The heat of combustion is understood as the ratio of the released heat to the mass of the fuel element that was consumed, or otherwise, the amount of heat released by a unit mass of fuel during its complete combustion. The heat of combustion of fuel is an integral characteristic. To determine the heat of combustion, you can use the calorimetry method. The essence of this method is that the heat source is placed in a closed vessel, the reaction is carried out, and the heat released is removed until the temperature of the combustion products becomes equal to the initial temperature of the heat source. The measured amount of heat is divided by the mass of fuel in the calorimeter.
The heat of combustion of a CT defined in this way is different from the change in chemical energy AU X on the difference in heat capacities of the initial and final substances.
Indeed, before being placed in the calorimeter, the fuel had an energy reserve of one kilogram:
Where U XT- chemical energy of TT; c t is the heat capacity of the heat transformer; T- temperature at which calorimetry begins and ends.
After combustion, the energy reserve of the fuel is equal to U x ap 4- c np dT + Q, Where U x a- a certain amount of chemical energy, counted from the previous level. Hence,
where 
The value of c t - c ir = first of all depends on the conditions of calorimetry (it is carried out at constant pressure or volume).
Each chemical compound has its own level of chemical energy, which is characterized by the heat of formation.
The heat of formation is understood as the amount of heat released (-D Hf) or absorbed (+D Hf) when a chemical compound is formed from simple substances.
To carry out a thermodynamic calculation of the composition and parameters of the working process of combustion products, relative enthalpy values (the difference in the enthalpy values of substances in different states) with a certain conditional reference point are used. This conditional reference point can be arbitrary, but it is the same for all substances participating in the process - the standard state. For H 2, 0 2, N 2, F 2, C1 2, the enthalpy of the gaseous molecular state is taken as the starting point, i.e., the heat of formation of these substances is zero. These gaseous molecular compounds are stable at T 0 = 293.15 K ir= 0.1 MPa. Solid p-graphite is taken as the standard state of the substance (in the USA, solid carbon in the form of diamond is accepted). For C, the allotropic form of p-graphite is adopted, for metals such as Al, Mg, Li, Be and others - crystalline forms.
The heat of formation is considered positive when the formation of a substance from simple substances occurs with the absorption of heat (endothermic reactions), and is considered negative when the formation of a substance occurs with the release of heat (exothermic reactions). As an example, Table 5.1 shows the values of the standard heat of formation of some substances.
If, as a result of combustion, a substance is formed from simple substances that are in a standard state, the heat of formation of combustion products is equal in absolute value and opposite in sign to the heat of combustion.
Yes, in reaction
the heat of formation of H 2 0 will be negative, and the heat of combustion of fuel 2H 2 + 0 2 will be positive.
Then 
where A/Go is the heat of formation of substances taken in the standard state. In the notation for heat of formation, D indicates a change in energy level relative to the standard state. The index "°" at the top indicates standardity, and the subscript "0" at the bottom, expressed as a number, gives the absolute temperature of the original components in the standard state. Enthalpies of elements in standard state at temperature T 0 are taken as the reference point for the enthalpy of fuel components and combustion products.
The standard heat of formation is understood as the heat of formation of a substance from simple substances (elements) in a standard state under standard conditions:
Table 5.1
Standard heat of formation values for some substances
The standard heat of formation is determined experimentally. As temperature T 0 most often used T 0= 298.15 K, and also T 0= 293.15 K, T 0= O K. In this case, the heats of formation of the elements themselves in the standard - stable - and most common natural states are taken equal to zero. The expression for calculating the initial value of molar enthalpy can be written as
Where (Н° t -Нт 0)- change in enthalpy as a result of chemical reactions.
When taken as standard temperature T 0 = O K heat of formation AN)t 0 converted into a pure measure of chemical energy.
