Measuring Changes of
Heat and Temperature over
time.
“It was the heat of the
moment”
Measuring heat
• We cannot measure heat directly so
we use changes in temperature as
an indirect measure.
• Heat Energy is also related to mass.
• Heat Energy is also related to the
type of substance or reaction taking
place.
Laws of Thermodynamics
• 1 – The energy in the universe is
constant
• 2 – The universe is becoming more
and more chaotic or less ordered.
• 3 – As substances cool their entropy
(or chaos) will approach zero.
Specific Heat
• The quantity of heat needed to raise
the temp. of 1 gram of a substance
1 oC.
• Units J / (g)oC
• Energy is measured in joules.
• Is related to an object’s mass.
Common Objects
SAMPLE
SPECIFIC HEAT J/goC
Water
4.1796
Gold
0.129
Iron
0.4498
Olive Oil
2.0
Heat Exchange (Change in
Temperature)
• Q = (cp) x (m) x (∆T)
• Q = change in heat energy
• cp = specific heat (relative to
sample)
• m = mass of sample
• ∆T = Tf - Ti
Exothermic v. Endothermic
• Exothermic reactions release heat.
• Endothermic reactions absorb heat.
• (Q+) = heat is absorbed by the
system
• (Q-) = heat is released by the system
Exercise
• 300 g of Gold (c = 0.129 J/goC) is
cooled from 295 K to 273 K. How
much heat is removed from the
system?
Phase Changes and Heat
• Temperature does not change
during a phase change so a new
equation is needed.
• The amount of heat needed is
dependent on two things: the
amount of the substance and what
the substance is.
• So……
Cont’d
• During a phase change…
• Q = (m) x (heat of vaporization or
fusion)
• Hv and Hf hold true in both
directions.
• Liquid water to ice = -333.5 J/g
• Ice to liquid water = 333.5 J/g
Exercise
• What is the heat of fusion for a 400g
substance that requires 500 J to
melt?