Chapter 17 –
Thermochemistry
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-is the study of energy ( heat)
relationships in chemical and physical
reactions
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• Thermochemistry is concerned with
the flow of heat from the system to
it’s surroundings, and vice-versa.
• almost nothing happens in chemistry
unless there is an energy advantage; all
reactions are an attempt to arrive at a
lower energy within the system.
a) 1st law of Thermodynamics: Law of
conservation of energy.
• Energy is never created nor destroyed , it
only changes form.
This law states that energy can change form
but is never really lost in any closed system.
b) 2nd law of Thermodynamics:
• Heat Energy always travels spontaneously
from a warmer body (body with higher
temperature) to a colder body.
-dropping a hot rock in water
-frostbite
• temperature is simply an indirect
measure of the average kinetic energy
of the particles in a sample of matter.
i)
Celsius scale sets the zero
value at the freezing point of water at
101.325 kPa and 100 at the boiling point
of water at the same pressure.
ii)
Kelvin scale sets the zero value at
absolute zero.
Remember: 0K = -273.15 °C
K = t + 273.15 °C
Energy and Heat
• Energy - capacity for doing work or
supplying heat
– if within the chemical substancescalled chemical potential energy
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• Heat - represented by “q”, is energy
that transfers from one object to
another, because of a temperature
difference between them.
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• Essentially all chemical reactions, and
changes in physical state, involve
either:
– release of heat, (exothermic) or
– absorption of heat (endothermic)
• heat flowing into a system from its
surroundings:
– q has a positive value
– called endothermic
– Absorbs in the form of heat
• system gains heat as the
surroundings cool down
• heat flowing out of a system into it’s
surroundings:
– q has a negative value
– called exothermic
– Releases in the form of heat
• system loses heat as the
surroundings heat up
• 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
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Heat Capacity and Specific
Heat
• A calorie is defined as the quantity of
heat needed to raise the temperature
of 1 g of pure water 1 oC.
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• The calorie is related to the joule, the
SI unit of heat and energy
– 4.184 J = 1 cal
• Heat Capacity - the amount of heat
needed to increase the temperature
of an object exactly 1 oC
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• Specific Heat Capacity - the
amount of heat it takes to raise the
temperature of 1 gram of the
substance by 1 oC often called
simply “Specific Heat”
• mass; measured in g or kg. The greater
the mass, the more heat required to
change the temperature.
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• For water, C = 4.18 J/(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!
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To calculate, use the formula:
q = m C T
heat abbreviated as “q”
m = mass
T = change in temperature (Tf - Ti)
C = Specific Heat
Units are either J/(g oC) or kJ/(kg oC)
– J/(g K) or kJ/(kg K)
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Table of Specific Heats
Ex: Determine the heat required to raise the
temperature of 100.g of water from 298.0 K to
373.0 K .
Q = m c ΔT
Q =100.g (4.184 J/g K)( 373.0 K –298.0 K)
Q = 418.4 J/K (75 K)
Q = 31350 J
Q = 31.4 kJ
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Molar heat capacity
• Cp0
• of a substance is the amount of energy
required to raise the temperature of one
mole of a substance by one Celsius
degree.
• Molar heat capacity
• = (specific heat) (molar mass)
• Calculate the molar heat capacity of
water, given that the specific heat water
is 4.184 J/g°C.
Cp0 = (specific heat)(molar mass)
= (4.184 J/g°C)(18.02g/mol)
= 75.40 J/mol °C