Thermochemistry Thermochemistry Study of energy changes in chemical processes 2H2 O2 2H2O energy Energy kinetic potential First Law of Thermodynamics E q w Enthalpy H Thermochemical Equations Mary J Bojan H for chemical reactions Calorimetry Heat Capacity Hess s Law Heat of formation Hf Standard State H f Foods and Fuels Chem 110 1 First Law of Thermodynamics Law of conservation of energy Energy can be converted from one form to another but total energy remains constant Mary J Bojan Chem 110 2 Energy Kinetic Energy energy of motion Mechanical Electrical Light Sound Heat moving mass 1 2mv2 moving charge photons molecules moving uniformly molecules moving randomly Potential Energy stored energy Mechanical Chemical Nuclear mass in a place where a force can act bonds binding energy Mary J Bojan Chem 110 3 Energy Electronic Energy The potential energy associated with the electrons in atoms and molecules Chemical energy the Potential Energy associated with bonding H H H H O O 3 bonds Mary J Bojan 4 bonds Chem 110 4 Changes in Energy E internal energy total energy E associated with a system the sum of all sources of kinetic potential energy the capacity to do work or transfer heat E Efinal Einitial q w w work action of force through a distance often P V work done to the system q heat thermal energy energy transferred due to a difference in temperature heat added to the system Mary J Bojan Chem 110 5 Changes in Energy Energy lost by the is gained by the The system SYSTEM SURROUNDINGS what you are interested in a atom molecule or chemical reaction energy System Surroundings E energy Surroundings System E Mary J Bojan Chem 110 6 Energy changes Cool the water Which one has more energy Heat the water Mary J Bojan Chem 110 7 State Function a function whose value does not depend on the pathway used to get to the present state State Function only depends on the current state composition T P does not depend on past history E Mary J Bojan Chem 110 8 State Functions State functions are written as uppercase letters E H P V T S q and w are not state functions but E q w is a state function Changes in state functions are path independent Mary J Bojan Chem 110 9 Energy E and Enthalpy H Energy transferred at constant V E When changes occur at constant pressure E q w w P V work done on system at constant P arises from expansion or contraction of the system V Vfinal Vinitial E qp wexpansion Mary J Bojan Chem 110 10 Enthalpy quantitative definition E qp wp qp P V P constant so qp E P V H Enthalpy qualitative definition H is heat transferred at constant P For many chemical processes P V is small and Like E H is a state function Mary J Bojan Chem 110 11 Examples of enthalpy energy transfers accompany physical changes we ve seen before freezing melting add heat to ice temperature does not change during melting vaporizing condensing add heat to water temperature does not change during vaporization Mary J Bojan Chem 110 12 Thermochemical Equations A balanced chemical equation that also includes the energy change H2 g 1 2 O2 g H2O g H 241 8kJ H enthalpy heat given off or absorbed in the reaction Mary J Bojan Chem 110 13 ENTHALPY OF REACTION H H products H reactants If H 0 exothermic heat released If H 0 Endothermic heat absorbed Mary J Bojan Chem 110 14 ENTHALPY 1 Enthalpy is an extensive property 2 H for a reaction is equal in magnitude and opposite in sign to H for the reverse reaction 3 H for a reaction depends on the states of reactants and products gas liquid solid Is this reaction 1 exothermic 2 endothermic How much heat is given off per mole of H2 How much heat is given off per mole of O2 Mary J Bojan Chem 110 16 If I convert water to H2 O2 the reaction will be 1 2 exothermic endothermic How much heat will be needed to convert 9g of water into H2 O2 How much heat will be given off if liquid water is formed instead of gaseous water Hvap of water 44kJ mol How much heat will be given off if 10g of H2 is consumed Mary J Bojan Chem 110 17 CALORIMETRY Experimental measure of heat flow q C m T q heat flow C specific heat heat capacity per gram m mass T Tfinal Tinitial For H2O C 4 184 J g C Molar heat capacity 75 2 J mole C Note H2O is usually part of the surroundings qsurr Csurr m T Mary J Bojan Chem 110 18 Measuring heat of reaction Mix 50ml of 1M NaOH 50ml of 1M HCl What is H rxn Ti Tf Mary J Bojan Chem 110 19 Measuring heat of reaction 1 Write the balanced reaction 2 T Tf Ti 3 Is qp H rxn positive or negative 4 qp C m T V 100ml Then assume d 1g ml m 100ml 1g ml 100g qp 4 184J goC 100g T Mary J Bojan Chem 110 20 Hess Law H for a sum of steps is the same as H for the overall process True because H is a state function A B H1 B C H2 A B B C H1 H2 Hrxn Mary J Bojan Chem 110 21 Hess Law Example Given the following information A 2 H2 g F2 g 2 HF g HA 537kJ B 2 H2 g O2 g 2 H2O g HB 572kJ Determine H for reaction C C 2 F2 g 2 H2O g 4 HF g O2 g HC SOLUTION find combination of reactions such that nA mB C Then n HA m HB HC Mary J Bojan Chem 110 22 Hess Law Example Given the following information A 2 H2 g F2 g 2 HF g HA 537kJ B 2 H2 g O2 g 2 H2O g HB 572kJ Determine H for reaction C C 2 F2 g 2 H2O g 4 HF g O2 g HC SOLUTION 2A 1B C 2xA 4 H2 g 2 F2 g 4 HF g 1x B 2 H2O g 2 H2 g O2 g H 2x 537kJ H 572kJ 2 F2 g 2 H2O g 4 HF g O2 g Then 2 537kJ 1 572kJ 502kJ Mary J Bojan Chem 110 23 You try Given the following information 2SO2 g O2 g 2SO3 g H 196kJ 2S s 3 O2 g 2SO3 g 790kJ What is Hrxn for S s O2 g SO2 g Mary J Bojan Chem 110 24 Heat of Formation Hf enthalpy of formation combine Elements H f standard enthalpy of formation Compound Hf STANDARD STATE P 1 atm T 25 C 298K most stable state gas liquid solid For an element in its standard state H f 0 by definition Mary J Bojan Chem 110 25 Heat of Formation For which of the following reactions at 25 C and 1 atm is Hrxn H f a H2 g F2 g 2HF g b NO …
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