The relationship between the molar enthalpy of fuel and the heat of formation is based on Hess's law, which is a special case of the law of conservation of energy. Hess's law states that the final value of the heat of formation during a chemical transformation does not depend on which reaction sequence took place, but is determined only by the state parameters of the initial characteristics and final reaction products. In accordance with this law, the heat of formation (or enthalpy change AN) process associated with chemical transformations or changes in state can be calculated by the relation
where v is the number of moles of the substance; A N) t- heat of formation of a substance at temperature T, equal to the change in enthalpy during its formation from elements taken at this temperature in standard states.
Example. Determine the heat of formation of diethyl cyclohexane if it is known that its combustion in an oxygen atmosphere releases 6320 kJ/mol:
Since the combustion reaction of diethylcyclohexane is exothermic, the heat of formation of the reaction is:
The heat of oxygen formation under the specified conditions (T = 293.15 K; R= 0.1 MPa) is taken equal to zero (standard conditions).
When calculating the heat of formation and enthalpy, attention should be paid to reference data on the thermal effects of chemical reactions, since, along with the generally accepted values of thermal effects, there are values of thermal effects during the formation of water in the form of steam. In this case, the value of the heat of formation of water should be reduced by 44.2 kJ/mol, which corresponds to the heat of vaporization.
The heat of formation during complete combustion of fuel, when the reaction products are complete oxides of elements (H 2 0, C0 2, A1 2 0 3, etc.), is the heat of combustion of the fuel.
There are various experimental methods for determining the heat of formation, for example the calorimetry method or the spectral method. If the heat of formation is determined by the calorimetric method, then the standard temperature is taken T 0 = 298 K or T 0 = 293 K. The spectral method has greater accuracy and is simpler. The essence of this method is that in order to remove parts of a dissociating molecule to an infinite distance from each other, it is necessary to use energy (heat of formation). Due to the fact that with such a division of atoms, energy is quantized, the change in the distance between nuclei at a sufficiently high temperature and the transition of energy from one level to another corresponds to its own line in the radiation spectrum. In this case, as the distance between the atoms increases, the bands approach each other and tend to a certain limit. The position of the point of merging of the bands gives the energy characteristic of the dissociating substance, i.e., it allows one to determine the heat of formation of the substance in the standard state. To experimentally determine the heat of combustion, fuel is burned in an environment with an excess of oxidizer. The heat of combustion of fuel is distinguished when water is released in the form of liquid or steam. The heat of combustion during the formation of water in the form of a liquid corresponds to the case when the heat released during the condensation of water vapor contained in the combustion products is taken into account.
In rocket engines, fuel combustion occurs, as a rule, when there is a lack of oxidizer. The thermal effect of the combustion reaction under these conditions without the addition of oxygen from the environment is called caloric value. There are higher and lower calorie contents of fuel when water is released in the form of liquid and in the form of steam.
Since the determination of the heat of combustion of fuel is usually carried out calorimetrically in a Cracker bomb (a bomb of constant volume), the experimental values of both the heat of combustion and caloric content correspond to the heat release during the formation of water in the form of a liquid. In a solid propellant rocket engine, the combustion products of fuel throughout the flow path have a temperature that excludes the possibility of water condensation, and therefore the highest calorific value cannot be realized. If there are compounds in the combustion products that, at a certain temperature in the flow part of the solid propellant rocket engine, can undergo phase transitions, it is necessary to take into account the heat of their condensation. Typically, the heat of phase transitions is reflected in tables of the dependence of enthalpy on temperature. The enthalpy of a multicomponent fuel, consisting of several compounds, is determined by its mass composition and the initial values of the total enthalpy of the components contained in the fuel. If the fuel contains t 1,%, enthalpy compounds H v t p,%, compounds with enthalpy I p it. d., then the total enthalpy is equal to 
In order to be able to compare the thermal effects of various reactions and carry out thermochemical calculations, the concept of thermal effect under standard conditions was introduced. The following standard states are currently accepted:
For individual crystalline and liquid substances - the real state (the most stable modification) at a given temperature and pressure of 1 bar.
