# lecture 17.1- endothermic vs. exothermic

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Section 17.1 Lecture for Honors & Prep Chemistry

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• Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes.
• There are two types of energy Kinetic Energy and Potential Energy As temperature increases, the kinetic energy (motion) of particles increases. Potential energy is stored energy. Potential energy is stored in chemical bonds.
• There are two types of energy Kinetic Energy and Potential Energy Energy can be converted between the two types but it cannot be created or destroyed = The law of conservation of energy
• Heat (q) is energy that flows from a warmer object to a cooler object. When the warmer object loses heat, its temperature decreases and molecules move slower. When the cooler object absorbs heat, its temperature rises and its molecules speed up. Heat flows until both objects have the same temperature.
• HEAT vs. TEMPERATURE Energy measures average kinetic energy Depends on the Independent of amount of substance sample size A drop of boiling water on your hand doesnt hurt as much as a pot of boiling water. Same temperature (100C), but different amounts of heat.
• Cold and hot Cold is the sensation of molecules slowing down (decreasing temperature) Hot is the sensation of molecules speeding up (increasing temperature) Hot and cold are relative. A warm cup can feel very hot if your hand has been in snow.
• In thermochemistry, it is useful to divide the universe into two parts The system- The thing we are studying The surroundings- Everything else universe = system + surroundings
• Enthalpy (H) is the heat content of a system at constant pressure. Most processes involve a change in enthalpy (H) which can be measured in a lab. H is measured in J or kJ
• The change in enthalpy for a reaction is called the enthalpy (heat) of reaction (Hrxn).
• Hrxn is the difference between the enthalpy of products and the enthalpy of the reactants. The total enthalpy change is a sum of all bonds that are broken or formed during a reaction. Bond breaking = requires energy Bond forming = releases energy
• A process or reaction is either EXOTHERMIC or ENDOTHERMIC Heat (energy) goes Heat (energy) goes out of the system into the system H is negative H is positive energy is given off energy is absorbed
• EXOTHERMIC = Products have lower energy than reactants ENDOTHERMIC = Products have higher energy than reactants
• TNT Higher energy lower energy + energy released!
• You can tell if a reaction is endothermic or exothermic by touch. Remember- the system determines the sign of H and we are part of the surroundings YOU ARE HERE
• A reaction that feels cold has heat going from your finger INTO THE SYSTEM = endothermic A hot reaction has heat coming OUT OF THE SYSTEM = exothermic
• Equations for endothermic processes have energy as a reactant. sunlight + 6CO2(g) + H2O(l) C6H12O6(aq) + 6O2(g) Equations for exothermic processes have energy as a product. Propane(g) + O2(g) CO2(g) + H2O(g) + heat

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