Download system = part of the universe that contains the reaction or process

April 30, 2014
Thermochemistry is the study of heat changes
(energy!) that accompany chemical reactions and
phase changes
• Heat is exchanged between the system and
surroundings
system = part of the universe that contains
the reaction or process being studied
surroundings = rest of the universe that
interacts with the system
April 30, 2014
April 30, 2014
Enthalpy (H) is the heat content of a system at
constant pressure. (Enthalpy is potential energy)
Change in enthalpy for a reaction is called the
enthalpy (or heat) of reaction ( Hrxn).
Hrxn = Hfinal - Hinitial
Hrxn = Hproducts - Hreactants
The second equation shows you how you can
think about enthalpy of a reaction as the
difference in energy of the products and
reactants.
April 30, 2014
Energy required to
break bonds
<
Energy released to
form bonds
is
gy d
er
En ease
l
re
Enthalpy
Reactants
Exothermic Reaction
H<0, -
H
Products
What happens to the surrounding?
April 30, 2014
Energy required to
break bonds
>
Energy released to
form bonds
Enthalpy
Products
Reactants
is
y
d
rg be
e
r
En so
ab
Endothermic Reaction
H>0, + H
What happens to the surrounding?
April 30, 2014
Is thi
April 30, 2014
Thermochemical equations: balanced chemical
equation that includes the physical states of all
reactants and products and the energy change,
usually expressed as the change in enthalpy ( H).
Example:
CH4(g) + 2O2 (g)
CO2(g) + 2H2O(g)
Hrxn= -802.3kJ
April 30, 2014
Coffee cup calorimeter
Bomb calorimeter
Reaction
happens in
water
-qsystem = qwater
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Calorimetry Lab Calculations
Step 1: Finding how much heat was transferred from
metal to water (remember...heat flows from higher
temperature to lower temperature)
Step 2: Using heat transferred, calculate specific
heat of metal.
April 30, 2014
Sample Calculations
Mass of water = 75g
Initial Temperature of water = 21.0ºC
Final Temperature of water = 25.5ºC
Mass of metal = 50.5g
Initial temperature of metal = 100ºC
Final Temperature of metal = 25.5ºC
April 30, 2014
Hess's Law
• Used when it is impractical to measure ΔH using a
calorimeter
• Hess's Law states if you can add two or more
thermochemical equations to produce a final
equation, the sum of the enthalpy changes of each
individual reaction is the enthalpy for the final
reaction
∆ Hoverall=)))H
∆ 3).).).
∆ 1)+)))H
∆ 2)+)))H
Imagine that chemical reactions happen
in small steps. You add all of the steps
together along with their enthalpy
changes
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April 30, 2014
Example 17:
Calculate ΔH for the formation of sulfur trioxide
2S(s) + 3O2(g) ! 2SO3(g) ΔH =?
Given:
S(s) + O2(g) ! SO2(g) ΔH = -297kJ
2SO3(g) ! 2SO2(g) + O2(g) ΔH = 198 kJ
April 30, 2014
Hess's Law Rules
1. If the reaction is reversed, the sign of ΔH is also
reversed
2. If the coefficients are multiplied by a factor,
then the ΔH is multiplied by the same factor
3. The state of matter (s, l, g, a q) of reactants and
products affects whether they cancel out or not.
April 30, 2014
Example 18:
Calculate the enthalpy of reaction of
2C(s) + O2(g) + 2H2O(l) ! H2CO2(l) ΔH=?
Given:
C(s)+1/2O2(g) + H2O(l)!CO(g)+ H2O(l) ΔH=-110.5kJ
CO(g)+ H2O(l)!H2CO2(g) ΔH=33.7 kJ
H2CO2(g)! H2CO2(l) ΔH=-62.9 kJ
April 30, 2014
Example 19:
Calculate the enthalpy of reaction of
2H2O2(l) ! H2O(l) + O2 ΔH=?
Given:
2H2(g) + O2(g)!2H2O(l) ΔH=-572 kJ
H2(g) + O2 !H2O2(l) ΔH=-188 kJ
April 30, 2014
Not practical to calculate every ΔH to use in Hess's
Law.
Use Standard Enthalpy of Formation!
Standard Enthalpy of Formation: Change in
enthalpy when one mole of a compound forms from
it's constituent elements in their standard states.
• standard state: normal physical state at 1 atm and
25ºC
• For a pure elements in its standard state ΔH = 0
C(s) + 2H2(g)
CH4(g)
∆ H= -74.6 kJ/mol
April 30, 2014
Using Standard Enthalpies of formation
• You can use standard enthalpies of formation to
calculate enthalpy of reaction under standard
conditions (ΔHºrxn) using Hess's Law
Example 20:
H2S(g) + 4F2(g)
2HF(g) + SF6(g) ΔHºrxn =?
Given:
• 1/2H2(g) + 1/2F2(g)
HF(g) ΔHºf=-273kJ
• S(s) + 3F2(g)
SF6(g)
ΔHºf= -1220kJ
• H2(g) + S(s)
H2S(g)
ΔHºf= -21kJ
ΔHºrxn=
ΔHºf(product) -
ΔHºf(reactants)
April 30, 2014
ΔHºrxn=
ΔHºf(product) -
ΔHºf(reactants)
April 30, 2014
Example 21: Using the chart given to you, calculate
ΔHºrxn for the following reactions:
a) CH4(g) + 2O2(g)
b) CaCO3
CO2(g) + 2H2O(l)
CaO(s) + CO2(g)
April 30, 2014
Calculating ΔH using bond energy (BE) data
• you can estimate ΔH for gaseous molecules
• Bond formation is ____________
• Breaking bonds is ____________
• ΔH = BEbreaking - BEforming
Example 21: Determine ΔH for the following
reaction given the bond energies
N2(g) + 3H2
2NH3(g)
N=N 941 kJ/mol
H-H
436 kJ/mol
N-H
391 kJ/mol
April 30, 2014