Chapter 17
Thermochemistry
17.1 The Flow of Energy
• Thermochemistry - study of heat changes
during chemical and physical reactions.
– System - what you are looking at
– Surroundings - everything else
– Universe = system + surroundings
• Endothermic – heat energy in, system absorbs heat
from the surroundings.
• Exothermic – heat energy out, system loses heat to
surroundings
17.1 The Flow of Energy
•
Energy - capacity to do work or supply
heat.
1. potential energy - stored energy
a. positional - where its at
b. compositional - what it is
– chemical potential energy energy stored in chemicals
17.1 The Flow of Energy
2. kinetic energy - energy of motion
a. heat (q) - energy transferred between two
objects of differing temperature.
• Enthalpy (H) - heat content of a system
at constant pressure.
• q (at constant pressure) = H
b. light, electrical energy, mechanical, ...........
How does the dam energy work?
1.
2.
3.
4.
5.
6.
Mechanical energy of moving water
Potential energy of dammed water
Mechanical of falling water
Mechanical of moving turbine
Mechanical energy of spinning magnetic in generator
Electrical energy of moving electrons
17.1 The Flow of Energy
Law of conservation of energy - in ordinary
reactions energy is not created or destroyed
but converted from one form to another.
Temperature verses Heat
What contains more heat?
a glass of boiling water or an iceberg
What does your body sense?
temperature or heat
Temperature verses Heat
• Heat – type of energy transferred because of
a difference in temperature.
– Can’t be measured directly
• Temperature – measure of the average
kinetic energy of the particles in a sample of
matter.
– Determines the direction of heat transfer
17.1 Units of Energy
• calorie (cal) - heat needed to raise 1 g of
water 1 C.
• Calorie (Cal) - food Calorie - 1000 cal
• Joule (J) - SI unit of energy
– 1Cal = 1kcal = 1000 cal = 4184 J = 4.184 kJ
Specific Heats of
common substances
17.1 Specific Heat
• Heat Capacity – heat it takes to
change a substances temperature by
1 C.
• Specific heat – amount of heat
required to raise the temperature of
1 g of a substance 1ºC.
– Characteristic intensive property of a
substance.
– A substance with a high specific heat is
harder to heat up.
Material
Specific
Heat
Capacity
[J/(gC)]
Air
1.00
Aluminum
0.897
Brass
0.380
Carbon
dioxide
0.832
Copper
0.387
Ethyl
alcohol
2.44
Gold
0.129
Granite
0.803
Ice
2.03
Iron
0.449
Lead
0.129
Silver
0.235
Steam
2.01
Water
4.184
Zinc
0.386
Specific Heats of
common substances
17.1 Specific Heat
• Which has the highest on the list?
– Water!!
• This allows bodies of water to store
large quantities of energy.
– Cities by water have cooler
summers, warmer winters, and
wind!
• Land has a much lower specific heat
capacity
Material
Specific
Heat
Capacity
[J/(gC)]
Air
1.00
Aluminum
0.897
Brass
0.380
Carbon
dioxide
0.832
Copper
0.387
Ethyl
alcohol
2.44
Gold
0.129
Granite
0.803
Ice
2.03
Iron
0.449
Lead
0.129
Silver
0.235
Steam
2.01
Water
4.184
Zinc
0.386
17.2 Enthalpy Change
To study the energy changes that accompany
reactions, chemists use a property called enthalpy.
– Enthalpy (H) - heat content of a system at constant
pressure.
Although you cannot measure the actual energy or
enthalpy of a substance, you can measure the
change in enthalpy, which is the heat absorbed or
released in a chemical reaction.
q (at constant pressure) = H
17.2 Enthalpy Change
Enthalpy change of a reaction (Hrxn)
Hrxn = Hfinal - Hinitial
Because reactants are present at the beginning
and products are at the end,
Hrxn = Hproducts - Hreactants
H = positive(+) is endothermic
H = negative(-) is exothermic
17.2 Enthalpy Change
Exothermic Reaction (H = –)
Endothermic Reaction (H = +)
Products
Energy
Energy
Reactants
Products
Reactants
The energy is a measure of the chemical
potential energy of the reactants and
products!
17.2 Thermochemical Equations
• All reactions involve energy.
• Thermochemical equation – balanced
chemical equation that includes states and
energy.
2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g) + 10942kJ
Endothermic - energy a reactant
Exothermic - energy is a product
17.2 Heat of reaction
• Heat of reaction – energy change for a
chemical reaction exactly as it’s written.
– Energy is written outside of the reaction
– In a heat of reaction, sign determines if a
reaction is endothermic or exothermic!
2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g) H= -10942kJ
Is this reaction endothermic or exothermic?
Exothermic!
17.2 Heat of reaction
• Heat of reaction – energy change for a
chemical reaction exactly as it’s written.
2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g) H= -10942kJ
How much energy is released if 4 moles of octane
(C8H18) burns?
−10942 kJ
4 mol C8 H18 ×
=
2 molC8 H18
21884 kJ