Formula for the amount of heat required to evaporate a body. Calculation of the amount of heat during heat transfer, specific heat capacity of a substance

Exercise 81.
Calculate the amount of heat that will be released during the reduction of Fe 2 O 3 metallic aluminum if 335.1 g of iron was obtained. Answer: 2543.1 kJ.
Solution:
Reaction equation:

= (Al 2 O 3) - (Fe 2 O 3) = -1669.8 -(-822.1) = -847.7 kJ

Calculation of the amount of heat that is released when receiving 335.1 g of iron is made from the proportion:

(2 . 55,85) : -847,7 = 335,1 : X; x = (0847.7 . 335,1)/ (2 . 55.85) = 2543.1 kJ,

where 55.85 atomic mass of iron.

Answer: 2543.1 kJ.

Thermal effect of reaction

Task 82.
Gaseous ethyl alcohol C2H5OH can be obtained by the interaction of ethylene C 2 H 4 (g) and water vapor. Write the thermochemical equation for this reaction, having first calculated its thermal effect. Answer: -45.76 kJ.
Solution:
The reaction equation is:

C 2 H 4 (g) + H 2 O (g) = C2H 5 OH (g); = ?

The values ​​of standard heats of formation of substances are given in special tables. Considering that the heats of formation of simple substances are conventionally assumed to be zero. Let's calculate the thermal effect of the reaction using a consequence of Hess's law, we get:

= (C 2 H 5 OH) – [ (C 2 H 4) + (H 2 O)] =
= -235.1 -[(52.28) + (-241.83)] = - 45.76 kJ

Reaction equations in which their aggregate states or crystalline modification, as well as the numerical value of thermal effects are indicated next to the symbols of chemical compounds, are called thermochemical. In thermochemical equations, unless specifically stated, the values ​​of thermal effects at constant pressure Q p are indicated equal to the change in enthalpy of the system. The value is usually given on the right side of the equation, separated by a comma or semicolon. The following abbreviated designations for the state of aggregation of a substance are accepted: G- gaseous, and- liquid, To

If heat is released as a result of a reaction, then< О. Учитывая сказанное, составляем термохимическое уравнение данной в примере реакции:

C 2 H 4 (g) + H 2 O (g) = C 2 H 5 OH (g); = - 45.76 kJ.

Answer:- 45.76 kJ.

Task 83.
Calculate the thermal effect of the reduction reaction of iron (II) oxide with hydrogen based on the following thermochemical equations:

a) EO (k) + CO (g) = Fe (k) + CO 2 (g); = -13.18 kJ;
b) CO (g) + 1/2O 2 (g) = CO 2 (g); = -283.0 kJ;
c) H 2 (g) + 1/2O 2 (g) = H 2 O (g); = -241.83 kJ.
Answer: +27.99 kJ.

Solution:
The reaction equation for the reduction of iron (II) oxide with hydrogen has the form:

EeO (k) + H 2 (g) = Fe (k) + H 2 O (g); = ?

= (H2O) – [ (FeO)

The heat of formation of water is given by the equation

H 2 (g) + 1/2O 2 (g) = H 2 O (g); = -241.83 kJ,

and the heat of formation of iron (II) oxide can be calculated by subtracting equation (a) from equation (b).

=(c) - (b) - (a) = -241.83 – [-283.o – (-13.18)] = +27.99 kJ.

Answer:+27.99 kJ.

Task 84.
When gaseous hydrogen sulfide and carbon dioxide interact, water vapor and carbon disulfide CS 2 (g) are formed. Write the thermochemical equation for this reaction and first calculate its thermal effect. Answer: +65.43 kJ.
Solution:
G- gaseous, and- liquid, To-- crystalline. These symbols are omitted if the aggregative state of the substances is obvious, for example, O 2, H 2, etc.
The reaction equation is:

2H 2 S (g) + CO 2 (g) = 2H 2 O (g) + CS 2 (g); = ?

The values ​​of standard heats of formation of substances are given in special tables. Considering that the heats of formation of simple substances are conventionally assumed to be zero. The thermal effect of a reaction can be calculated using a corollary of Hess's law:

= (H 2 O) + (СS 2) – [(H 2 S) + (СO 2)];
= 2(-241.83) + 115.28 – = +65.43 kJ.

