Measuring Changes of Heat and Temperature over time. “It was the heat of the moment”
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Transcript of Measuring Changes of Heat and Temperature over time. “It was the heat of the moment”
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 ObjectsSAMPLE 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?