Chapter 5 Thermochemistry 5 1 Chapter 5 Thermochemistry Chapter 5 5 1 Energy 5 2 Enthalpy 5 3 Energy Temperature Changes and Changes of State 5 4 Enthalpy Changes and Chemical Reactions 5 5 Hess s Law 5 6 Standard Heats of Reaction Chapter In Context In this chapter we begin an exploration of thermochemistry the study of the role that energy in the form of heat plays in chemical processes We will investigate the energy changes that take place during phase changes and the chemical reactions you have studied in previous chapters and learn why some chemical reactions occur while others do not In the chapter that follows we will study energy changes at the molecular level and the consequences those energy changes have on the properties of atoms and elements Environmental Studies Industry A major portion of our economy is based on extracting potential energy from fossil fuels that have been built up over millions of years through photosynthesis where plants chemically store energy obtained from sunlight The energy is stored in the form of chemical compounds that are high in chemical potential energy Chemicals that are high in chemical potential energy can be made to react to give off heat and that heat energy can in turn be used to run an engine or heat a home During all these processes energy is transformed from one form to another but is never really removed or added to That is energy is constant our economy is based on finding sources of one type of energy chemically stored potential energy and turning it into another kind of energy heat thermal energy Chapter Goals Recognize different types of energy Understand the principles of thermodynamics including heat and work Define enthalpy and understand its relationship to chemical systems Relate energy to temperature change Relate energy to physical change Relate energy to chemical change 5 1 5 2 5 1 Thermochemistry Chapter 5 Energy OWL Opening Exploration 5 1 Types of Energy Chemical reactions involve reactants undergoing chemical change to form new substances products Reactants Products What is not apparent in the above equation is the role of energy in a reaction For many reactions energy often in the form of heat is absorbed that is it acts somewhat like a reactant You might write an equation for those reactions that looks like this Energy Reactants Products In other reactions energy is produced that is it acts like a product Reactants Products Energy In many reactions such as the combustion of gasoline in a car or natural gas on a stovetop burner energy is the most important product Energy is defined most simply as the ability to do work Work is defined in many ways the simplest definition being the force involved in moving an object some distance From a chemist s point of view energy is best viewed as the ability to cause change and thermochemistry is the study of how energy in the form of heat is involved in chemical change Kinetic and Potential Energy Energy takes many forms such as mechanical electrical or gravitational These are categorized into two broad classes kinetic energy energy associated with motion and potential energy energy associated with position Most of the events we see around us involve conversion of energy from one form to another Consider the use of a small photocell to run a fan Figure 5 1 Calculating Kinetic and Potential Energy Kinetic energy is calculated from the equation 2 KE mv m mass v velocity Potential energy calculations depend on the forces that exist between particles Because different types of particles experience different types of forces it is not possible to use a single equation to calculate potential energy Figure 5 1 A photoelectric cell drives this small fan In this example light radiant energy is absorbed by the photocell which converts it into an electric current That electric current is then used to drive the fan The energy conversions occurring are therefore radiant kinetic and potential electrical kinetic and potential mechanical kinetic Measuring Energy Energy Units Energy is measured in different units For example heating fuel is typically measured in British Thermal Units BTUs and food energy content is measured in Calories Energy Chapter Goals Revisited Recognize different types of energy Use and interconvert energy units Chapter 5 Thermochemistry 5 3 associated with most chemical processes is reported in terms of joules J and kilojoules kJ or calories cal and kilocalories kcal The food energy unit Calorie is equal to 1 kcal One joule is equal to the energy needed to accelerate a 1 kg object by 1 m2 s2 One calorie is the energy needed to raise the temperature of one gram of pure water by one degree Celsius Table 5 2 shows conversion factors for joules calories BTU and kilowatt hours the energy unit used in measuring electrical energy Table 5 2 Energy unit conversion factors J kJ cal 1J 1 0 001 0 2390 1 kJ 1000 1 239 0 1 cal 4 184 1 4 184 10 3 1 kcal 4184 4 184 1000 1 kWH 3 6 106 3 6 103 8 604 105 1 BTU 1055 1 055 252 kcal 2 390 10 4 0 2390 0 001 1 860 4 0 252 kWh 2 778 10 7 2 778 10 4 1 162 10 6 1 162 10 3 1 2 93 10 4 2 1 J 1 kg m s 2 BTU 9 479 10 4 0 9479 3 968 10 3 3 968 3143 1 EXAMPLE PROBLEM Energy Unit Conversion A barrel contains 42 gallons of oil This is the equivalent of 4 50 1010 J of energy How many kilowatt hours of electrical energy does this barrel represent SOLUTION The conversion factor table tells us that 1 J 2 778 10 7 kWh of energy The conversion is therefore 2 778 10 7 kWh 450 1010 J 1250 kWh 1J OWL Example Problem 5 2 Energy Unit Conversion Tutor 5 3 Energy Unit Conversion Principles of Thermodynamics Thermochemistry is part of the field of thermodynamics the study of the relationships between heat energy and work and the conversion of one into the other When considering chemical events it is useful to define the system the item or reaction of interest and separate that from the surroundings everything else An isolated system is one in which neither matter nor energy can be passed to or from the surroundings A closed system is one in which energy but not matter can be passed to or from the surroundings In almost all cases in chemistry the system of interest is closed and the internal energy the energy of the system changes when energy in the form of heat q is added or lost and work w is done by or on the system While the total internal energy of a system cannot be measured directly the change in internal energy Esystem is calculated from the following