Transcript HEAT
Thermochemistry INTRO TO THERMOCHEMISTRY • • • 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 Thermochemistry • The study of energy changes that occur during chemical reactions and changes of state. 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. 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. 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 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. An Ice Cold Spoon energy transfer A Hot Spoon If 2 objects remain in contact, heat will flow from a warmer object to the cooler object until the temperatures are equal. AKA: HEAT 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 • 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 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 2 Hot Spoons (KE =½mv2) Thermal energy is also related to the type of material Different type of materials may 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’s heat capacity 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 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 Units for Measuring Heat The Joule is the SI system unit for measuring heat: kg m 1 J N m 2 s 2 The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree 1calorie 4.18 Joules Heat DEFINITION THE FLOW OF THERMAL ENERGY FROM SOMETHING WITH A HIGHER TEMP TO SOMETHING WITH A LOWER TEMP UNITS MEASURED IN JOULES OR CALORIES THROUGH WATER OR AIR = CONVECTION TYPES THROUGH SOLIDS = CONDUCTION TRANSFERRED ENERGY BY COLLISION WITH PHOTON = RADIANT ENERGY 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 q = mCT Specific Heat The amount of heat required to raise the temperature of one gram of substance by one degree Celsius. Calculations Involving Specific Heat q c m T p OR q c m T c = Specific Heat q = Heat lost or gained T = Temperature change m = Mass p Table of Specific Heats 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. 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 Endothermic Reactions 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 Exothermic Reactions HEAT HEAT HEAT HEAT 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. Calorimeter Key Points • Δ = Change • q=mcΔT • q= heat capacity • q= heat • KE = ½ mv2 • m = mass • Heat Capacity • c = specific heat • 1 J = 0.2390 cal • ΔT = change in temperature • 4.184 J = 1 cal • Calorimetry 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 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- 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 exoHeat lost is the in-to direction; ie endo- Exothermic and Endothermic Processes Enthalpy Change qsurr mcT qsys H qsurr mcT 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 Heat of Combustion • The heat of reaction for the complete burning of one mole of a substance.