Endothermic/Exothermic
Reactions
And Thermochemical Equations
Endothermic
„ Endothermic
reaction – absorbs energy
„ (energy goes “in” = “endo”)
E
n
e
r
g
y
P
Activation Energy
R
Reaction Progress
Exothermic
„ Exothermic
Reaction – releases energy
„ (“exo = exiting”; energy is released)
E
n
e
r
g
y
Activation Energy
R
P
Reaction Progress
„ Quick
review… what is a generic formula
for combustion?
„ Fuel (CxHy) + oxygen --> carbon dioxide
+ water + energy
„ Example:
– CH4 + 2 O2 Æ CO2 + 2 H2O + Energy
„ Why
is ENERGY written as a product?
„ EXOTHERMIC!
_C2H6 + __O2 Æ __CO2 + __H2O + ?kJ of E
Balance the equation
„ 2C2H6
+ 7O2 Æ 4CO2 + 6H2O + ?kJ of E
But what about the E… doesn’t it have to be
balanced too?
„ Fig
16 p 205
XWith the info
we have so
far, which
column would
help us more?
„ SO
this chart tells us that when ONE mole
of ethane is burned, 1560kJ are released
(hence 1560kJ/mol)
„ But how many moles are we referring to
in that last equation? How do we
account for that?
„ 2molC2H6 x 1560kJ/1mol = 3120kJ
2 C2H6 + 7 O2 Æ 4 CO2 + 6H2O + 3120 kJ
Example 1
„ Propane
is a common gas used in
barbeque grills. Write the balanced
equation for the combustion of propane
(C3H8). One mole of propane releases
2200kJ when burned. Include energy in
your equation.
„ C3H8
+ 5O2 → 3CO2 + 4H2O + 2200kJ
Example 2
„ Butane
(C4H10) is the fluid in many
common hand-held lighters. When one
mole of butane is burned 2859kJ of
energy is released. Write the balanced
thermochemical equation for the
combustion of butane.
„ 2C4H10 + 13O2 → 8CO2 + 10H2O + 5718kJ
Heat of combustion
„ Heat
of combustion is a measure of the
amount of energy per gram
„ If an experiment is performed and the
following data are obtained, calculate the
heat of combustion (J/g) of butane
(C4H10).
Experimental Data
„ mass
of water heated
„ initial temperature of H2O
„ final temperature of H2O
„ initial mass of butane
„ final mass of buntane
„ specific heat of water
99.5 g
15 oC
28 oC
15.5 g
14.0 g
4.18 J/goC
„ Q=mc∆T
= (99.5g)(4.18J/g°C)(13 °C) =
5406.83 J
Amount of butane burned:15.5g -14.0g =
1.5g
Heat of comb = 5407J = 3605 J/g
1.5g
Molar heat of combustion
„ Heat
of combustion in J/mol
„ Using the information from the previous
example, calculate the molar heat of
combustion of butane (C4H10)
„ 1.5g x 1 mol = 0.0259mol
58g
5407 J = 209,764 J/mol
0.0259mol