The flow of energy

H E AT A N D W O R K

THE FLOW OF ENERGY

OBJECTIVES

• When you complete this presentation, you will be able to• define energy, heat, and work• describe the flow of energy in exothermic and

endothermic processes• use the proper units when calculating energy, heat, and

work• perform specific heat calculations

ENERGY TRANSFORMATIONS

• Energy is the capacity for doing work or supplying heat.• Energy is detected only by its effects:• the motion of a bounced basketball• The heat generated by a chemical reaction

• Thermochemistry is the study of changes in chemical reactions and changes of state.• Chemical potential energy is the energy stored

in chemical bonds.

ENERGY TRANSFORMATIONS

• Chemical potential energy is seen in the combustion of methane in a laboratory burner.• Methane, CH4, is added to the burner with oxygen gas,

O2.• A spark is added to the mixture and there is a flame.• C-H (in CH4), and O-O (in O2) bonds are broken.• C-O (in CO2) and O-H (in H2O) bonds are formed.• Energy is released.

ENERGY TRANSFORMATIONS

• Looking at the reaction CH4 +2 O2 → CO2 + 2 H2O

ENERGY TRANSFORMATIONS

• Looking at the reaction CH4 +2 O2 → CO2 + 2 H2O

First, we break bonds.

ENERGY TRANSFORMATIONS

• Looking at the reaction CH4 +2 O2 → CO2 + 2 H2O

Then, we make bonds.+ Energy

ENERGY TRANSFORMATIONS

• Heat, symbolized by q,• is the transfer of energy from one object to another• due to the temperature difference between the two

objects.• Heat always flows from a warm object to a

cooler object.• If the two objects are in contact, and remain in

contact, they will come to the same temperature.

EXOTHERMIC AND ENDOTHERMIC PROCESSES

• A chemical reaction is part of a system.• Everything outside the system is called the

surroundings.

Surroundings System

EXOTHERMIC AND ENDOTHERMIC PROCESSES

• Energy is not created or destroyed.• The total amount of energy in the system and the

surroundings must remain the same.

Surroundings System

energy

EXOTHERMIC AND ENDOTHERMIC PROCESSES

• If the chemical reaction in the system uses energy• then energy is transferred from the surroundings into

the system.• This is an endothermic process

• In an endothermic process, the system gains heat while the surroundings cool down.

SystemSurroundin

gs

• If the chemical reaction in the system makes energy• then energy is transferred from the system out to the

surroundings.• This is an exothermic process

• In an exothermic process, the system cools down while the surroundings gain heat.

energy

EXOTHERMIC AND ENDOTHERMIC PROCESSES

SystemSurroundin

gs

• We measure heat flow in two common units:• the calorie (cal)• the joule (J)

• The calorie (with a lower case “c”) is ...• a non-SI unit.• the amount of heat to increase the temperature of water

by 1°C.• equal to 0.001 Calories (food calories or Cal).• There are 1,000 cal in 1 Cal.

UNITS FOR MEASURING HEAT FLOW

• We measure heat flow in two common units:• the calorie (cal)• the joule (J)

• The joule is ...• the SI unit for heat (and energy and work).• the energy required to apply 1 newton of force (about 3.6

ounces) over a distance of 1 meter.• equal to 0.2390 cal.• There are 4.184 J in 1 cal.

UNITS FOR MEASURING HEAT FLOW

• We measure heat flow in two common units:• the calorie (cal)• the joule (J)

• We will be using the joule exclusively in this course.• You will not be required to convert from J ➔ cal

or from cal ➔ J.

UNITS FOR MEASURING HEAT FLOW

• The amount of heat needed to increase the temperature of an object by exactly 1°C is called the heat capacity of that object.• The heat capacity of an object depends on both

its ...• mass• chemical composition

• It takes more heat to increase the temperature of 1 kg of water than it does to increase the temperature of 1 g of water.• It takes more heat to increase the temperature of 1

kg of water than it does to increase the temperature of 1 kg of iron.

HEAT CAPACITY AND SPECIFIC HEAT

• The amount of heat needed to increase the temperature of an object with a mass of exactly 1 g by exactly 1°C is called the specific heat capacity or the specific heat, C, of that object.• Different objects have different specific heats.

HEAT CAPACITY AND SPECIFIC HEAT

Substance C(J/g∙°C) C(cal/g∙°C)Water 4.18 1.00

Ice 2.1 0.50Steam 1.7 0.4

Aluminum, Al 0.90 0.21Iron, Fe 0.46 0.11

Silver, Ag 0.24 0.057

• Water has a very high specific heat.• C = 4.18 J/g•°C.

• This means that you must add a lot of heat to water to increase its temperature.• It also means that we get a lot of heat out of

water when we decrease its temperature.• Farmers use this to protect crops in danger of freezing.

HEAT CAPACITY AND SPECIFIC HEAT

• Water has a very high specific heat.• C = 4.18 J/g•°C.

• This means that you must add a lot of heat to water to increase its temperature.• It also means that we get a lot of heat out of

water when we decrease its temperature.• This is why the filling in a hot apple pie is more likely to

burn your tongue than is the crust.

HEAT CAPACITY AND SPECIFIC HEAT

• To calculate the specific heat, C, of a materialyou divide the amount of heat input, q,

bythe mass, m, times the temperature change, ∆T.

C =

• We can use this to calculate the specific heat of any material.

HEAT CAPACITY AND SPECIFIC HEAT

qm∙∆T

Example 1:The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat of copper?m = 95.4 g∆T = Tf – Ti = 48.0°C – 25.0°C = 23.0°Cq = 849 J

C = = 0.387 J/g∙°C

qm∙∆T

849 J(95.4 g)(23.0°C)

=

HEAT CAPACITY AND SPECIFIC HEAT

Sample Problems: Find the specific heat of each of the following metals that are heated from an initial temperature of 25.0°C to the indicated final temperature using the indicated amount of heat.1. 37.0 g of iron heated to 50.0°C using 415 J of

heat.

2. 15.0 g of benzene heated to 52.0°C using 705 J of heat.

3. 78.0 g of calcium heated to 30.0°C using 252 J of heat.

HEAT CAPACITY AND SPECIFIC HEAT

C = 0.449 J/g∙°C

C = 1.74 J/g∙°C

C = 0.646 J/g∙°C

• Energy is the capacity for doing work or supplying heat.• Thermochemistry is the study of energy changes

in chemical reactions and changes of state.• Chemical potential energy is the energy stored in

chemical bonds.• Heat, symbolized by q, is the transfer of energy

from one object to another due to the temperature difference between the two objects.

SUMMARY

• A chemical reaction is part of a system.• Everything outside the system is called the

surroundings.• Energy is not created or destroyed, which means

that the total amount of energy in the system and the surroundings must remain the same.

SUMMARY

• If the chemical reaction in the system uses energy, then energy is transferred from the surroundings into the system (an endothermic reaction).• If the chemical reaction in the system makes

energy, then energy is transferred from the system into the surroundings (an exothermic reaction).

SUMMARY

• We measure heat flow in two common units: the calorie (cal), which we will not use and the joule (J), which we will use.• The amount of heat needed to increase the

temperature of an object by exactly 1°C is called the heat capacity of that object.• The heat capacity of an object depends on both

its mass and its chemical composition.

SUMMARY

• The amount of heat needed to increase the temperature of an object with a mass of exactly 1 g by exactly 1°C is called the specific heat capacity or the specific heat, C, of that object.• To calculate the specific heat, C, of a material you

divide the amount of heat input, q, by the mass, m, times the temperature change, ∆T.

SUMMARY