Section 5.3 Energy, Temperature Changes and Changes of State

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 JT

mass 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)