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

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Presentation transcript:

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. 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. 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 o C. Units J / (g) o C Energy is measured in joules. Is related to an object’s mass. The quantity of heat needed to raise the temp. of 1 gram of a substance 1 o C. Units J / (g) o C Energy is measured in joules. Is related to an object’s mass.

Common Objects SAMPLESPECIFIC HEAT J/g o C Water Gold0.129 Iron Olive Oil2.0

Heat Exchange (Change in Temperature) Q = (c p ) x (m) x (∆T) Q = change in heat energy c p = specific heat (relative to sample) m = mass of sample ∆T = T f - T i Q = (c p ) x (m) x (∆T) Q = change in heat energy c p = specific heat (relative to sample) m = mass of sample ∆T = T f - T i

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 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 = J/g o C) 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…… 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) H v and H f hold true in both directions. Liquid water to ice = J/g Ice to liquid water = J/g During a phase change… Q = (m) x (heat of vaporization or fusion) H v and H f hold true in both directions. Liquid water to ice = J/g Ice to liquid water = J/g

Exercise What is the heat of fusion for a 400g substance that requires 500 J to melt?