lecture 17.1- endothermic vs. exothermic
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Section 17.1 Lecture for Honors & Prep ChemistryTRANSCRIPT
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Thermochemistry is the study of heatchanges that accompany chemicalreactions and phase changes.
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As temperature increases, the kineticenergy (motion) of particles increases.
There are two types of energyKinetic Energy and Potential Energy
Potential energy is stored energy.Potential energy is stored in chemicalbonds.
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There are two types of energyKinetic Energy and Potential Energy
Energy can be converted between thetwo types but it cannot be created ordestroyed= The law of conservation of energy
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Heat (q) is energy that flows from awarmer object to a cooler object.
• When the warmer object loses heat, itstemperature decreases and moleculesmove slower.
• When the cooler object absorbs heat, itstemperature rises and its molecules speed up.
• Heat flows until both objects havethe same temperature.
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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 doesn’thurt as much as a pot of boiling water.Same temperature (100ºC), but differentamounts of heat.
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Cold and hot
• Cold is the sensation of moleculesslowing down (decreasing temperature)
• Hot is the sensation of moleculesspeeding up (increasing temperature)
Hot and cold are relative. A warm cup can feelvery hot if your hand has been in snow.
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In thermochemistry, it is useful todivide the universe into two parts
The system- The thing we are studyingThe surroundings- Everything else
universe = system + surroundings
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• Enthalpy (H) is the heat content ofa system at constant pressure.
• Most processes involve a change in enthalpy (ΔH)which can be measured in a lab.
• ΔH is measured in J or kJ
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The change in enthalpy for a reaction is called theenthalpy (heat) of reaction (∆Hrxn).
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∆Hrxn is the difference between the enthalpy ofproducts and the enthalpy of the reactants.
The total enthalpy change is a sum of all bondsthat are broken or formed during a reaction.
Bond breaking = requires energy
Bond forming = releases energy
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A process or reaction is either
EXOTHERMIC or ENDOTHERMICHeat (energy) goes Heat (energy) goesout of the system into the systemΔH is negative ΔH is positiveenergy is given off energy is absorbed
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ENDOTHERMIC =Products have higherenergy than reactants
EXOTHERMIC =Products havelower energy thanreactants
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Higher energy → lower energy +
energy released!
TNT
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You can tell if a reaction isendothermic or exothermic by touch.
Remember- the system determines thesign of ΔH and we are part of thesurroundings
YOUAREHERE
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A reaction that feelscold has heat goingfrom your fingerINTO THE SYSTEM= endothermic
A hot reaction has heatcoming OUT OF THESYSTEM = exothermic
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Equations for endothermicprocesses have energy as a reactant. sunlight + 6CO2(g) + H2O(l)
C6H12O6(aq) + 6O2(g)
Equations for exothermic processeshave energy as a product.
Propane(g) + O2(g) CO2(g) + H2O(g) + heat