Chapter 6 Thermochemistry 1 Important Terminology Make sure that you know these definitions and are able to apply them thermodynamics the study of thermochemistry the study of the relationship between energy the capacity to heat the flow of energy caused by work the result of w w Energy is something an object or a set of objects Heat and work are the ways that an objects or sets of objects kinetic energy the energy associated with the Ek potential energy energy due to or thermal energy energy associated with the chemical energy energy associated with the relative positions of Chemical Energy is a form of system That part of the universe that surroundings Universe 6 2 Example internal energy of the system the sum total of all of the Often we cannot know but we can know state function A property of a system that is determined by Important A state function is Examples of state functions 2 Units Joule J 1J This should be easy to remember because Ek Example What is the kinetic energy of a 2 kg bowling ball moving at a velocity of 1 m s calorie cal 1 cal Example How many Joules of heat are required to raise the temperature of 1 0 g of water by 1 C Calorie Cal 1 Cal Example There are 1350 Calories in a Big Mac meal 1 Big Mac 1 Large Fries 1 Large Soft drink This would supply enough heat to 6 3 3 First Law of Thermodynamics First Law 1 2 3 4 5 6 Algebraic expression of the first law E where E E q w 4 Sign Convention very important Two important points 1 Delta means X 2 The sign is determined from the point of view If q is positive then If q is negative then If w is positive then If w is negative then 6 4 Example A substance gains 25 J of heat and does 30 J of work What is the E for this system 5 Calorimetry Calorimetry Calorimeter Bomb Calorimeter Coffee Cup Calorimeter Heat capacity Specific heat capacity c Important q Molar heat capacity Cm Example How many joules of heat must be removed from 100 g of Hg to cool it from 20 C to its freezing point of 39 C specific heat of Hg 0 140 J g C Pb 0 128 J g C Al 0 902 J g C 4 18 J g C H2O 100 g each of Pb Al and H2O are placed in the sun for one hour They all absorb an equivalent amount of heat from the sun Which one has the highest temperature Question Given this table of specific heats 6 5 Example A 30 0 g sample of water at 280 K is mixed with 50 0 g of water at 330 K Calculate the final temperature of the mixture assuming no heat loss to the surroundings Example A 50 0 g chunk of lead is heated to 100 0 C and then dropped into a beaker with 100 0 g of H2O at 25 0 C Assuming that no heat is lost to the surroundings determine the final temperature of the mixture The specific heat of water is 4 18 J g C the specific heat of lead is 0 128 J g C 6 Pressure Volume Work Many chemical reactions are carried out in an open container under conditions This type of work is called work or work definition w definition P rearrange F substitute w 6 6 for a cylindrical piston w and A x h so w The First Law Under Constant Volume Conditions and E Where qv w What is an example of a Constant Volume Calorimeter A useful conversion factor Example The volume of an ideal gas is decreased from 9 75 L to 1 00 L at a constant pressure of 1 50 atm What is the work associated with this process Example A certain reaction performed at 1 00 atm pressure gives off 6 20 kJ of heat During the reaction the volume expands from 1 00 L to 44 2 L What is E for this reaction 7 Enthalpy H Derivation of an Important Relationship start with the first law E solve for q q 6 7 Under constant volume conditions qv and where qv means Many chemical reactions occur in open containers under conditions expansion work under constant pressure w so under constant pressure conditions plug P V into the equation qp qp qp where qp means Now we define the thermodynamic property H or H where H H or H E P V if H 0 then the reaction is if H 0 then the reaction is Question What are two important differences between H and q Answer 1 2 6 8 8 Thermochemical Equations For N2 g O2 g 2NO g this reaction is found to be endothermic Thermochemical equation Important The stoichiometric coefficients in a thermochemical equation refer to Three rules for thermochemical equations 1 The conditions must Shortcut 2 When the reaction is reversed 3 The enthalpy change H is for To illustrate N2 g O2 g 2NO g H must specify conditions 2NO g N2 g O2 g H reverse eqn sign changes NO g N2 g O2 g H H is for the specified number of moles The first equation means that when 1 mol of N2 reacts with 1 mol of O2 to form 2 mol of NO g the enthalpy change is 180 5 kJ when it is measured under standard conditions Example What is the amount of heat required to produce 55 0 g of NO gas from N2 and O2 N2 g O2 g 2NO g H 180 5 kJ 6 9 Example How much heat is given off when 75 0 g of aluminum oxide is formed from aluminum and oxygen 4Al s 3 O2 g 2 Al2O3 s 9 H 3351 kJ Hess s Law Hess s Law states Example problem Use Hess s law to calculate H for the reaction CO2 g H2 g CO g H2O g H from the following thermochemical equations CO g 1 2 O2 g CO2 g 2H2O l 2H2 g O2 g H2O g H2O l H 283 0 kJ H 571 6 kJ H 44 0 kJ 10 Enthalpies of Formation A formation reaction Standard enthalpy of formation Example for CO g H f 110 5 kJ mol Write the corresponding thermochemical equation Important Sometimes fractional coefficients are Example for KClO3 s H f 397 73 kJ mol Write the corresponding thermochemical equation 6 10 Question Which substances always have zero as their standard enthalpy of formation Why As a consequence of Hess s law we can use H f values to calculate the H rxn H rxn where np and nr represent the Just remember minus Example Use the following standard enthalpies of formation to calculate the standard molar enthalpy of combustion of glucose C6H12O6 substance H f C6H12O6 s CO2 g H2O l 1260 393 5 285 8 Example The chromium metal used in chrome steel is obtained by heating the ore chromite FeCr2O4 with …