For individual gases - a hypothetical state that occurs during isothermal expansion of a gas to an infinitesimal pressure with subsequent compression of 1 bar, but according to the ideal gas isotherm Fig. 3
1 – real gas
2 – ideal gas
The thermal effect under standard conditions is calculated using the standard heats of formation and combustion. Standard heat of formation is the thermal effect of the reaction of the formation of 1 mole of a given substance from simple substances (or elements) at a pressure of 1.013 * 10 5 Pa and provided that all participants in the reaction are in stable states of aggregation.
For the convenience of comparing standard heats of formation, they are referred to a base temperature of 298 K. The standard state of a pure liquid or crystalline (solid) substance is taken to be its most stable physical state at a given temperature and normal atmospheric pressure. As a standard state for a gas, a hypothetical state is accepted, in which the gas at p = 1.013 * 10 5 Pa obeys the laws of ideal gases, and its enthalpy is equal to the enthalpy of a real gas. The standard heats of formation of simple substances (elements) in a stable state of aggregation are taken as zero. The heat of formation is referred to 1 mole of a substance, indicating its state of aggregation.
Standard calorific value is the heat released during the combustion of 1 mole of a substance in an oxygen atmosphere at a standard pressure of 1.013 * 10 5 Pa to the simplest oxides. In this case, all participants in the reaction must be in stable states of aggregation. Like standard heats of formation, standard heats of combustion are referred to a base temperature of 298 K. Combustion products under these conditions are CO 2 (g), H 2 O (l), SO 2 (g), N 2, etc. Standard heats combustion of the simplest oxides in stable states is taken as zero.
Heat capacity
True heat capacity body (C) is the ratio of the infinitesimal amount of heat δQ received by the body to the corresponding temperature increment: C = δQ/dT. The heat capacity of a body with a mass equal to one is called specific. Molar heat capacity is more convenient to use. Molar heat capacity C M is the amount of heat obtained by 1 mole of a substance when its temperature increases by one.
Sometimes used average heat capacity. The average molar heat capacity (C) in the temperature range from T 1 to T 2 is the heat capacity that is equal to the ratio of the amount of heat (Q) obtained by 1 mole of a substance to the temperature increment (∆T). In this temperature range, C=Q/∆T is constant.
Molar values of heat capacity are expressed in J/(mol K), and specific values in J/(g K). True heat capacity depends on the nature of the substance, temperature and conditions under which heat transfer to the system occurs. If the system is enclosed in a constant volume, then the amount of heat required to increase the temperature by one unit will be expressed by the equality:
where C V is the isochoric heat capacity.
If a system contracts or expands and the pressure remains constant, then
where C P is isobaric heat capacity.
Heat capacities at constant volume and constant pressure differ by the amount of work required to change the volume of the system. Since in the process p=const the work of isobaric expansion of 1 mole of an ideal gas is performed, a greater amount of heat is required to increase the temperature of the system by one unit, therefore C P > C V:
C P = C V + R - Mayer's equation,
where R is the universal gas constant. In liquids and solids, due to the small change in volume when heated, C P ≈C V.
Let's talk about what the heat of formation is, and also define those conditions that are called standard. In order to understand this issue, let’s find out the differences between simple and complex substances. To consolidate the concept of “heat of formation,” let’s consider specific chemical equations.
Standard enthalpy of formation of substances
The reaction of carbon with hydrogen gas releases 76 kJ of energy. In this case, this figure is the thermal effect. But this is also the heat of formation of a methane molecule from simple substances. "Why?" - you ask. This is explained by the fact that the initial components were carbon and hydrogen. 76 kJ/mol will be the energy that chemists call the “heat of formation.”
Data tables
In thermochemistry, there are numerous tables that indicate the heats of formation of various simple substances. For example, the heat of formation of a substance whose formula is CO 2 in the gaseous state is 393.5 kJ/mol.
Practical significance
Why are these quantities needed? The heat of formation is a quantity that is used when calculating the thermal effect of any chemical process. In order to carry out such calculations, the application of the law of thermochemistry will be required.
