Chemistry 1A Chapter 6 Page 1 Chapter 6 Thermochemistry Homework Read Chapter 6 Work out sample practice exercises in the sections as you read Suggested Problems 35 43 47 55 61 67 73 75 79 81 85 93 Chemical Thermodynamics Chemical thermodynamics is the study of the changes and the transfers of energy that accompany chemical and physical processes It is VERY IMPORTANT to always identify the phases in chemical reactions This unit also will occasionally require balancing reactions with fractions Thermodynamics addresses 3 fundamental questions 1 Will two or more substances spontaneously react when mixed under specified conditions or will it be nonspontaneous will not react 2 What energy changes and transfers are associated with the reaction 3 To what extent does a reaction occur equilibrium learn in Chem 1B Systems tend toward 1 Minimizing potential energy 2 Maximizing entropy degrees of freedom or ways of being Thermochemistry tells us nothing of the reaction rate Reactions may be spontaneously favored under specified conditions but may not react at an observable rate or reactions may need a large initial amount of energy to initiate and reach the activation energy Thermodynamic examples Methane will spontaneously react and combust in oxygen but the reaction does not initiate without a spark Ice will spontaneously melt into water but only if the temperature is above freezing We will study physical and chemical processes accompanied by the transfer of heat or work Nuclear energy where the atom changes is different Chemistry 1A Chapter 6 Page 2 Example 1 Systems tend toward minimizing potential energy and maximizing entropy Identify the side that minimizes the potential energy Identify the side that maximizes ways of being entropy Assume the same temperature and atmospheric pressure for each a Top of waterfall or Bottom of waterfall b 1 mole H2 molecules or 2 mole of H atoms c H2O g at 100 C or H2O l at 100 C Nature of Energy Energy is the capacity to do work or transfer heat Energy a quantity can exist in a variety of forms and can be transformed from one form to another It is measured by the amount of work done usually in joules or watts Chemistry generally uses joules SI units of energy 1 Joule 1 kg m2 s2 Energy Conversions 1 calorie cal 1 Calorie Cal food 1 kilowatt hour kWh 4 184 joules J exact 1000 cal 1 kcal 4184 J 3 60 x 106 J Some Forms of Energy Electrical kinetic energy associated with the flow of electrical charge Mechanical kinetic energy associated with moving parts Heat or thermal energy kinetic energy associated with molecular motion Light or radiant energy kinetic energy associated with energy transitions in an atom Nuclear potential energy in the nucleus of atoms Chemical potential energy due to the structure of the atoms the attachment between atoms the atoms positions relative to each other in the molecule or the molecules relative positions in the structure Chemistry 1A Chapter 6 Page 3 Energy can be classified into two major categories potential and kinetic Potential Energy is the energy by virtue of its position of composition a For an object that has height potential energy is the mass x gravity 9 8 m s2 x height Ep mgh b Another form is electrostatic potential energy from interactions with charged particles Eel kQ1Q2 d where k 8 99 x 109 J m C2 and C is coulomb that is a unit of charge Q is charge and d is distance Like charges repel and opposites attract c Potential energy also comes from the arrangement of chemical compositions When bonds break and new ones form in a chemical reaction the energy change generally is due to the changes in potential energy composition Kinetic Energy is the energy of motion For a moving object kinetic energy is half the mass x velocity squared Ek mv2 Example 2 What is the potential energy of a 400 g ball on top of a building that is 30 0 m tall Example 3 What is the kinetic energy of a 400 g ball moving at 30 0 m s Some Thermochemistry terms System the portion we study Surroundings everything else affected by the portion we study Universe the combined system plus surroundings In the lab experiments our universe will be a calorimeter and its contents Chemistry 1A Chapter 6 Page 4 Laws of Thermodynamics Laws of thermodynamics are useful in predicting outcomes but unlike a theory do not explain the expected behavior Zeroth Law Temperature Concept Temperature measures the intensity of hotness or coldness of an object When two objects are brought together heat always flows spontaneously from a hotter object to a colder object until thermal equilibrium is reached Think of the heat flow direction when placing your hand in ice water verses hot water First Law Law of Conservation of Energy Energy cannot be created or destroyed The energy of the universe is constant This law works for ordinary chemical and physical processes It was later combined with another conservation of matter to include nuclear reactions after E mc2 It was changed to the Law of Conservation of Matter and Energy Example 4 What is the difference between temperature and heat Example 5 Review your temperature conversions between F C and Kelvin a 98 6 F C K b 788 K C F When you see X where X can be anything remember that it is just the change from the final minus the initial values X Xfinal Xinitial Definitions Heat is the flow of energy caused by temperature differences Enthalpy H the energy accounting for heat flow in constant pressure processes Units are generally kJ mol When the energy is measured in J or kJ for a particular system qp may be used Chemistry 1A Chapter 6 Page 5 Internal Energy E or U the sum of all the kinetic and potential energies of all components This includes all motions of vibration rotation movement through space of the object its atoms nuclei and even including subatomic particles like electrons We cannot truly measure all of this so we generally try to solve for the difference or change in internal energy in our studies E Efinal Einitial When solving for E we work with a constant volume system Units are generally kJ mol When the energy is measured in J or kJ for a particular system q v may be used Work w Work is force acting over a distance work force x distance the energy used to cause an object to move Pressure Volume work involves the compression or expansion of gases Work P V if using atm and liter convert to J with R R 101 3 J L atm Work nRT use the R with joules R 8 314 J mol K For most reactions the energy from work is quite small