Chapter 17
Thermochemistry
Basics

Thermochemistry
– Study of heat changes in a chemical reaction

Calorimeter
– Instrument used to measure heat changes
(actually measure temp changes because heat cannot be measured directly!)
– Styrofoam cup, “real” calorimeter (metal)

Temperature (T)
– Measure of average KE of sample
– Increased KE = Increased temp

Heat
– Measure of TOTAL KE of sample
– Depends on the speed of the particles, the number of particles
(the size or mass), and the type of particles in an object.

Specific Heat (s)
– Amount of energy needed to raise 1g of sample 1°C
Calorimetry Calculations
q = msΔT
 Example Problems

Heat of Reaction

Heat of Reaction (q)
– Energy given off or absorbed during a
reaction

Thermochemical Equation
– A reaction with heat information listed

Enthalpy (H)
– Measure of heat of reaction; usually per mol
of substance tested
Heat of Reaction
ΔH = change in enthalpy
 ΔH = ΔHproducts – ΔHreactants
 If ΔH is negative = exothermic
 If ΔH is positive = endothermic
 Example:

– 2H2O (g)  2H2 (g) + O2 (g) ΔH =+483.6kJ
 Positive ΔH means endothermic rxn – needs energy
Heat of Reaction

Stability of products vs. reactants
– If products have higher energy = endothermic
– If reactant have higher energy = exothermic
ENDO – PUT
IN ENERGY
T
e
m
p
Reaction Progress
EXO – ENERGY
GIVEN OFF
Heat of Reaction

Heat of Formation (ΔHf)
– Energy change when forming 1mol of a
compound from elements in most stable form

Heat of Combustion (ΔHcombus)
– Energy change during burning in O2

Heat of Reaction (ΔHrxn)
– -ΔH are ‘favored’ (expect to happen)
– Exothermic reactions
Calculating ΔH
ΔHrxn = [Σ(coefficient)(ΔHf products)] –
[Σ(Coefficient)(ΔHf reactants)]
 Practice problems (using ΔH charts)

Hess’ Law

Hess’ Law
– One big reaction that is the sum of several
smaller reactions
– Rules:
 Flip a reaction = change sign of ΔH
 Multiply by a coefficient = multiply ΔH
Hess’ Law

Practice Problems
Driving Forces for Reactions

Enthalpy (ΔH)
– Exothermic (-) are favored

Entropy (ΔS)
– Disorder
– Gas – high entropy
– More disorder (+) are favored
 Example of MORE disorder:

Boiling (l -> g)

Melting (s -> l)
Entropy

Practice Problems (Using ΔS charts)
Free Energy

Gibb’s Free Energy (ΔG)
– Available energy
– Is the FINAL answer whether a reaction will
occur or not
– ΔG = ΔH – TΔS
 Units must match!!
 If ΔG is negative  Reaction does occur
spontaneously
 If ΔG is positive  Reaction does not occur
spontaneously
Free Energy

Calculations (Using equation)
Free Energy

Calculations (Using ΔG charts)
Summary
-ΔH are favored
 +ΔS are favored
 -ΔG are SPONTANEOUS (do occur)
