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Thermochemistry

Thermochemistry

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Thermochemistry

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  1. Thermochemistry

  2. INTROTOTHERMOCHEMISTRY • Chemical reactions involve changes in energy • Breaking bonds requires energy • Forming bonds releases energy • The study of the changes in energy in chemical reactions is called thermochemistry. • The energy involved in chemistry is real and generally a measurable value • Energy units are numerous, but we will concentrate on the Joule (SI base unit) and the calorie (little c, big C is the food Calorie or a kilocalorie) • 1 calorie = 4.184 Joules

  3. Thermochemistry • The study of energy changes that occur during chemical reactions and changes of state.

  4. The Universe • Can be divided into 2 “parts” • System- the part you are investigating • Surroundings- the rest of the universe • In a thermo-chemical experiments the region in immediate vicinity of the system are the surroundings.

  5. Chemical Potential Energy • the energy stored in the chemical bonds of a substance • the kinds of atoms and their arrangement in the substance determine the amount of energy stored in the substance.

  6. There are three methods used to transfer heat energy. • Conduction–transfer of heat through direct contact • Convection– transfer of heat through a medium like air or water • Radiant– transfer of heat by electromagnetic radiation

  7. A Hot Spoon An Ice Cold Spoon energy transfer Heat • Symbol = q • energy that transfers from one object to another because of temperature difference • Heat always flows from a warmer object to a cooler object.

  8. If 2 objects remain in contact, heat will flow from a warmer object to the cooler object until the temperatures are equal. AKA: HEAT

  9. WHAT IS HEAT? • Hot & cold, are automatically associated with the words heat and temperature • Heat & temperature are NOT synonyms • The temperature of a substance is directly related to the energy of its particles, specifically its: • Kinetic Energy defines the temperature • Particles vibrating fast = hot • Particles vibrating slow = cold

  10. Kinetic energy is transferred from one particle to the next (a.k.a. conduction) • Sometimes this energy can be transferred from one object to another and influence physical properties • The more energy an object has the more energy is transferred

  11. 2 Hot Spoons • Thermal energy is the total energy of all the particles that make up a substance. • Kinetic energy from vibration of particles • Potential energy from molecular attraction (within or between the particles) • Thermal energy is dependent upon the amount or mass of material present • (KE =½mv2) Thermal energy is also related to the type of material

  12. Different type of materialsmay have the same temp, same mass, but different connectivity. • They are affected by the potential energy stored in chemical bonds or the IMFs holding molecules together • It is possible to be at same temp (same KE)but have very different thermal energies. • The different abilities to hold onto or release energy is referred to as the substance’sheat capacity

  13. Unit for Measuring Heat Flow • calorie (cal) = heat needed to raise the temperature of 1 g of pure water by 1º C • 1 Calorie = 1 kilocalorie = 1000 calories • calorie = c, dietary calorie = C • 1 Calorie = energy in food • a doughnut has 500 Calories or 500,000 cal

  14. Unit for Measuring Heat Flow • Joule, the SI unit of energy • One joule of heat raises the temperature of 1 g of pure water 0.2390º C • 1 J = 0.2390 cal • 4.184 J = 1 cal

  15. Units for Measuring Heat The Joule is the SI system unit for measuring heat: The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree

  16. Heat Capacity • The measure of how well a material absorbs • or releases heat energy is its heat capacity • Physical property unique to a particular material • The heat capacity depends on both its mass and its chemical composition. • The greater the mass; the greater the heat capacity. • It can be thought of as a reservoir to hold heat, how much it holds before it overflows is its capacity • Water takes 1 calorie of energy to raise temp 1 °C • Steel takes only 0.1 calorie of energy to raise temp 1 °C

  17. q = mCT

  18. Specific Heat The amount of heat required to raise the temperature of one gram of substance by one degree Celsius.

  19. Calculations Involving Specific Heat OR c = Specific Heat q = Heat lost or gained T = Temperature change m = Mass

  20. Table of Specific Heats

  21. Law of Conservation of Energy • In any chemical or physical process, energy is neither created or destroyed • If energy of the system decreases the energy of the surroundings must increase by the same amount so that the total energy of the universe remains unchanged.

  22. Endothermic Process • a reaction in which heat energy is absorbed • process in which energy is absorbed as it proceeds, and surroundings become colder • the system gains heat as the surroundings cool down • typically feels cooler the longer the reaction proceeds • q is positive

  23. Endothermic Reactions

  24. Exothermic Process • a reaction in which heat energy is generated • process in which energy is released as it proceeds, and surroundings become warmer • typically feel warm as the reaction proceeds • the system loses heat as the reaction progresses • q is negative

  25. Exothermic Reactions

  26. SURROUNDINGS HEAT HEAT HEAT HEAT SYSTEM SYSTEM EXOTHERMIC ENDOTHERMIC

  27. Calorimetry • The precise measurement of heat flow into or out of a system for chemical and physical purposes • The heat released by the system is equal to the heat absorbed by its surroundings. • Conversely, the heat absorbed by a system is equal to the heat released by it’s surroundings.

  28. Calorimeter

  29. Δ = Change q= heat capacity KE = ½ mv2 Heat Capacity 1 J = 0.2390 cal 4.184 J = 1 cal q=mcΔT q= heat m = mass c = specific heat ΔT = change in temperature Calorimetry Key Points

  30. Enthalpy • H • The heat content of a system at a constant pressure. • The heat released or absorbed by a reaction at constant pressure is the change in enthalpy. • Heat and Enthalpy are used interchangeably. • So…… q = ΔH

  31. CHANGE IN HEAT ENERGY (ENTHALPY) • The energy used or produced in a chemical reaction is called the enthalpy of the reaction • Burning a 15 gram piece of paper produces a particular amount of heat energy or a particular amount of enthalpy • Enthalpy is a value that also • contains a component of direction • (energy in or energy out) • Heat gained is the out-of direction; ie exo-

  32. CHANGE IN HEAT ENERGY (ENTHALPY) • The energy used or produced in a chemical reaction is called the enthalpy of the reaction • Burning a 15 gram piece of paper produces a particular amount of heat energy or a particular amount of enthalpy • Enthalpy is a value that also • contains a component of direction • (energy in or energy out) • Heat gained is the out-of direction; ie exo- • Heat lost is the in-to direction; ie endo-

  33. Exothermic and Endothermic Processes

  34. Enthalpy Change

  35. Thermochemical Equations • The enthalpy change for a reaction can be written as a reactant or product. • The Heat of Reaction is the enthalpy change for the chemical equation exactly as it is written. CaO(s) + H2(l) Ca(OH)2 (s) + 65.2 kJ

  36. Heat of Combustion • The heat of reaction for the complete burning of one mole of a substance.