Thermochemistry
It is the basic law that explains the energy processes observed during a chemical reaction. During interaction, qualitative transformations are observed in the reacting system. Some substances disappear, and new components appear in their place. This process is accompanied by a change in internal energy in the system, which manifests itself in the form of work or heat. The work associated with expansion for chemical transformations has a minimum indicator. The heat released when one component transforms into another substance can be large.
If we consider various transformations, for almost all of them absorption or release of a certain amount of heat is observed. To explain the phenomena occurring, a special section was created - thermochemistry.
Hess's law
Thanks to this, it became possible to calculate the thermal effect depending on the conditions of the chemical reaction. Calculations are based on the basic law of thermochemistry, namely Hess's law. Let us give its formulation: the thermal effect of a chemical transformation is associated with the nature, initial and final state of the substance, it is not associated with the path of interaction.
What follows from this formulation? In the case of obtaining a certain product, there is no need to use only one reaction option; the reaction can be carried out in a variety of ways. In any case, no matter how you obtain the desired substance, the thermal effect of the process will be a constant value. To determine it, you need to sum up the thermal effects of all intermediate transformations. Thanks to Hess's law, it became possible to perform calculations of numerical indicators of thermal effects, which is impossible to do in a calorimeter. For example, the heat of formation of carbon monoxide is calculated quantitatively according to Hess’s law, but you will not be able to determine it through ordinary experiments. This is why special thermochemical tables are so important, in which digital values for various substances determined under standard conditions are entered.
Important points in calculations
Considering that the heat of formation is the thermal effect of a reaction, the substance in question is of particular importance. For example, when making measurements, it is customary to consider graphite, not diamond, to be the standard state of carbon. They also take into account pressure and temperature, that is, the conditions in which the reacting components were initially located. These physical quantities can have a significant impact on the interaction; they increase or decrease the amount of energy. In order to perform basic calculations, it is customary in thermochemistry to use specific indicators of pressure and temperature.
Standard terms
Since the heat of formation of a substance is a determination of the magnitude of the energy effect precisely under standard conditions, we will highlight them separately. The temperature for calculations is chosen to be 298 K (25 degrees Celsius), the pressure is 1 atmosphere. In addition, an important point that is worth paying attention to is the fact that the heat of formation for any simple substances is zero. This is logical, because they do not form themselves, that is, no energy is spent for their emergence.
Elements of thermochemistry
This section of modern chemistry is of particular importance, because it is here that important calculations are carried out and specific results used in thermal power engineering are obtained. In thermochemistry, there are many concepts and terms that are important to operate in order to obtain the desired results. Enthalpy (ΔH) indicates that the chemical interaction occurred in a closed system, there was no influence on the reaction from other reagents, and the pressure was constant. This clarification allows us to talk about the accuracy of the calculations performed.
Depending on what kind of reaction is being considered, the magnitude and sign of the resulting thermal effect can differ significantly. Thus, for all transformations that involve the decomposition of one complex substance into several simpler components, the absorption of heat is assumed. Reactions of combining multiple starting substances into one, more complex product are accompanied by the release of a significant amount of energy.
Conclusion
When solving any thermochemical problem, the same algorithm of actions is used. First, the value of the heat of formation is determined from the table for each initial component, as well as for the reaction products, not forgetting about the state of aggregation. Next, armed with Hess’s law, they create an equation to determine the desired value.
Particular attention should be paid to taking into account the stereochemical coefficients present in front of the starting or final substances in a particular equation. If there are simple substances in the reaction, then their standard heats of formation are equal to zero, that is, such components do not affect the result obtained in the calculations. Let's try to use the information received on a specific reaction. If we take as an example the process of formation of pure metal from iron oxide (Fe 3+) by interaction with graphite, then in the reference book you can find the values of the standard heat of formation. For iron oxide (Fe 3+) it will be -822.1 kJ/mol, for graphite (a simple substance) it is zero. As a result of the reaction, (CO) is formed, for which this indicator has a value of 110.5 kJ/mol, and for the released iron, the heat of formation corresponds to zero. The recording of the standard heat of formation of a given chemical interaction is characterized as follows:
ΔH about 298 = 3× (-110.5) - (-822.1) = -331.5 + 822.1 = 490.6 kJ.
