Chem 1211 1st Edition Lecture 14 Outline of Last Lecture I Titration Outline of Current Lecture I Specific Heat II State Functions and Changes of State III Thermochemical Equations Current Lecture I II Specific Heat a ConcepTest Calculate the amount of heat required to raise the temperature of 100 g of water from 0 degrees C to 100 degrees Celcius a ANSWER 49 kJ multiply mass x specific heat x change in temperature and you get the energy in joules The specific heat of water is 4 184 b ConcepTest At room temperature a typical dorm room contains 30 kg of air and 300 g of water vapor at 50 relative humidity How much energy is required to warm the room 10 degrees Celsius if the specific heat for water vapor is 1 86 J gC and for air is 1 0 J gC a ANSWER 305 kj b REASONING Heat air 30 kg x 1000g 1kg x 1 0 J gC x 10 degrees C 300 000 J 1 Heat H2O 300g x 1 86 J gC x 10 degrees C 5 580 J 2 Answer 305 580 J 305 kJ State Functions and Changes of State a State functions set of conditions and properties that only depend on the state of the system 1 Do not depend on the pathway by which systems arrived at the state 2 Indicated by capital letters 3 Includes temperatures like These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute III a T temperature b P pressure c V volume d E energy e H enthalpy heat f S enthropy disorder 4 Often interested in changes in state functions of change in x xfinal xinitial 5 Heat vs Temperature Plot a Temperature on x axis degrees Celcius energy on y axis in kJ b Warm ice c Melt ice d Heat liquid water e Evaporate water f Heat steam b Enthalpy Changes and Reactions 1 Reactions typically generate heat 2 Amounts of heat are exact and related to potential energy of reactants and products 3 Exothermic reactions have products of lower potential energy than reactants 4 The slope on the plot defines how much energy is in the molecule c Exothermic a process where heat is evolved 1 On a graph you end at a lower potential energy d The barrier to the reaction happing spontaneously is the high point of the graph 1 Heat needed to make the reaction happen is independent from the energy given of Thermochemical Equations a Balanced chemical reaction plus the change in H a Example C5H12 8 O2 5 CO2 6 H2O 3523 J 1 New idea there is an amount of energy associated with each reaction b Heat released during reaction 3523 kJ assuming stoichiometric units are moles 1 1 mol of C5H12 reacts with 8 mole of 02 to produce 5 mol of CO2 6 mole of H20 b Change in H 0 negative number designates an exothermic reaction c Change in H 0 positive number designates an endothermic reaction d Hess s Law the enthalpy change for a reaction is the same whether it occurs by one step of by any hypothetical series of steps a Hess s Law is true because H is a state function b The change in HO values for reactions 2 and 3 can determine the change in H0 for any reaction 1 c Example 1 4 FeO O2 2Fe2O3 change in H 2 2 FeO 2 Fe O2 change 544 kJ 3 4Fe 3O2 2 Fe2O3 change 1648kJ a Manipulate the equations to find the answer you are interested in d Hess s Law you can add equations 1 A B C delta H 100kg 2 2A 2B 2C delta H 200kg a C A B delta H e Hess s Law using Delta Hf 1 For chemical reaction at standard conditions standard enthalpy change is sum of standard molar enthalpies of formation of products each multiplied by its coefficient in balanced chemical equation minus corresponding sum for reactants 2 If you know the heat of formation a Heat of formation delta Hf is making a compound from an element b Add up the heats of the starting materials reactants c Add up the heats of products d Subtract the reactants from the product e That gives you the heat of reaction f Standard enthalpies of formation delta Hf 1 Standard molar enthalpy of formation of elements in their most stable forms at 298 15K and 1 000 atm are 0 a Delta Hf elements 0 2 Others are measured
View Full Document
Unlocking...