Specific heat of combustion of ice. Molecular physics

The energy that a body gains or loses during heat transfer is called the amount of heat. Denoted by the letter Q and measured in joules (J).

The amount of heat required to heat the body (or released by it when it cools),
depends on the kind of substance of which it is composed, on the mass of this body and on the change in its temperature.

To calculate the amount of heat required to heat the body or released by it during cooling, you need to multiply the specific heat capacity of the substance by the mass of the body and the difference between its higher and lower temperatures.

Where c is the specific heat capacity of a given substance, m is its mass, t 1 is the initial temperature of the body, t 2 is its final temperature.

The physical quantity showing how much heat is required to change the temperature of a body from a given substance weighing 1 kg by 1 ° C is called specific heat capacity. It is measured in J / (kg ºС).

As a rule, metals have a low specific heat capacity, so they heat up quickly and cool down just as quickly.

The transition of a substance from a solid to a liquid state is called melting. The temperature at which a substance melts is called the melting point of the substance. The transition of a substance from a liquid to a solid state is called solidification or crystallization. The temperature at which a substance solidifies (crystallizes) is called the solidification or crystallization temperature. Substances solidify at the same temperature at which they melt. The melting and crystallization temperature depends on atmospheric pressure: the higher the pressure, the higher the melting point. Therefore, in the table, the melting point values are presented at normal atmospheric pressure.

The physical quantity showing how much heat must be imparted to a crystalline body weighing 1 kg in order to completely transfer it to a liquid state at the melting point is called the specific heat of fusion. It is denoted by the letter λ and is measured in J / kg.

The amount of heat required to melt a substance of mass m, taken at the melting point, is calculated by the formula: Q=λ·m.

To calculate the amount of heat in these processes, the values of specific quantities are given in tables.

The melting process always proceeds with the absorption of energy, the reverse process occurs with the release of energy. In this case, since the temperature remains constant during the melting process, the average kinetic energy of the chaotic motion of molecules does not change, but the potential energy of their interaction changes.

molecular interaction.

In a heated vessel, both ice and water are simultaneously present - two aggregate states of the same substance, until all the ice has melted. Next, the resulting water is heated. Since the specific heat capacity of water is greater than the specific heat capacity of ice, the water heats up more slowly, the slope of the line is smaller.

In the previous paragraph, we considered the graph of melting and solidification of ice. The graph shows that while the ice is melting, its temperature does not change (see Fig. 18). And only after all the ice has melted, the temperature of the resulting liquid begins to rise. But after all, even during the melting process, the ice receives energy from the fuel burning in the heater. And from the law of conservation of energy it follows that it cannot disappear. What is the energy consumption of the fuel during melting?

We know that in crystals the molecules (or atoms) are arranged in a strict order. However, even in crystals they are in thermal motion (oscillate). When the body is heated, the average speed of the molecules increases. Consequently, their average kinetic energy and temperature also increase. On the graph, this is section AB (see Fig. 18). As a result, the range of vibrations of molecules (or atoms) increases. When the body is heated to the melting temperature, the order in the arrangement of particles in crystals will be violated. Crystals lose their shape. A substance melts, changing from a solid state to a liquid state.

Consequently, all the energy that a crystalline body receives after it has already been heated to the melting point is spent on the destruction of the crystal. In this regard, the body temperature ceases to rise. On the graph (see Fig. 18) this is the BC section.

Experiments show that for the transformation of various crystalline substances of the same mass into a liquid at a melting point, a different amount of heat is required.

The specific heat of fusion is denoted by λ (Greek letter "lambda"). Its unit is 1 J/kg.

Determine the specific heat of fusion in the experiment. Thus, it was found that the specific heat of melting of ice is 3.4 10 5 - . This means that for the transformation of a piece of ice weighing 1 kg, taken at 0 ° C, into water of the same temperature, 3.4 10 5 J of energy is required. And in order to melt a bar of lead weighing 1 kg, taken at its melting point, it will take 2.5 10 4 J of energy.

Therefore, at the melting point, the internal energy of a substance in the liquid state is greater than the internal energy of the same mass of substance in the solid state.