2H 2 S (g) + CO 2 (g) = 2H 2 O (g) + CS 2 (g); = +65.43 kJ.

Answer:+65.43 kJ.

Thermochemical reaction equation

Task 85.
Write the thermochemical equation for the reaction between CO (g) and hydrogen, as a result of which CH 4 (g) and H 2 O (g) are formed. How much heat will be released during this reaction if 67.2 liters of methane were produced in terms of normal conditions? Answer: 618.48 kJ.
Solution:
Reaction equations in which their aggregate states or crystalline modification, as well as the numerical value of thermal effects are indicated next to the symbols of chemical compounds, are called thermochemical. In thermochemical equations, unless specifically stated, the values ​​of thermal effects at constant pressure Q p equal to the change in enthalpy of the system are indicated. The value is usually given on the right side of the equation, separated by a comma or semicolon. The following abbreviated designations for the state of aggregation of a substance are accepted: G- gaseous, and- something, To- crystalline. These symbols are omitted if the aggregative state of the substances is obvious, for example, O 2, H 2, etc.
The reaction equation is:

CO (g) + 3H 2 (g) = CH 4 (g) + H 2 O (g); = ?

The values ​​of standard heats of formation of substances are given in special tables. Considering that the heats of formation of simple substances are conventionally assumed to be zero. The thermal effect of a reaction can be calculated using a corollary of Hess's law:

= (H 2 O) + (CH 4) – (CO)];
= (-241.83) + (-74.84) ​​– (-110.52) = -206.16 kJ.

The thermochemical equation will be:

22,4 : -206,16 = 67,2 : X; x = 67.2 (-206.16)/22?4 = -618.48 kJ; Q = 618.48 kJ.

Answer: 618.48 kJ.

Heat of formation

Task 86.
The thermal effect of which reaction is equal to the heat of formation. Calculate the heat of formation of NO based on the following thermochemical equations:
a) 4NH 3 (g) + 5O 2 (g) = 4NO (g) + 6H 2 O (l); = -1168.80 kJ;
b) 4NH 3 (g) + 3O 2 (g) = 2N 2 (g) + 6H 2 O (l); = -1530.28 kJ
Answer: 90.37 kJ.
Solution:
The standard heat of formation is equal to the heat of reaction of the formation of 1 mole of this substance from simple substances under standard conditions (T = 298 K; p = 1.0325.105 Pa). The formation of NO from simple substances can be represented as follows:

1/2N 2 + 1/2O 2 = NO

Given is reaction (a), which produces 4 mol of NO, and given reaction (b), which produces 2 mol of N2. Oxygen is involved in both reactions. Therefore, to determine the standard heat of formation of NO, we compose the following Hess cycle, i.e., we need to subtract equation (a) from equation (b):

Thus, 1/2N 2 + 1/2O 2 = NO; = +90.37 kJ.

Answer: 618.48 kJ.

Task 87.
Crystalline ammonium chloride is formed by the reaction of ammonia and hydrogen chloride gases. Write the thermochemical equation for this reaction, having first calculated its thermal effect. How much heat will be released if 10 liters of ammonia were consumed in the reaction, calculated under normal conditions? Answer: 78.97 kJ.
Solution:
Reaction equations in which their aggregate states or crystalline modification, as well as the numerical value of thermal effects are indicated next to the symbols of chemical compounds, are called thermochemical. In thermochemical equations, unless specifically stated, the values ​​of thermal effects at constant pressure Q p equal to the change in enthalpy of the system are indicated. The value is usually given on the right side of the equation, separated by a comma or semicolon. The following have been accepted: To-- crystalline. These symbols are omitted if the aggregative state of the substances is obvious, for example, O 2, H 2, etc.
The reaction equation is:

NH 3 (g) + HCl (g) = NH 4 Cl (k). ;

The values ​​of standard heats of formation of substances are given in special tables. Considering that the heats of formation of simple substances are conventionally assumed to be zero. The thermal effect of a reaction can be calculated using a corollary of Hess's law:

= ?
= (NH4Cl) – [(NH 3) + (HCl)];

The thermochemical equation will be:

= -315.39 – [-46.19 + (-92.31) = -176.85 kJ.

