SLU CHEM 107-001
Summer 2017
Dr. Eric C. Booth
Energy
General Kinds of Energy
 Potential
energy
 energy
waiting to
be released
 example: energy of
rock before it falls
off the hill
 Kinetic
energy
 energy
of
movement
 example: energy of
rock as it falls off
the hill
Specific Types of Energy
 Mechanical:
expanding gas
 Electrical: electrons (volts): battery
 Light: photons (frequency): fireflies
 Heat: ice
 energy
flows from high T to low T
 Chemical
 converts
to all above forms
 associated with absorbing or giving
off heat
Joules, calories and Calories
 Joule:
SI unit of energy
≈
energy released by 1 kg mass
dropping through 10.2 cm: not much
 chemists use kJ for reactions
 calorie
(lowercase) = 4.184 J
 energy
 Calorie
 the
to change T of 1 g of H2O 1 °C
(uppercase) = 1000 calories
“food calorie”
Joules, calories and Calories
 Joule:
SI unit of energy
≈
energy released by 1 kg mass
dropping through 10.2 cm: not much
 chemists use kJ for reactions
16 m 40 s
 calorie (lowercase) = 4.184 J
 Energy
 Calorie
 the
to change T of 1 g of H2O 1 °C
(uppercase) = 1000 calories
“food calorie”
How Much Heat Can a
Substance Hold?
 Specific
heat (heat capacity)
describes substance’s capability to
contain quantity of heat energy
 Over normal T ranges, substances’
specific heats distinct, unchanging
 Heat capacity related to both the
type and mass of a material
units  J g-1 °C-1
 large masses absorb more heat
 thus,
Relating T, Heat Changes
 Heat
transfer
 change in T
 Large heat capacity
 small T change
shuttle’s H2
fuel keeps engine
from melting
 example:
 Small
heat capacity
 large T change
 example:
an empty
pot placed on a
stove’s burner
State Changes
 To
get things to melt or boil, you
have to add thermal energy to
overcome attractive forces holding
molecules together
 To get things to freeze or
condense, you have to remove
thermal energy, until attractive
forces can “hold” molecules better
Heat of Fusion
 Amount
of energy that must be
removed from a particular liquid
substance to make it a solid
OR
 Amount of energy that must be
added to a particular solid
substance to make it a liquid
Melting & Freezing: Example
 If
the heat of fusion for pure H2O
is 334 J/g, how much energy
must, at 0 °C, be:
 removed
 added
to freeze 32.1 g of water?
to melt 5.7 g of ice?
Heat of Vaporization
 Amount
of energy that must be
added to a particular liquid
substance to make it a gas
OR
 Amount of energy that must be
removed from a particular
gaseous substance to make it a
liquid
Evaporate, Condense: Ex.
 If
the heat of vaporization for H2O
is 2260 J/g, how much energy
must, at 100 °C, be:
 removed
 added
to liquify 100 g of steam?
to boil 234 mL of water?
Heating, Cooling Curves
 Plots
of heat added vs. temperature
of substance
 Contain “steps” @ phase changes
 Substance’s temperature cannot
change at these points, because:
 molecules
that get hotter will escape
(steam from boiling water)
 molecules that get colder will “fall
out” (rain from condensing clouds)
Combined Energy
Calculations
 To
calculate the energy required
to change the temperature of a
substance, you must:
 use
heat capacity to calculate
energy between phase changes
 use heats of fusion & / or vaporization to calculate energy required
at phase changes
Combined Energy Example
 How
much energy does it take
to turn 120 g of water at 25 °C
into steam at 100 °C?
Heat in Reactions
 Reactant,
product energies differ
 Some reactions give out heat
(product energy < reactant energy )
 exothermic
(heat leaves): negative
 Other
reactions take in heat
(product energy > reactant energy )
 endothermic
 Matter,
(heat goes in): positive
energy change form in rxn.
 Specific amounts heat exchanged,
specific amounts material reacting
Heat in Reactions: Example
 819
kJ are released in the reaction
2 Na (s) + Cl2 (g)  2NaCl (s),
when 45.980 g of Na are reacted
with 70.906 g of chlorine.
• How much energy gets released
when 68.97 g of sodium metal react
completely with chlorine gas?