Analyzing the numerical result obtained according to Hess’s law, we can make a logical conclusion that this process is an endothermic transformation, that is, it involves the expenditure of energy on the reaction of the reduction of iron from its trivalent oxide.
Standard enthalpy of formation (standard heat of formation)
The standard heat of formation is understood as the thermal effect of the reaction of the formation of one mole of a substance from simple substances and its components that are in stable standard states.
For example, the standard enthalpy of formation of 1 mole of methane from carbon and hydrogen is equal to the thermal effect of the reaction:
C(tv) + 2H 2 (g) = CH 4 (g) + 76 kJ/mol.
The standard enthalpy of formation is denoted by Δ H fO. Here the index f means formation, and the crossed out circle, reminiscent of a Plimsol disk, means that the value refers to the standard state of matter. Another designation for standard enthalpy is often found in the literature - ΔH 298.15 0, where 0 indicates pressure equal to one atmosphere (or, somewhat more precisely, standard conditions), and 298.15 is temperature. Sometimes index 0 is used for quantities related to pure substance, stipulating that it is possible to designate standard thermodynamic quantities with it only when a pure substance is chosen as the standard state. For example, the state of a substance in an extremely dilute solution can also be accepted as standard. “Plimsoll disk” in this case means the actual standard state of matter, regardless of its choice.
The enthalpy of formation of simple substances is taken equal to zero, and the zero value of the enthalpy of formation refers to the state of aggregation, stable at T = 298 K. For example, for iodine in the crystalline state Δ H I2(tv) 0 = 0 kJ/mol, and for liquid iodine Δ H I2(l) 0 = 22 kJ/mol. The enthalpies of formation of simple substances under standard conditions are their main energy characteristics.
The thermal effect of any reaction is found as the difference between the sum of the heats of formation of all products and the sum of the heats of formation of all reactants in a given reaction (a consequence of Hess’s law):
Δ H reaction O = ΣΔ H f O (products) - ΣΔ H f O (reagents)
Thermochemical effects can be incorporated into chemical reactions. Chemical equations that indicate the amount of heat released or absorbed are called thermochemical equations. Reactions accompanied by the release of heat into the environment have a negative thermal effect and are called exothermic. Reactions accompanied by the absorption of heat have a positive thermal effect and are called endothermic. The thermal effect usually refers to one mole of reacted starting material whose stoichiometric coefficient is maximum.
Thermochemical equations
The most important quantity in thermochemistry is the standard heat of formation (standard enthalpy of formation). The standard heat (enthalpy) of formation of a complex substance is the thermal effect (change in standard enthalpy) of the reaction of the formation of one mole of this substance from simple substances in the standard state. The standard enthalpy of formation of simple substances in this case is taken equal to zero.
In thermochemical equations, it is necessary to indicate the aggregative states of substances using letter indices, and the thermal effect of the reaction (ΔH) must be written separately, separated by a comma. For example, the thermochemical equation
4NH 3 (g) + 3O 2 (g) → 2N 2 (g) + 6H 2 O (l), ΔH = -1531 kJ
shows that this chemical reaction is accompanied by the release of 1531 kJ of heat, at a pressure of 101 kPa, and refers to the number of moles of each substance that corresponds to the stoichiometric coefficient in the reaction equation.
In thermochemistry, equations are also used in which the thermal effect is related to one mole of the formed substance, using fractional coefficients if necessary.
Hess's law
Thermochemical calculations are based on Hess’s law: The thermal effect (∆H) of a chemical reaction (at constant P and T) depends on the nature and physical state of the starting substances and reaction products and does not depend on the path of its occurrence.