To calculate the amount of heat Q required to melt a crystalline body of mass m, taken at its melting temperature and normal atmospheric pressure, the specific heat of fusion λ must be multiplied by the mass of the body m:

From this formula, it can be determined that

λ = Q / m, m = Q / λ

Experiments show that during the solidification of a crystalline substance exactly the same amount of heat is released that is absorbed during its melting. So, during the solidification of water weighing 1 kg at a temperature of 0 ° C, an amount of heat equal to 3.4 10 5 J is released. Exactly the same amount of heat is required for the melting of ice weighing 1 kg at a temperature of 0 ° C.

When a substance solidifies, everything happens in the reverse order. The speed, and hence the average kinetic energy of molecules in a cooled molten substance, decrease. Attractive forces can now keep slowly moving molecules close to each other. As a result, the arrangement of particles becomes ordered - a crystal is formed. The energy released during crystallization is used to maintain a constant temperature. On the graph, this is the EF section (see Fig. 18).

Crystallization is facilitated if any foreign particles, such as dust particles, are present in the liquid from the very beginning. They become centers of crystallization. Under normal conditions, there are many centers of crystallization in a liquid, near which the formation of crystals occurs.

Table 4
Specific heat of fusion of certain substances (at normal atmospheric pressure)

During crystallization, energy is released and transferred to surrounding bodies.

The amount of heat released during the crystallization of a body of mass m is also determined by the formula

In this case, the internal energy of the body decreases.

Example. To prepare tea, the tourist put ice weighing 2 kg and having a temperature of 0 ° C into the pot. How much heat is needed to turn this ice into boiling water at 100°C? The energy spent on heating the kettle is not taken into account.

What amount of heat would be needed if, instead of ice, a tourist took water of the same mass at the same temperature from the hole?

Let's write down the condition of the problem and solve it.

Questions

How to explain the process of body melting on the basis of the doctrine of the structure of matter?
What is the fuel energy spent on during the melting of a crystalline body heated to the melting point?
What is the specific heat of fusion?
How to explain the process of hardening on the basis of the doctrine of the structure of matter?
How is the amount of heat required to melt a crystalline body taken at the melting point calculated?
How to calculate the amount of heat released during the crystallization of a body that has a melting point?

Exercise 12

Exercise

Place two identical cans on the stove. Pour water weighing 0.5 kg into one, put several ice cubes of the same mass into the other. Note how long it takes for the water in both jars to boil. Write a short account of your experience and explain the results.
Read the paragraph “Amorphous bodies. Melting of amorphous bodies". Prepare a report on it.

Melting

Melting It is the process of changing a substance from a solid to a liquid state.

Observations show that if crushed ice, having, for example, a temperature of 10 ° C, is left in a warm room, then its temperature will rise. At 0 °C, the ice will begin to melt, and the temperature will not change until all the ice has turned into a liquid. After that, the temperature of the water formed from the ice will rise.

This means that crystalline bodies, which include ice, melt at a certain temperature, which is called melting point. It is important that during the melting process the temperature of the crystalline substance and the liquid formed during its melting remains unchanged.

In the experiment described above, the ice received a certain amount of heat, its internal energy increased due to an increase in the average kinetic energy of the movement of molecules. Then the ice melted, its temperature did not change, although the ice received a certain amount of heat. Consequently, its internal energy increased, but not due to the kinetic, but due to the potential energy of the interaction of molecules. The energy received from the outside is spent on the destruction of the crystal lattice. Similarly, the melting of any crystalline body occurs.

Amorphous bodies do not have a specific melting point. As the temperature rises, they gradually soften until they turn into a liquid.

Crystallization

Crystallization is the process by which a substance changes from a liquid state to a solid state. Cooling, the liquid will give off a certain amount of heat to the surrounding air. In this case, its internal energy will decrease due to a decrease in the average kinetic energy of its molecules. At a certain temperature, the process of crystallization will begin, during this process the temperature of the substance will not change until the entire substance passes into a solid state. This transition is accompanied by the release of a certain amount of heat and, accordingly, a decrease in the internal energy of the substance due to a decrease in the potential energy of interaction of its molecules.

Thus, the transition of a substance from a liquid state to a solid state occurs at a certain temperature, called the crystallization temperature. This temperature remains constant throughout the melting process. It is equal to the melting point of this substance.

The figure shows a graph of the dependence of the temperature of a solid crystalline substance on time in the process of heating it from room temperature to the melting point, melting, heating the substance in the liquid state, cooling the liquid substance, crystallization and subsequent cooling of the substance in the solid state.

Specific heat of fusion

Different crystalline substances have different structures. Accordingly, in order to destroy the crystal lattice of a solid at its melting point, it is necessary to inform it of a different amount of heat.

