 Study
of energy relationships in a
chemical system
 We
will look at energy in terms of three
concepts
1. Temperature
2. Heat
3. Enthalpy (H)
1.
2.
3.
Temperature
- measurement of avg. KE of particles in
an object
Heat
- heat is the transfer of energy btw 2
objects due to a ∆ temperature
Enthalpy ( H)
- total energy in a chemical system
- we will be concerned with the
TRANSFER of enthalpy (∆ H)
 Temperature
( Celsius and Kelvin)
 Energy (joules or calories)
• Ability of a system to do work or supply ( or
produce) heat
 Heat
(CANNOT MEASURE)
• We can only measure through changes in
temperature
 Calorie
(cal) is the amount of energy
needed to raise one gram of water by 1 °C.
• kcal = energy needed to raise 1000 g of water 1
°C.
• food calories = kcals.
Energy Conversion Factors
1 calorie (cal)
1 Calorie (Cal)
=
=
4.184 joules (J)
1000 calories (cal)
1 kilowatt-hour (kWh)
=
3.60 x 106 joules (J)
Tro's "Introductory Chemistry", Chapter 3
6
 Example
• Window in the winter time
 Energy
always flows in the same
direction
→
 When
does the energy flow stop?
 Energy
is stored in chemicals, found in
bonds that hold atoms together
 As
chemical rxn takes place, bonds
break, new bonds are created, energy is
exchanged
 Change
in energy is change in enthalpy
 Energy
can be released or absorbed
 Heat
releasing (Exothermic)
Reactants --> Products + E
 Heat
H = (-)
Absorbed (Endothermic)
Reactants + E --> Products
H = (+)
a. How much heat is released when 275
grams of sulfur is burned?
b. How much heat is released when 25 mol
of sulfur is burned in excess oxygen?
c. How much heat is released when 150.0
grams of sulfur dioxide is produced?
 Some
chemical reactions we cannot carry
out in a calorimeter they release or
absorb tons of energy
 We
must indirectly calculate the change
in energy for these reactions
 We
use Hess’ Law
 Before
we discuss Hess’ Law we must first
talk about Energy
 Energy
is a state function
• This means that the pathway one uses to get from
one energy level to another is not important
 Change
in enthalpy for any equation can
be calculated without actually carrying
out the reaction
 Just
simply add up the H of other related
experiments
N2(g) + 2O2(g) --> 2NO2 (g)
N2(g) + 2O2 (g) --> N2O4 (g)
H = 67.8 kJ
H = 9.67 kJ