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?