Thermochemical equation of the reaction. Thermochemical equations. Options for test tasks

Video tutorial 2: Calculations using thermochemical equations

Lecture: Thermal effect of a chemical reaction. Thermochemical equations

Thermal effect of a chemical reaction

Thermochemistry is a branch of chemistry that studies thermal, i.e. thermal effects of reactions.

As you know, each chemical element has n-amount of energy. We face this every day, because... Each meal our body stores energy from chemical compounds. Without this, we will not have the strength to move or work. This energy maintains a constant t of 36.6 in our body.

At the time of reactions, the energy of the elements is spent either on destruction or on the formation of chemical bonds between atoms. Energy must be expended to break a bond and released to form it. And when the energy released is greater than the energy expended, the resulting excess energy turns into heat. Thus:

The release and absorption of heat during chemical reactions is called thermal effect of reaction, and is designated by the letters Q.

Exothermic reactions– in the process of such reactions, heat is released, and it is transferred to the environment.

This type of reaction has a positive thermal effect +Q. As an example, let's take the combustion reaction of methane:

Endothermic reactions– in the process of such reactions, heat is absorbed.

This type of reaction has a negative thermal effect -Q. For example, consider the reaction of coal and water at high t:

The thermal effect of a reaction directly depends on temperature as well as pressure.

Thermochemical equations

The thermal effect of a reaction is determined using a thermochemical equation. How is it different? In this equation, next to the symbol of an element, its state of aggregation (solid, liquid, gaseous) is indicated. This must be done because The thermal effect of chemical reactions is influenced by the mass of the substance in the aggregate state. At the end of the equation, after the = sign, the numerical value of thermal effects in J or kJ is indicated.

As an example, a reaction equation is presented showing the combustion of hydrogen in oxygen: H 2 (g) + ½O 2 (g) → H 2 O (l) + 286 kJ.

The equation shows that 286 kJ of heat is released per 1 mole of oxygen and 1 mole of water formed. The reaction is exothermic. This reaction has a significant thermal effect.

When any compound is formed, the same amount of energy will be released or absorbed as is absorbed or released during its decomposition into primary substances.

Almost all thermochemical calculations are based on the law of thermochemistry - Hess's law. The law was derived in 1840 by the famous Russian scientist G. I. Hess.

Basic law of thermochemistry: the thermal effect of a reaction depends on the nature and physical state of the starting and final substances, but does not depend on the path of the reaction.

By applying this law, it will be possible to calculate the thermal effect of an intermediate stage of a reaction if the overall thermal effect of the reaction and the thermal effects of other intermediate stages are known.

Knowledge of the thermal effect of a reaction is of great practical importance. For example, nutritionists use them when preparing a proper diet; in the chemical industry, this knowledge is necessary when heating reactors and, finally, without calculating the thermal effect it is impossible to launch a rocket into orbit.

Task 88.

The thermal effect of which reaction is equal to the heat of formation of methane? Calculate the heat of formation of methane based on the following thermochemical equations:

A) H 2 (g) + 1/2O 2 (g) = H 2 O (l); = -285.84 kJ;
b) C (k) + O 2 (g) = CO 2 (g); = -393.51 kJ;
c) CH 4 (g) + 2O 2 (g) = 2H 2 O (l) + CO 2 (g); = -890.31 kJ.
Answer: -74.88 kJ.

Solution:
. 105 Pa). The formation of methane from hydrogen and carbon can be represented as follows:

C (graphite) + 2H 2 (g) = CH 4 (g); = ?

Based on these equations according to the conditions of the problem, taking into account that hydrogen burns to water, carbon to carbon dioxide, methane to carbon dioxide and water and, based on Hess’s law, thermochemical equations can be operated in the same way as algebraic ones. To obtain the desired result, you need to multiply the hydrogen combustion equation (a) by 2, and then subtract the sum of the hydrogen (a) and carbon (b) combustion equations from the methane combustion equation (c):

CH 4 (g) + 2O 2 (g) - 2 H 2 (g) + O 2 (g) - C (k) + O 2 (g) =
= 2H 2 O (l) + CO 2 - 2H 2 O - CO 2;
= -890,31 – [-393,51 + 2(-285,84).

