Section 10.2

The Flow of Energy

1. To understand how heat energy flows and how it is measured

2. To understand how substances differ in their capacity to absorb and release heat

3. To solve problems involving heat energy

Learning Objectives

Section 10.2

The Flow of Energy

Recall: Exothermic vs. Endothermic processes

•Heat flows when there is a gradient•from higher to lower temperature•An exothermic process gives off heat (system loses energy)•An endothermic process absorbs head (system gains energy)

Section 10.2

The Flow of Energy

A. Thermodynamics

• Thermodynamics – study of energy • First law of thermodynamics

– Energy of the universe is constant:just as matter is conserved, energy is conserved

Section 10.2

The Flow of Energy

• Internal energy, E – sum of kinetic and potential energies of all the “particles” in a system

A. Thermodynamics

– Internal energy can be changed by two types of energy flow: • Heat (q)

• Work (w)

E = q + w

Section 10.2

The Flow of Energy

• Thermodynamic quantities always consist of 2 parts:

A. Thermodynamics

– A number (magnitude of the change)– A sign (indicates the direction of flow)

• Reflects the system’s point of view

XX

Q = energy

flowing

(heat)

Section 10.2

The Flow of Energy

B. Measuring Energy Changes - Units

• Common energy units for heat

– Calorie (c) – the amount of energy (heat) required to raise the temperature of one gram of water by 1oC.

* Note: Food calorie (C) = 1000 calories or 1kCal *

– Joule (J) – 1 calorie = 4.184 joules

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

The energy (heat) required to change the temperature of a substance depends on:

1. The amount of substance being heated (mass in g)

2. The temperature change (no. of degrees, ∆T)

∆T = Tfinal – Tinitial

cont.

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

3. The type of substance, because different substances react differently to heat:

1 calorie raises a gram of water by 1oC

1 calorie raises a gram of gold by 32oC!

Section 10.2

The Flow of Energy

Water has a relatively high specific heat – it absorbs a lot of heat for a small change in temperature…

Section 10.2

The Flow of Energy

Great Britain

Chicago Latitude: 41.9o London Latitude: 51.5o

Section 10.2

The Flow of Energy

Measuring Energy Changes – Ex.: WaterHow much energy is needed to raise the temperature of

7.40 g water from 29.0 oC to 46.0 oC?

4.184 joules (1 cal) are needed to raise one gram of water by one degree centigrade; therefore

7.40 g water require 7.40g x 4.184 joules to be heated by one degree centigrade.

Since we want an increase of 17 degrees, we must also multiply by 17.0:

4.184 J / g oC x 7.40 g x 17.0 oC = 526 J

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

• Specific heat capacity is the energy required to change the temperature of one gram of a substance by one Celsius degree.

* KNOW THE VALUE FOR WATER! *

Section 10.2

The Flow of Energy

B. Measuring Energy Changes

• To calculate the energy required for a reaction:

Q = s m t

Qlost = -Qgained