CH 20.2 Power Point

Measuring and Using Energy Changes
Section 20.2
Main Idea
 Energy stored in chemical bonds can be converted to
other forms and used to meet the needs of individuals
and of societies.
The heat generated in chemical reactions can be
measured by a technique called calorimetry and a
device called a calorimeter.
The amount of heat energy of a reaction is calculated
using the following equation:
q = m x c x ΔT
q= heat lost or gained by sample
m= mass of the sample
c= specific heat of sample
ΔT = temperature change of the water
(ΔT = Tfinal – Tinitial)
(ΔT negative endothermic)
You will be using a calorimeter where a hot substance
and cold substance are brought together.
Example problem
How much heat is absorbed by a reaction that lowers the
temperature of 500.0g of water in a calorimeter by
Use q = m x c x ΔT
m= 500.0 g
ΔT = 1.10°C
What is c? Specific heat. Look up for water…
c= 4.18 J/g•°C
q= 500.0g x 4.18 J/g•°C x 1.10°C
q= 2,299J
Another problem…
Aluminum reacts with iron (III) oxide to yield aluminum
oxide and iron. Calculate the heat given off in the
reaction if the temperature of the 100.00 g of water in
the calorimeter increases by 3.00°C.
Use q = m x c x ΔT
Another problem…
Burning a small quantity of hexane raises the
temperature of 424g of water in a calorimeter from
18.4°C to 32.7°C. How much heat was released in the
q= (m)(c)(ΔT)
q= 25,400J
In this experiment you will determine the specific heat
of an unknown metal. The metal will be heated to a
known temperature then placed in a styrofoam cup
that contains a known amount of water a room
Since Styrofoam is an insulator, it acts as a calorimeter.
Heat will be transferred from the hot metal to the
water, but heat should not be lost to the surroundings.
You will observe what happens to the temperature of the
metal and the water. Heat will be transferred from a
hot substance to a cold substance until the
temperatures are equal.
Sound familiar…
At the end of the experiment, the metal and water will
be the same temperature. The heat lost by the metal
will be gained by the water.
You can then calculate the specific heat of the metal by
knowing the mass of the metal, mass of the water and
the change of temperature.
Energy Value of Food
 A calorie if the heat required to raise the temperature of 1 g
of liquid water by 1°C.
A kilocalorie is a unit = 1000 calories
One calorie = 4.184 J
One Joule = 0.239 calorie
The calories on a nutrition label for the food you eat is
actually a Calorie (1 kcal)
 Different macromolecules provide different amounts
of energy per gram:
1 g fat = 9 Calories
1 g carbohydrate OR protein = 4 Calories
CALORIMETRY can be used to measure the energy
content in food. Practice Problems, p. 718 #s 8-10