6.4
Quantifying Heat
Thermal Energy
Microscopically, thermal energy is the kinetic energy associated with the random motion of atoms and molecules. Temperature is a quanti…
Internal energy is defined by the interconversion of heat and work. But how is heat measured?
Every substance possesses thermal energy. When a system experiences a temperature difference with its surroundings, thermal energy flows from the matter of higher temperature to the matter of lower temperature. This thermal energy transfer is also called heat transfer and is measured in Joules.
Temperature, on the other hand, describes a property of thermal energy and is measured in Celsius or Kelvin. Thus, the temperature is substance independent, unlike heat, which depends on the number and type of molecules in a system.
Heat can be measured by a technique called calorimetry, which uses the relationship between the heat and a change in temperature of a system.
When a piece of iron at 114 °C is placed into room temperature water, heat flows from the iron to the water until they are the same temperature. Once there is no net flow of heat, they are in thermal equilibrium.
In an isolated system, the water gains exactly as much heat as the iron loses. But the final temperature is only one degree above the water’s starting temperature, while the iron has cooled by 93 °C.
The amount of heat transferred is proportional to the temperature change ΔT.
The proportionality constant C or the “heat capacity”, is the amount of heat needed to increase a system’s temperature by one kelvin, or one degree Celsius.
Heat capacity depends on the type of the substance: a system with a large heat capacity like water needs to absorb more heat to raise its temperature than a system with a smaller heat capacity, such as iron.
Heat capacity also depends on the mass of the substance. Specific heat capacity, Cs, or molar heat capacity, Cm, describes the amount of heat needed to increase the temperature of 1 gram, or 1 mole of a substance by 1 °C. Thus, the unit is given in either Joules per gram degree Celsius or as Joules per mole degree Celsius.
Thus, if specific heat capacities and temperature change of a system are known, the value for heat can be calculated from their product.
Q1: What is the difference between heat and temperature?
Temperature describes a property of thermal energy and is measured in Celsius or Kelvin, making it substance-independent. Heat, measured in Joules, is the transfer of thermal energy between two bodies at different temperatures and depends on the number and type of molecules in a system. While temperature tells you how hot something is, heat quantifies the energy flowing between objects.
Q2: How does heat capacity differ from specific heat capacity?
Heat capacity is an extensive property that depends on both the type and mass of a substance, so larger objects have greater heat capacity. Specific heat capacity is an intensive property describing the heat needed to raise 1 gram of a substance by 1°C, independent of mass. For example, a large iron pan requires more total energy to heat than a small iron pan, but both have the same specific heat capacity for iron.
Q3: What happens when objects at different temperatures come into contact?
Heat flows spontaneously from the hotter object to the cooler object until they reach thermal equilibrium, where no net heat transfer occurs. In an isolated system, the heat lost by the hot object equals the heat gained by the cold object. For example, when hot iron is placed in room-temperature water, thermal energy transfers from iron to water until both reach the same final temperature.
Q4: How is heat calculated from mass, specific heat, and temperature change?
Heat is calculated using the equation q = m × c × ΔT, where m is mass in grams, c is specific heat capacity in J/g°C, and ΔT is the temperature change. If a substance gains thermal energy, q is positive; if it loses thermal energy, q is negative. This relationship allows you to determine any of these quantities if the other three are known.
Q5: Why does water require more heat to change temperature than iron?
Water has a relatively high specific heat capacity of about 4.2 J/g°C, while most metals like iron have much lower specific heats, usually less than 1 J/g°C. This means water needs to absorb significantly more heat energy to raise its temperature by one degree compared to iron. The difference reflects how molecular structure affects thermal energy absorption.
Q6: What is calorimetry and how does it measure heat?
Calorimetry measures heat using the relationship between heat transfer and temperature change in a system. When a substance at one temperature is placed in contact with a substance at a different temperature, the heat absorbed or released can be calculated from the resulting temperature change and the substance's specific heat capacity. This technique allows quantification of thermal energy transfer in isolated systems.
Q7: What does thermal equilibrium mean in heat transfer?
Thermal equilibrium occurs when two objects in contact reach the same temperature and no net heat flows between them. At this point, the thermal energy of both objects remains constant. In an isolated system, thermal equilibrium is achieved when the heat lost by the hotter object exactly equals the heat gained by the cooler object.