Molecular Basis of Specific Heat

Molecules and Specific Heat

On a cold winter night, the latest snowfall has downed power lines around your neighborhood. The temperature inside your house is dropping, and you pile more blankets on your bed. The only heat source is a gas oven. Your mom asks if you want a hot water bottle or a hot brick to warm your bed. Which should you choose, and why?

Temperature is a measure of the average kinetic energy of the molecules in a particular substance. The molecules in 4 degrees Celsius water will be moving much slower than the molecules in water at 24 degrees Celsius.

Absolute zero is the temperature at which particles are completely still. Although this theoretical temperature exists, absolute zero does not exist in nature: even the darkest, most distant regions of the universe are at 3 K. When a substance is cooled to a few thousandths of a degree above absolute zero, atoms condense and behave very differently.

The SPECIFIC HEAT of a substance is defined as the amount of heat required to raise the temperature of a known mass by one degree. In SI units, specific heat is often measured in kJ/kg K. Joules are a unit of energy, derived from Watts and seconds. K stands for Kelvin. The specific heat of a substance can be calculated from an experiment in which a pure sample of a known mass is heated from an initial temperature to a final temperature. The amount of Joules delivered to the object can be calculated from the Watts and length of time the material is heated. A low specific heat indicates that it requires relatively few Joules to increase the temperature of a substance. A high specific heat indicates that it requires a relatively larger number of Joules to increase the temperature of a substance. Solids and liquids have one number for specific heat. For gases, specific heat has to be determined under two conditions; once when pressure is held constant and a second time when volume is held constant. These two specific heat measurements are different.

The specific heat of different substances has been measured and is available in tables. When measured as a function of mass, the specific heat for different pure solids is very different. However, when it is measured as a function of moles at room temperature and above, the differences largely disappear. This is true for solids at room temperature and above; for cold temperatures, quantum effects become important. The specific heat for gases is generally low. The specific heat of dry soil is lower than that of wet soil. This is due to the special chemical properties of water.

The specific heat of water is 4.2 kJ/kg K, higher than any other commonly available substance. This is due to the unique chemical properties of water. Each molecule of water is made up of one oxygen and two hydrogen atoms. Oxygen attracts the electrons in the bond between oxygen and hydrogen more strongly. Because electrons are negatively charged, each oxygen atom in water has a partial negative charge. The hydrogen atoms in water end up with a partial positive charge. Water molecules orient themselves such that the oxygen atoms point toward hydrogen atoms. This is called hydrogen bonding. Disrupting this arrangement takes energy. This additional energy required to increase the speed of water molecules and thus the temperature of water means that water has a high specific heat.

So, if both the water and the brick have the same mass and are at the same temperature, choose the water. The higher specific heat of water means that it contains more energy and will keep you warmer longer at night.

Multiple Choice

In the scenario described in Paragraph 1, what is happening to the air molecules inside your house?

• They are speeding up over time.

• They are escaping through cracks around the windows.

• They are slowing down over time.

• They are becoming more easily visible.

Multiple Choice

In a water molecule, the two hydrogen atoms have a partial ______ charge.

• negative

• positive

• neutral

• up

Multiple Choice

Water molecules stick to each other using -

• covalent bonds.

• heat bonds.

• charged bonds.

• hydrogen bonds.

Multiple Choice

The specific heat of solid Substance A is 0.1 kJ/kg K. The specific heat of solid Substance B is 1 kJ/kg K. Which substance will require a higher number of Joules to heat from 10 degrees C to 20 degrees C?

• Substance A

• Substance B

• Both require the same

• Can't be determined from the information provided

Multiple Choice

The specific heat of dry soil is lower than that of wet soil. Which of the following statements explains that finding?

• Dry soil is denser, and therefore easier to heat up.

• Dry soil contains less water.

• Dry soil is lighter, and therefore easier to heat up.

• Dry soil is a solid, whereas wet soil is a liquid.

Multiple Choice

What is the approximate specific heat of water in kJ/kg K?

• 0.42

• 1.42

• 4.2

• 14.2