Chapter 6 Thermochemistry: Energy Flow and Chemical

Change

Outline

1. Forms of Energy and Their Interconversion

2. Enthalpy: Heats of Reaction and Chemical

Change

3. Calorimetry: Laboratory Measurement of

Heats of Reaction

4. Stoichiometry of Thermochemical

Equations

5. Hess’s Law

6. Standard Enthalpies of Reaction

2

Energy is the capacity to do work.

thermal

chemical

potential

kinetic

nuclear

solar 3

When energy is transferred from one

object to another, it appears as work

and/or heat

5

System vs Surroundings

System: the specific part of the universe that

is of interest in the study.

6

Internal Energy

7

A bulk property of matter

that, on a microscopic

level, represents all

energy within a system

In the system below, the

gas molecules have

internal energy due to:

Surroundings

Surroundings

Internal Energy

The internal energy of a system can only

change by exchange of heat (q) and

mechanical work (w) with the surroundings.

8

DEsystem = q + w

System

ΔE

ENERGY IN + ENERGY OUT -

+q: heat added

+w: work done

on the system

-q: heat lost

-w: work done

by the system

Surroundings

Surroundings

Internal Energy

9

System

ΔE

ENERGY IN + ENERGY OUT -

+q: heat added

+w: work done

on the system

-q: heat lost

-w: work done

by the system

Energy transferred as heat only

10

The system releases

heat.

The system absorbs

heat.

Energy transferred as work only

11

Units of Energy

Joule (J): SI unit of energy

◦ 1 𝐽 = 1 𝑘𝑔 × 𝑚2

𝑠2

calorie (cal): amount of energy

needed to raise the temperature of

water by 1ºC

◦ 1 𝑐𝑎𝑙 = 4.184 𝐽

nutritional Calorie: shows the energy

available from food; a kilocalorie

12

Learning Check: DEsys = q + w

When gasoline burns in a car engine,

the heat released causes the products

CO2 and H2O to expand, which pushes

the pistons outward. If the expanding

gases do 451 J of work on the pistons

and the system loses 325 J to the

surroundings as heat, calculate the

change in energy in J, kJ, and cal.

(1 cal = 4.184 J) 13

-776 J, -0.776 kJ, -0.185 kcal

Energy diagrams for the transfer of internal

energy between a system and its surroundings

Energy of system decreases Energy of system increases

14

1st Law of Thermodynamics

Law of Conservation of Energy

Energy can be converted from one form to

another, but cannot be created or

destroyed.

The total energy of a system and its

surroundings remains constant.

15

DEuniverse = DEsystem + DEsurroundings = 0

DEsystem = - DEsurroundings

16

State functions are properties that are

determined by the state of the system,

regardless of how that condition was

achieved.

Potential energy of hiker 1 and hiker 2

is the same even though they took

different paths.

DE = Efinal - Einitial

DP = Pfinal - Pinitial

DV = Vfinal - Vinitial

DT = Tfinal - Tinitial

17

DE is a state function – a function that

only depends on the initial and final

states and not on the path.

DE is a state function – a function that

only depends on the initial and final

states and not on the path.

18

In summary

Internal energy is transferred as heat

(q) or work (w).

Energy is always conserved.

Internal energy is a state function. 19

Food (quantity) Cal

beer (12 oz) 150

coffee (6 oz) ~0

milk (1 cup) 150

egg (1 large) 80

butter (1 tbsp) 100

apple (8 oz) 125

bread, white (2 slices)

130

brownie (40 g) 190

hamburger (4 oz) 326

fried chicken (1 drumstick)

195

carrots (1 cup) 70

20

Activity Cal/h*

sleeping 80

driving a car 120

standing 140

eating 150

walking 2.5 mph 210

mowing lawn 250

swimming 0.25 mph

300

roller skating 350

tennis 420

bicycling 13 mph 660

running 10 mph 900

*for a 160-lb person

From: http://catalog.flatworldknowledge.com/bookhub/4309?e=averill_1.0-ch05_s04