Thermochemistry

1
Thermochemistry

• Chapter 6

2
The Nature of Energy
Work force used to move an object a distance.
Heat transfer of energy due to temperature
differences

• 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 systemnot on the
  changes it has or will experience.
• Internal energy
• Pressure
• Volume
• Energy

System
Surroundings
Heat and work are NOT state functions!
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
Heat Lost Heat Gained
Draw
a graph of energy vs. reaction time for a
reaction that gives energy to the surroundings
AND a second one for a reaction that absorbs
energy from the surroundings
6
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

7
PV Work

• Work (w) Forcedisplacement
• W F d F Dh
• W P A Dh

P Force/Area
When pressure of system does not change W -PDV
VOLUME!
8
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 Latm 101.3 J)

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

Compare the equation for total internal energy
with the equation for enthalpy listed above. How
can you alternately define Enthalpy?
Answer Now
10
Enthalpies of Formation

• The enthalpy of formation (Hfo) for an element in
  its standard state is ZERO.

Standard State is at 1 atm and 25oC
The more negative the value of Hfo, the more
stable the compound.
DHrxn Snp(DHfoprod)-Snr(DHforct)
11
Sample Problems

• Try Me 1
• 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)

12
Experimental Determination of Heat Enthalpy

• 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.

13
Another way to find DHrxn

• Experimentally!
• DHrxn DHproducts DHreactants
• How do you measure this stuff?
• CALORIMETRY!

14
Hesss Law

• Enthalpy is a state function
• The value will be the same regardless of how many
  steps are needed to complete the reaction.

Hesss Law States The enthalpies of individual
steps in a reaction mechanism can be added
together to calculate the enthalpy of the overall
reaction.
15
Fundamentals for Applying Hesss 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?
16
Try Me!

• Overall N2O4(g)gtN2(g) 2O2(g)
• Reaction Mechanism
• NO2(g) ? ½ N2(g) O2(g) H-84.75 kJ
• 2NO2(g) ? N2O4(g) H-145.5 kJ

This is fun! Lets do some more!