thermochemistry. energy is the capacity to do work thermal energy is the energy associated with the...
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Thermochemistry
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Energy is the capacity to do work
• Thermal energy is the energy associated with the random motion of atoms and molecules
• Chemical energy is the energy stored within the bonds of chemical substances
• There are also other forms of energy, such as nuclear, potential, and electrical
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Heat is the transfer of thermal energy between two bodies that are at different temperatures.
Energy Changes in Chemical Reactions
Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
900C400C
greater thermal energy6.2
Which one has greater thermal energy?
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Law of Conservation of Energy:
• Energy can not be created or destroyed. It can only change form.
• What types of energy are involved when you eat a burger?
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Thermochemistry is the study of heat change in chemical reactions.
The system is the specific part of the universe that is of interest in the study.
open
mass & energyExchange:
closed
energy
isolated
nothing
SYSTEMSURROUNDINGS
6.2
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Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.
Endothermic process is any process in which heat has to be supplied to the system from the surroundings.
2H2 (g) + O2 (g) 2H2O (l) + energy
Here’s a video of this reaction: Can you tell energy is a product?
H2O (g) H2O (l) + energy
energy + 2HgO (s) 2Hg (l) + O2 (g)
6.2
energy + H2O (s) H2O (l)
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Exothermic and Endothermic
• Exothermic
• Chemical Thermal
• Releases heat!
• Endothermic
• Thermal Chemical
• Absorbs heat!
Firewood burning Ice melting
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Product: 127 kJ/mol
Reactants: 520.5 kJ/mol
Reactants: 139.3 kJ/mol
Products: 720 kJ/mol
H = H (products) – H (reactants)
H = heat given off or absorbed during a reaction
Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.
Exothermic: negative ΔH Endothermic: positive ΔH
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Reaction Endo or Exo?
N2(g) + O2 (g) 2NO (g) ∆H = 168 kJ/mol
2H2(g) + O2 (g) 2H2O (l) ∆H = -571 kJ/mol
2 P(s) + 5 Cl2(g) 2 PCl5(s) ∆H = -880 kJ/mol
H2O(l) H2O(g) ∆H = 44 kJ/mol
Endo!
Exo!
Exo!
Try It!
Endo!
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The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.
Heat (q) absorbed or released:
q = mct
t = tfinal - tinitial
6.4
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I Do: How much heat is given off in Joules when an 869 g iron bar cools from 940C to 50C? Is this process endothermic or exothermic?
q = mct
6.4
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We Do: 71.5g of aluminum metal is heated from 20°C to 50°C in order to increase its malleability. What amount of heat is absorbed by the aluminum?
Is this process endothermic or exothermic?_____________
q = mct
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You Do: A 2.0-gram sample of graphite cools from 10°C to 0°C. How much heat was released from the graphite sample?
Endothermic or exothermic? _______________
q = mct
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Determining Endo/Exothermicity
• To determine if a reaction is exothermic or endothermic, we measure the temperature of the surroundings.
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CalorimetryCalorimetry is the quantitative measurement of the heat
required or evolved during a chemical process.
For example, if the energy from an exothermic chemical reaction is absorbed in a container of water, the change in temperature of the water provides a measure of the amount of heat added.
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The enthalpy of solution (Hsoln) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.
Hsoln = Hsoln - Hcomponents
6.6
Which substance(s) could be used for melting ice?
Which substance(s) could be used for a cold pack?
LiCl and CaCl2
NaCl, KCl, NH4Cl, NH4NO3