8.6 The Thermodynamic Standard State
The value of H reported for a reaction depends on the number of
moles of reactants....or how much matter is contained in the system
C3H8(g) + 5O2(g)
> 3CO2(g) + 4H2O(g)
2 C3H8(g) + 10 O2(g)
> 6 CO2(g) + 8 H2O(g)
H = - 2043 kJ
H = - 4086 kJ
Also, the reactants and products must have their physical states
specified [(g), (l), (s) ] since heat is required to change temperatures
and states. For reaction of 1 mole of propane above, H = - 2219 kJ if
the water is produced as a liquid.
C3H8(g) + 5O2(g)
> 3CO2(g) + 4H2O(l)
H = - 2219 kJ
Thus it is convenient to define a standard state.... a pure
substance in a specified state, normally its most stable form at
1 atm pressure, 25 oC and, if in solution, at a concentration of 1
mol/L.
From now on we will be using measurements of H made under these
standard state conditions as indicated by addition of a superscript o to
the symbol of H.
Thus, Ho is the standard enthalpy of reaction.
The first thermchemical equation on the previous side can now be written:
C3H8(g) + 5O2(g)
> 3CO2(g) + 4H2O(g)
Ho = - 2043 kJ
1
8.7 Enthalpies of Physical and Chemical Change
Almost every change in a chemical system involves a change in
enthalpy , H. The changes are either :
physical (same compound different state) or chemical (different
compound) changes.
1. Physical Changes
[ Lab #2, part ll ]
Each of these physical state changes require a change in enthalpy, so we can
talk about the enthalpy of fusion ( Hfusion), the enthalpy of vaporization ( Hvap),
or the enthalpy of sublimation ( Hsubl).
For physical changes in water:
Melt ice
H2O (s) ----> H2O (l)
vaporize water H2O (l) ----> H2O (g)
sublimation
H2O (s) ----> H2O (g)
Hfusion = 6.01 kJ/mol
Hvap = 40.7 kJ/mol
Hsub = 46.71 kJ/mol
Draw and describe Figure 8.5 (textbook) in class [ on blackboard]
If the reaction is reversed, the value has the same magnitude but is opposite
in sign.
H2O (l) ----> H2O (s)
H = - 6.01kJ/mol
2. Chemical Changes: Endothermic and Exothermic Reactions
[ Lab #3]
A reaction is endothermic if heat moves from the surroundings into the system
during the reaction. H is positive.
A reaction is exothermic if the system releases heat to the surroundings. H
is negative.
2 Fe (s) + Al2O3 (s)
2 Al (s) + Fe2O3 (s)
2 Al (s) + Fe2O3 (s)
2 Fe (s) + Al2O3 (s)
Ho = + 852 kJ
Ho = - 852 kJ
2
Problem 8.8 (b)
How much heat (in kJ) is evolved or absorbed when 4.88g of barium
hydroxide octahydrate react with ammonium chloride as seen in
Solution to problem done in class on blackboard
Ans: q (heat absorbed) = 1.24 kJ
Note:
H is (+) Heat moves from surroundings
system
8.8 Calorimetry and Heat Capacity
We need to calculate the quantity of heat (q) flowing during a reaction.
We can measure the amount of heat transferred during a chemical
reaction using a device called a calorimeter. There are two types
The first type is open to the atmosphere and consists of a well
insulated reaction vessel, a stirrer, and a thermometer. The reaction
vessel is open to the atmosphere (constant pressure, but V) and
usually involves reactions in solution. The reaction is the system and
the water is the surrounding.
We measure qp to calculate H of the reaction.
In the lab Exp. #2 and #3
You use a simple cup
calorimeter
3
The second type is a bomb calorimeter which measures heat evolved
during a combustion reaction. It is preformed at Constant Volume,
therefore we are calculating E of the reaction of the reaction by
measuring heat evolved (qv).
With both calorimeters we measure temperature change ( T) to calculate heat flow
during a reaction (qreaction)
The exact amount of heat absorbed ( or given off) is equal to
the substances heat capacity (C) times the temperature
change .
q=Cx T
Heat Capacities of substances depends on its size and composition.
Two types of heat capacities:
a) Specific heat (units: J/ g . oC)
- the amount of heat necessary to raise the temperature of 1 g of
substance by 1 oC.
i.e., H2O (l) is 4.179 J/ g . oC
q = (specific heat ) x (Mass) x T
b) Molar heat capacity (Cm) (Units: J/ mol . oC)
- amount of heat necessary to raise the temperature of 1 mol of
substance by 1 oC
i.e., Fe (s) is 25.1 J/ mol . oC
q = (Cm) x (moles of substance) x T
4
Relation between Molar heat capacity (Cm) and specific heat
Cm = specific heat x (molar mass)
Cm (H2O) = 4.179 J/ g . oC x 18.02 g/mol
= 75.31 J/ mol . oC
I.e., For water (l)
Problem 8.10
Assuming Coca-Cola has the same specific heat as water 4.18 J/(g
°C), calculate the amount of heat in kJ transferred when one can (about
350 g) is cooled from 25 °C to 3 °C.
Ans: q = -32 kJ
.. solution to problem done in class
Problem 8.12 [ similar to lab #3, Part l]
When 25.0 mL of 1.0 M H2SO4 is added to 50.0 mL of 1.0 M NaOH at
25.0 °C in a calorimeter, the temperature of the aqueous solution
increases to 33.9 °C. Assuming the specific heat of the solution is 4.18
J/(g·°C), that its density is 1.00 g/mL, and that the calorimeter itself
absorbs a negligible amount of heat, calculate H (in kJ) for the
reaction:
H2SO4 (aq) + 2 NaOH (aq)
Ans: -1.1 x 102 kJ
2 H2O (l) + Na2SO4 (aq)
solution to problem done in class
5
This document was created with Win2PDF available at http://www.daneprairie.com.
The unregistered version of Win2PDF is for evaluation or non-commercial use only.