Download Thermochemistry

Thermochemistry
• Thermochemistry is the study of heat
changes that occur during chemical
reactions.
• Heat (q) - energy that is transferred
from one object to another because of a
temperature difference between them.
• Heat always flows from a warmer object to
a cooler object.
• Heat cannot be measured directly.
• Temperature – the average kinetic
energy of the particles in a sample of
matter
• Energy - the capacity for doing work or
supplying heat.
Energy in Reactions
• Endothermic process - system absorbs
heat from the surroundings. (Energy
must be added to the reaction for the
reaction to take place)
• Exothermic process - system releases
heat to the surroundings.
Ex. Endothermic or exothermic process?
-evaporating alcohol
-leaves burning
-boiling water
-water cooling
-melting ice
-freezing water
To calculate the heat energy required for a
temperature change, use the following
formula:
Q
=
(m)(C)(DT)
Q = heat energy absorbed or released (J)
• + q = endothermic (heat absorbed)
• - q = exothermic (heat released)
m = mass (g)
C = specific heat of the object (J/goC)
•
•
DT=
The amount of heat it takes to raise the
temperature of 1 g of a substance 1oC.
Water has a uniquely high specific heat
compared to other substances.
change in temperature of object (oC)
Example
• How much energy is required to
heat an iron nail with a mass of 7.0
g from 25oC until it becomes red hot
at 752oC?
C Fe= 0.46J/goC
DT = 752-25 = 727oC
q = mC DT
q = 7.0g(0.46J/goC) 727oC
q = 2300 J
Phase Change
• Heat energy can be used to not only
change the temperature of matter, but
also its phase.
• The energy goes into separating or
organizing the molecules into a new state
• The amount of heat energy necessary to
cause a phase change can be calculated
using the formula:
Q = mLf (solid/liquid)
or
Q = mLv (liquid/gas)
• Q = heat required for phase change (J)
• m = mass (kg)
• Lf = latent heat of fusion (J/kg)
• Energy required to change one kilogram of a
material from a solid to a liquid
• Water Lf = 334,000 (J/kg)
• Lv = latent heat of vaporization (J/kg)
• Energy required to change one kilogram of a
material from a liquid to a gas
• Water Lv = 2,260,000 (J/kg)
Example
• Find the amount of heat needed
for melting 1.3 kg ice at 0 ºC?
Notice you calculate each
change separately and
then add them all up.
Enthalpy (D H)
• The amount of heat energy
absorbed or lost by a system
• Exothermic reactions have - D H
• Endothermic reactions have + D H
• D H is also called the heat of
reaction.
Calculating Heat Changes
• Bond energy is the amount of energy
required to break or make a bond
• Breaking a bond is endothermic (+ D H)
• Forming a bond is exothermic (- D H)
• D H = Hproducts – Hreactants
Example 1
• Calculate the energy of the reaction
using heats of formation.
H2(g) + Br2(g)  2 HBr(g)
•
•
•
•
•
•
D H HBr = -36.29 kJ/mol
All elements and diatomic molecules
have an enthalpy of zero.
D H = Hproducts – Hreactants
D H = [2 x -36.29 kJ/mol] – [0]
D H = -72.58 kJ/mol
Exothermic
Example 2
• Calculate the energy of the reaction
using heats of formation.
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
• D H CH4 = -74.9 kJ/mol
• D H CO2 = -393.5 kJ/mol
• D H H2O = -285.8 kJ/mol
• D H = Hproducts – Hreactants
• D H = [-393.5 kJ/mol + (2 x -285.8
kJ/mol)] – [-74.9 kJ/mol + (2 x 0)]
• D H = -890.2 kJ/mol
• Exothermic