1. Chapter 17 Thermochemistry
17.1 the Flow of Energy – Heat at Work
17.2 Measuring and Expressing Enthalpy Changes
17.3 Heat in Changes of State
17.4 Calculating Heats of Reaction

2. 17.1 The flow of energy – heat at work
In what direction does heat flow?
What happens in endothermic and exothermic processes?
In what units is heat flow measured?
On what two factors does the heat capacity of an object depend?

3. Energy Transformations:
Energy is the capacity for doing work or supplying heat.
Energy has neither mass or volume.
Energy is detected by its effect.
Thermochemistry – is the study of energy changes that occur
during chemical reactions and changes in state.
5. Chemical Potential Energy – The energy stored in the chemical
bonds of a substance.
6. Energy changes occur as either heat transfer or work, or a
combination of both.
7. Heat – symbol = q – energy transfers from one object to another
from higher amounts of energy to less from warm to cold

4. Exothermic and Endothermic Processes:
Chemical reactions either release or absorb heat.
The “system” is the focus of a study and concern is the flow of heat
either to or from the system.
3. The “surroundings” is the area around the system that gives or takes
heat from the system.
4. Law of Conservation of Energy – energy can not be created or
destroyed, the energy however can change form.
5. Endothermic – The system gains heat, cooler than surrounding, +q
6. Exothermic – The system loses heat, hotter than surrounding, -q

5. Units for measuring Heat:
Heat flow is measured two ways.
a. calorie
b. joule
2. A calorie (cal) is the quantity of heat needed to raise the
temperature of 1g of water 1O C
3. The word calorie is written with a small “c”
4. Dietary Calorie is written with a large “C”
5. 1Calorie = 1000calories
6. Joule – the SI unit for energy
a. 1J = cal
b J = 1cal

6. Heat Capacity and Specific Heat:
Heat Capacity – the amount of heat needed to increase the
temperature of an object exactly 1OC
2. The heat capacity of an object depends on both its mass and
chemical composition.
3. Specific Heat – is the amount of heat it takes to raise the
temperature of 1g of a substance 1OC
4. Water has a large specific heat. What does that mean?
5. To calculate Specific heat
C = q/m x DT
6. What are the units of specific heat?

7. In what direction does heat flow?
What happens in endothermic and exothermic processes?
In what units is heat flow measured?
On what two factors does the heat capacity of an object depend?

8. 17.2 Measuring and Expressing Enthalpy Changes:
What basic concepts apply to calorimetry?
How can you express the enthalpy change for a reaction in a
chemical equation?

9. qsys = DH = -qsurr = -m x C x DT
Calorimetry:
Is the precise measurement of the heat flow into and out of a system
for chemical or physical processes.
The heat released by the system is equal to the heat absorbed by
its surroundings
Calorimeter – a device used to measure the release or absorption of
heat in chemical or physical processes.
Enthalpy – (H) The heat content of a system at constant pressure
Change in enthalpy – (DH) The heat released or absorbed by a reaction
at constant temperature. q = DH
Equations:
qsurr = m x C x DT
qsys = DH = -qsurr = -m x C x DT
The sign of DH is negative for an exothermic reaction and positive for
an endothermic reaction.

10. Constant – Volume Calorimeters:
Bomb Calorimeter – a device used for calorimetry experiments in
which the volume remains constant.

11. Thermochemical Equations:
In a chemical equation, the enthalpy change for the reaction can
be written as either a reactant or a product.
2. Thermochemical Equation: an equation that includes the
enthalpy change.
3. Heat of Reaction: is the enthalpy change for the chemical
equation as it is exactly written.
4. Problems involving enthalpy changes are similar to stoichiometry
problems.
5. Heat of Combustion: is the heat of reaction for the complete
burning of one mole of a substance.

12. What basic concepts apply to calorimetry?
How can you express the enthalpy change for a reaction in a chemical equation?

13. 17.3 Heat in Changes of State:
How does the quantity of heat absorbed by a melting solid compare to
the quantity of heat released when the liquid solidifies?
-How does the quantity of heat absorbed by a vaporizing liquid compare
to the quantity of heat released when the vapor condenses?
-What thermochemical changes can occur when a solution forms?

14. Heats of Fusion and Solidification:
All solids absorb heat as they melt to become liquids.
A gain of heat causes a change in state instead of a change
in temperature.
3. Molar Heat of Fusion: the heat absorbed by one mole of a solid as
it melts to a liquid at a constant temperature. DHfus
4. Molar Heat of Solidification: the heat lost when one mole of a
liquid solidifies at a constant temperature. DHsolid
5. The quantity of heat absorbed by a melting solid is exactly the
same as the quantity of heat released when the liquid solidifies.
D Hfus = - DHsolid

15. Heats of Vaporization and Condensation:
When liquids absorb heat at their boiling points, they become vapors.
Molar Heat of Vaporization: the amount of heat necessary to vaporize
one mole of a given liquid. DHvap
3. When vapor condenses, heat is released.
4. Molar Heat of Condensation: the amount of heat released when 1 mole
of vapor condenses at the normal boiling point. DHcond
5. The quantity of heat absorbed by a vaporizing liquid is exactly the
same as the quantity of heat released when the vapor condenses.
DHvap = -DHcond

16. Heat of Solution:
During the formation of a solution, heat is either released or
absorbed.
2. Good examples; hot and cold packs
3. Molar Heat of Solution: the enthalpy change caused by the
dissolution of one mole of a substance.

17. -How does the quantity of heat absorbed by a melting solid compare to the quantity of heat released when the liquid solidifies?
-How does the quantity of heat absorbed by a vaporizing liquid compare to the quantity of heat released when the vapor condenses?
-What thermochemical changes can occur when a solution forms?

18. 17.4 Calculating Heats of Reaction:
-What are the two ways that you can determine the heat of reaction
when it cannot be directly measured?

19. Hess’s Law:
Hess’s Law: allows you to determine the heat of reaction indirectly
so that we can determine the enthalpy change.
2. Hess’s Law of Heat Summation: states that if you add two or more
thermochemical equations to give a final equation, then you also
add the heats of reaction to give the final reaction.

20. Standard Heats of Formation:
Enthalpy changes depend on conditions of the process.
Standard State: a common set of conditions used as a reference
point that refer to the stable form of a substance at 25OC
and 101.3kPa
3. Standard Heat of Formation: is the change in enthalpy that
accompanies the formation of one mole of a compound from its
elements with all substances in their standard states. DHfO
4. The standard heat of formation for diatomic molecules = 0
5. For a reaction that occurs at standard conditions, you can
calculate the heat of reaction by using standard heats of
formation and is called the standard heat of reaction.
DHO = D HfO (products) DHfO (reactants)