Thermodynamics

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Discusses thermodynamic laws and quantities including enthalpy, entropy and free energy with solved problems.
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Thermodynamics

  1. 1. WHY SOME REACTIONS HAPPEN AND SOME DON’T ! Copyright Sautter 2015
  2. 2. CHEMICAL THERMODYNAMICS • WHAT IS REQUIRED FOR A REACTION TO BE SPONTANEOUS ? • SPONTANEOUS REFERS TO A REACTION OCCURRING ON ITS OWN WITHOUT ENERGY INPUT. SPONTANEOUS DOES NOT REFER TO REACTION RATE! THE REACTION MAY BE EXTREMELY SLOW BUT AS LONG AS IT CONTINUES TO OCCUR IT IS SAID TO BE SPONTANEOUS. • UNDERSTANDING CHEMICAL THERMODYNAMICS REQUIRED THE KNOWLEDGE OF THREE FUNDAMENTAL LAWS AND THE NATURE OF HEAT ENERGY. 2
  3. 3. CHEMICAL THERMODYNAMICS • THE ENERGY CONTENT OF A SYSTEM CAN BE IN THE FORM OF KINETIC OR POTENTIAL ENERGY. POTENTIAL ENERGY IS FOUND IN THE BONDS THAT JOIN ATOMS TOGETHER TO FORM MOLECULES. • KINETIC ENERGY IS THE ENERGY OF MOLECULAR MOTIONS. UNDERSTANDING THE KINETIC ENERGY CONTENT OF A SYSTEM REQUIRES US TO RECALL THE KINETIC MOLECULAR THEORY OF MATTER. IT BASIC TENENT IS THAT ALL MOLECULES ARE IN CONSTANT, RANDOM MOTION, EXCEPT AT ABSOLUTE ZERO (-273 CELSIUS). 3
  4. 4. CHEMICAL THERMODYNAMICS • MOLECULAR MOTIONS CONSIST OF FOUR TYPES: • (1) TRANSLATIONAL MOTION – THE PLACE TO PLACE MOTION OF THE MOLECULES THROUGH SPACE. • (2) ROTATIONAL MOTION – MOLECULES ROTATE OR SPIN AS THEY MOVE THROUGH SPACE • (3) VIBRATIONAL MOTION – THE BONDS BETWEEN MOLECULES ARE MUCH LIKE SPRINGS AND THE ATOMS THAT ARE BONDED TOGETHER IN MOLECULES MOVE BACK AND FORTH ALONG THE BOND AXIS WITH PARTICULAR VIBRATIONAL FREQUENCIES. • (4) ELECTRONIC ENERGY – THE ELECTRONS WITHIN THE ATOMS CONTAIN THEIR OWN ENERGIES OF MOTION. • THE SUM OF ALL THESE ENERGIES FOR THE MOLECULES IN A SYSTEM IS TERMED THE INTERNAL ENERGY (E) OF THE SYSTEM. 4
  5. 5. CHEMICAL THERMODYNAMICS • HEAT ENERGY MAY BE ADDED OR REMOVED FROM A CHEMICAL SYSTEM. THOSE SYSTEMS WHICH ABSORB HEAT ENERGY ARE CALLED ENDOTHERMIC. THOSE WHICH RELEASE ENERGY ARE EXOTHERMIC. • THE FIRST LAW OF THERMODYNAMICS STATES THAT ALL ENERGY TRANSFERS MUST ACCOUNT FOR THE TOTAL AMOUNT OF ENERGY. NO ENERGY MAY BE LOST OR GAINED, IT MAY ONLY BE CHANGED IN FORM. THIS, OF COURSE, IS THE LAW OF CONSERVATION OF ENERGY. • THE QUANTITY OF HEAT ENERGY ADDED OR REMOVED FROM A SYSTEM CAN BE MEASURED IN JOULES, CALORIES OR BTUs. GENERALLY JOULES AND KILOJOULES ARE THE PREFERRED UNITS OF ENERGY. 5
  6. 6. CHEMICAL THERMODYNAMICS • HOW CAN HEAT ENERGY BE MEASURED? • MEASURING HEAT REQUIRES THREE FUNDAMENTAL QUANTITIES. • (1) THE AMOUNT OF SUBSTANCE HEATED (MASS IN GRAMS OR KILOGRAMS) • (2) THE KIND OF SUBSTANCE HEATED (THIS IS MEASURED USING THE SPECIFIC HEAT OF THE SUBSTANCE) • (3) THE TEMPERATURE CHANGE OF THE SUBSTANCE (IN DEGREES CELSIUS OR KELVIN) • THE EQUATION RELATING THESE VARIABLES IS: Q = M x C x  T WHERE Q = HEAT LOST OF GAINED, M = MASS OF SUBSTANCE HEATED OR COOLED, C = THE SPECFIC HEAT OF THE SUBSTANCE  T = THE TEMPERATURE CHANGE OF THE SUBSTANCE6
