Chapter 6 Thermochemistry Sections 6 1 Chapter 6 Homework Due 10 23 This week No SLW No Lab 6 1 Forms of Energy and Their Interconversions In this section we will study thermochemistry the loss or gain of energy that takes place during a chemical or physical change Defining The System and Its Surroundings In order to observe and measure a change in energy we must define the system the part of the universe that we will focus on When we define the system everything else is defined as the surroundings the surroundings the system Energy Transfer to and from the System Each particle in a system has potential and kinetic energy the sum of these energies for all the particles in the system is the internal energy E We determine the energy change E that accompanies a chemical change by measuring the difference between the system s internal energy before Einitial and after Efinal the change Reactant Products E Efinal Einitial Eproducts Ereactants where the capital Greek letter delta means change in The Law of Energy Conservation first law of thermodynamics Energy transfers between systems and surroundings behave according to the law of conservation of energy The law of conservation of energy states that the total energy of the universe is constant energy can be converted from one form to another but cannot be created or destroyed Another way of saying this is the change in energy of the system Esystem plus the change in energy of the surroundings Esurroundings must equal zero Esystem Esurroundings 0 Esystem Esurroundings Because energy must always be conserved the change in energy of the system is always accompanied by an equal and opposite change in the energy of the surroundings Figure 6 1 Esystem Esurroundings Esystem Efinal Einitial Esystem Efinal Einitial Note The sign of E indicates whether energy is lost to E or gained from E the surroundings Heat and Work Two Forms of Energy Transfer Energy transfer outward from the system or inward from the surroundings can appear in two forms heat and work Heat or thermal energy q is the energy transferred between a system and its surroundings as a result of a difference in their temperatures only Work w is the energy transferred when an object is moved by force The total change in a system s internal energy is the sum of the energy transferred as heat and or work E q w Define sign of energy change from system s perspective Let s consider four simple cases of energy transfer Figure 6 2 Energy Transfer as Heat Only w 0 E q 0 q 1 q heat flow out of system and gained by surroundings 2 q heat gained by system and lost by surroundings Energy Transfer as Work Only q 0 E 0 w w 3 w work done by the system thus energy lost from system 4 w work done on the system thus energy gained by system When a gas system expands against a piston surroundings in an internal combustion engine the sign of the work being done by the gas is A positive B negative C neutral D initially positive then negative E cannot be determined from the information given Work is being done by the gas system on the piston surroundings Energy must be flowing out of the system Sign of the work must be negative Units of Energy The SI unit of energy is the joule J derived unit from three base units 1 J kg m2 s2 Both heat and work are expressed in joules The calorie is an older unit originally defined as the quantity of energy needed to raise the temperature of 1 g of water by 1 C specifically 14 5 C to 15 5 C 1 cal 4 184 J or 1 J 0 2390 cal Note This is not a nutritional calorie Cal 1 Cal 1000 cal Sample Problem 6 1 If the expanding combustion gases do 451 J of work on the pistons of an engine and the system loses 325 J as heat calculate the change in energy E in J kJ and kcal Define system and surroundings system reactants and products surroundings pistons engine car etc q 325 J w 451 J E q w 325 J 451 J 776 J 776 J x 1 kJ 0 776 kJ 1000 J 776 J x 1 cal x 1 kcal 0 185 kcal 4 184 J 1000 cal What is the change in internal energy E in Joules of a system that releases 576 J of internal energy to its surroundings and has 255 cal of work done on it A 321 J B 321 J C 491 J D 831 J E 1643 J q 576 J w 255 cal x 4 184 J 1066 9 J 1 cal E q w 576 J 1066 9 J 490 9 J 491 J State Functions and Path Independence of the Energy Change Energy E is a state function that is a property dependent only on the current state of the system not on the path the system took to reach that state The change in energy E of a system is also a state function it depends only on its initial and final states A given E can occur through countless combinations of heat q and work w Note Both q and w are not state functions They depend on how the change occurs Other examples of state functions Pressure P and Volume V