gas in which the stoichiometric coefficient for hydrogen gas is 1 H2 g O2 g H2O l S products reac tan ts S 70 J J 1 J J kJ 131 205 163 5 0 1635 Kmol Kmol 2 Kmol Kmol Kmol 2 Use the following tabulated data Hf H2O l 286 kJ mol to determine the change in entropy for the surroundings for this change under standard conditions Work H sys Ssur Tsurroudings kJ mol 0 96 kJ 298K molK 286 Ssur 3 Use your answers from 2 and 3 to determine the change in entropy for the universe and predict if this change is spontaneous under standard conditions Work Suniverse Ssys Ssur Suniverse 0 1635 kJ kJ kJ 0 96 0 797 molK molK molK Based only on the entropy of the universe this reaction is spontaneous Revised SH 7 17 13 LaBrake Vanden Bout 2013
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