1. Chapter 17
Thermochemistry

2. 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

3. 17.1 The Flow of Energy Energy - capacity to do work or supply heat.
potential energy - stored energy
positional - where its at
compositional - what it is
chemical potential energy - energy stored in chemicals

4. 17.1 The Flow of Energy kinetic energy - energy of motion
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,

5. How does the dam energy work?
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

6. 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.

7. Temperature verses Heat
What contains more heat?
a glass of boiling water or an iceberg
What does your body sense?
temperature or heat

8. 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

9. 17.1 Units of Energy
calorie (cal) - heat needed to raise 1 g of water 1 C.
Calorie (Cal) - food Calorie cal
Joule (J) - SI unit of energy
1Cal = 1kcal = 1000 cal = 4184 J = kJ

10. Specific Heats of common substances 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
Silver
0.235
Steam
2.01
Water
4.184
Zinc
0.386
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.

11. Specific Heats of common substances 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
Silver
0.235
Steam
2.01
Water
4.184
Zinc
0.386
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

12. q (at constant pressure) = H
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

13. 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

14. 17.2 Enthalpy Change
Energy
Reactants
Products
Exothermic Reaction (H = –)
Energy
Reactants
Products
Endothermic Reaction (H = +)
The energy is a measure of the chemical potential energy of the reactants and products!

15. 17.2 Thermochemical Equations
All reactions involve energy.
Thermochemical equation – balanced chemical equation that includes states and energy.
2C8H18(g) O2(g)  16CO2(g) H2O(g) kJ
Endothermic - energy a reactant
Exothermic - energy is a product

16. 17.2 Heat of reaction Exothermic!
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) O2(g)  16CO2(g) H2O(g) H= kJ
Is this reaction endothermic or exothermic?
Exothermic!

17. 17.2 Heat of reaction
Heat of reaction – energy change for a chemical reaction exactly as it’s written.
2C8H18(g) O2(g)  16CO2(g) H2O(g) H= kJ
How much energy is released if 4 moles of octane (C8H18) burns?
4 mol C 8 H 18 × −10942 kJ 2 mol C 8 H 18
= 21884 kJ