Chapter 3 Matter and Energy Section 4 Thermal

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 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 fiveninths 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 1 o. C 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. ) a) copper b) iron c) aluminum d) 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?