Ch 5 Principles of Chemical Reactivity Energy and Chemical Reactions Thermodynamics the science of heat and work the study of energy changes that accompany physical and chemical processes usually involves heat hence thermo 5 1 Energy Some Basic Principles Energy capacity to do work 2 basic categories of energy 1 Potential energy energy that results from an object s position 2 Kinetic energy energy associated with motion Law of Conservation of Energy Energy can neither be created nor destroyed The total energy of the universe is constant SI unit for energy is the joule J Systems and Surroundings System is defined as an object or collection of objects being studied e g in the lab it can be what you are doing in the beaker Surroundings include everything outside the system e g outside of the beaker Energy flows between the two Directionality and Extent of Transfer of Heat Thermal Equilibrium Energy transfer as heat will occur spontaneously from an object at a higher temperature to an object at a lower temperature Transfer of energy as heat continues until both objects are at the same temperature and thermal equilibrium is achieved At thermal equilibrium the object with a temperature increase has gained thermal energy the object with a temperature decrease has lost thermal energy Thermal Equilibrium Clicker Q What is loosing heat A water B metal Exothermic When energy leaves the system and goes into the surroundings the process is said to be EXOTHERMIC H enthalpy negative q heat negative In the case of thermal energy the temperature of the system decreases qsystem 0 and thus the T of the surroundings increases Tsystem Tfinal Tinitial 0 Exothermic Reactants products heat Key phrases heat evolved heat energy transferred from system to surroundings C 5 H 12 8 O 2 g 5 CO 2 g 6 H 2 O 3523 kJ 1 mole 8 moles 5 moles 6 moles C5 H12 8 O 2 g 5 CO 2 g 6 H 2O H orxn 3523 kJ Endothermic When energy enters the system from the surroundings the process is said to be ENDOTHERMIC H enthalpy positive q heat positive In the case of thermal energy the temperature of the system increases qsystem 0 and the surroundings decreases Tsystem 0 Endothermic Reactants heat products Key phrases heat absorbed heat energy transferred from surroundings to system H2O H2 O2 H 241 82 kJ mol H2O 241 82 kJ mol H2 O2 Clicker Q A exothermic B endothermic CO g Cl2 g COCl2 g heat evolved q is 108 kJ 5 2 Specific Heat Capacity Heating and Cooling When an object is heated or cooled the quantity of energy transferred depends on 3 things 1 Quantity mass of material 2 Magnitude of T change 3 Identity of material gaining or losing energy q m C DT Specific heat capacity c the energy transferred as heat that is required to raise the T of 1 g of a substance by 1 K or 1 C units of J gK or J g C intensive property q m C DT q energy gained or lost m mass C specific heat capacity J g C or J gK or J molK etc T change in T Tfinal T initial Molar heat capacity heat capacities can be expressed on a per mole basis J mol K or J mol C In the laboratory a student finds that it takes 62 4 Joules to increase the temperature of 10 2 grams of solid iron from 23 2 to 37 9 C What is specific heat capacity c of Fe Clicker Q Specific heat capacity of Cu is 0 385 J gK How much energy is required to heat 200 g of Cu from 25 0 C to 50 0 C enter in J Determine the final temperature of a 25 0 g block of metal that absorbs 255 cal of energy The initial temperature of the block was 17 0 C J The specific heat capacity of the metal is 2 72 g C Clicker Q A sample of solid platinum c 0 519 J g C is heated with an electrical coil If 25 5 Joules of energy are added to a 10 9 gram sample and the final temperature is 38 0oC what is the initial temperature of the platinum Quantitative Aspects of Energy Transferred as Heat 55 0 g of iron at 99 8 C is placed into 225 g water initially at 21 0 C At thermal equilibrium the water and iron are both at 23 1 C What is the specific heat capacity of the metal 55 0 g of iron at 99 8 C is placed into 225 g water initially at 21 0 C final T 23 1 C What is the specific heat capacity of the metal Clicker Q A 20 0 g piece of metal at 203 C is dropped into 100 0 g of water 4 184 J g C at 25 0 C The water T rises to 29 0 C Calculate the specific heat of the metal assume no heat losses to the surroundings You mix 2 water containers one is 100 0 g of water at 90 0 C and the other is 50 0 g at 50 0 C What is the final temperature of the water specific heat for water is 4 184 J g C 5 3 Energy and Changes of State Change of state solid to liquid to gas etc Solid to liquid to gas requires an input in energy to overcome the attractive forces of the molecules atoms So endothermic Temperature is constant throughout a change of state Solid liquid melting T What is endothermic What is exothermic Heat of vaporization Hvap the heat required to convert a substance from the liquid to the gas state all at the same T units do not include T because it happens at constant T melting T for water Hfus 333 J g Heat of fusion Hfus the heat required to convert a substance from the solid to the liquid state all at the same T units do not include T for water Hvap 2256 J g 2260 J 1 00 g H O at 100 0 o C 1 00 g H 2 O at 100 0 o C 2 g 2260 J Heating curve for water Heating curve 27 Do blank heating curve 1 Where is T bp mp 2 Where is Hfus used 3 Where is c solid used Energy Change Calculations Heating Cooling q heating or cooling m C DT Heat absorbed or lost in a Phase change q phase change D phase change H n n moles or grams Watch units can be J g or J mol or cal g or cal mol etc Heat Transfer Involving Liquid q m C T Heat must be added to a liquid to raise its T The specific heat J g C or molar heat capacity J mol C of a liquid is the amount of heat that must be added to the stated mass of liquid to raise its T by 1 C 30 The molar heat or enthalpy of vaporization Hvap …