PHASE CHANGES

9-7. Solve problems involving heat flow and temperature changes, using
known values of specific heat (q= m ●c ● Δ T) & latent heat of phase
change (q= m Δ H). (CSS: 7d)
PHASE CHANGES

Phases of Matter

Phases of Matter
Solid Liquid Gas
•Motion of particles is
restricted (only
vibrational)
•Particles are more
closely packed
•Lowest kinetic energy
•Strong intermolecular
forces
•Have definite volume
•Particles are in constant
motion
•Lower kinetic energy
than gas particles
•Attractive forces
between particles effect
their behavior
•Particles are not fixed,
but are closer together
than in a gas
•Particles are in
constant, rapid motion
•Highest kinetic energy
state of matter
•Attractive forces do not
effect their behavior
•Particles are far apart

Phase Change Names
“Fusion”
“Vaporization”

Why phase changes require heat
energy
Heat is required for phase changes because it
breaks/ weakens the attraction between
molecules.

ΔH = Change in Heat (aka Enthalpy)
∆H fusion = “Latent Heat of
Fusion”
• heat required to melt or
freeze a substance
• +ΔH = melting
(endothermic)
• ─ΔH = freezing
(exothermic)
∆H vaporization = “Latent Heat
of Vaporatization”
• heat required to vaporize
or condense a substance
• +ΔH = vaporization
(endothermic)
• ─ΔH = condensation
(exothermic)

Phase Diagram (water)

Heat = Temperature
As you can see from the graph (plateaus) , it is possible to add
heat to water, but see no temperature change.
Two things can happen when you heat a substance:
1) The substance can experience a raise in temperature. That
is, the heat can be used to speed up the molecules of the
substance.
2) The substance can change state. Although heat is absorbed by
this change of state, the absorbed energy is not used to speed
up the molecules. The energy is used to change the bonding
between the molecules.

Latent Heat of Fusion (Δ H Fus ) for
Water
• Amount of heat (q) required to change a mass (m) of a
substance from a solid to a liquid or liquid to a solid:
Energy (J or cal)
q = m ·Δ H Fus
Mass (g)
“Latent Heat of
Fusion”
(J/g or cal/g)
Δ H fus water = + 334 J/g
Δ H fus water = - 334 J/g

Latent Heat of Vaporization for
Water
• Amount of heat (q) required to change a mass (m) of
a substance from a liquid to a gas or gas to a liquid:
q = m ·Δ H Vap
Δ H vap water = + 2260 J/g
Δ H vap water = ─2260 J/g

Fusion and Vaporization for Water
q= mΔ H vap= +/- 2260 Joules/ g
q= m Δ H fus = +/- 334 Joules/ g
For (a- b) (c-d) & (e-f) on graph
use : q = m · c · Δ T
To find energy needed to raise temperature

Sample Problem: How much heat in KJ is required to
change 50 grams of ice at -10 C to a liquid at 40 C?
3
2
1
Step 1: Find heat it takes to raise ice from -10 ° C to 0 ° C (q = m · c · Δ T)
q= (50g) (4.18 J/g ° C ) (10 ° C ) = 2090 J
Step 2: Find heat to do a phase change from solid- liquid (q = m Δ H Fus)
q = (50 g) (334 J/ g) = 16700 J
Step 3: Find heat it takes to raise temp from 0 ° C to 40° C (q = m · c · Δ T)
q= (50 g) (4.18 J/g ° C ) (40 ° C )= 8360 J
Total (add steps 1+2+ 3)= 27150 J or 27.150 KJ