Transcript Thermochemistry

Thermochemistry
Chapter 17
The Flow of Energy –
Heat and Work
Essential Question:
What is heat, and how is heat involved in
endothermic and exothermic reactions?
Energy Transformations

Energy is the capacity to do work or supply
heat.

Energy does not have mass or volume.

Energy is detected only because of its
effects.
Thermochemistry

Thermochemistry is the study of energy
changes in chemical reactions and in
changes of state.

All substances have energy stored in the
chemical bonds that hold the atoms
together.

This energy is called chemical potential
energy.
Heat ( q )

Heat is energy that transfers from one object
to another because of a temperature
difference.

Heat always flows from a warmer object to
a colder object.

This continues until the objects are the same
temperature.
Conservation of Energy

System – the focus of attention

Surroundings – everything else

Law of Conservation of Energy – in any
chemical or physical process, energy is
neither created nor destroyed.
Endothermic and Exothermic
Processes

Endothermic Process – energy is absorbed
from the surroundings. The system gains
heat as the surroundings cool down.

Exothermic Process – energy is released
into the surroundings. The system loses heat
as the surroundings heat up.
Units for Measuring Heat

Two common units – calories and joules.

Calorie = the amount of energy needed to
raise the temperature of one gram of pure
water by one degree Celsius.

1 Calorie = 1 kilocalorie = 1000 calories

1 Joule = 0.2390 cal or 4.184 Joules = 1 cal
Heat Capacity and
Specific Heat

Heat Capacity = heat needed to raise the
temperature of an object by 1 °C

Specific Heat (Capacity) = heat needed to
raise the temperature of one gram of a
substance by 1 °C.

Water has high specific heat; metals have
low specific heat.
Q = m c ΔT

Heat is equal to mass times specific heat
times the change in temperature.

Caution! Filling Is Hot!!

Don’t lick the flag pole during the winter!!
Measuring and Expressing
Enthalpy Changes
Essential Question:
How do chemical equations express heat
changes, and what law is made use of in
calorimetry?
Calorimetry

The heat released by the system is equal to
the heat absorbed by the surroundings.

The insulated device used to measure this is
called a calorimeter.

The change in heat content is called a
change in enthalpy.

Heat = enthalpy change ( q = ΔH )
Calorimetry
Heat absorbed by the surroundings is
equal to the heat lost by the system:
qsys = ΔH = −qsurr = −m × C × ΔT
Thermochemical Equations

In a chemical equation, heat can be written as
either a reactant or as a product.
Exothermic Reactions
The heat released is written as a product:
CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ
The change in enthalpy is a negative value:
CaO(s) + H2O(l) → Ca(OH)2(s)
ΔH = −65.2 kJ
Endothermic Reactions
The heat absorbed is written as a reactant:
2NaHCO3(s) + 129 kJ → Na2CO3(s) + H2O(g) + CO2(g)
The change in enthalpy is a positive value:
2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g) ΔH = 129 kJ
Heat of Combustion

…is the heat of reaction for the complete
burning of one mole of a substance.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 890 kJ
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
ΔH −890 kJ
Heat in Changes of State
Essential Question:
How does the quantity of heat absorbed by a
melting solid compare to the heat released
in solidification? and how does the heat
absorbed by a vaporizing liquid compare
with the heat released when the vapor
condenses?
Heats of Fusion and
Solidification

Heat absorbed by one mole of a solid as it
melts = molar heat of fusion

Heat lost when one mole of a liquid
solidifies = molar heat of solidification
For a given substance:
Δ Hfus = Δ Hsolid
Heats of Vaporization and
Condensation

Heat necessary to vaporize one mole of a
liquid = molar heat of vaporization

Heat released when one mole of a vapor
condenses = molar heat of condensation
For a given substance:
Δ Hvap = Δ Hcond
Heat of Solution

During the formation of a solution, heat is
either released or absorbed.