22,4 : -176,85 = 10 : The heat released during the reaction of 10 liters of ammonia in this reaction is determined from the proportion:

Answer: X; x = 10 (-176.85)/22.4 = -78.97 kJ; Q = 78.97 kJ.

78.97 kJ.

>>Physics: Calculation of the amount of heat required to heat a body and released by it during cooling
To learn how to calculate the amount of heat that is necessary to heat a body, let us first establish on what quantities it depends.
From the previous paragraph we already know that this amount of heat depends on the type of substance the body consists of (i.e. its specific heat):
Q depends on c

If we want to heat the water in the kettle so that it becomes only warm, then we will not heat it for long. And in order for the water to become hot, we will heat it longer. But the longer the kettle is in contact with the heater, the more heat it will receive from it.

Consequently, the more the body temperature changes when heated, the greater the amount of heat that needs to be transferred to it.

Let the initial temperature of the body be tbegin, and the final temperature be tend. Then the change in body temperature will be expressed by the difference:

Finally, everyone knows that for heating For example, 2 kg of water requires more time (and therefore more heat) than to heat 1 kg of water. This means that the amount of heat required to heat a body depends on the mass of that body:

So, to calculate the amount of heat, you need to know the specific heat capacity of the substance from which the body is made, the mass of this body and the difference between its final and initial temperatures.

Let, for example, you need to determine how much heat is needed to heat an iron part weighing 5 kg, provided that its initial temperature is 20 °C, and the final temperature should be equal to 620 °C.

From Table 8 we find that the specific heat capacity of iron is c = 460 J/(kg°C). This means that heating 1 kg of iron by 1 °C requires 460 J.
To heat 5 kg of iron by 1 °C, 5 times more heat will be required, i.e. 460 J * 5 = 2300 J.

To heat iron not by 1 °C, but by A t = 600°C, another 600 times more amount of heat will be required, i.e. 2300 J X 600 = 1,380,000 J. Exactly the same (modulo) amount of heat will be released when this iron cools from 620 to 20 °C.

So, to find the amount of heat required to heat a body or released by it during cooling, you need to multiply the specific heat capacity of the body by its mass and by the difference between its final and initial temperatures:

??? 1. Give examples showing that the amount of heat received by a body when heated depends on its mass and temperature changes. 2. What formula is used to calculate the amount of heat required to heat a body or released by it when cooling?

S.V. Gromov, N.A. Rodina, Physics 8th grade

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HEAT EXCHANGE.

1. Heat exchange.

Heat exchange or heat transfer is the process of transferring the internal energy of one body to another without doing work.

There are three types of heat transfer.

1) Thermal conductivity- This is heat exchange between bodies during their direct contact.

2) Convection- This is heat exchange in which heat is transferred by gas or liquid flows.

3) Radiation– This is heat exchange through electromagnetic radiation.

2. Amount of heat.

The amount of heat is a measure of the change in the internal energy of a body during heat exchange. Denoted by the letter Q.

Unit for measuring the amount of heat = 1 J.

The amount of heat received by a body from another body as a result of heat exchange can be spent on increasing temperature (increasing the kinetic energy of molecules) or changing the state of aggregation (increasing potential energy).

3.Specific heat capacity of the substance.

Experience shows that the amount of heat required to heat a body of mass m from temperature T 1 to temperature T 2 is proportional to the mass of the body m and the temperature difference (T 2 - T 1), i.e.

Q = cm(T 2 - T 1 ) = smΔ T,

With is called the specific heat capacity of the substance of the heated body.

The specific heat capacity of a substance is equal to the amount of heat that must be imparted to 1 kg of the substance to heat it by 1 K.

Unit of measurement of specific heat capacity =.

The heat capacity values ​​for various substances can be found in physical tables.

Exactly the same amount of heat Q will be released when the body is cooled by ΔT.

4.Specific heat of vaporization.

Experience shows that the amount of heat required to convert a liquid into steam is proportional to the mass of the liquid, i.e.

Q = Lm,

where is the proportionality coefficient L is called the specific heat of vaporization.

The specific heat of vaporization is equal to the amount of heat required to convert 1 kg of liquid at boiling point into steam.

