1. Thermochemistry
Chapter 6

2. Section 1: The Nature of Energy
Energy is the capacity to do work or produce heat.
Law of Conservation of Energy
Total energy of the universe is constant.
Energy lost = Energy gained by something else.
Potential Energy = energy due to position= mgh
Kinetic Energy = energy of motion = ½ mv2

3. Heat is a transfer of energy
System vs Surroundings
State Function – a property that depends only on the present state of the system…not on the changes it has or will experience.
Internal energy
Pressure
Volume
Enthalpy
System
Surroundings

4. Thermodynamic Quantities
Consist of two parts
1) the number – indicates how much
2) the sign- indicates direction of flow
HEAT q
WORK w
ENTHALPY H
Internal
Energy E
Negative Values = flow out of system
Positive Values = Flow into system

5. Exothermic and Endothermic
Exothermic Reaction
Feels hot
Heat transferred to surroundings (lost by system)
Negative enthalpy and heat values
Endothermic Reaction
Feels cold
Heat transferred to system (gained by system)
Positive enthalpy and heat values

6. PV Work Work (w) = Force*displacement W = F * d = F * Dh
W = P * A * Dh
P = Force/Area
When pressure of system does not change
W = -PDV
VOLUME!

7. Try Me Problem
A balloon is inflated to its maximum capacity by heating. If the volume changes from 4.0 x 106L to 4.5 x 106L by addition of 1.3 x 108J energy as heat. Assuming that the balloon expands against constant 1.0 atm pressure. Calculate Internal Energy.
(1 L*atm = J)

8. Section 2: Enthalpy & Calorimetry
Specific/Molar Heat of Combustion
q = mcDT q = ncDT
-The heat needed to raise the temperature of 1 g (or 1 mol) of substance 1 degree K.

9. How is enthalpy different?
Enthalpy, H, is the amount of energy capable of doing work in a system.
The amount of energy contained within the bonds of chemicals involved in the system.
H = E + PV
Answer
Now
Use the equation above and the
total internal energy equation
to determine an alternate
equation for enthalpy

10. A Third Way to find DHrxn
DHrxn = DHproducts – DHreactants
How do you measure this stuff?
CALORIMETRY!

11. Section 3: Hess’s Law Enthalpy is a state function Hess’s Law States:
The value will be the same regardless of how many steps are needed to complete the reaction.
Hess’s Law States:
The enthalpies of individual steps
in a reaction mechanism can be
added together to calculate the
enthalpy of the overall reaction.

12. Fundamentals for Applying Hess’s Law
Reverse the reaction, reverse the sign on enthalpy.
Multiply the reaction by a coefficient, multiply the enthalpy by the same coefficient.
Add the reactions together, add the enthalpies together.
why?

13. Try Me! Overall: N2O4(g)=>N2(g) + 2O2(g) Reaction Mechanism:
NO2(g)  ½ N2(g) + O2(g) H= kJ
2NO2(g)  N2O4(g) H= kJ
This
is fun! Let’s do some more!

14. Section 4: Enthalpies of Formation
The enthalpy of formation (Hfo) for an element in its standard state is ZERO.
Standard State
1 atm and 25oC
is at
The more negative the value of Hfo, the more stable the compound.
DHrxn = S[np)DHfoprod)]-S[np(DHforct)]

15. Sample Problems Try Me 1 Try Me 2 Find the enthalpy for the reaction:
4NH3(g) + HCl(l)  4NH4Cl(s)
Try Me 2
Find the enthalpy for the reaction:
2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(s)