Honors Chemistry
Presentation 09-08
The Flow of Energy
Page 1 of 5
Objectives:

When you complete this presentation, you will be able to
o define energy, heat, and work
o describe the flow of energy in exothermic and endothermic processes
o use the proper units when calculating energy, heat, and work
o perform specific heat calculations
Energy Transformations

Energy is the capacity for doing work or supplying heat.
o 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.

Chemical potential energy is seen in the combustion of methane in a
laboratory burner.
o Methane, CH4, is added to the burner with oxygen gas, O2.
o A spark is added to the mixture and there is a flame.
o C-H (in CH4), and O-O (in O2) bonds are broken.
o C-O (in CO2) and O-H (in H2O) bonds are formed.
o Energy is released.

Looking at the reaction CH4 +2 O2 → CO2 +2 H2
Honors Chemistry
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Presentation 09-08
The Flow of Energy
Page 2 of 5
Heat, symbolized by q,
o is the transfer of energy from one object to another
o 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.
Endothermic and Exothermic Processe

A chemical reaction is part of a system.

Everything outside the system is called the surroundings.

Energy is not created or destroyed.

The total amount of energy in the system and the surroundings must remain
the same.

If the chemical reaction in the system uses energy
o 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.

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.
Honors Chemistry
Presentation 09-08
The Flow of Energy
Page 3 of 5
Units for Measuring Heat Flow

We measure heat flow in two common units:
o the calorie (cal)
o the joule (J)

The calorie (with a lower case “c”) is ...
o a non-SI unit.
o the amount of heat to increase the temperature of water by 1°C.
o equal to 0.001 Calories (food calories or Cal).
o There are 1,000 cal in 1 Cal.

The joule is ...
o the SI unit for heat (and energy and work).
o the energy required to apply 1 newton of force (about 3.6 ounces)
over a distance of 1 meter.
o equal to 0.2390 cal.

There are 4.184 J in 1 cal.

We will be using the joule, J, exclusively in this course.

You will not be required to convert from J ➔cal or from cal ➔J.
Heat Capacity and Specific Heat

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 ...
o mass
o 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.

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.
Honors Chemistry
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Presentation 09-08
The Flow of Energy
Page 4 of 5
Different objects have different specific heats.
Substance
C(J/g∙°C)
C(cal/g∙°C)
Water
4.18
1.00
Ice
2.1
0.50
Steam
1.7
0.4
Aluminum, Al
0.90
0.21
Iron, Fe
0.46
0.11
Silver, Ag
0.24
0.57
Water has a very high specific heat.
o 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.
o Farmers use this to protect crops in danger of freezing.
o This is why the filling in a hot apple pie is more likely to burn your
tongue than is the crust.

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.
C=

q
m ∙ ∆T
We can use this to calculate the specific heat of any material.
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?
Honors Chemistry
Presentation 09-08
The Flow of Energy
Page 5 of 5
Sample Problems:
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.