Section 4: Thermal Energy Chapter 3: Matter and Energy Section 4: Thermal Energy

Learning Objectives Convert between Fahrenheit, Celsius, and Kelvin temperature scales. Relate energy, temperature change, and heat capacity.

Thermal Energy The atoms and molecules that compose matter are in constant random motion they contain thermal energy The temperature of a substance is a measure of its thermal energy.

Thermal Energy The hotter an object, the greater the random motion of the atoms and molecules that compose it, and the higher its temperature.

Thermal Energy Heat, which has units of energy, is the transfer or exchange of thermal energy caused by a temperature difference. when a piece of cold ice is dropped into a cup of warm water, heat (thermal energy) is transferred from the water to the ice.

Thermal Energy Temperature, by contrast, is a measure of the thermal energy of matter (not the exchange of thermal energy). Measures the average kinetic energy of the molecules of matter.

Thermal Energy Both cups of water are at the same temperature… Which has a higher average kinetic energy? Which contains more thermal energy?

Temperature Scales The Fahrenheit scale was set according to the following standards 0 °F to the freezing point of a concentrated saltwater solution 96 °F to normal body temperature.

Temperature Scales On the Fahrenheit (°F) scale water freezes at 32 °F water boils at 212 °F Room temperature is approximately 72 °F.

Temperature Scales On the Celsius (°C) scale: water freezes at 0 °C water boils at 100 °C Room temperature is approximately 22 °C

Temperature Scales The Kelvin (K) scale avoids negative temperatures by assigning 0 K to the coldest temperature possible, absolute zero. Absolute zero is the temperature at which molecular motion stops. On the Kelvin (K) scale, water freezes at 273 K water boils at 373 K. Room temperature is approximately 295 K

The Fahrenheit degree is five- ninths the size of a Celsius degree. The Celsius degree and the Kelvin are the same size.

Temperature Scales We can convert between Fahrenheit, Celsius, and Kelvin temperature scales using the following formulas:

Practice Convert –25 °C to kelvin.

Practice Convert 358 K to Celsius.

Practice Convert 55 °F to Celsius.

Practice Convert 139 °C to Fahrenheit.

Practice Convert 310 K to Fahrenheit.

Practice Convert –321 °F to kelvin.

Heat Capacity Heat capacity: The quantity of heat (usually in joules) required to change the temperature of a given amount of the substance by 1 °C

Heat Capacity Specific heat capacity: the amount of heat required to raise the temperature of 1 g of any substance by 1oC Specific heat capacity has units of joules per gram per degree Celsius, J/g °C

Heat Capacity Specific heat capacity is an intensive property! Specific heat describes how well an object retains heat A substance with a low specific heat is quickly heated, but also quickly cools A substance with a high specific heat takes a long time to warm up, but will also retain that heat for a longer period

Heat Capacity Styrofoam is a very poor conductor of heat; it is a good insulator. It has a high specific heat. Metals are good conductors of heat. They have low specific heats.

Check-in: If you want to heat a metal plate to as high a temperature as possible for a given energy input, what metal should you use? (Assume all the plates have the same mass.) copper iron aluminum it would make no difference

Heat Capacity Calculations q is the amount of heat in joules. m is the mass of the substance in grams. C is the specific heat capacity in joules per gram per degree Celsius. T is the temperature change in Celsius. The symbol Δ means the change in, so ΔT means the change in temperature.

Example Gallium is a solid at 25.0°C and melts at 29.9°C. If you hold gallium in your hand, it can melt from your body heat. How much heat must 2.5 g of gallium absorb from your hand to raise its temperature from 25.0°C to 29.9°C? The specific heat capacity of gallium is 0.372 J/g°C.

Practice The temperature of a lead fishing weight rises from 26°C to 38°C as it absorbs 11.3 J of heat. What is the mass of the fishing weight in grams?

Practice A chemistry student finds a shiny rock that she suspects is gold. She determines that its mass is 14.3 g. She then finds that the temperature of the rock rises from 25°C to 52°C upon absorption of 174 J of heat. Find the heat capacity of the rock and determine whether the value is consistent with the heat capacity of gold (which is listed in Table 3.4).

Practice A 328 g sample of water absorbs 5.78 × 103 J of heat. Calculate the change in temperature for the water. If the water is initially at 25.0°C, what is its final temperature?

Check-in: The heat capacity of substance A is twice that of substance B. If samples of equal mass of the two substances absorb the same amount of heat, which substance undergoes the larger change in temperature?