Victoria Seaton
December 15, 2009
Chemistry 12
B-Block
Mr. Therrien
Endothermic and Exothermic Reactions

An exothermic process is one that gives off heat, which is transferred to the
surroundings. An endothermic process is a reaction which heat has to be supplied
to the system from the surroundings
Endothermic Reactions
In an endothermic reaction, heat is taken from the
surroundings. The surrounding’s temperature
decreases, and the system absorbs that heat.
Exothermic Reactions
In an exothermic reaction, heat is released from the
product and raises the temperature of the
surroundings.
The ΔH˚ value will be on the reactant side of the
chemical equation.
The ΔH˚ value will be on the product side of the
chemical equation.
For an endothermic reaction:
Reactants + Heat
Products
ΔH˚ (enthalpy change) > 0
Hproduct > Hreactant
Enthalpy content has increased
For an exothermic reaction:
Reactants
Products + Heat
ΔH˚ (enthalpy change) < 0
Hproduct < Hreactant
Enthalpy content has decreased
Endothermic reactions absorb energy in the form
of heat. An example of this is: melting ice. The
process of adding heat to an object can cause a
reaction, which is an endothermic reaction.
Melting ice will turn to water, and then,
eventually, steam.
Exothermic reactions exude energy in the form of
heat. An example of this is: freezing water. The
process of taking heat away from an object can cause
a reaction, which is an exothermic reaction. Freezing
water will turn into ice.
H2O(l)
H2O(s) + Heat kJ
H2O(l)
H2O(s)
ΔH˚ = -Heat kJ
Molar Enthalpy of Fusion (Hfus)
Heat is absorbed by one mole of a certain
substance when melting at the same temperature.
Molar enthalpy of freezing is the same amount of
heat that is released when a substance solidifies.
Molar Enthalpy of Vaporization (Hvap)
Heat is absorbed by one mole of a certain substance
while vaporizing at the same temperature.
Hfus:

Hvap:


Hfr:

A positive
number
Absorbs heat

1 mole (s) + Hfus
1 mole (l) + Hfr
A negative
number
Releases heat
1 mole (l)
1 mole (s)

A positive
number
Absorbs heat
Hcond:
 A negative
number
 Releases heat
1 mole (l) + Hvap
1 mole (g) + Hcond
1 mole (g)
1 mole (l)
Phase Change q = nH
Description
Solid to Liquid
Term
Melting
Energy Flow
Endothermic
Temperature Change
Liquid to Solid
Freezing
Exothermic
None
Liquid to Gas
Vaporizing
Endothermic
Gas to Solid
Condensation
Exothermic
Phase change with no temperature change?
The energy, or heat, coming into the phase change, breaks the bonds between molecules of the substance.
If the molecules are broken, they are at a higher potential energy state. The molecules are not moving any
faster, so the average kinetic energy remains the same.
q = enthalpy change
n = moles
H = energy (heat)
Example: Calculate the enthalpy change for the melting of a 30 g ice cube.
Q = nH
X mol of H2O(s) 30g = 1 mol/18g of H2O = 1.6667 mol
= (1.6667mol)(6.01)
= 10.04 kJ/mol
Endothermic Reaction