GSU CHEM 1211K - Chapter 6 – Thermochemistry
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Chapter 6 Thermochemistry Nature of Energy o Even though chemistry is the study of matter energy affects matter o Energy is anything that has the capacity to do work o Work is a force acting over a distance Energy Work Force x Distance o Heat is the flow of energy caused by a difference in temperature o Heat and Work are the two different ways that an object can exchange energy with other objects Classification of Energy o Kinetic Energy is energy of motion or energy that is being transferred o Thermal Energy is the energy associated with temperature Thermal energy is a form of kinetic energy o Potential Energy is energy that is stored in an object or energy associated with the composition and position of the object Energy stored in the structure of a compound is potential Some Forms of Energy o Electrical Kinetic energy associated with the flow of electrical charge o Heat or Thermal Energy Kinetic energy associated with molecular motion o Light or radiant energy Kinetic Energy associated with energy transitions in an atom o Nuclear Potential energy in the nucleus of atoms o Chemical Potential energy due to the structure of the atoms the attachment betwee atoms the atoms positions relative to each other in the molecule or the molecules relative position in the structure Conservation of Energy o The 1st law of thermodynamics Law of Conservation of Energy states that energy cannot be created nor destroyed When energy is transferred between objects or converted from one form to another You can therefore never design a system that will continue to product energy without some source of energy Units of Energy o 1 Joule J is equal to the energy expended or work done in applying a force of one newton through a distance of 1 meter 1 J 1 N M 1kg m m s2 1 kg m2 s2 1 N 1kg m s2 o Calorie cal is the amount of energy needed to raise the temperate of one gram of water 1 C Kcal energy needed to raise 1000 g of water 1 C o Energy Conversion Factors System and Surroundings 1 Calorie cal 4 184 joules J 1 Calorie cal 1kcal 1000 cal 1 kcal 4184 J 1 kilowatt hour kWh 3 60 106J o We define the system as a part of the universe on which we wish to focus attention o Surroundings include everything else in the universe Energy Flow and Conservation of Energy o Conservation of energy requires that the sum of the energy changes in the system and the surroundings must be zero Internal Energy o The internal energy is the sum of the kinetic and potential energies of all of the particles that compose the system State function is a mathematical function whose result only depends on the initial and final conditions not on the process used Energy Flow o Energy Exchange o Energy is exchanged between the system and surroundings through heat and work Q heat thermal energy W work energy Q and W are NOT state functions their value depends on the process o Heat Exchange o Heat is the exchange of thermal energy between the system and surroundings o Temperature is the measure of the amount of thermal energy within a sample of matter o Heat flows from matter with high temperature to matter with low temperature until both objects reach the same temperature Thermal Equilibrium Quantity of Heat Energy Absorbed Heat Capacity o When a system absorbs heat its temperature increases o The increase in temperature is directly proportional to the amount of heat absorbed o The proportionality constant is called the heat capacity Units of C are J C or J K o Q C T o The larger the heat capacity of the object being studied the smaller the temperature rise will be for a given amount of heat Factors affecting Heat Capacity o The heat capacity of an object depends on its amount of water 200g of water requires twice as much heat to raise its temperature by 1 C as does 100g of water o The heat capacity of an object depends on the type of material 1000 J of heat energy will raise the temperature of 100g of sand 12 C but only raise the temperature of 100g of water by 2 4 C Specific Heat Capacity o The specific heat capacity is the amount of heat energy required to raise the temperature of one gram of a substance 1 C Cs Units are J g C o The molar heat capacity is the amount of heat energy required to raise the temperature of one mole of a substance 1 C Classifying Heat Energy o The heat absorbed by an object is proportional to its mass and the specific heat of the material Heat mass x specific heat x temp change q m Cs T Heat Transfer Final Temperature o When two objects at different temperatures are placed in contant heat flows from the material at the higher temperature to the material at the lower temperature until both materials reach the same final temperature o The amount of heat energy lost by the hot material equals the amount of heat gained by the cold material Pressure Volume Work o PV work is work caused by a volume change against an external pressure o When gases expand V is but the system is doing work on the surroudings so wgas is o As long as the external pressure is kept constant Workgas External Pressure Change in Volumegas w P V To convert the units to joules use 101 3J 1 atm L E Calorimetry at Constant Volume o Because E q w we can determine E by measuring q Measuring and w o In practice it is easiest to do a process in such a way that there is no change in volume so w 0 At constant volume Esystem qsystem o The surroundings is called a bomb calorimeter and is usually made of a sealed insulated container filled with water qsurroundings qcalorimeter qsystem Bomb Calorimeter system o Used to measure E because it is a constant volume o The heat capacity of the calorimeter is the amount of heat absorbed by the calorimeter for each degree rise in temperature and is called the colorimeter constant o Ccal kJ C Enthalpy o The enthalpy H of a system is the sum of the internal energy of the system and o H E PV o The enthalpy change H of a reaction is the heat evolved in a reaction at the product of pressure and volume H is a state function constant pressure Hreaction qreaction at constant pressure o Usually H and E are similar in value the difference for reactions that produce or use large quantities of gas Endothermic and Exothermic Reactions o When H is heat is being released by the system Reactions that release heat are called exothermic reactions During the course of a reaction old bonds are broken and new bonds are The products of the reaction have less chemical potential energy than the reactants The difference in energy is

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