Explain how water's ability to absorb heat is used to measure energy changes in chemical reactions?

Question

Q=mcdT

Answer 1

Since water's ability to absorb heat is known, we can use the water in a calorimeter when an unknown substance is being tested.

Water has the unique property of being a liquid through 0 C to 100 C. This means there is generally no change in phase (heat of fusion or heat of vaporization) when using water in a calorimeter. It simplifies the calculations.

The specific heat of water is 4.194 j/g*C. We need to know the mass of water in the calorimeter (density of water is 1.0 g/mL), the original temperature of the water, and the final temperature of the water. We also need to know the mass of the material being tested (or the specific heat of the material), the original temperature, and the final temperature (same as final temperature of the water). Since there is generally no phase change in the water we can use the equation:

Q = mc(Tf-To)

to find the heat absorbed by the water. The heat absorbed by the water is equal to the heat given off by the unknown material. Use the same equation for the unknown material, but change the sign of "Q" since it is being given off:

-Q = mc(Tf-To)

this will allow you to calculate either the mass or the specific heat of the unknown material - depending on which variable you need to find.

Answer 2

According to the kinetic-molecular theory, heat energy acquired by a body is transformed into increased kinetic energy of the molecules of the body. We observe this increased kinetic energy whenever a solid, a liquid, or a gas expands on heating. A further increase in kinetic energy will eventually cause the particles of a solid or liquid to become a gas.

Recall that when an ice cube (a solid) is heated, it melts and becomes liquid water. When the water is heated, it vaporizes and becomes gaseous water. According to the kinetic-molecular theory, as increasing amounts of heat are supplied to a piece of ice, the water molecules move more rapidly until they gain sufficient energy to overcome the attractive forces holding them together. This permits the ice to liquefy and become water. Similarly, as still more energy is received, the water molecules move at even greater speeds. The attractive forces in the liquid are weakened and the water is converted into gaseous water.