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5
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Energy: the capacity to do work or transfer heat
Kinetic Energy: the energy that results from motion
1
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Chemical Energy: energy stored within the structural units (molecules or
polyatomic ions) of chemical substances
a
...
2
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Oppositely charged particles attract each other
b
...
• Like charges result in a positive value for Eel, which indicates repulsion
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Ex) Dropping an object and allowing it to fall converts potential energy to kinetic energy
...
*The total amount of energy in the universe is constant
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Endothermic Process: a process that absorbs heat
*The energy of the products in an endothermic reaction is higher than the energy of the reactants
...
*Another unit used to express energy is a calorie (cal)
...
184J
o Not an SI unit
• 1 Cal = 1000cal
• 1 Cal = 4184J
5
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Open System: a system that can exchange mass and energy with its surroundings
ex) water in an open container
2
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3
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Ex) Going from the first floor to the fourth…either
going straight from 1st to 4th or going from 1st to 6th and then
down to 4th…the change in elevation is not affected because
it only relies on the initial and final state
First Law of Thermodynamics: energy can be converted
from one form to another, but cannot be created or destroyed
• ΔU = change in internal energy
• Uf = internal energy of a system in the final state
• U = internal energy of a system in the initial state
• Uf = internal energy of a system in the final state
• Ui = internal energy of a system in the initial state
*The internal energy of a system has two components: kinetic energy and potential energy
...
*When a system releases or absorbs heat, its internal energy changes
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g
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g
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3: Enthalpy
*q is determined by measuring temperature changes
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Pressure-Volume (PV) Work:
• P = external, opposing pressure
• ΔV = change in the volume of the container
*When a chemical reaction is carried out at constant volume, then no PV work can be done
because ΔV=0
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*While q is not a state function, qV is
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The change in
enthalpy, ΔH, is equal to the heat exchanged between the system and surroundings at constant
pressure qP
...
• U = internal energy of the system
• P = pressure of the system
• V = volume of the system
*Because U and PV have energy units, enthalpy also has energy units…meaning they are all
state functions
...
*If the reaction occurs under constant-volume conditions, then the heat change, qV, is equal to
ΔU
...
Enthalpy of Reaction ΔH:
• Endothermic processes ΔH < 0
Exothermic processes ΔH > 0
Thermochemical Equation: a chemical equation that includes the enthalpy change
1
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2
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3
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Consequently, the magnitude of ΔH for the equation remains the same,
but its sign changes
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4: Calorimetry
Calorimetry: the measurement of heat changes
Specific Heat (s): the amount of heat required to raise the temperature of 1 gram of a substance
by 1ºC; units are J/g*ºC
Heat Capacity (C): the amount of heat required to raise the temperature of an object by 1 ºC;
units are (J/ ºC)
• s = specific heat
• M = mass of the substance undergoing the temperature change
• ΔT = temperature change: ΔT = T -T
• s = specific heat
• M = mass of the substance undergoing the temperature change
• ΔT = temperature change: ΔT = Tfinal-Tinitial
• C = heat capacity
• ΔT = temperature change
*The sign convention for q is the same as that for an enthalpy change: q is positive for
endothermic processes and negative for exothermic processes
Specific Heat Values of Some Common Substances
Substance Specific Heat (J/g ⋅ °C)Substance
Specific Heat (J/g ⋅ °C)
Al(s)
0
...
444
Au(s)
0
...
139
C(graphite) 0
...
184
C(diamond)0
...
46
Cu(s)
0
...
2
Heat of
neutralization
Heat of ionization
Heat of fusion
Heat of
vaporization
+56
...
01
+44
...
*Energy content is a positive quantity
...
5: Hess’s Law
Hess’s Law: the change in enthalpy that occurs when reactants are converted to products in a
reaction is the same whether the reaction takes place in one step or in a series of steps
*In general, we apply Hess’s law by arranging a series of chemical equations (corresponding to a
series of steps) in such a way that they sum to the desired overall equation
...
6: Standard Enthalpies of Formation
Standard Enthalpy of Formation (
): the heat change that results when 1 mole of a
compound is formed from its constituent elements in their standard states
*The phrase “in their standard states” refers to the most stable form of an element under standard
conditions, meaning at ordinary atmospheric pressure
...
Standard Enthalpy of Reaction (
): the enthalpy of a reaction carried out under standard
*The standard enthalpy of formation of any element in its most stable form is 0
...
”
• The result has units kJ/mol, where “per mole” means per mole of reaction as written