Download Thermochemistry Ch. 20

Thermochemistry
Ch. 20
Chemical Energy
• Thermochemistry- study of
energy released/absorbed
during chemical reactions
• Transfer of heat between
the system and its
surrounding
• System- the reaction being
observed
• Surroundings- everything
the system is in contact with
• Universe Systems +
Surroundings
What is Energy?
• Types of energy:
–
–
–
–
Potential vs. Kinetic
Radiant
Thermal
Chemical
• Energy- ability to do
work/supply heat
• Heat (q)- movement of
energy from higher
concentrations (Hot) to
lower concentrations
(Cold)
Enthalpy
• Enthalpy (H)- potential heat
contained in a substance or a
system
• ΔH- transfer of heat energy
under constant pressure;
measured in kilojoules (kJ)
• +ΔH absorption of energy by
the system from the
surroundings
Endothermic
• -ΔH release of energy by the
system to the surroundings
Exothermic
Endothermic Reactions
• A + B + Energy C +D; +ΔH
• Products have more
energy than the reactants
• Products need to absorb
energy to form; energy
supply must be constant
• Electrolysis of H2O:
2H2O 2H2 + O2
– H2 and O2 have more energy
than H2O
– Once the energy is cut off,
the reaction stops
ΔH +572 kJ
Exothermic Reactions
• A + B  C + D + Energy; -ΔH
• Reactants have more energy
than products
• As reactants breakdown to
form produces, excess energy
is released (heat or work)
• Hydrogen-Oxygen Fuel Cell
2H2 + O2  2H2O
– Redox reaction powers battery
– Electricity is used to do work;
some energy lost as heat
ΔH -572 kJ
What does the ΔH of the forward
and reverse reaction prove?
Law of Conservation of Energy
Starting a Reaction
• All reactions need additional
energy to occur
• Activation energy- energy
required to start a reaction
• Endothermic:
AE  energy supplied till
product is formed
• Exothermic:
AE energy supplied till reaction
is self-sustaining
If exothermic reactions have an
AE, why are the exothermic?
Overall ΔH is negative;
energy released is larger
than AE
Lowering AE
• High AE can limit when
or how a reaction can
occur
• Catalysts:
– Lower the AE by making
the reaction more
efficient
– Lowers the “randomness”
factor
3H2 + N2 –Fe 2NH3
How do catalysts support
complex life on Earth?
Enzymes (biological
catalysts) speed up
reactions in a cell
Order vs. Disorder
• All reactions are controlled by
two simple rules:
1) Systems move from high energy to
low energy
ex. Movement of heat
2) Systems move from order to
disorder
ex. Smashed glass
• Entropy (S): degree of disorder in
a system
– Is NOT conserved; lost order is not
recovered
– Motivates reactions to happen
spontaneously
– Increases with # mols, # molecules
formed, phases changes (melting
or evaporation)
Spontaneous vs. Non-Spontaneous
• Both exothermic and endothermic reactions
can be spontaneous
• Spontaneity determined by level of entropy or
energy:
Reaction Type
Entropy
Spontaneous?
Exothermic (losing energy)
increasing
Yes
Exothermic (losing energy)
decreasing
Yes at low temps
Endothermic (gaining energy)
increasing
Yes at high temps
Endothermic (gaining energy)
Decreasing
No
Measuring Energy
• Calorimeter: measures
change in temperature of
a liquid surrounding a
thermochemical reaction
H20 4.184 J/g∙oC
• Specific Heat Capacity (C): the
amount of heat needed to
raise the temp of 1 g of
substance 1oC; J/g∙oC
• Low C substance heats
up/cools down quickly
• High C substance heats
ups/cools down slowly
Specific Heat Capacity (C)
• Metals have low C; Non-metals have high C
• C= q / (m)(ΔT)
q= heat absorbed by the substance; J
m= the mass of the substance; g
ΔT= change in temp of the substance; oC
q= 794 J m= 89.1 g ΔT=51.1oC-22.0oC= 29.1oC
C= 794/(89.1)(29.1)= 0.306 J/g∙oC
Using a Calorimeter
• Heat (q) from the reaction will be absorbed by the
water; so using the Specific Heat Capacity of
water, we can calculate the energy of the reaction
• qwater= (m)(ΔT)(Cwater)
m= mass of the water; g
ΔT= temp change of the water; oC
Cwater= 4.184 J/g∙oC
• The heat absorbed by the water is the energy
released by the reaction
qreaction = -(qwater)
+qreaction Endothermic -qreaction Exothermic
Calorimeter Practice
• A 1.75 g sample of acetic acid, CH3CO2H, was burned in
oxygen in a calorimeter. The calorimeter contained 925
g of water its contents increased from 22.2oC to 26.5oC.
What is the molar heat of combustion of acetic acid?
• qwater= (mwater)(ΔT)(Cwater)  (925g)(26.5-22.2)(4.184)
• qwater= 16,641.86 J
• qreaction= - qwater  -16,641.86 J -16.6 kJ
60.5g CH CO H
qreaction= -16.6 kJ__________
= 575.3 kJ/mol
1.75 g CH CO H 1mol CH CO H
3
3
2
3
2
2
Energy in Food
• Body burns certain
amount of energy
everyday
– Height/Weight
– Activity Level
– Male/Female
• calorie: 4.184 J
• kilocalorie: 1000
calories
• Calorie: energy unit
for food; 4.184 kJ
• If you are going for a
long hike what food
would you bring?
Economics of Energy
• Redox reaction
electricity
• Thermochemistry heat
• No energy system is 100%
efficient; most energy lost
through heat
• Modern systems are based
on fossil-fuels which are
only 63% efficient
• Each stage of energy
captures lowers that 63%
more through their own
inefficiencies
• Modern coal plant is only
36% efficient
Homework
• Group PPTs will look into the modern methods of
conserving energy and alternative energy sources:
1) Recycling
2) Clean Coal Burning
3) Solar Energy
4) Geothermal
5) Wind Energy
6) Nuclear Power
-Present on the pros and cons of the technology
-How does the energy production compare to Fossil
Fuels
-How easily does the technology fit into society