CH 4 (g) = C (k) + 2H 2 (k); = +74.88 kJ.2

Since the heat of formation is equal to the heat of decomposition with the opposite sign, then

(CH 4) = -74.88 kJ.

Answer: -74.88 kJ.

Task 89.
The thermal effect of which reaction is equal to the heat of formation of calcium hydroxide? Calculate the heat of formation of calcium hydroxide based on the following thermochemical equations:

Ca (k) + 1/2O (g) = CaO (k); = -635.60 kJ;
H 2 (g) + 1/2O 2 (g) = H 2 O (l); = -285.84 kJ;
CaO (k) + H 2 O (l) = Ca (OH) 2 (k); = -65.06 kJ.
Answer: -986.50 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 calcium hydroxide from simple substances can be represented as follows:

Ca (k) + O 2 (g) + H 2 (g) = Ca (OH) 2 (k); = ?

Based on the equations that are given according to the conditions of the problem and taking into account that hydrogen burns to water, and calcium, reacting with oxygen, forms CaO, then, based on Hess’s law, thermochemical equations can be operated in the same way as algebraic ones. To obtain the desired result, you need to add all three equations together:

CaO (k) + H 2 O (l) + Ca (k) + 1/2O (g) + H 2 (g) + 1/2O 2 (g = (OH) 2 (k) + CaO (k) + H 2 O (l);
= -65.06 + (-635.60) + (-285.84) = -986.50 kJ.

Since the standard heats of formation of simple substances are conventionally assumed to be zero, the heat of formation of calcium hydroxide will be equal to the thermal effect of the reaction of its formation from simple substances (calcium, hydrogen and oxygen):

== (Ca(OH) 2 = -986.50 kJ.2

Answer: -986.50 kJ.

Task 90.
The thermal effect of the combustion reaction of liquid gasoline with the formation of water vapor and carbon dioxide is equal to -3135.58 kJ. Make up a thermochemical equation for this reaction and calculate the heat of formation of C 6 H 6 (l). Answer: +49.03 kJ.
Solution:
Reaction equations in which their state of aggregation or crystal 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 Qp 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, g - liquid, j - crystalline. These symbols are omitted if the aggregative state of the substances is obvious, for example, O 2, H 2, etc.
The thermochemical equation of the reaction is:

C 6 H 6 (l) + 7/2O 2 = 6CO 2 (g) + 3H 2 O (g); = -3135.58 kJ.

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:

6 (CO 2) + 3 =0 (H 2 O) – (C 6 H 6)

(C 6 H 6) = -;
(C 6 H 6) = - (-3135.58) = +49.03 kJ.

Answer:+49.03 kJ.

Heat of formation

Task 91.
Calculate how much heat will be released during the combustion of 165 liters (n.s.) of acetylene C 2 H 2 if the combustion products are carbon dioxide and water vapor? Answer: 924.88 kJ.
Solution:
Reaction equations in which their state of aggregation or crystal 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 Qp 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-- 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:

C 2 H 2 (g) + 5/2O 2 (g) = 2CO 2 (g) + H 2 O (g); = ?

2(CO 2) + (H 2 O) – (C 2 H 2);
= 2(-393.51) + (-241.83) – (+226.75) = -802.1 kJ.

The heat released during the combustion of 165 liters of acetylene by this reaction is determined from the proportion:

22.4: -802.1 = 165: x; x = 165 (-802.1)/22.4 = -5908.35 kJ; Q = 5908.35 kJ.

Answer: 5908.35 kJ.

Task 92.
When ammonia gas burns, it produces water vapor and nitrogen oxide. How much heat will be released during this reaction if 44.8 liters of NO were obtained, based on normal conditions? Answer: 452.37 kJ.
Solution:
The reaction equation is:

NH 3 (g) + 5/4O 2 = NO (g) + 3/2H 2 O (g)

= (NO) + 3/2 (H 2 O) – (NH 3);
= +90.37 +3/2 (-241.83) – (-46.19) = -226.185 kJ.

The thermochemical equation will be:

We calculate the heat released during the combustion of 44.8 liters of ammonia from the proportion:

22.4: -226.185 = 44.8: x; x = 44.8 (-226.185)/22.4 = -452.37 kJ; Q = 452.37 kJ.

