엔탈피

Enthalpy is a type of energy that flows between objects of different temperatures.

In order to understand enthalpy, one must be familiar with the first law of thermodynamics, which states that the energy cannot be created or destroyed, it can only change forms. And the total amount of energy in a system is constant.

The concept of enthalpy is evident in a pot of water on a stove. Heat, denoted by the letter Q, spontaneously flows from the hot stove to cooler water. In response, the temperature of the water rises. However, since this is an open system, there is some heat lost to the surroundings.

On the other hand, a system can be thermodynamically isolated, like a thermos filled with hot water, where heat is not transferred between the system and its surroundings. If you drop a piece of a cold metal into this system, the heat spontaneously flows from the hot water to the cooler metal. So, if we apply the first law of thermodynamics to this isolated system, we can say that the heat lost by the water, or Qout, equals the heat absorbed by the metal, or Qin.

In this video, we will demonstrate this simple heat transfer experiment that tests the first law of thermodynamics.

Before delving into the protocol, let’s review some important concepts related to this experiment. As we discussed, heat, or Q, is a type of energy that is spontaneously transferred from a hot to a cold object.

Heat is often confused with temperature, which is the measure of the average kinetic energy of all of the individual molecules in a substance. For example, consider a large and small piece of hot aluminum at thermal equilibrium. They both have the same temperature, however the smaller piece of metal has less thermal energy than the other because it has fewer molecules and less mass.

The relationship between heat and temperature is given by this formula: Q = mCΔT. Therefore, the amount of heat required to raise the temperature depends on mass, m, which makes sense, as less heat is required to raise the temperature of 1 gram of aluminum as opposed to 1 kg.

The other factor is C, or the heat capacity, which depends on the material. For example, wood has a higher heat capacity than aluminum. This means that less heat is needed to increase the temperature of 1 kg of aluminum than 1 kg of wood.

C is a constant that is defined as the amount of heat required to raise the temperature of a unit mass of a substance by one degree. These values have been calculated empirically for many common materials, like water.

In the next section, we will see how to experimentally calculate C for lead using a calorimeter, which provides a thermodynamically isolated system.

First, obtain two Styrofoam cups, which will act as the insulated calorimeter in this experiment. Cut a small portion off of the top of one cup, so that it can act as a lid for the other. Punch a small hole in the lid so that the thermometer will fit through tightly

Pour 220 mL of water into the unmodified cup, then place the lid on top. Measure the temperature of the water.

Next, fill a beaker with enough water so that a lead sample can be fully submerged.Place the beaker on a hot plate, and bring the water to a boil.

Weigh a lead sample, and record the mass. Then, attach a string and suspend it using a ring stand. Submerge the lead sample in the boiling water until it is completely covered with water.

Wait five minutes to allow the sample to reach thermal equilibrium with the boiling water. Remove the sample from the boiling water, and record its initial temperature.

Quickly place the hot sample into the cup, and place the lid on top. Slide the thermometer back through the hole in the lid.

Swirl the coffee cup with the lead sample to ensure a uniform temperature. Watch the temperature on the thermometer as it changes, and record the final stabilized temperature.

From the first law of thermodynamics, we know that in this experiment, the hot piece of lead transferred heat to the colder water. If we assume that the calorimeter provides a thermodynamically isolated system, then the heat output from the lead equals the heat input to the water. Using the formula Q = mCΔT, we get the following equation.

From the experiment, we know the mass of the lead and the water, and the temperature change of the lead and water. The heat capacity of water is also known. Thus, the heat capacity of lead can be calculated.

This is in excellent agreement with the known heat capacity of lead, 0.128. This result validates the first law of thermodynamics.

Heat transfer and the conservation of energy principles apply to several day-to-day events, but often go unnoticed. Here are some examples.

A simple experiment using water and ice demonstrates the first law of thermodynamics and heat transfer by thermal conduction. Initially, the glass of water is at room temperature and is cooled with the addition of ice. Eventually, the ice melts and the water and melted ice reach the same temperature, as heat was transferred from the water to the ice.

However, because the system is not isolated from the surroundings, eventually the warmer room transfers heat to the water raising the temperature.

Another example of heat transfer is the one between the sun and the Earth. However, this happens via thermal radiation, since the sun is at a much higher temperature than the Earth, the heat flows from the sun to the Earth. However, not all heat is transferred to the Earth, as some is lost to other bodies in the universe and to the surroundings.

You’ve just watched JoVE’s introduction to heat and the first law of thermodynamics. You should now understand the basic concept of heat and the conservation of energy. Thanks for watching!