Transcript Chapter 17 notes

Ch. 17
THERMOCHEMISTRY
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Thermochemistry
• Thermochemistry: study of energy
changes that occur during a chemical rxn
and changes in state.
• Enthalpy (H): heat content of a system
(constant pressure)
• Heat always flows from warmer to cooler
objects.
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System v. Surrounding
• System: part of universe you are studying
– Could be a test tube, entire lab, or planet.
• Surrounding: everything else.
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Bond energy
• Breaking bonds requires(absorbs) energy
while forming bonds releases energy
• Energy stored in chemical bonds is
chemical potential energy.
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H
• Delta H = enthalpy or heat of reaction
• ΔH = H products – H reactants
– If products have more stored energy, answer
is positive (energy was put into system)
– If reactants have more stored energy, answer
is negative (energy was released from
system)
– Units are calories (cal) or Kilojoules (kJ)
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Exothermic Rxns
• Exothermic System releases energy
– Release means loses energy to surroundings
– Means it gets warmer to us (we are
surroundings)
– Number that represents enthalpy is negative
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Exothermic systems
• Ways to indicate an exothermic system
–
H = - # in kJ or cal (negative number)
– CaO + H2O → Ca(OH)2 + 65.2 kJ
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Exothermic Rxn
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Endothermic Reactions
• Endothermic System absorbs energy from
surroundings
• Gets cooler to us (we are surroundings)
• Or energy must be provided from a heat source
(hotplate)
– Number that represents enthalpy is positive
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Endothermic Systems
• Ways to indicate an endothermic system
–
H = + # in kJ or cal (positive number)
– 2NaHCO3 + 129 kJ → Na2CO3 + H2O + CO2
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Endothermic Rxn
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Phase change Calculations
When a substance goes through a phase change, a
certain amount of energy is released or absorbed. This
can be calculated:
Qphase change=mHf or v
The H value is a constant and will be provided. For
example: Hf of ice=334 J/g or Hv of water=2260 J/g
Temp is constant!
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Hf of ice=334 J/g Hv of water=2260 J/g
• Hf is used when going from solid to a liquid
• Hv is used when going from a liquid to a gas
• When going in reverse, the H value is
negative
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Hf of ice=334 J/g
Hv of water=2260 J/g
Calculate the energy needed to have 54
grams of water go from liquid to vapor
Calculate the energy needed to have 32
grams of water go from solid to liquid
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Calculating Heat
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Heat Equation
• c is also writen as Cp units are :
J/goC or
cal/goC
1 Calorie=1 kilocalorie=1000 calories
1 Joule (J)=0.2390 cal
4.184 J = 1 cal
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• Phase Change or Heat Graph
– Moving left to right(a-e) → is ENDOTHERMIC
• Must put energy in to melt/boil
• Energy goes into breaking bonds if temp is same
– Moving Right to left (e-a) ← is EXOTHERMIC
• Energy is release in condensation/freezing
• Energy is released as bonds form keeping temp
same
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Phase changes
• Endothermic: melting, boiling
• Exothermic: condensation, freezing
•
(B-C) Enthalpy of fusion: Q= m fusion
•
(D-E)Enthalpy of vaporization:Q=m vap
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Phase change Equation
• For phase changes, use a different equation
because there is no temp. change
–
Q=m
–
Q=m
fusion
for water see back of P.T.
vap
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Summary of Equations
• Depending on what is happening to temp
dictates which equation to use.
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1. What amount of heat is required to raise the
temperature of 85.9 g of water by 7°C? The specific
heat of water is 4.184 J/g°C.
2. What amount of heat is required to increase the
temperature of 75.0 g of water from 22.3°C to 36.1°C?
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When 2 different substances of different temperatures are added to one system,
the final Q values are the same. When solving the problems:
m1C1T1=m2C2T2
A copper cylinder has a mass of 76.8 g and a specific heat of 0.092 cal/g·C. It is
heated to 86.5° C and then put in 68.7 g of turpentine whose temperature is
19.5° C. The final temperature of the mixture is 31.9° C. What is the specific
heat of the turpentine?
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