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Thermochemistry Introduction Thermochemistry The study of energy changes during chemical reactions or changes of state. First Law of Thermodynamics Energy is neither created nor destroyed during chemical or physical changes. Also called the Law of Conservation of Energy What is Energy? Energy is the ability to do work. Some Types of Energy: Kinetic Energy of motion Potential Energy stored by object due to its position or composition Chemical Potential Energy is stored in chemical bonds Energy We’re Going to Focus On Chemical Energy (energy in bonds) Heat (energy due to atomic/molecular motion) Heat (q) The flow or transfer of energy due to a temperature difference. Heat always flows from an object of higher temp to one of lower temp until both are the same temp. Why? Heat flows from the hot pack to the woman’s cooler back. Units for Energy: calorie (cal) Amount of energy required to raise the temperature of 1g of water 1oC 1000 calorie = 1 kcal = 1 nutritional Calorie (Cal) Joule (J) 4.184 J = 1.000 cal Energy in Food: Nutrition Labels The United States This is in nutrition (food) calories, Cal. It is actually 453,000 cal, or 453 kcal. 1 Cal = 1000 cal England Germany Australia New Zealand China Unit Conversion Problems Convert 345.7 J into calories. 1.000cal 345.7 J 4.184 J Answer: 82.62 calories Unit Conversion Problems Convert 325 Calories to Joules 1000cal 4.184 J 325Cal 1Cal 1cal Answer: 1.36 x 106 J Some Basic Definitions System: The Surroundings part of the universe we’re focusing on Often the materials involved in a chemical reaction or phase change Surroundings: Everything Surroundings else in the universe Surroundings Surroundings System Surroundings Surroundings Describing the Direction of Heat Flow: Exothermic: Heat flows out of the system and into the surroundings The system loses energy The surrounding gain energy qsys is negative qsurr is positive We System see an increase in temperature in the surroundings. An exothermic reaction would feel hot to the touch because heat is being transferred to your hand. Surroundings Describing the Direction of Heat Flow: Endothermic: Energy flows into the system from the surroundings The system gains energy The surroundings lose energy qsys is positive qsurr is negative We see a decrease in temperature of the surroundings An endothermic reaction would feel cold to the touch because it is absorbing heat from your hand. System Surroundings General Outcome: qsurr = -qsys Signs are used to indicate the direction of energy flow. According to the law of conservation of energy, the amount of heat lost or gained by the system MUST be exactly the same as the energy gained or lost by the surroundings. However, the signs are opposite. Chemical (Potential) Energy Think about this: Is the breaking of bonds an endothermic or exothermic process? Endothermic! It requires energy to occur. Chemical (Potential) Energy Breaking bonds is ALWAYS endothermic (it always requires energy!) So, qsys is positive for breaking bonds. Chemical (Potential) Energy Think about this: Is the formation of bonds endothermic or exothermic? Exothermic! Energy is released when bonds form. Pow!!! Chemical (Potential) Energy Forming bonds is ALWAYS exothermic(it always releases energy!) So, qsys is negative for forming bonds. Pow!!! Your turn Would you expect a synthesis reaction to typically be an endothermic or exothermic reaction? Exothermic because bonds are being formed. Would you expect a decomposition reaction to typically be an endothermic or exothermic reaction? Endothermic broken. because bonds are being Your turn Is the melting of ice endothermic or exothermic? Endothermic because intermolecular forces are being broken. Is the condensing of steam endothermic or exothermic? Exothermic because intermolecular forces are being formed. (This is why burns from steam are so bad.) Your turn HW: Thermochemistry Introduction ws