Topic: Enthalpy and Table I
Do Now: packet p.1
L
S
The universe wouldn’t exist without energy. It
provides all our work, our heat, can be stored to use
later and it never goes away (conservation of energy)
The total amount of energy a substance contains
depends on many factors, some of which are not
totally understood…it’s impossible to know the total
heat content of a substance.
So scientists measure scientists can measure the
amount of heat lost or gain by a system
endothermic

endothermic
Energy absorbed = _______________
exothermic
Chemistry is the study of matter and energy. So far we have learned about the
types of matter and how they can physically combined to make mixutures or
chemically combine in chemical reactions to from new compounds.
Every physical or chemical change is accompanied by energy change
exothermic
G
 Energy released = _________________
H = Enthalpy = heat
ΔH = Heat of Reaction
The heat gained or lost by the
system (at constant pressure)
ΔH = Hproducts – Hreactants
ΔH=Q
Energy is stored in chemical
bonds
For any reaction occurring at constant pressure ΔH = Q
• Q = mCT
– Q = Energy change
– m = mass of water
– C = specific heat of water
– T = temperature change = Tf – Ti
4Fe(s) + 3O2(g)  2Fe2O3(s) + 1625 kJ
4Fe(s) + 3O2(g)  2Fe2O3(s)
OR
NH4NO3(s) + 27 kJ  NH4+(aq) + NO3-(aq)
OR
NH4NO3(s)  NH4+(aq) + NO3-(aq)
H = -1625 kJ (heat is lost)
H = 27 kJ (heat is gained)
If ΔH is negative…
- ΔH = exothermic – heat is lost
- Hproducts < Hreactants
- PE of Products < PE of Reactant
- Example: 4Fe(s) + 3O2(g)  2Fe2O3(s) + 1625 kJ
- ΔH = -1625kJ
If ΔH is positive…
+ ΔH = endothermic – heat is gained
- Hproducts > Hreactants
- PE of Products > PE of Reactant
Example: NH4NO3(s) + 27 kJ  NH4+(aq) + NO3-(aq)
- ΔH = +27kJ
Since energy is conversed…the system changes in one
direction and the surrounding have to change in the
opposite direction
Universe
Environment
System
A
B
A. Reaction is
Exothermic,
environment gets
warmer
_________________
B. Reaction is
Endothermic,
environment gets
colder
_________________
Since there are different types of
reactions, you have various ΔH’s
• ΔHcomb = enthalpy (heat) of combustion – the
enthalpy change for the complete bunring of one
mole of the substance
• ΔHformation = enthalpy (heat) of formation – the
enthalpy change for the formation of a compound
from its constituent elements
• ΔHsolution = enthalpy (heat) of solution – the
enthalpy change when 1 mole of an ionic substance
is dissolved in water.
Look at Table I: Heats of Reaction
Lets label the various types from above
Table I
• Rxns 1-6: combustion rxns
-H = heat of combustion
• Rxns 7-18: formation
(synthesis) rxns
– Substance is formed from
its elements
– H = heat of formation
• Rxns 19-24: dissolving
equations
– H = heat of solution
Many other processes other than
chemical reactions absorb or release
energy
like, Changes of state
Hvaporization = molar heat of vaporization
= amount of heat required to vaporize
one mole of a liquid
Hfusion = molar heat of fusion
= amount of heat required to melt one
moles of a solid
Energy depends on amount
• Remember – it takes more energy to
heat up water in the ocean than to make
a cup of tea
CH4(g) + 2O2(g)  CO2(g) + 2H2O (l) H = -890.4 kJ
1 mole of methane + 2 mole of oxygen →
1 mole of carbon dioxide gas & 2 moles of liquid water
exothermic
reaction is ____________
(negative sign for ΔH)
890.4 kJ energy released per mole of CH4(g) burned
What would happen if we had 2 moles of methane?
Twice as much energy would be released
2 x 890.4 kJ = 1780.8 kJ will be released
Reactions: Energy depends on
direction too!
• N2(g) + 3H2(g)  2NH3(g) H = -91.8 kJ
91.8 kJ
• 2NH3(g)  N2(g) + 3H2(g) H = _______
If reverse equation, reverse sign of H
Table I
• Exothermic: ΔH = • Pay attention to number of moles, may
need to divide or multiply ΔH
• Pay attention to direction, if in reverse
direction, switch the sign
• Check POINT
• 1. According to Ref. Table I, the
decomposition of which
compound is exothermic?
• A) C2H6
B)NO
• C) CO2
D) NH3
• 2. According to Ref. table I, what
is the heat of reaction for the
formation 1 mole of H2O (l) from
hydrogen and oxygen gas at
STP?
• A) -571.6kJ B) -483.6 kJ
• C)-55.8kJ
D) -285.8kJ
If we can’t calc. H for one
individual substance how are we
able to calc. ΔH???!
FYI: Hess’s Law
(not on regents)
Can add 2 or more equations by adding the
H’s
Enables you to calculate H for # of rxns
Say you’re interested in
2S(s) + 3O2(g)  2SO3(g)
2S(s) + 3O2(g)  2SO3(g)
Have H’s for the following:
a) 2 S(s) +2O2(g) 2SO2(g) H = -297 kJ x (2)
b) 2SO3(g)  2SO2(g) + O2(g)
H = 198 kJ
2SO2(g) + O2(g)  2SO3(g) 
H = -198 kJ
H = -594KJ + (- 198kJ)
H = -792 kJ