Answer: 452.37 kJ

Equations of chemical reactions, which indicate their thermal

effects are called thermochemical equations.

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Thermochemical equations have a number of features:

a) Since the state of the system depends on the aggregate states of substances

in general, in thermochemical equations using letter subscripts

(j), (g), (p) and (d) indicate the states of substances (crystalline, liquid, dissolved and gaseous). For example,

b) So that the thermal effect of the reaction is expressed in kJ/mol of one of the starting substances or reaction products, in thermochemical equations

Fractional odds are allowed. For example,

=−46.2 kJ/mol.

c) Often the heat of reaction (thermal effect) is written as ∆H

The upper index 0 means the standard value of the thermal effect (the value obtained under standard conditions, i.e., at a pressure of 101 kPa), and the lower index means the temperature at which the interaction occurs.

The peculiarity of thermochemical equations is that when working with them, you can transfer the formulas of substances and the magnitude of thermal effects from one part of the equation to another. As a rule, this cannot be done with ordinary equations of chemical reactions.

Term-by-term addition and subtraction of thermochemical equations is also allowed. This may be necessary to determine the thermal effects of reactions that are difficult or impossible to measure experimentally.

11.Formulate Hess’s law and a corollary to Hess’s law.

Hess's law is formulated as follows: the thermal effect of a chemical reaction does not depend on the path of its occurrence, but depends only on the nature and physical state (enthalpy) of the starting substances and reaction products.

Corollary 1. The thermal effect of the reaction is equal to the difference between the sums of the heats of formation of the reaction products and the heats of formation of the starting substances, taking into account their stoichiometric coefficients.

Corollary 2. If the thermal effects of a number of reactions are known, then it is possible to determine the thermal effect of another reaction, which includes substances and compounds included in the equations for which the thermal effect is known. At the same time, with thermochemical equations you can perform a variety of arithmetic operations (addition, subtraction, multiplication, division) as with algebraic equations.

12.What is the standard enthalpy of formation of a substance?

The standard enthalpy of formation of a substance is the thermal effect of the reaction of formation of 1 mole of a given substance from the corresponding amount of simple substances under standard conditions.

13.What is entropy? How is it measured?

Entropy is a thermodynamic function of the state of the system, and its value depends on the amount of the substance (mass) under consideration, temperature, and state of aggregation.

Units J/C

14.Formulate the 2nd and 3rd laws of thermodynamics.

Second law of thermodynamics

In isolated systems (Q= 0, A= 0, U= const) spontaneously occur

only those processes that are accompanied by an increase in the entropy of the system, i.e. S>0.

The spontaneous process ends when the maximum at

given entropy conditions S max, i.e. when ∆S= 0.

Thus, in isolated systems, the criterion for a spontaneous process is an increase in entropy, and the limit of such a process is -∆S = 0.

Third law of thermodynamics

The entropy of each chemical element in an ideal crystalline state at a temperature close to absolute zero is close to zero.

The entropy of nonideal crystals is greater than zero, since they can be considered

as mixtures with entropy of mixing. This is also true for crystals that have defects in the crystal structure. This leads to the principle

the unattainability of absolute zero temperature. Currently achieved

lowest temperature 0.00001 K.

From the lesson materials you will learn which chemical reaction equation is called thermochemical. The lesson is devoted to studying the calculation algorithm for the thermochemical reaction equation.

Topic: Substances and their transformations

Lesson: Calculations using thermochemical equations

Almost all reactions occur with the release or absorption of heat. The amount of heat that is released or absorbed during a reaction is called thermal effect of a chemical reaction.

If the thermal effect is written in the equation of a chemical reaction, then such an equation is called thermochemical.

In thermochemical equations, unlike ordinary chemical ones, the aggregate state of the substance (solid, liquid, gaseous) must be indicated.

For example, the thermochemical equation for the reaction between calcium oxide and water looks like this:

CaO (s) + H 2 O (l) = Ca (OH) 2 (s) + 64 kJ

The amount of heat Q released or absorbed during a chemical reaction is proportional to the amount of substance of the reactant or product. Therefore, using thermochemical equations, various calculations can be made.

