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152 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
Practice Problems
1. How much heat does a 23.0-g ice cube absorb as its temperature increases from Ϫ17.4°C to 0.0°C? Give the answer in both
joules and calories.
2. A sample of an unknown metal has a mass of 120.7 g. As the
sample cools from 90.5°C to 25.7°C, it releases 7020 J of
energy. What is the specific heat of the sample? Identify the
metal among those in Table 16-2 in your textbook.
3. A 15.6-g sample of ethanol absorbs 868 J as it is heated. If the
initial temperature of the ethanol was 21.5°C, what is the final
temperature of the ethanol?
15.2 Heat in Chemical Reactions and Processes
Heat changes that occur during chemical and physical processes can
be measured using an insulated device called a calorimeter. In one
type of calorimeter, the temperature change of a known mass of
water is used to determine the amount of energy released or
absorbed by a system undergoing a chemical or physical change.
The following example problem shows you how to determine the
specific heat of an unknown substance by using calorimetry data.
Example Problem 15-2
Using Data from Calorimetry
A calorimeter contains 195 g of water at 20.4°C. A 37.8-g sample of
an unknown metal is heated to 133°C and placed into the water in the
calorimeter. Heat flows from the metal to the water until both reach a
final temperature of 24.6°C. What is the specific heat of the metal?
First, calculate the heat gained by the water.
q w ϭ c w ϫ m w ϫ ⌬T w
q w ϭ 4.184 J/(g и °C) ϫ 195 g ϫ (24.6°C Ϫ 20.4°C)
q w ϭ 4.184 J/(g и °C) ϫ 195 g ϫ 4.2°C
q w ϭ 3430 J
Note that three significant figures are retained for precision in the
calculation. At the end of the problem, you should round off the
final answer to two significant figures.
152 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 15
Practice Problems
1. How much heat does a 23.0-g ice cube absorb as its temperature increases from Ϫ17.4°C to 0.0°C? Give the answer in both
joules and calories.
2. A sample of an unknown metal has a mass of 120.7 g. As the
sample cools from 90.5°C to 25.7°C, it releases 7020 J of
energy. What is the specific heat of the sample? Identify the
metal among those in Table 16-2 in your textbook.
3. A 15.6-g sample of ethanol absorbs 868 J as it is heated. If the
initial temperature of the ethanol was 21.5°C, what is the final
temperature of the ethanol?
15.2 Heat in Chemical Reactions and Processes
Heat changes that occur during chemical and physical processes can
be measured using an insulated device called a calorimeter. In one
type of calorimeter, the temperature change of a known mass of
water is used to determine the amount of energy released or
absorbed by a system undergoing a chemical or physical change.
The following example problem shows you how to determine the
specific heat of an unknown substance by using calorimetry data.
Example Problem 15-2
Using Data from Calorimetry
A calorimeter contains 195 g of water at 20.4°C. A 37.8-g sample of
an unknown metal is heated to 133°C and placed into the water in the
calorimeter. Heat flows from the metal to the water until both reach a
final temperature of 24.6°C. What is the specific heat of the metal?
First, calculate the heat gained by the water.
q w ϭ c w ϫ m w ϫ ⌬T w
q w ϭ 4.184 J/(g и °C) ϫ 195 g ϫ (24.6°C Ϫ 20.4°C)
q w ϭ 4.184 J/(g и °C) ϫ 195 g ϫ 4.2°C
q w ϭ 3430 J
Note that three significant figures are retained for precision in the
calculation. At the end of the problem, you should round off the
final answer to two significant figures.