  7. 7. CHEMICAL THERMODYNAMICS • A COMMON DEVICE USED TO MEASURE THE HEAT OF REACTION IS A BOMB CALORIMETER. IN THIS APPARATUS, A REACTION IS CARRIED OUT IN A CHAMBER IMMERSED IN A CONTAINER OF WATER. HEAT FLOW OUT OF THE REACTION CHAMBER AND INTO THE WATER. BASED ON THE TEMPERATURE CHANGE OF THE WATER AND ASSOCIATED EQUIPMENT THE HEAT OF REACTION ( E) CAN BE CALCULATED. • A BOMB CALORIMETER IS A CONSTANT VOLUME MEASUREMENT MEANING THAT ALL RELEASED HEAT IS USED TO RAISE THE INTERNAL ENERGY OF THE SYSTEM AND NONE IS USED TO DO WORK. • IN OTHER THERMODYNAMIC SYSTEM CONSTANT PRESSURE PROCESSES OCCUR. HERE THE WORK ASSOCIATED WITH VOLUME CHANGES OF THE GASES MUST BE CONSIDERED. 7
  8. 8. IGNITER POWER SUPPLY THERMOMETER STIRRER REACTION CHAMBER INSULATED CONTAINER WATER CHAMBER BOMB CALORIMETER (CONSTANT VOLUME) HEAT FLOWS OUT OF THE REACTION CHAMBER INTO THE SURROUNDING WATER 8
  9. 9. CHEMICAL THERMODYNAMICS • CALORIMETRIC CALCULATIONS : When 0.1567 moles of ammonia react with HCl gas, in a calorimeter with water equivalent of 200 grams (the equipment’s heat storing capacity equals that or 200 grams of water) the temperature of the 2000 grams of water in the calorimeter increases from 26.13 to 29.27 0C. What is E for the reaction ? • Solution: Q = M x C x  T, mass = (2000 + 200) grams C* for water = 4.18 joules / gram x degree Celsius  T = (29.27 – 26.13) = 3.14 0C Q (heat lost from reaction) = 2200 x 4.18 x 3.14 = 28875 j  E (heat of reaction) = 28.875 KJ / 0.1567 moles = 184.4 KJ/mole Since heat is lost by the reaction it is exothermic and  E = - 184.4 KJ/m *specific heat varies depending on the substance. C for water is a value required for many problems and is 4.18 joules / gram x degree Celsius 9
  10. 10. CONSTANT VOLUME VS CONSTANT PRESSURE SYSTEMS • IN CONSTANT VOLUME SYSTEMS ALL ADDED OR REMOVED HEAT ENERGY ALTERS THE INTERNAL ENERGY OF THE SYSTEM AND NONE IS USED TO DO WORK. • WORK FROM GASEOUS EXPANSION OR COMPRESSION IS CALCULATED BY MULTILPYING PRESSURE TIMES THE RESULTING VOLUME CHANGE OF THE SYSTEM (W = P x  V). IN A CONSTANT VOLUME SYSTEM SUCH AS THE BOMB CALORIMETER,  VOLUME IS ZERO AND THEREFORE THE WORK DONE IS ZERO. • THE FIRST LAW OF THERMODYNAMICS IN EQUATION FORM IS:  E = Q + W, WHERE  E IS THE INTERNAL ENERGY CHANGE, Q IS HEAT ADDED OR REMOVED AND W IN WORK DONE BY OR ON THE SYSTEM. IF W = 0 THEN  E = Q AND ALL HEAT ADDED OR REMOVED RESULTS IN A INTERNAL ENERGY CHANGES. 10
