Chapter 2 Energy and Matter
2.4
Specific Heat
1
Specific Heat
Specific heat
• is different for different substances.
• is the amount of heat that raises the temperature of 1 g
of a substance by 1°C.
• in the SI system has units of J/gC.
• in the metric system has units of cal/gC.
2
Examples of Specific Heats
TABLE 2.6
cal/g°C
0.214
0.0920
0.0308
0.108
0.0562
0.125
0.488
0.588
0.207
0.100
3
Learning Check
A. When ocean water cools, the surrounding air
1) cools.
2) warms.
3) stays the same.
B. Sand in the desert is hot in the day, and cool
at night. Sand must have a
1) high specific heat.
2) low specific heat.
4
Solution
A. When ocean water cools, the surrounding air
2) warms.
B. Sand in the desert is hot in the day, and cool
at night. Sand must have a
2) low specific heat.
5
Heat Equation
Rearranging the specific heat expression gives the
heat equation.
Heat = g x °C x
J =J
g°C
The amount of heat lost or gained by a substance is
calculated from the
• mass of substance (g).
• temperature change (T).
• specific heat of the substance (J/g°C).
6
Learning Check
What is the specific heat of a metal if 24.8 g absorbs
275 J of energy and the temperature rises from 20.2C to
24.5C?
7
Solution
What is the specific heat of a metal if 24.8 g absorbs
275 J of energy and the temperature rises from 20.2C to
24.5C?
Given: 24.8 g metal, 275 J of energy, 20.2C to 24.5C
Need: J/gC
Plan:
SH = Heat/gC
ΔT = 24.5C – 20.2C = 4.3 C
Set Up:
275 J
(24.8 g)(4.3C)
= 2.6 J/gC
8
Learning Check
How many kilojoules are needed to raise the
temperature of 325 g of water from 15.0°C to 77.0°C?
1) 20.4 kJ
2) 77.7 kJ
3) 84.3 kJ
9
Solution
How many kilojoules are needed to raise the
temperature of 325 g of water from 15.5°C to
77.5°C?
3) 84.3 kJ
77.0°C – 15.0°C = 62.0°C
325 g x 62.0°C x 4.184 J x 1 kJ
g °C
1000 J
= 84.3 kJ
10