Heat is ... How much heat will be released during combustion?

All substances have internal energy. This value is characterized by a number of physical and chemical properties, among which special attention should be paid to heat. This value is an abstract mathematical value that describes the forces of interaction of matter molecules. Understanding the mechanism of heat exchange can help to answer the question of how much heat was released during cooling and heating of substances, as well as their combustion.

History of the discovery of the phenomenon of heat

Initially, the phenomenon of heat transfer was described very simply and understandably: if the temperature of the substance rises, it receives heat, and in the case of cooling it releases it into the environment. However, heat is not an integral part of the fluid or body in question, as thought three centuries ago. People naively believed that the substance consists of two parts: its own molecules and heat. Now very few people remember that the term "temperature" in Latin means "mixture", and, for example, they spoke of bronze as "the temperature of tin and copper".

In the 17th century, two hypotheses appeared that could explain the phenomenon of heat and heat transfer. The first proposed in 1613 by Galileo. His formulation sounded like this: "Heat is an unusual substance that can penetrate into any body and come out of it." Galileo called this substance heat. He argued that the heat can not disappear or be destroyed, but only able to move from one body to another. Accordingly, the more heat in the substance, the higher its temperature.

The second hypothesis appeared in 1620, and the philosopher Bacon suggested it. He noticed that under strong hammer blows, the iron had heated up. This principle also acted when the fire was kindled by friction, which led Bacon to the idea of the molecular nature of heat. He argued that when a mechanical action on the body of its molecules begin to fight against each other, increase the speed of movement and thereby raise the temperature.

The result of the second hypothesis was the conclusion that heat is the result of the mechanical action of molecules of matter with each other. Lomonosov tried to prove this theory over a long period of time.

Heat is a measure of the internal energy of a substance

Modern scientists came to the following conclusion: thermal energy is the result of the interaction of molecules of matter, i.e., the internal energy of the body. The velocity of the particles depends on the temperature, and the magnitude of the heat is directly proportional to the mass of the substance. So, a bucket of water has more thermal energy than a filled cup. However, a saucer with a hot liquid can have less heat than a basin with a cold one.

The theory of heat, proposed in the 17th century by Galileo, was refuted by scientists J. Joule and B. Rumford. They proved that thermal energy does not possess any mass and is characterized exclusively by the mechanical movement of molecules.

How much heat will be released when the substance is burned? Specific heat of combustion

To date, universal and widely used energy sources are peat, oil, coal, natural gas or wood. When burning these substances, a certain amount of heat is released, which is used for heating, starting mechanisms, etc. How can this value be calculated in practice?

For this purpose, the concept of specific heat of combustion is introduced . This value depends on the amount of heat that is released when 1 kg of a certain substance is burned. It is denoted by the letter q and is measured in J / kg. Below is a table of q values for some of the most common fuels.

Engineers in the construction and calculation of engines need to know how much heat will be released when a certain amount of matter burns. For this, indirect measurements can be made using the formula Q = qm, where Q is the heat of combustion of the substance, q is the specific heat of combustion (tabulated value), and m is the specified mass.

The formation of heat in combustion is based on the phenomenon of energy release during the formation of chemical bonds. The simplest example is the combustion of carbon, which is contained in any type of modern fuel. Carbon burns in the presence of atmospheric air and combines with oxygen, forming carbon dioxide. The formation of a chemical bond proceeds with the release of thermal energy into the environment, and this energy has been adapted to use for its own purposes.

Unfortunately, the thoughtless expenditure of such valuable resources as oil or peat can soon lead to depletion of sources of extraction of these fuels. Already today there are electrical appliances and even new models of cars, whose work is based on such alternative sources of energy as sunlight, water or the energy of the earth's crust.

Heat transfer

The ability to exchange thermal energy within the body or from one body to another is called heat transfer. This phenomenon does not occur spontaneously and arises only at a temperature difference. In the simplest case, the thermal energy is transferred from the more heated body to the less heated until equilibrium is established.

The bodies need not touch the heat transfer phenomenon. In any case, the establishment of equilibrium can occur and at a small distance between the objects in question, but at a slower rate than when they are in contact.

Heat transfer can be divided into three types:

1. Thermal conductivity.

2. Convection.

3. Radiation exchange.

Thermal conductivity

This phenomenon is based on the transfer of thermal energy between atoms or molecules of matter. The reason for the transfer is the chaotic motion of molecules and their constant collision. Due to this, heat transfers from one molecule to another along a chain.

Observe the phenomenon of thermal conductivity can be on ignition of any iron material, when the redness on the surface gradually spreads and gradually damps (a certain amount of heat is released into the environment).

J. Fourier derived a formula for the heat flux, which collected all the quantities that affect the degree of thermal conductivity of the substance (see the figure below).

In this formula, Q / t is the heat flux, λ is the coefficient of thermal conductivity, S is the cross-sectional area, and T / X is the ratio of the temperature difference between the ends of the body located at a certain distance.

Thermal conductivity is a tabular value. It is of practical importance for the insulation of an apartment building or the thermal insulation of equipment.

Radiant heat transfer

Another way of heat transfer, which is based on the phenomenon of electromagnetic radiation. Its difference from convection and thermal conductivity lies in the fact that energy transfer can occur in vacuum space. However, as in the first case, a temperature difference is necessary.

Radiant exchange is an example of the transfer of thermal energy of the Sun to the surface of the Earth, for which mainly infrared radiation is responsible. To determine how much heat gets to the earth's surface, numerous stations were built that monitor the change in this indicator.

Convection

Convection movement of air flows is directly related to the phenomenon of heat transfer. Regardless of how much heat we have reported to liquids or gas, the molecules of matter begin to move faster. Because of this, the pressure of the entire system decreases, and volume, on the contrary, increases. This is the cause of the movement of warm air currents or other gases upward.

The simplest example of using the phenomenon of convection in everyday life can be called heating the room with the help of batteries. They are located at the bottom of the room for a reason, but for the heated air to rise where it leads to a circulation of flows through the room.

How can you measure the amount of heat?

The heat of heating or cooling is calculated mathematically by means of a special instrument - a calorimeter. The installation is represented by a large thermally insulated vessel that is filled with water. A thermometer is lowered into the liquid to measure the initial temperature of the medium. Then the heated body is lowered into the water to calculate the temperature change of the liquid after equilibrium is established.

By increasing or decreasing t of the medium, it is determined how much heat to heat the body should be expended. A calorimeter is the simplest device that can record a temperature change.

Also, using a calorimeter, you can calculate how much heat is released when the substances are burned. For this, a "bomb" is placed in a vessel filled with water. This "bomb" is a closed vessel in which the test substance is located. Special electrodes for arson are added to it, and the chamber is filled with oxygen. After complete combustion of the substance, a change in the temperature of the water is recorded.

During such experiments it was established that the sources of thermal energy are chemical and nuclear reactions. Nuclear reactions take place in the deep layers of the Earth, forming the main reserve of heat of the whole planet. They are also used by man to generate energy during thermonuclear fusion.

Examples of chemical reactions are the burning of substances and the splitting of polymers to monomers in the human digestive system. The quality and quantity of chemical bonds in a molecule determines how much heat is released in the end.

What is the measure of heat?

The unit of measurement of heat in the international SI system is joule (J). Also, non-system units - calories - are used in everyday life. 1 calorie equals 4.1868 J in accordance with the international standard and 4.184 J based on thermochemistry. Previously, there was a British thermal unit BTU, which is rarely used by scientists. 1 BTU = 1.055 J.