Enthalpy, Exothermic and
Endothermic Processes
Direction of heat flow in chemical reactions
Enthalpy(H)
• ENTHALPY – a measure of the flow of heat
energy under conditions of constant pressure.
HOT WATER
COLD WATER
(H = q at constant pressure)
Concept of system and surrounding
Describing the direction of heat flow
• SYSTEM – The part of the universe being
studied; in a chemical reaction the system is
chemicals.
• SURROUNDINGS – rest of the universe which
is not the system; (beaker, air, desk top, you)
EXOTHERMIC (∆H = -)
R → P + HEAT
• Exothermic Process – heat energy leaves
(exits) the system; Heat flows from the system
to the surroundings; ∆H = • Exothermic rxns feel hot
• Temp rxn > Temp surroundings
Endothermic Reaction
R + HEAT → P
• Endothermic process – heat energy flows into
(enters) system from surroundings; ∆H = +
• Endothermic reactions feel cold
• Temp rxn < Temp surr
• Key idea: Heat flows from hot to cold. Heat
flow is defined from the prospective of the
chemicals. What do the chemicals need to do
(gain or lose heat?) to return to RT
REACTION PROFILES DIAGRAMS
EXOTHERMIC
ENDOTHERMIC
Demonstration of NH4Cl in water
 Chemicals (NH4Cl + H2O) = System
 Beaker, desk top, air, you = Surroundings
 Feels: COLD
 Temp chem < T Surrounding
 Heat flows from Surroundings to System
 ENDOTHERMIC ; ∆H = +
Demonstration of CaCl2 in water
 Chemicals (CaCl2 + H2O) = System
 Beaker, desktop, air, you = Surroundings
 Feels: Hot
 Temp chem > Temp Surr
 Heat flows from system → Surroundings
 Exothermic ; ∆H = -
Hand-Warmer Demonstration
 Super-saturated solution
 Contains more sodium acetate dissolved in water volume
than is stable at room temperature.
 Prepared by heating a saturated solution to high
temperature and allowing solution to cool slowly.
Video link seed crystal
Fun with sodium Ac video
Reaction of Potassium
Permanganate and Glycerin
Initial Observations
• Thick viscous liquid is
dropped on pile of dark
purple/black crystals
Video link
Final observations
• Bright purple, flames,
smoke, very hot
Exothermic Reaction
• Temp of system >> Temp surroundings
• Heat flows from system
surrounding
∆H = -
Initial Observations
Final Observations
• Place small puddle of
water on board
• Ba(OH)2∙ 8 H2O +
NH4SCN are both white,
odorless solids
• Upon stirring mixture
becomes slushy, smell of
ammonia
• Beaker feels very cold
• Beaker sticks to board so
tightly that beaker can lift
board
Video link
Interpretation of Beaker Frozen to
Board Demonstration
Surroundings
(air)
system
Water (part of
surroundings)
T sys < T surr
Chemicals (system) are colder
HEAT
than surroundings. So much
Heat flows from water
(surroundings) to system, that
Surroundings
water freezes to beaker and
(board & water)
board together.
Reaction Profile Diagram for
Beaker/Board Demo
∆H = +
Heat Energy Changes in chemical
reactions are the result of Making and
Breaking chemical bonds
Making and Breaking chemical bonds means
changing the distance (and thus amount of
attraction) between + and – charges
Breaking Bonds Costs Energy
Energy is released when
bonds are formed
Net energy change = difference of cost of
breaking old bonds vs energy released when
new bonds are formed
2A) O2 (g) → 2 O (g)
• ∆H = + ; Bond is breaking; energy
absorbed.
2B) Cl + Cl → Cl2
• ∆H = - ; Bond is forming, energy is
released
2C) H2O (l) → H2O (g)
∆H = +
Break Hydrogen-bonds
2D) H2 (g) + O2 (g) → 2H2O
∆H = -
2D) 2H2O (l) → H2 (g) + O2 (g)
• ∆H = + ; energy added from battery