Ch 5 Thermochemistry Review
As a student you should be able to…..
1) Use appendix C to calculate the heat of a rxn at standard conditions.
∆H = ∑ H prod - ∑ H react
2) Use appendix C to calculate the entropy of rxn at standard conditions
∆S = ∑ S prod - ∑S react
3) Use Gibbs Free Energy Equation to calculate the Change in Energy of a system to
determine if the rxn is spontaneous or nonspontaneous at the stated conditions.
∆G = ∆H –T (∆S /1000)
4) Use Hess’s Law to ID intermediates, catalysts, and the delta H of a net rxn.
5) Be able to interpret and PE diagram for a rxn or rxn mechanism
6) Be able to look at a chemical rxn and tell whether it is exothermic and endothermic based
on the delta H or PE diagram
7) Using stoichiometry, calculate the heat generated or absorbed by a chemical rxn.
Vocabulary: PE diagrams, catalyst, activated complex/intermediate, activation energy,
enthalpy, entropy, standard heat of formation, heat of a reaction, forward vs reverse reactions.
Rxns absorb heat (Endothermic) or they create/produce heat (Exothermic)
Heat is energy also called enthalpy
Heat Temp
Units of heat= calorie or Joule
1 calorie = 4.18 Joules
The heat of a rxn can be called many things
change in enthalpy
heat of rxn
heat of solution
heat of neutralization
heat of ppt
heat of combustion etc
The above terms all involve knowing the change in heat for a rxn.
∆H = ∑ H prod - ∑ H react
∆H = (-) exothermic and heat is a product (Created/produced)
∆H =(+) endothermic and heat is a reactant (Absorbed)
If you have the standard heats of formation ( ∆Hf˚ )for the reactant & products (Appendix C) you can calculate the
heat of a reaction without experimental data.
In nature exothermic reactions are preferred. This is known as the first law of thermodynamics or the Law of
________________.
In nature products that are more random/chaotic are preferred. This is known as the 2
the Law of ________________
∆S = (+) products are increasing in randomness.(more chaotic)
∆S =(-) products are decreasing in randomness. (less chaotic)
∆S = ∑ S prod - ∑S react
nd
law of thermodynamics or
In nature
∆H = (-) Will be spontaneous under all temperature conditions
∆S =(+)
∆H = (+)
∆S =(-)
Will be nonspontaneous under all temperature conditions
∆H = (+)
∆S =(+)
Depends on temperature…….@ HIGH temperature this rxn will be spontaneous
∆H = (-)
∆S =(-)
Depends on temperature……..@ LOW temperatures this rxn will be spontaneous
WHEN THE ∆G = (+)
WHEN THE ∆G = (-)
RXN A NO GO
RXN WILL PROCEED.
Hess’s Law,
When the steps to a chain rxn (rxn mechanism) are added up, you can add up the ∆H of each step to calculate the
change in enthalpy of the net rxn
When doing this you need to ID any intermediates or catalysts.
General rules
Rxn, A ---> 6 B
∆H = -5 KJ
What happens if the rxn is reverse?
What happens if you triple the rxn?
What happens if you cut the rxn in half?
Use Hess’s Law to figure out the delta H of a net rxn by manipulating the known reactions so
that when added together they will equal the NET.
Interpret a PE diagram