Chemical Reactions Thermochemistry Part 1 Prepare for Recitation December 1st ALEKS Objective 13 due November 29th ANGEL Quiz 13 December 1st Lecture 37 November 16th Lecture 38 November 18th Thanksgiving Break Lecture 39 November 28th Read Ch 4 6 10 5 Reactions 5 3 5 7 8 8 Thermo Additional Preparation Packet Rxns 12 18 BLB 10 57 BLB 5 4 17 29 37 39 53 55 83 85 Packet 5 1 7 BLB 8 65a 67ac 92a 18 72ab 74 Packet 8 12 14 CH110 FA11 SAS 1 Chemical Reactions Thermochemistry Part 1 KEY QUESTIONS THE BIG PICTURE How can chemical thermodynamics help me to understand the energy of reactions What is enthalpy and why is it often more useful than energy in chemistry Remember Energy is stored in molecules How can we use it What is the relationship between a balanced chemical equation and a thermochemical equation How do thermochemical equations help me understand reaction chemistry By the end of Today s Lecture you should know First Law of Thermodynamics Heat work Energy and Enthalpy Thermochemical Equations Heat of Formation CH110 FA11 SAS 2 First Law of Thermodynamics Conservation of Energy Energy converted from one form to another When it is converted the total energy KINETIC ENERGY POTENTIAL ENERGY mechanical mechanical mass in a place where force can act moving mass mv2 joule kg m2 s2 electrical chemical bonds moving charge nuclear light photons binding energy sound molecules moving uniformly heat molecules moving randomly CH110 FA11 SAS 3 Change in Internal Energy E Efinal Einitial E is a state function State function Energy is conseved All energy lost by a system under observation is gained by the surroundings System energy Surroundings Surroundings energy System Internal energy is the capacity to Thermal energy is energy transferred due to difference in temperature CH110 FA11 SAS 4 Practice Problem 100 mL of water in a beaker is cooled from 90oC to 25oC A second 100 mL of water in a separate beaker is warmed from 1oC to 25oC Both beakers are open to the atmosphere Which system has more energy A B C D The system that was cooled has more The system that was heated has more They both have the same energy I need more information to answer the question CH110 FA11 SAS 5 Heat Work and Changes in Energy During energy conversion some heat is always produced E q w The work of expansion is very common Demo Sublimation of CO2 E q p v What is the sign of w Which ends up with higher E V v 0 0 w w p v P V q CH110 FA11 SAS q 6 Energy and Enthalpy During energy conversion some heat is always produced When changes occur at constant pressure E qp wexpansion E qp p v H qp H is a state function Change in enthalpy H is the quantity of thermal energy transferred to a system at H E if for chemical changes this is usually true Formally H qp CH110 FA11 SAS 7 Examples of Enthalpy Examples of physical changes Freezing and melting add heat to ice at 0oC and it melts BUT Hfusion heat of fusion Vaporizing and condensing heat water at 100oC and it vaporizes BUT Hvaporization heat of vaporization Examples of chemical changes Reaction to change chemical composition Hrxn heat of reaction Formation of a compound from elements Hf heat of formation CH110 FA11 SAS 8 Thermochemical Equations A balanced chemical equation that also includes the Ho delta H standard Units of Ho Remember the demo show Na2O2 s 2H2O l 2Na 2OH H2O2 aq Sodium peroxide 2H2O2 aq 2 H2O O2 When bonds are broken energy is so this reaction gives off have high conc of O2 cotton has low ignition temperature so flame CH110 FA11 SAS 9 Enthalpy of Reaction H H products H reactants Endothermic H 0 Exothermic H 0 2 H2 g O2 g 2 H2O g heat Hrxno 483 6 kJ Characteristics of Enthalpy 1 Enthalpy is an extensive property 2 H for a reaction depends on states of reactants and products 3 H for a reaction is in magnitude but in sign to H for reverse reaction CH110 FA11 SAS 10 The Hydrogen Balloon 2 H2 g O2 g 2H2O g Hrxno 483 6 kJ The reaction to form water from hydrogen and oxygen is thermic How much heat is given off per mole of H2 consumed in the production of water How much heat is given off per mole of O2 consumed in the production of water The reaction to form H2 and O2 gas from water vapor is thermic How much heat will be needed to convert 9g of water vapor into H2 and O2 gas CH110 FA11 SAS 11 Exo vs Endothermic Reactions 2 HI g products H Hrxn 53 kJ H2 g I2 g reactants THERMIC REACTION CH4 g and 2 O2 g reactants H Hrxn 890 kJ CO2 g 2 H2O l products THERMIC REACTION CH110 FA11 SAS 12 Exo vs Endothermic Demonstration Endothermic Ba OH 2 8H2O s 2 NH4 SCN s Ba SCN 2 aq 2 NH3 g 10 H2O l mix two solids highly endothermic reaction increase in entropy is driving force go from two ordered crystals to ions and gases in solution Exothermic Na2SO3 aq NaOCl aq Na2SO4 aq NaCl aq highly exothermic reaction oxidation reduction drives the reaction can you identify this CH110 FA11 SAS 13 Heat Capacity and Calorimetry Experimental measurement of heat flow heat surroundings conservation of energy mc Tsurroundings So temperature increase in the surroundings means heat is by system and vice versa Measure T for surroundings in a controlled environment calorimeter q m C T For H2O C 4 184 CH110 FA11 SAS J g C 14 How is q Measured CH110 FA11 SAS 15 Quantitative Heat Capacity A calorimeter with 200 g H2O is used for reaction If T rises from 25 0oC to 33 0oC how much energy is released heat capacity of H2O l 4 184 J oC g CH110 FA11 SAS 16 H of Formation combine Elements Compound Combination reaction to form substance from elements Enthalpy of the reaction per mole of substance formed units The most stable form of carbon at room temperature pressure is graphite diamond When all substances in standard states 25oC 1 atm stable state then Hfo Hfo for stable form of element CH110 FA11 SAS 17 Example of Hfo table 5 3 OR Appendix C What does the Hfo for ethanol C2H5OH actually mean Write the balanced chemical reaction for formation of 1 mole of PRODUCT C graphite H2 g O2 g 1 C2H5OH l Hfo 277 7 kJ mol ethanol Note standard state P 1 atm T 25oC CH110 FA11 SAS 18 What You Should Know Chemical Thermodynamics What is the First Law of Thermodynamics What is the relationship between heat work and internal energy What is the difference between internal energy and enthalpy Thermochemical Equations What is the difference between a balanced chemical equation and a thermochemical equation How do you use reaction enthalpy How do you determine reaction enthalpy calorimetrically Heat of Formation What is the
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