Let's look at examples of problem solving.

Task 1:Determine the amount of heat spent on the decomposition of 3.6 g of water in accordance with the TCA of the water decomposition reaction:

You can solve this problem using the proportion:

during the decomposition of 36 g of water, 484 kJ were absorbed

during decomposition 3.6 g of water was absorbed x kJ

In this way, an equation for the reaction can be written. The complete solution to the problem is shown in Fig. 1.

Rice. 1. Formulation of the solution to problem 1

The problem can be formulated in such a way that you will need to create a thermochemical equation for the reaction. Let's look at an example of such a task.

Problem 2: When 7 g of iron interacts with sulfur, 12.15 kJ of heat is released. Based on these data, create a thermochemical equation for the reaction.

I draw your attention to the fact that the answer to this problem is the thermochemical equation of the reaction itself.

Rice. 2. Formalization of the solution to problem 2

1. Collection of problems and exercises in chemistry: 8th grade: for textbooks. P.A. Orzhekovsky and others. “Chemistry. 8th grade” / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006. (p.80-84)

2. Chemistry: inorganic. chemistry: textbook. for 8th grade general education establishment /G.E. Rudzitis, F.G. Feldman. - M.: Education, OJSC “Moscow Textbooks”, 2009. (§23)

3. Encyclopedia for children. Volume 17. Chemistry / Chapter. ed.V.A. Volodin, Ved. scientific ed. I. Leenson. - M.: Avanta+, 2003.

Additional web resources

1. Solving problems: calculations using thermochemical equations ().

2. Thermochemical equations ().

Homework

1) p. 69 problems No. 1,2 from the textbook “Chemistry: inorganic.” chemistry: textbook. for 8th grade general education institution." /G.E. Rudzitis, F.G. Feldman. - M.: Education, OJSC “Moscow Textbooks”, 2009.

2) pp. 80-84 No. 241, 245 from the Collection of problems and exercises in chemistry: 8th grade: for textbooks. P.A. Orzhekovsky and others. “Chemistry. 8th grade” / P.A. Orzhekovsky, N.A. Titov, F.F. Hegel. - M.: AST: Astrel, 2006.

Thermal effects of chemical reactions

Thermal effect of the reaction - the amount of heat that is released or absorbed by a system as a result of a chemical reaction. This can be DN (P,T = const) or DU (V,T = const).

If heat is released as a result of a reaction, i.e. the enthalpy of the system decreases ( DH< 0 ), then the reaction is called exothermic.

Reactions accompanied by the absorption of heat, i.e. with increasing enthalpy of the system ( DH > 0), are called e ndothermic.

Like other state functions, enthalpy depends on the amount of substance, so it is scaled ( DN) usually referred to 1 mol of substance and expressed in kJ/mol.

Typically, system functions are determined by standard conditions, which, in addition to the standard state parameters, includes the standard temperature T = 298.15 K (25°C). Temperature is often indicated as a subscript ().

Thermochemical equations

Thermochemical reaction equations- equations that indicate the thermal effect, reaction conditions and aggregative states of substances. Usually the enthalpy of the reaction is indicated as the thermal effect. For example,

C (graphite) + O 2 (gas) = CO 2 (gas), DH 0 298 = -396 kJ.

The thermal effect can be written in the reaction equation:

C (graphite) + O 2 (gas) = CO 2 (gas) + 396 kJ.

In chemical thermodynamics, the first form of notation is used more often.

Features of thermochemical equations.

1. The thermal effect depends on the mass of the reactant, po-

Therefore, it is usually calculated per mole of substance. In this regard, in thermochemical equations one can use fractional odds. For example, for the case of the formation of one mole of hydrogen chloride, the thermochemical equation is written as follows:

½H 2 + ½Cl 2 = HCl, DH 0 298 = -92 kJ

or H 2 + Cl 2 = 2HCl, DH 0 298 = -184 kJ.

2. Thermal effects depend on the state of aggregation of the reagents; it is indicated in thermochemical equations by indices: and - liquid, G- gaseous, T - hard or To - crystalline, R– dissolved.