Thermochemistry

Thermochemistryq=mcDT

Specific Heatis the amount of heat required to

raise the temperature of one gram of a substance by one degree Celsius. ◦Represented by a “C”

Because different substances have different compositions, each substance has its own specific heat

Specific Heats at 298 K

Substance Specific heatJ/(g•°C)

Water (l) 4.184Water (s) 2.03Water (g) 2.01Ethanol (l) 2.44Beryllium (s) 1.825Magnesium (s) 1.023Aluminum (s) 0.897Concrete (s) 0.84Granite (s) 0.803Calcium (s) 0.647Iron (s) 0.449Strontium (s) 0.301Silver (s) 0.235Barium (s) 0.204Lead (s) 0.129Gold (s) 0.129

*(s) = solid, (l) = liquid, (g) = gas

Specific Heats at 298 K

Substance Specific heatJ/(g•°C)

Water (l) 4.184Water (s) 2.03Water (g) 2.01Ethanol (l) 2.44Beryllium (s) 1.825Magnesium (s) 1.023Aluminum (s) 0.897Concrete (s) 0.84Granite (s) 0.803Calcium (s) 0.647Iron (s) 0.449Strontium (s) 0.301Silver (s) 0.235Barium (s) 0.204Lead (s) 0.129Gold (s) 0.129

*(s) = solid, (l) = liquid, (g) = gas

A low specific heat is matter that loses or gains heat quickly

Examples:• tiles on the space

shuttle• concrete that we

walk on

Specific Heats at 298 K

Substance Specific heatJ/(g•°C)

Water (l) 4.184Water (s) 2.03Water (g) 2.01Ethanol (l) 2.44Beryllium (s) 1.825Magnesium (s) 1.023Aluminum (s) 0.897Concrete (s) 0.84Granite (s) 0.803Calcium (s) 0.647Iron (s) 0.449Strontium (s) 0.301Silver (s) 0.235Barium (s) 0.204Lead (s) 0.129Gold (s) 0.129

*(s) = solid, (l) = liquid, (g) = gas

A high specific heat is matter that loses or gains heat slowly

Examples:• water - has a

uniquely high specific heat compared to other substances.

How do we calculate the amount of heat a substance loses or gains?

We use the mass, known specific heat and the change in temperature.

Heat Curve DiagramTe

mpe

ratu

re (°

C)

Solid warming

Solid melting Liquid warming

Liquid boiling (vaporizing) Gas warming

q = (m)(C)(DT) q = the heat absorbed or releasedC = specific heat of the substancem = mass of the sample in gramsDT = the change in temperature

DT = Tfinal – Tinitial

Suppose that the temperature of a 5.00 x 103 g block of concrete sidewalk increased by 6.0 °C. How much heat did the concrete absorb?

q = m = C = DT =

In the construction of bridges and skyscrapers, gaps must be left between adjoining steel beams to allow for the expansion and contraction of the metal due to heating and cooling. The temperature of a sample of iron with a mass of 10.0 g is changed from 50.4 °C to 25.0 °C with the release of 114 J. What is the specific heat of iron?

q = m = C = DT =

If it takes 3590 calories to heat up a sample of water by 12.2 oC, what is the mass of the water?

q = m = C = DT =

If 5750 J of energy are added to a 455 g piece of glass at 24.0 oC, what is the final temperature of the glass?

q = m = C = DT =

How do food chemists obtain Calorie information that appears on our packaged food?

They use combustion reactions that they carry out in calorimeters.◦Calorimeter – an insulated device

used for measuring the amount of heat absorbed or released during a chemical or physical process