Section 5.3 Energy, Temperature Changes and Changes of State
Feb 10, 2016
Section 5.3 Energy, Temperature Changes
and Changes of State
In this section:
a.heat transfer: specific heat capacityb.calculating temperature changesc.changes of stated.heating curves
HEAT: What happens to thermal (heat) energy?
Three possibilities:• Warms another object• Causes a change of state• Is used in an endothermic reaction
Heat is added to a substance; how much does its temperature increase?
Specific heat capacity: energy required to raise the temperature of a 1 g sample by 1 oC
Determining specific heat capacity
Using the following data, determine the specific heat capacity of silver.
q = 150 J m = 5.0 g Ag Tfinal = 145.0 °C Tinitial = 20.0 °C
Trends in specific heat capacity
Heat and Temperature:Key Equation
# ( / ) ( ) ( )o oJ specific heat J g C mass g T C
Calculating temperature change.
( )( ) ( / )o
heat JTmass g specific heat J g C
Calculate the final temperature reached when 324 J of heat is added to a 24.5-g iron bar initially at 20.0 °C.
What happens to thermal (heat) energy?
When objects of different temperature meet:
• Warmer object cools• Cooler object warms• Thermal energy is transferred
•qwarmer = -qcooler
specific heat x mass x T = specific heat x mass x T
warmer object cooler object
Heat transfer between substances:
woodq =
Cuq =
Specific heats: Cu = 0.385 J/goC Wood = 1.8 J/goC
Predicting Thermal Equilibrium: Conceptually Easy Example:
If we mix 250 g H2O at 95 oC with 50 g H2O at 5 oC, what will the final temperature be?
HEAT: What happens to thermal (heat) energy?
Three possibilities:• Warms another object• Causes a change of state• Is used in an endothermic reaction
Thermal Energy and Phase ChangesFirst: What happens? Heating Curve
Thermal Energy and Phase Changes
Thermal Energy and Phase Changes
Warming:
• Molecules move more rapidly
• Kinetic Energy increases
• Temperature increases
Melting/Boiling:
• Molecules do NOT move more rapidly
• Temperature remains constant
• Intermolecular bonds are broken
• Chemical potential energy (enthalpy) increases
But what’s really happening?
Energy and Phase Changes: Quantitative TreatmentMelting:
Heat of Fusion (Hfus) for Water: 333 J/g
Boiling:
Heat of Vaporization (Hvap) for Water: 2256 J/g
Total Quantitative AnalysisHow much energy is required to convert 40.0 g of ice at –30 oC to steam at 125 oC?
Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC
Warm ice: (Specific heat = 2.06 J/g-oC)
Melt ice:
Warm water (s.h. = 4.18 J/g-oC)
Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC
Boil water:
Warm steam (s.h. = 1.92 J/g-oC)