Specific heat of fusion is the amount of heat that must be imparted to 1 kg of a crystalline substance in order to turn it into a liquid at its melting point. Experience shows that the specific heat of fusion is specific heat of crystallization .

The specific heat of fusion is denoted by the letter λ . Unit of specific heat of fusion - [λ] = 1 J/kg.

The values of the specific heat of fusion of crystalline substances are given in the table. The specific heat of melting of aluminum is 3.9 * 10 5 J / kg. This means that for the melting of 1 kg of aluminum at the melting temperature, it is necessary to spend an amount of heat of 3.9 * 10 5 J. The increase in internal energy of 1 kg of aluminum is equal to the same value.

To calculate the amount of heat Q, required to melt a substance with a mass m, taken at the melting point, follows the specific heat of fusion λ multiply by the mass of the substance: Q = λm.

The same formula is used when calculating the amount of heat released during the crystallization of a liquid.

Summary of the lesson “Melting and crystallization. Specific heat of fusion”.

In physics, melting is the transition of a substance from a solid to a liquid state. Classical examples of the melting process are the melting of ice and the transformation of a solid piece of tin into liquid solder when heated with a soldering iron. The transfer of a certain amount of heat to the body leads to a change in its state of aggregation.

Why does solid become liquid?

Heating a solid body leads to an increase in the kinetic energy of atoms and molecules, which at normal temperature are clearly located at the nodes of the crystal lattice, which allows the body to maintain a constant shape and size. When certain critical speeds are reached, atoms and molecules begin to leave their places, bonds are broken, the body begins to lose its shape - it becomes liquid. The melting process does not occur abruptly, but gradually, so that for some time the solid and liquid components (phases) are in equilibrium. Melting refers to endothermic processes, that is, to those that occur with the absorption of heat. The opposite process, when a liquid solidifies, is called crystallization.

Rice. 1. The transition of a solid, crystalline, state of matter into a liquid phase.

It was found that until the end of the melting process, the temperature does not change, although heat is supplied all the time. There is no contradiction here, since the incoming energy during this period of time is spent on breaking the crystalline bonds of the lattice. After the destruction of all bonds, the influx of heat will increase the kinetic energy of the molecules, and, consequently, the temperature will begin to rise.

Rice. 2. Graph of body temperature versus heating time.

Determination of specific heat of fusion

The specific heat of fusion (denoted by the Greek letter “lambda” - λ) is a physical quantity equal to the amount of heat (in joules) that must be transferred to a solid body weighing 1 kg in order to completely transfer it to the liquid phase. The formula for the specific heat of fusion is:

$$ λ =(Q \over m)$$

m is the mass of the melting substance;

Q is the amount of heat transferred to the substance during melting.

Values for different substances are determined experimentally.

Knowing λ, we can calculate the amount of heat that must be imparted to a body of mass m for its complete melting:

In what units is the specific heat of fusion measured?

Specific heat of fusion in SI (International System) is measured in joules per kilogram, J / kg. For some tasks, an off-system unit of measurement is used - kilocalorie per kilogram, kcal / kg. Recall that 1 kcal = 4.1868 J.

Specific heat of fusion of some substances

Information on specific heat values for a particular substance can be found in book reference books or in electronic versions on Internet resources. They are usually presented in the form of a table:

Specific heat of fusion of substances

One of the most refractory substances is tantalum carbide - TaC. It melts at a temperature of 3990 0 C. TaC coatings are used to protect metal molds in which aluminum parts are cast.

Rice. 3. Metal melting process.

What have we learned?

We learned that the transition from solid to liquid is called melting. Melting occurs by transferring heat to a solid. The specific heat of fusion shows how much heat (energy) is needed for a solid substance weighing 1 kg to convert it into a liquid state.

Topic quiz

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Everyone knows that water can be found in nature in three states of aggregation - solid, liquid and gaseous. During melting, solid ice turns into a liquid, and upon further heating, the liquid evaporates, forming water vapor. What are the conditions for melting, crystallization, evaporation and condensation of water? At what temperature does ice melt or steam form? We will talk about this in this article.

This is not to say that water vapor and ice are rare in everyday life. However, the most common is the liquid state - ordinary water. Experts have found that our planet is more than 1 billion cubic kilometers of water. However, no more than 3 million km 3 of water belong to fresh water bodies. A fairly large amount of fresh water "rests" in glaciers (about 30 million cubic kilometers). However, melting the ice of such huge blocks is far from easy. The rest of the water is salty, belonging to the seas of the oceans.

