KEY2CHEM

Transfer of Energy as Heat

On average, particles in a substance at a higher temperature have higher kinetic energy than particles in a substance at a lower temperature. If particles between the two substances collide, energy is transferred from the higher-temperature object to the lower-temperature object as heat. Eventually the two substances in contact with one another will come to the same final temperature; this is referred to as thermal equilibrium.

Example 1.

Two $$100.0\text{ g}$$ blocks of aluminum are placed in thermal contact with one another. If the initial temperature of one block is $$100.0^{\circ}\text{ C}$$ and the initial temperature of the other block is $$50.0^{\circ}\text{ C}$$, what is the final temperature?

A. $$50.0^{\circ}\text{ C}$$

B. $$75.0^{\circ}\text{ C}$$

C. $$100.0^{\circ}\text{ C}$$

Solution

B. $$75.0^{\circ}\text{ C}$$

Since the two blocks have the same mass and are the same substance (will have the same specific heat capacity), the final temperature is exactly halfway between the two initial temperatures. Energy is transferred from the $$100.0^{\circ}\text{ C}$$ block to the $$50.0^{\circ}\text{ C}$$ block until thermal equilibrium is achieved.

Example 2.

Which statement is true?

A. Heat is a substance; a hot object contains more heat than a cold object.

B. Energy is transferred as heat when an object transfers kinetic energy via collisions of particles to another object having lower kinetic energy.

C. A cold object will transfer kinetic energy to a hot object spontaneously.

Solution

B. Energy is transferred as heat when an object transfers kinetic energy via collisions of particles to another object having lower kinetic energy.

Although heat is not a substance, kinetic energy can be transferred via particle collisions as heat. An object at a higher temperature has more kinetic energy on average than a lower-temperature object, so the higher-temperature object will spontaneously transfer energy as heat to the lower-temperature object.

Example 3.

A $$5 \text{ g}$$ block of iron ($$c = 0.450 \text{ J/g}\cdot ^{\circ}\text{C}$$) at $$200^{\circ}\text{C}$$ is dropped into $$15\text { g}$$ of water ($$c = 4.184 \text{ J\g}\cdot ^{\circ}\text{C}$$) at $$15^{\circ}\text{C}$$. What is true about the final temperature?

A. It will be exactly halfway between the initial temperatures

B. It will be closer to the initial temperature of the water.

C. It will be closer to the initial temperature of the iron.

Solution

B. It will be closer to the initial temperature of the water.

The final temperature (when the iron and water come to thermal equilibrium) will be closer to the initial temperature of the water. Since the water has a larger mass and larger specific heat capacity, it requires more heat to change its temperature.