Notes: Laws of Thermodynamics
1
Overview:

____________________ (W) is a force acting over a distance

____________________is the system and surroundings

____________________is the part of the universe you are focusing on (molecules, atoms or ions
undergoing a change)

____________________is everything else

_________________________is a property or system that depends only on initial and final conditions
Energy:

____________________ (E) is the capacity to do work or transfer heat (q)

Two forms of energy are ____________________and ____________________

The ______________________________________states that energy can not be created or destroyed

__________________Energy is the sum of kinetic energy and potential energy in a system. ________
Three Laws of Thermodynamics:

First: Energy cannot be created or destroyed or the _____________________________________
___________________________________________

Second: In any spontaneous process there is an ________________________________________of
the universe. The entropy (disorder) of the universe is _____________________________

Third: The entropy of a perfect crystal at 0K is __________________________
Enthalpy:

____________________ (H) is a state function (independent of the path) and is related to heat

________________________________________

________________________________________

H depends on the ____________________ ____________________ of the substances
EXOTHERMIC
ENDOTHERMIC
heat flows
heat flows
heat is a
heat is a
system has _______________energy
system has _____________ energy
Notes: Laws of Thermodynamics
2
ΔH is _______________to indicate
direction of heat flow
ΔH is _______________to indicate direction of heat flow
energy released making bonds ____
than energy absorb to break bonds
energy absorb to break bonds _____ than energy released
making bonds
product’s bond energy ____
reactant’s bond energy
reactant’s bond energy ____ product’s bond energy

H is an ____________________ property – it depends on the number of moles present
Exothermic Reaction
_______________
_______________
_______________
Endothermic Reaction
_______________
_______________
_______________
Hess’s Law:

The enthalpy of a reaction _______________ depend on the path, only initial and final conditions. It
is a ______________________________

The enthalpy of a reaction is the _______________of the enthalpies of simpler reactions

If a reaction is _______________, the sign of the enthalpy is _______________
Notes: Laws of Thermodynamics
3

If the coefficients in a balanced reaction are _______________by an integer, the enthalpy is
_______________by the same integer

Nitrogen and oxygen react to produce nitrogen dioxide
N2 (g) + O2 (g)  2NO (g)
H = 180 KJ
2NO (g) + O2 (g)  NO2 (g)
H = -112 kJ
N2 (g) + O2 (g)  2NO2 (g)
H = 68 kJ

Notice that the compounds on both sides of the yield sign cancel out.

The heat of the reaction is the sum of the two steps.
Boron and hydrogen react to produce diboron hexahydride (diborane):
Reaction
H
2B (s) + 3/2O2 (g)  B2O3 (s)
-1273 kJ
B2H6(g) + 3O2(g)  B2O3(s) + 3 H2O(g)
-2035 kJ
H2 (g) + ½O2(g)  H2O(l)
-286 kJ
H2O(l) + H2O (g)
44kJ
2B(s) + 3H2(g)  B2H6(g)

Label the reactions A, B, C …

Determine how to add the reactions to obtain the desired overall reaction

Multiply reactions by integers to make coefficients work (also multiply H)

Flip reactions to make intermediate compounds cancel (change the sign of H)
Notes: Laws of Thermodynamics
4
Heat of Formation:

You can use the heat of formation to determine the heat of a reaction.

This is a short cut which used the property of state functions (like Hess’s Law)

Simply determine the heat of formation DHf for each substance from the provided charts

Multiply the coefficient by the heat for each substance

Determine the sum of the heat of formation for the reactants and then the products

The heat of formation for an element in its standard state is 0kJ.

Subtract the heat of formation for the reactants from the products
H   nH  products   nH 
reactants
Calculate the change in enthalpy for the following using the ΔHof in the appendix
4NH3(g) + 7O2(g)  4NO2(g) + 6H2O(l)