Heat energy, also known as thermal energy, represents the total energy associated with the random movement of atoms and molecules within a substance. This energy is transferred between systems or objects due to a difference in their temperatures. Heat naturally flows from a higher temperature object to a lower temperature object until both reach thermal equilibrium. Calculating the quantity of energy transferred or released during a process is fundamental to the study of thermodynamics. This calculation is determined through specific mathematical formulas based on whether the substance is changing temperature or changing its physical state.
Finding Heat Energy from Temperature Shifts
The most common way to calculate heat energy transfer occurs when a substance absorbs or releases energy, resulting in a temperature change without a change in its physical state. This calculation relies on the formula \(Q = mc\Delta T\), where \(Q\) represents the heat energy transferred, typically measured in Joules.
The variable \(m\) is the mass of the substance, measured in kilograms or grams. A greater mass requires a proportionally larger amount of energy to achieve the same temperature increase. The variable \(c\) is the specific heat capacity, a characteristic property of the material. Specific heat capacity is defined as the energy needed to raise the temperature of one unit of mass of a substance by one degree.
Different substances possess different specific heat capacities. Water has a notably high specific heat capacity of about 4,186 Joules per kilogram per degree Celsius, meaning it takes a large amount of energy to warm up. The final variable, \(\Delta T\), represents the change in temperature, calculated as the final temperature minus the initial temperature.
If the final temperature is higher than the initial temperature, \(\Delta T\) is positive, indicating that the substance absorbed heat energy. Conversely, a negative \(\Delta T\) means the substance released heat energy into the surroundings.
Calculating Heat Energy During Phase Transitions
A different calculation is required when heat energy is added to or removed from a substance changing its physical state. During these phase transitions, the temperature remains constant even though energy is continuously being transferred. This energy, which does not cause a temperature rise, is known as latent heat.
The heat energy absorbed or released during a phase change is calculated using the formula \(Q = mL\). Here, \(Q\) is the energy transferred, \(m\) is the mass, and \(L\) is the specific latent heat. The \(L\) value represents the energy required per unit of mass to complete the state change, which is used to overcome the forces holding the molecules together rather than increasing molecular speed.
There are two primary types of specific latent heat: the Latent Heat of Fusion (\(L_f\)) and the Latent Heat of Vaporization (\(L_v\)). \(L_f\) is the energy required to change a substance between a solid and a liquid state. \(L_v\) is the energy needed to change a substance between a liquid and a gas state.
Practical Measurement Using Calorimetry
While the formulas provide a theoretical calculation, scientists often find heat energy experimentally using calorimetry. Calorimetry is the science focused on measuring the amount of heat absorbed or released during a physical change or chemical reaction. The apparatus used is a calorimeter, an insulated container designed to minimize heat exchange with the outside environment.
The fundamental concept behind calorimetry is the conservation of energy: heat lost by a hot object must be gained by a colder object within the insulated system. In practice, a reaction or hot object is placed into a known mass of water inside the calorimeter, and the resulting temperature change of the water is measured. The heat gained by the water is calculated using the \(Q = mc\Delta T\) formula, based on the water’s mass, specific heat capacity, and temperature change.
Since the heat lost by the substance or reaction is equal to the heat gained by the water, the value of \(Q\) calculated for the water directly reveals the heat energy transferred by the sample. This experimental method is used in various applications, including determining the energy content of food or finding the unknown specific heat capacity of a new material.