I 1 2 3 4 5 6 B 1 2 Chapter 10 Energy Changes in Chemical Reactions Energy and Energy Changes every change that matter undergoes is accompanied either by the absorption or the release of energy A Definitions System is a specific part of the universe that is under study Surroundings refers to all the rest of the universe Heat refers to the transfer of thermal energy between two bodies that are at different temperatures Thermochemistry is the study of the heat associated with chemical reactions Exothermic a chemical reaction that is accompanied by the release of energy CH4 g 2 O2 g CO2 g 2 H2 g energy Endothermic A chemical reaction that is accompanied by the absorption of thermal energy CaCO3 s energy CaO s CO2 g Units of Energy The SI unit for energy is the joule J The calorie is used in the USA to express the energy content of foods The rest of the world uses the joule 1 cal 4 184 J 1 Cal sometimes called the dietetic calorie 1000 cal 3 In calculating energy changes in chemical reactions the joule is used 1 A 1 2 3 1 2 3 4 II Introduction to Thermodynamics Thermodynamics is the study of the inter conversion of heat and other types of energy Types of Systems Open system can exchange mass and energy heat with the surroundings Closed system can transfer energy to the surroundings but not mass Isolated system cannot transfer energy or mass to the surroundings 0000000000000000000000000000000000000000000000000000000000000000000000 open system closed system isolated system B States and State Functions In thermodynamics change in the state of a system is studied State functions are properties that depend only on the initial and final state of the system and not on path by which the system acquired its final state Energy pressure volume and temperature are all state functions Energy is a state function The net increase in potential energy is the same regardless of how you get from the ground floor to the fourth floor of the builiding at the right Whether you go up the stairs or take an elevator your potential energy change is the same The change in potential energy is the difference between the final and initial potential energy of the system 2 C The First Law of Thermodynamics energy can be converted from one form to another but cannot be created or destroyed 1 2 3 a b 4 a b c The energy content of a given amount of matter cannot be measured but changes in energy can be measured Change in internal energy U can be calculated by the equation U Uf Ui where f final state i initial state Internal energy has two components kinetic and potential energy Kinetic energy is due to molecular motion and movement of electrons within molecules Potential energy is due to 1 attractive forces between nuclei and electrons in molecules 2 repulsive forces between nuclei and electrons within molecules 3 interactions between molecules Changes in energy can be measured For example S s O2 g SO2 g energy released 296 kJ mol The energy of the reactants and the product is not known but the change in energy is 296 kJ mol when sulfur combines with oxygen to form sulfur dioxide This means that the energy of the product is 296 kJ mol less than the energy of the reactants U is 296 kJ mol The energy released by the reaction is absorbed by the surroundings therefore the over all energy of the universe did not change that is energy was conserved The change in energy of the system and the surroundings is the same magnitude but of opposite sign Usys Usurr 0 Usys Usurr d Energy released by one must be absorbed by the other 3 D 1 3 4 A 1 2 Work and Heat Energy is the capacity to do work If a system performs work on the surroundings its internal energy decreases If the surroundings does work on the system the internal energy of the system increases 2 The overall change in the system s internal energy is given by the equation U q w Sign conventions for q and w q is heat absorbed or released w is work done on or by the system endothermic process exothermic process q heat in q heat out work done on the system w energy of the system work done by the system w energy of the system In chemistry we are interested in the energy change of the system rather than the energy change of the surroundings Unless specifically stated U refers to the internal energy change of the system Sample calculation Determine whether work is done on or by the system if 928 kJ of heat are released and the change in internal energy U is 1 47 x 103 kJ III Enthalpy In order to determine w we have to know if the reaction takes place at constant volume or at constant pressure conditions Reactions at Constant Volume or Constant Pressure Sodium azide is used in automobile airbags Upon impact it decomposes according to the equation 2 NaN3 s 2 Na s 3 N2 g This reaction can be carried out at constant volume in which case the nitrogen produced is confined to a container of fixed volume and the pressure inside the container will increase as N2 forms 4 3 4 a b c 5 Or it can be carried out at constant pressure which is what occurs in an automobile airbag As nitrogen is generated the conatiner airbag expands and in this expansion does work on the surroundings This type of pressure volume work is referred to as PV work The amount of work performed by the system is given by the equation w P V where P is the external pressure in atm and V is the change in volume in liters To convert L atm to J the conversion 101 3 J is used 1L atm Sample Calculations Calculate the work done by the system when a gas expands from 1 25 L to 3 87 L against an external pressure of 1 75 atm Calculate the work done on the system when a gas is compressed from 87 5 mL to 72 9 mL by a pressure of 2 76 atm What will be the volume of a 200 0mL sample of gas if the gas is compressed by application of 7 83 atm of pressure and 150 0 J of work is done on the system When a chemical reaction is carried out at constant volume no PV work is done because there is no change in volume U q P V P V 0 U qv qv U 5 6 B 1 2 3 4 C Most chemical reaction occur in open containers at the prevailing atmospheric pressure that is at constant pressure conditions The internal energy change at contstant pressure is given by qP U P V the subscript p indicates constant pressure Enthalpy and Enthalpy Changes Enthalpy H is defined as the heat change between the system and the surroundings at constant pressure and is given by the …