title: lesson 3 hess’s law and enthalpy of formation and combustion learning objectives: –...
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Title: Lesson 3 Hess’s Law and Enthalpy of Formation and Combustion
Learning Objectives:– Define Hess’s Law– Using calculations show that enthalpies for formation of
products are the same regardless of the route taken– Calculate change in enthalpy of reactions using enthalpy of
formation or combustion data
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How much energy, in joules, is required to increase the temperature of 2.0 g of aluminium from 25 to 30 °C? (Specific heat of Al = 0.90 J g–1 K–1).
A. 0.36B. 4.5C. 9.0D. 54
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Calculations linked to Enthalpy Cycles (Hess’s Law)
Enthalpy of Formation Enthalpy of Combustion Bond Enthalpy
You must know the difference between these!
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Hess’ Law The enthalpy change of a reaction is independent of the
pathway of that reaction i.e. All that matters is the start and finish points
Note: add when going ‘with’ an arrow, subtract when going against an arrow.
A + B
C + D
E + F
∆H1
∆H2 ∆H3
∆H1 = ∆H2 + ∆H3
A + B
C + D
E + F
∆H1
∆H2 ∆H4
∆H1 = ∆H2 - ∆H3 + ∆H4
G + H
∆H3
The first law of thermodynamics relates to the conservation of energy. It is sometimes expressed in the following form: Energy cannot be created or destroyed, it can only change form.
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Why is this useful
It is not always possible to directly measure the enthalpy change we want. It may be an endothermic reaction that needs a
constant heat supply It may be that the reaction doesn’t ‘stop’ where
you need it to It may be that the reaction is simply too slow
Hess cycles allow us to measure enthalpy changes indirectly
HR
HESS’S LAW
THE TOTAL ENTHALPY CHANGE OF A REACTION
H1
REACTANTS PRODUCTS
INTERMEDIATES
H2
By Hess’s Law: HR =
Intermediates =
IS INDEPENDENT OF THE REACTION ROUTE.
oxides if ΔHC known,or gaseous atoms if E[X-Y] (Bond Enthalpy) known
elements if ΔHf known,
Alternative route
H1 + H2
Direct measurement of ΔHR may not be possible because :
1. Reaction incomplete
2. Other reactions occur
3. Reaction too slow
EXAMPLE OF CYCLE 1
HR
HESS’S LAW
H1
REACTANTS PRODUCTS
INTERMEDIATES
H2
Therefore By Hess’s Law: HR =
Intermediates =oxides if ΔHC known,
or gaseous atoms if E[X-Y] (Bond Enthalpy) known
elements if ΔHf known,
Alternative route
-H1 + H2
We can consider situations where the conversions are the other way round. In this case the intermediate will convert to both reactants and products. (Look at the direction of the arrows!)
EXAMPLE OF CYCLE 2
Pathways:
I R = H1
I P = H2
HR
HESS’S LAW
H1
REACTANTS PRODUCTS
INTERMEDIATES
H2
Intermediates =oxides if ΔHC known,elements if ΔHf known,
Alternative route
We can consider situations where the conversions are the other way round. In this case the reactants and products will both convert to intermediates. (Look at the direction of the arrows!)
Therefore By Hess’s Law: HR = H1 + -H2
EXAMPLE OF CYCLE 3
or gaseous atoms if E[X-Y] (Bond Enthalpy) known
Pathways:
R I = H1
P I = H2
LEARN!!!
STANDARD ENTHALPY CHANGE OF FORMATION, H f
The heat change when ONE MOLE of a substance is
FORMED from its ELEMENTS in their standard states
at 298K and 100kPa
NB Hof [ELEMENT in its standard state] = ZERO
Q
PC
STANDARD ENTHALPY CHANGE OF COMBUSTION, HC
The heat produced when ONE MOLE of a
substance is burned in excess oxygen measured
at 298K and 100kPa
Q
P
C
BOND ENTHALPY or BOND ENERGY E[X-Y]
The average heat needed when ONE MOLE of
covalent bonds are broken, measured in the
gaseous state at 298K and 100kPa
Q
P
C
EL
EM
EN
TS
OX
IDE
SG
AS
AT
OM
S
Hess’s Law states that the total enthalpy change is independent of the route taken
2NO2(g)
2NO(g) + O2(g)
N2(g) +2O2(g)
ΔHr = -66.4kJmol-1
Route 1
Route 2
-180.8kJ+114.4kJ
Route 2ΔHr = +114.4 + (-180.8) = -66.4kJmol-1
The total enthalpy change for route 1 is the same as for route 2
7.4 HESS’S LAW
http://www.youtube.com/watch?v=y01ePN0Hr-Y
This is a thermo-chemical cycle
Enthalpy Cycles and Hess’s Law
Think of a hike to your nice wood cabin in the French Alps.
The wood cabin is your destination – but you can either walk through the forest or over the mountains. Either way you will still end up at the cabin!
Calculate the potential energy of each climber taking route 1 and route 2
-137KJ
+125KJ
+87KJ
-193KJ
+102KJ
-163KJ+52KJ
-147KJ
-269KJ
+7KJ
Regardless of the route the climber and the miner took they ended up having the same amount of
potential energy!!
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Using Hess’s Law Although carbon and hydrogen do not combine directly to
form propane, C3H8, the enthalpy change for the reaction is:
This can be calculated from the enthalpy of combustion data of the elements and the compound
The steps in the cycle may be hypothetical and may refer to reactions that do not take place. The only requirement is that the individual chemical reactions in the sequence must balance.
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Solutions
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Complete the Hess’s Law Worksheet