Transcript Unit 7 Thermodynamics

Thermochemistry
Chapter 10
Energy
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Chemical potential energy (PE)
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Measuring energy
• Energy stored in bonds
• Unstable compounds have lots of PE
• More PE more reactive
• Experimentally using q=mcΔT and q=mΔH
• Theoretically using bond energy, heat of
formation, energy diagrams
Heat vs. Temperature
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Heat ≠ Temperature
• Temperature: average KE of molecule
• Heat: amount energy exchanged due to
temperature difference
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Heat depends on AMOUNT of substance
Heat either given off or absorbed (lose or
gain heat due to temp difference)
Heat is directional (moves from area of hot
to cold)
ENERGY LOST = ENERGY GAINED
Calorimetry
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q = mcΔT
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q = heat in Joules (J)
m = mass in grams (g)
c = specific heat capacity J/g˚C
ΔT= change in temp in ˚C or K
At constant pressure, heat = change in enthalpy
(ΔH)
Enthalpy: total energy of system (KE + PE)
Heat = Energy = Enthalpy (FYI: Used
interchangeably)
Energy Measurement and
conversions
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A joule is the SI unit of heat and energy
A calorie (cal) is the quantity of heat that
raises the temperature of 1 gram of pure
water 1 degree Celsius.
A food Calorie (Cal) is a kilocalorie (kcal).
1000 calories = 1 Calorie
1 Calorie = 1000 calories = 1 kcal = 4186
Joules
Specific heat capacity of water: 4.18 J/g◦C
or 1.00 cal/g◦C
Enthalpy
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Change in energy = change in
enthalpy (at constant pressure)
Enthalpy: measure of energy content
of substance
q = ΔH
Endothermic ΔH > 0
Exothermic ΔH < 0
Activated
Complex
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Reactants
Energy
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Products
Reaction
Coordinate
Energy Diagram
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Change in energy
is change in
enthalpy
Reaction is
exothermic
Activation energy
Activation Energy
Energy
H < 0
ALL reactions require
activation energy!
Reaction Coordinate
Endothermic Reaction
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Likelyhood of
endothermic rxn taking
place not likely (why?)
Non-spontaneous
Substances like to be
in a lower energy state
Exothermic rxns
usually spontaneous
Activation Energy
Energy
H > 0
Reactants
Reaction Coordinate
ENDOTHERMIC REACTION
Summary/ Review
Thermochemistry
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How does a reaction happen?
• Energy is added
• Activation energy!
• Used for BREAKING bonds between reactants
• Activated complex or intermediate formed
• New bonds formed that RELEASE energy
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Change in energy is enthalpy (ΔH)
Energy of Phase Changes
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A few definitions
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Melting point: temperature at which all energy
absorbed causes change of state from solid to
liquid
Freezing point: same temperature as melting point
causes change from liquid to solid
• Pure substances freezing and melting points are at
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the same temp, not true always for mixtures
Boiling point: temperature at which all energy
absorbed causes change of state from liquid to
gas or gas to liquid (condensation point)
Heating Curve
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1. Solid
2.Melting point
3.Liquid
4.Boiling point
5. Gas
Heating Curve
5
4
3
2
1
time
Phase Changes
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Changing temperature, calculating heat
exchanged use:
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Q = mCΔt
• Remember C is dependent on phase (state of
matter)
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Melting/ freezing/ boiling/etc, calculating
heat exchanged use:
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Q = mΔH
Units J/mol or J/gram
M = mass or moles (depending on ΔH)
Other Vocabulary:
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Heat of Reaction: Hrxn heat produced in a chemical reaction
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Heat of solution: Hsol heat produced by when something dissolves
Heat of Combustion: Hcomb heat produced by a combustion reaction
Heat of Neutralization: neutral heat produced in a neutralization
reaction (when an acid and base are mixed to get water, pH = 7)
Heat of Fusion: Hfus heat produced when something melts
Heat of Vaporization: Hvap heat produced when something evaporates
Heat of Sublimation: Hsub heat produced when something sublimes
Heat of formation: Hf change in enthalpy that accompanies the
formation of 1 mole of compound from it’s elements