  11. 11. CONSTANT VOLUME VS CONSTANT PRESSURE SYSTEMS • CONSTANT PRESSURE SYSTEMS OCCUR COMMONLY IN LAB SITUATIONS WHERE REACTIONS ARE OPEN TO THE ATMOSPHERE (USUALLY ABOUT 1 ATM.) • WHEN GASES EXPAND OR CONTRACT, THE WORK TERM IS EQUAL TO P x  V. • WHEN HEAT IS ADDED A SYSTEM, Q IS POSITIVE. WHEN HEAT IS RELEASED, Q IS NEGATIVE. • WHEN WORK IS DONE BY A SYSTEM (GAS EXPANDS AND WORK LEAVES THE SYSTEM), W IS NEGATIVE. WHEN WORK IS DONE ON A SYSTEM (SYSTEM IS COMPRESSED AND WORK IS ADDED TO THE SYSTEM), W IS POSITIVE. • WHEN ENERGY IS RELEASED BY A REACTION,  E = POSITIVE. WHEN ENERGY IS ASBORBED BY A REACTION,  E = NEGATIVE. 11
  12. 12. CONSTANT PRESSURE E = q + w = q + P V VOLUME CHANGE (V) CONSTANT PRESSURE AS GAS EXPANDS ADDED HEAT (+q) W = -P V WORK DONE BY THE SYSTEM MOVEABLE PISTON 12
  13. 13. CONSTANT VOLUME VS CONSTANT PRESSURE SYSTEMS • SUMMARY: HEAT ADDED, Q = +, HEAT LOST, Q = - WORK DONE ON SYSTEM, W = + WORK DONE BY THE SYSTEM, W = - INTERNAL ENERGY INCREASES,  E = + INTERNAL ENERGY DECREASES,  E = - • FOR CONSTANT PRESSURE REACTIONS THE HEAT THAT IS GAINED OR LOST BY THE REACTION IS CALLED ENTHALPY CHANGE ( H) AND  H =  E + W OR  H =  E + P V • IN MOST ALL REACTIONS AT ATMOSPHERIC PRESSURE, THE WORK FACTOR (P V) IS VERY SMALL COMPARED TO THE INTERNAL ENERGY CHANGE ( E) AND THEREFORE IN MOST CASES  H AND  E ARE PRACTICALLY THE SAME VALUE. 13
  14. 14. CONSTANT VOLUME VS CONSTANT PRESSURE SYSTEMS • For the decomposition of CaCO3 to CaO and CO2 what is the difference between  E and  H value at 25 0C and 1 atm.? CaCO3(S)  CaO(S) + CO2 (g) • SOLUTION:  H CAN BE CALCULATED USING HESSES LAW  H RXN =   HPRODUCTS -   HREACTANTS  H RXN = ((-635.5) + (-394)) - (-1207) = + 571 KJ  H =  E + P V, P V =  n RT, THEREFORE  H =  E +  n RT AND  E =  H -  n RT  n = MOLES OF GASEOUS PRODUCTS – MOLES OF GASEOUS REACTANTS (n = 1 –0) = 1  E = (+571) - ((1) x 0.00831 x (25 + 273)) = +571 – 2.48 = 569 KJ THE DIFFERENCE BETWEEN  H AND  E IS ONLY 2.48 K OR ONLY 0.43 % 14
  15. 15. CAUSES OF SPONTANEOUS CHANGE • FROM AN ENERGY STANDPOINT, REACTIONS ALWAYS TEND TO MOVE IN THE DIRECTION OF LOWEST ENERGY THAT IS EXOTHERMIC (ENERGY RELEASING) REACTONS ARE FAVORED. HOWEVER, THERE ARE NUMEROUS INSTANCES OF SPONTANEOUS ENDOTHERMIC (ENERGY ABSORBING) REACTIONS. HOW CAN THIS BE EXPLAINED? • THE ANSWER LIES IN THE SECOND LAW OF THERMODYNAMICS, A CONCEPT KNOWN AS ENTROPY. • ENTROPY IS ALSO REFERRED TO AS “RANDOMNESS” OR “DISORDER”. THE SECOND LAW STATES THAT ALL PROCESSES HAVE A NATURAL TENDENCY TO PROGRESS TO STATES OF INCREASED ENTROPY OR DISORDER. IN THIS PRINCIPLE LIES THE ANSWER TO SPONTANEOUS ENDOTHERMIC REACTIONS. 15
  16. 16. ENTROPY – A NATURAL TENDENCY FOR ALL CHANGE AN UNLIKELY EVENT SPONTANEOUS DECREASE IN ENTROPY A VERY LIKELY EVENT SPONTANEOUS INCREASE IN ENTROPY COINS SWEPT FROM A TABLE 16
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