A unit of measurement for the specific heat of vaporization.

During the reverse process, steam condensation, heat is released in the same amount that was spent on steam formation.

5.Specific heat of fusion.

Experience shows that the amount of heat required to transform a solid into a liquid is proportional to the mass of the body, i.e.

Q = λ m,

where the proportionality coefficient λ is called the specific heat of fusion.

The specific heat of fusion is equal to the amount of heat that is necessary to transform a solid body weighing 1 kg into a liquid at the melting point.

A unit of measurement for the specific heat of fusion.

During the reverse process, crystallization of the liquid, heat is released in the same amount that was spent on melting.

6. Specific heat of combustion.

Experience shows that the amount of heat released during complete combustion of fuel is proportional to the mass of the fuel, i.e.

Q = qm,

Where the proportionality coefficient q is called the specific heat of combustion.

The specific heat of combustion is equal to the amount of heat released during complete combustion of 1 kg of fuel.

Unit of measurement of specific heat of combustion.

7. Heat balance equation.

Heat exchange involves two or more bodies. Some bodies give off heat, while others receive it. Heat exchange occurs until the temperatures of the bodies become equal. According to the law of conservation of energy, the amount of heat that is given out is equal to the amount that is received. On this basis, the heat balance equation is written.

Let's look at an example.

A body of mass m 1, the heat capacity of which is c 1, has a temperature T 1, and a body of mass m 2, the heat capacity of which is c 2, has a temperature T 2. Moreover, T 1 is greater than T 2. These bodies are brought into contact. Experience shows that a cold body (m 2) begins to heat up, and a hot body (m 1) begins to cool. This suggests that part of the internal energy of the hot body is transferred to the cold one, and the temperatures are equalized. Let us denote the final overall temperature by θ.

The amount of heat transferred from a hot body to a cold one

Q transferred. = c 1 m 1 (T 1 – θ )

The amount of heat received by a cold body from a hot one

Q received. = c 2 m 2 (θ – T 2 )

According to the law of conservation of energy Q transferred. = Q received., i.e.

c 1 m 1 (T 1 – θ )= c 2 m 2 (θ – T 2 )

Let's open the brackets and express the value of the total steady-state temperature θ.

In this case, we obtain the temperature value θ in kelvins.

However, since Q is passed in the expressions.

and Q is received. is the difference between two temperatures, and it is the same both in Kelvin and in degrees Celsius, then the calculation can be carried out in degrees Celsius. Then

In this case, we obtain the temperature value θ in degrees Celsius.

The equalization of temperatures as a result of thermal conductivity can be explained on the basis of molecular kinetic theory as the exchange of kinetic energy between molecules upon collision in the process of thermal chaotic motion.

This example can be illustrated with a graph.

In this lesson we will learn how to calculate the amount of heat required to heat a body or released by it when cooling. To do this, we will summarize the knowledge that was acquired in previous lessons.

In addition, we will learn, using the formula for the amount of heat, to express the remaining quantities from this formula and calculate them, knowing other quantities. An example of a problem with a solution for calculating the amount of heat will also be considered.

This lesson is devoted to calculating the amount of heat when a body is heated or released when cooled.

The ability to calculate the required amount of heat is very important. This may be needed, for example, when calculating the amount of heat that needs to be imparted to water to heat a room.

Rice. 1. The amount of heat that must be imparted to the water to heat the room

Or to calculate the amount of heat that is released when fuel is burned in various engines:

Rice. 2. The amount of heat that is released when fuel is burned in the engine

This knowledge is also needed, for example, to determine the amount of heat that is released by the Sun and falls on the Earth:

Rice. 3. The amount of heat released by the Sun and falling on the Earth

• To calculate the amount of heat, you need to know three things (Fig. 4):
• body weight (which can usually be measured using a scale);
• the temperature difference by which a body must be heated or cooled (usually measured using a thermometer);

specific heat capacity of the body (which can be determined from the table).

Rice. 4. What you need to know to determine

The formula by which the amount of heat is calculated looks like this:

The following quantities appear in this formula:

The amount of heat measured in joules (J);

- The specific heat capacity of a substance is measured in ;

temperature difference, measured in degrees Celsius ().