Water surrounds modern man everywhere, during most daily procedures. Many believe that water resources are inexhaustible, and humanity will always be able to use the resources of the Earth's hydrosphere. However, this is not the case. The water resources of our planet are gradually depleted, and in a few hundred years, fresh water on Earth may not remain at all. Therefore, absolutely every person needs to take care of fresh water and save it. After all, even in our time there are states in which water supplies are catastrophically small.

Water properties

Before talking about the melting temperature of ice, it is worth considering the main properties of this unique liquid.

So, water has the following properties:

Lack of color.
Lack of smell.
Lack of taste (however, high-quality drinking water tastes good).
Transparency.
Fluidity.
The ability to dissolve various substances (for example, salts, alkalis, etc.).
Water does not have its own permanent shape and is able to take the shape of the vessel into which it enters.
The ability to be purified by filtration.
Water expands when heated and contracts when cooled.
Water can evaporate to become steam and freeze to form crystalline ice.

This list presents the main properties of water. Now let's figure out what are the features of the solid state of aggregation of this substance, and at what temperature ice melts.

Ice is a solid crystalline substance that has a rather unstable structure. It, like water, is transparent, colorless and odorless. Ice also has properties such as brittleness and slipperiness; it is cold to the touch.

Snow is also frozen water, but has a loose structure and is white in color. It snows every year in most countries of the world.

Both snow and ice are extremely unstable substances. It doesn't take much effort to melt the ice. When does it start melting?

In nature, solid ice exists only at temperatures of 0 °C and below. If the ambient temperature rises and becomes more than 0 °C, the ice begins to melt.

At the melting temperature of ice, at 0 ° C, another process occurs - freezing, or crystallization, of liquid water.

This process can be observed by all inhabitants of the temperate continental climate. In winter, when the temperature outside drops below 0 °C, it often snows and does not melt. And the liquid water that was on the streets freezes, turning into solid snow or ice. In the spring, you can see the reverse process. The ambient temperature rises, so the ice and snow melt, forming numerous puddles and mud, which can be considered the only disadvantage of spring warming.

Thus, we can conclude that at what temperature the ice begins to melt, at the same temperature the process of water freezing begins.

Quantity of heat

In a science such as physics, the concept of the amount of heat is often used. This value shows the amount of energy required for heating, melting, crystallization, boiling, evaporation or condensation of various substances. Moreover, each of these processes has its own characteristics. Let's talk about how much heat is required to heat ice under normal conditions.

To heat the ice, you must first melt it. This requires the amount of heat needed to melt the solid. Heat equals the product of the mass of ice and the specific heat of its melting (330-345 thousand Joules / kg) and is expressed in Joules. Suppose we are given 2 kg of solid ice. Thus, in order to melt it, we need: 2 kg * 340 kJ / kg = 680 kJ.

After that, we need to heat the resulting water. The amount of heat for this process will be a little more difficult to calculate. To do this, you need to know the initial and final temperature of the heated water.

So, let's say that we need to heat the water resulting from the melting of ice by 50 ° C. That is, the difference between the initial and final temperatures = 50 °C (initial water temperature - 0 °C). Then you should multiply the temperature difference by the mass of water and its specific heat capacity, which is equal to 4,200 J * kg / ° C. That is, the amount of heat required to heat water = 2 kg * 50 °C * 4,200 J*kg/°C = 420 kJ.

Then we get that for the melting of ice and the subsequent heating of the resulting water, we need: 680,000 J + 420,000 J = 1,100,000 Joules, or 1.1 Megajoules.

Knowing at what temperature ice melts, you can solve many difficult problems in physics or chemistry.

Finally

So, in this article, we learned some facts about water and its two states of aggregation - solid and liquid. Water vapor, however, is an equally interesting object to study. For example, our atmosphere contains approximately 25*10 16 cubic meters of water vapor. In addition, unlike freezing, the evaporation of water occurs at any temperature and is accelerated when it is heated or in the presence of wind.

We learned at what temperature ice melts and liquid water freezes. Such facts will always be useful to us in everyday life, since water surrounds us everywhere. It is important to always remember that water, especially fresh water, is an exhausting resource of the Earth and needs to be treated with care.