Download Thermochemistry

Thermochemistry
• Heat
• a form of energy.
• can be transferred between samples
• heat flows from matter at a higher temperature to
matter at a lower temperature
• Temperature
• a measure of the average kinetic energy of the
particles in a sample.
Thermochemistry
• Units of Heat
• Joule (SI unit)
• calorie
• cal
• the amount of energy required to raise the
temperature of one gram of water one degree Celsius.
• Calorie
• Cal
• a dietary calorie.
• kilocalorie, kcal (1,000 calories)
1 cal = 4.184 Joules
Thermochemistry
• Enthalpy
• the heat content of a system
• represented by H
• only changes in enthalpy can be measured
• ∴ ΔH is used
Thermochemistry
• Specific heat capacity, cp
• the amount of energy required to raise the
temperature of one gram of a substance one
degree Celsius
• used in equation
q = m x cp x ΔT
Thermochemistry
q = m x cp x ΔT
Energy,
or heat
(J)
mass
(g)
specific heat
(J/g•°C)
Δ temp
(°C)
• note that in the Metric System, Joules are the unit of
measure for heat.
Heat capacity, cp
For water, C = 4.18 J/(g oC),
and also C = 1.00 cal/(g oC)
Thus, for water:
it takes a long time to heat
up, and
it takes a long time to cool
off!
Water is used as a coolant!
q = m x cp x ΔT
1. A 45.0-gram sample of iron is heated from 25.0°C to
50.0°C. How much energy is required? (cp iron = 0.449
J/g°C)
q=?
m = 45.0 g
cp = 0.449 J/g°C
ΔT = 50.0°C – 25.0°C = 25.0°C
q = m cp ΔT
q = 45.0g (0.449 J/g°C) (25.0°C)
= 505.125 J
q = 505 J
2. What is the specific heat capacity of an object if a 12.5-gram
sample is heated from 12.0°C to 28.0°C using 100.0
joules?
q = 100.0 J
m = 12.5 g
cp = ?
ΔT = 28.0 – 12.0 = 16.0°C
q = m cp ΔT
q
cp =
mΔT
100 .0J
cp =
12.5g(16.0 C)
J
cp = 0.500 
gC
Heat - represented by “q”, is energy that
transfers from one object to another, because of
a temperature difference between them.
In studying heat changes, think of defining these
two parts:
the system – the part of the universe you
focus your attention on
the surroundings – everything else
If heat flows into a system from the
surroundings, the system gains energy, and
the change is said to be endothermic. Heat
has a positive value.
If heat flows out of a system to the
surroundings, the system loses heat, and the
change is said to be exothermic. Heat has a
negative value.
Every reaction has an energy change associated
with it.
Exothermic reactions release energy, usually in
the form of heat.
Endothermic reactions absorb energy.
Energy is stored in bonds between atoms.
The Law of Conservation of
Energy states that in any
chemical or physical process,
energy is neither created nor
destroyed.
All the energy is accounted for
as work, stored energy, or
heat.
Calorimetry - the accurate and precise
measurement of heat change for chemical and
physical processes.
For systems at constant pressure, the heat
content is the same as a property called
Enthalpy (H) of the system.
Changes in enthalpy = H
q = H These terms will be used
interchangeably.
Thus, q = H = m x C x T
H is negative for an exothermic reaction.
H is positive for an endothermic reaction.
Energy
C + O2  CO2 + 395 kJ
C + O2
395kJ
C O2
Reactants

Products
16
THIS IS AN EXOTHERMIC REACTION.
THE CHEMICAL BONDS OF THE PRODUCTS
CONTAIN LESS CHEMICAL POTENTIAL
ENERGY THAN THE BONDS OF THE
REACTANTS.
THE SYSTEM GIVES OFF ENERGY TO THE
SURROUNDINGS.
H IS NEGATIVE.
ANOTHER WAY OF SHOWING THIS IS THE
ENERGY CHANGE IS SHOWN AS A PRODUCT.
Energy
CaCO
kJ +CaO
CaCO
 CaO
CO2+ CO2
3 + 3176
CaO + CO2
176 kJ
CaCO3
Reactants

Products
18
THIS IS AN ENDOTHERMIC REACTIION.
THE CHEMICAL BONDS IN THE PRODUCTS
HAVE MORE CHEMICAL POTENTIAL ENERGY
THAN THE CHEMICAL BONDS IN THE
REACTANTS.
THE SYSTEM GAINS ENERGY FROM THE
SURROUNDINGS.
H IS POSITIVE.
Chemistry Happens in
MOLES
 An
equation that includes energy is
called a thermochemical equation
 CH4 + 2O2  CO2 + 2H2O + 802.2 kJ
 1 mole of CH4 releases 802.2 kJ of
energy.
 When you make 802.2 kJ you also
make 2 moles of water
Thermochemical Equations
A
heat of reaction is the heat
change for the equation,
exactly as written
• The physical state of reactants
and products must also be
given.
• Standard conditions for the
reaction is 101.3 kPa (1 atm.)
and 25 oC
21
CH4 + 2 O2  CO2 + 2 H2O + 802.2 kJ
 If 10.
3 grams of CH4 are burned
completely, how much heat will be
produced?
10. 3 g CH4
1 mol CH4
16.05 g CH4
802.2 kJ
1 mol CH4
= 514 kJ
22
SPECIAL THANKS TO: WWW.
pa016.k12.sd.us/Chapter%2011%20revised.ppt
THERMOCHEMISTRY
THE STUDY OF ENERGY
CHANGES THAT ACCOMPANY
CHEMICAL AND PHYSICAL
CHANGES