Let's consider the problem of calculating the amount of heat.

A copper glass with a mass of grams contains water with a volume of liter at a temperature. How much heat must be transferred to a glass of water so that its temperature becomes equal to ?

Rice. 5. Illustration of the problem conditions

First we write down a short condition ( Given) and convert all quantities to the International System (SI).

Solution:

First, determine what other quantities we need to solve this problem. Using the table of specific heat capacity (Table 1) we find (specific heat capacity of copper, since by condition the glass is copper), (specific heat capacity of water, since by condition there is water in the glass). In addition, we know that to calculate the amount of heat we need a mass of water. According to the condition, we are given only the volume. Therefore, from the table we take the density of water: (Table 2).

Table 1. Specific heat capacity of some substances,

Table 2. Densities of some liquids

Now we have everything we need to solve this problem.

Note that the final amount of heat will consist of the sum of the amount of heat required to heat the copper glass and the amount of heat required to heat the water in it:

Let's first calculate the amount of heat required to heat a copper glass:

Before calculating the amount of heat required to heat water, let’s calculate the mass of water using a formula that is familiar to us from grade 7:

Now we can calculate:

Then we can calculate:

Let's remember what kilojoules mean. The prefix "kilo" means .

Answer:.

For the convenience of solving problems of finding the amount of heat (the so-called direct problems) and quantities associated with this concept, you can use the following table.

This example can be illustrated with a graph.

In this lesson we will learn how to calculate the amount of heat required to heat a body or released by it when cooling. To do this, we will summarize the knowledge that was acquired in previous lessons.

In addition, we will learn, using the formula for the amount of heat, to express the remaining quantities from this formula and calculate them, knowing other quantities. An example of a problem with a solution for calculating the amount of heat will also be considered.

This lesson is devoted to calculating the amount of heat when a body is heated or released when cooled.

The ability to calculate the required amount of heat is very important. This may be needed, for example, when calculating the amount of heat that needs to be imparted to water to heat a room.

Rice. 1. The amount of heat that must be imparted to the water to heat the room

Or to calculate the amount of heat that is released when fuel is burned in various engines:

Rice. 2. The amount of heat that is released when fuel is burned in the engine

This knowledge is also needed, for example, to determine the amount of heat that is released by the Sun and falls on the Earth:

Rice. 3. The amount of heat released by the Sun and falling on the Earth

• To calculate the amount of heat, you need to know three things (Fig. 4):
• body weight (which can usually be measured using a scale);
• the temperature difference by which a body must be heated or cooled (usually measured using a thermometer);

specific heat capacity of the body (which can be determined from the table).

Rice. 4. What you need to know to determine

The formula by which the amount of heat is calculated looks like this:

The following quantities appear in this formula:

The amount of heat measured in joules (J);

- The specific heat capacity of a substance is measured in ;

temperature difference, measured in degrees Celsius ().

Let's consider the problem of calculating the amount of heat.

A copper glass with a mass of grams contains water with a volume of liter at a temperature. How much heat must be transferred to a glass of water so that its temperature becomes equal to ?

Rice. 5. Illustration of the problem conditions

First we write down a short condition ( Given) and convert all quantities to the International System (SI).

Solution:

First, determine what other quantities we need to solve this problem. Using the table of specific heat capacity (Table 1) we find (specific heat capacity of copper, since by condition the glass is copper), (specific heat capacity of water, since by condition there is water in the glass). In addition, we know that to calculate the amount of heat we need a mass of water. According to the condition, we are given only the volume. Therefore, from the table we take the density of water: (Table 2).

Table 1. Specific heat capacity of some substances,

Table 2. Densities of some liquids

Now we have everything we need to solve this problem.

Note that the final amount of heat will consist of the sum of the amount of heat required to heat the copper glass and the amount of heat required to heat the water in it:

Let's first calculate the amount of heat required to heat a copper glass:

Before calculating the amount of heat required to heat water, let’s calculate the mass of water using a formula that is familiar to us from grade 7:

Now we can calculate:

Then we can calculate:

Let's remember what kilojoules mean. The prefix "kilo" means .

Answer:.

For the convenience of solving problems of finding the amount of heat (the so-called direct problems) and quantities associated with this concept, you can use the following table.