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Thermochemistry and Energy Water freezing and boiling at the same time, really?! O Thermochemistry is the study of energy changes that occur during chemical reactions and phase changes (changes of state) The Nature of Energy O Energy is the ability to do work or produce heat O Heat (q or Q) is energy; flows due to temperature differences (always hot to cold) E E E E System E E E E E E System Law of Conservation of Energy O Energy cannot be created nor destroyed; only converted into different types. Kinetic vs. Potential Energy O Two main types of energy – kinetic and potential O Kinetic – energy of motion O Potential – energy due to position or energy stored in chemical bonds O Chemical potential energy - the energy stored in a substance because of its composition O Example: gasoline Kinetic vs. Potential Energy O Two main types of energy—kinetic and potential O Kinetic—energy of motion O Potential—energy due to position or energy stored in chemical bonds O Chemical potential every—the energy stored in a substance because of its composition O Example: gasoline Temperature vs. Heat O Temperature is a measure of the motion in a sample O Temperature is a measurement of heat. O Heat is the total energy of molecular motion, dependent upon amount, size, and type of particles. Heat is energy. Units of Heat O calorie - the amount of heat required to raise the temperature of one gram of pure water by one degree Celsius O Calorie – nutritional calorie; O 1 Calorie = 1000 calories = 1 kilocalories (kcal) O Joule – SI unit of heat O 1 calorie = 4.184 J Converting Energy Units Calorie/calorie/kilocalorie 1 C = 1000 cal = 1 kcal calorie/Joule 1 cal = 4.184 J Example 1 A cereal has 155 nutritional Calories per serving. How many calories, kilocalories and Joules is this? Example 2 A person on a diet consumed 1350 Calories in one day. How many calories, kilocalories and Joules is this? System and Surroundings O Universe = system + surroundings O System – the specific part of the universe you wish to study. In chemistry this is your chemical reaction/physical process O Surroundings – Everything else in the universe O When heat is transferred it can flow in or out of the system Endothermic vs. Exothermic O An Exothermic process is one that releases heat to its surroundings (feels warm) O Energyproducts < Energyreactants O An Endothermic process is one that absorbs heat from the surroundings (feels cold) O Energyproducts > Energyreactants Exothermic Process Eproducts < Ereactants Potential Endothermic Process Eproducts > Ereactants Potential Q and heat flow O Exothermic process, heat is released, q is negative (-) O Endothermic process, heat is absorbed, q is positive (+) Specific Heat = Cs O Specific heat of a substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius. O Unit for specific heat is J/goC O Each substance has a different specific heat O Water = 4.184 J/goC O Gold = 0.129 J/goC O Copper = 0.386 J/goC O The lower the specific heat the lower the amount energy is required to raise its temperature. Calculating Heat Released and Absorbed O Q = mCs(T) O Q = Heat (Joules or kilojoules) O m is mass (gram) O T is temperature change (°C) Tfinal -Tinitial O Cs is specific heat at a constant pressure (J/g°C) Example 3 If the temperature of 56.6g of ethanol increases from 45.0°C to 80.0°C, how much heat has been absorbed by the ethanol? Specific heat of ethanol =2.44J/g°C Example 4 A 4.00g sample of a substance was heated from 274K to 314K and absorbed 32J of heat, what is the specific heat of the substance? Example 5 If 98000J of energy are added to 6200g of water, what will the change in temperature of the water be? Specific heat of water is 4.184 J/g°C