Corollaries from Hess's law:
1. The thermal effects of forward and reverse reactions are equal in magnitude and opposite in sign.
2. The thermal effect of a chemical reaction (∆H) is equal to the difference between the sum of the enthalpies of formation of the reaction products and the sum of the enthalpies of formation of the starting substances, taken into account the coefficients in the reaction equation (that is, multiplied by them).
Hess's law can be written as the following mathematical expression:
15. The concept of the internal energy of a system, enthalpy and its changes in chemical processes.
Internal energy thermodynamic function of the state of the system, its energy determined by the internal state. Internal energy consists mainly of the kinetic energy of movement of particles (atoms, molecules, ions, electrons) and the energy of interaction between them (intra- and intermolecular). On internal energy influences the change in the internal state of the system under the influence of an external field; in internal energy includes, in particular, the energy associated with the polarization of the dielectric in an external electric field and the magnetization of a paramagnet in an external magnetic field. The kinetic energy of the system as a whole and the potential energy due to the spatial location of the system are not included in the internal energy. In thermodynamics, only the change in internal energy in various processes is determined. Therefore, the internal energy is specified up to a certain constant term, depending on the energy taken as the reference zero.
Internal energy U as a function of state is introduced by the first law of thermodynamics, according to which the difference between the heat Q transferred to the system and the work W performed by the system depends only on the initial and final states of the system and does not depend on the transition path, i.e. represents the change in state function Δ U
where U 1 and U 2- internal energy of the system in the initial and final states, respectively. Equation (1) expresses the law of conservation of energy as applied to thermodynamic processes, i.e. processes in which heat transfer occurs. For a cyclic process that returns the system to its initial state, Δ U=0. In isochoric processes, i.e. processes at a constant volume, the system does not perform work due to expansion, W=0 and the heat transferred to the system is equal to the increment of internal energy: Qv=Δ U. For adiabatic processes, when Q=0, Δ U=-W.
Internal energy system as a function of its entropy S, volume V and the number of moles m i of the i-th component is the thermodynamic potential. This is a consequence of the first and second laws of thermodynamics and is expressed by the relation:
where T is the absolute temperature, R- pressure, μ i - chemical potential of the i-th component. The equal sign refers to equilibrium processes, the inequality sign - to nonequilibrium ones. For a system with set values S, V, m i (closed system in a rigid adiabatic shell), the internal energy at equilibrium is minimal. Loss of internal energy in reversible processes at constant V And S equal to the maximum useful work (see Maximum work of the reaction).
Dependence of the internal energy of an equilibrium system on temperature and volume U=f(T, V) called the caloric equation of state. The derivative of internal energy with respect to temperature at constant volume is equal to isochoric heat capacity.
Enthalpy, Also thermal function And heat content- thermodynamic potential, characterizing the state of the system in thermodynamic equilibrium when choosing pressure, entropy and the number of particles as independent variables.
Simply put, enthalpy is that energy that is available to be converted into heat at a certain temperature and pressure.
If a thermomechanical system is considered as consisting of a macrobody (gas) and a piston with a load weighing Р = pS, balancing gas pressure R inside the vessel, then such a system is called expanded.
Enthalpy or energy of an expanded system E equal to the sum of the internal energy of the gas U and potential energy of the piston with load E sweat = pSx = pV
H = E = U + pV
Thus, enthalpy in a given state is the sum of the internal energy of the body and the work that must be expended so that the body has a volume V introduce into a pressurized environment R and being in equilibrium with the body. Enthalpy of the system H- similar to internal energy and other thermodynamic potentials - has a very definite meaning for each state, i.e. it is a function of the state. Therefore, in the process of changing state
Δ H = H 2 − H 1
The change in enthalpy (or the thermal effect of a chemical reaction) does not depend on the path of the process, being determined only by the initial and final state of the system. If the system somehow returns to its original state (circular process), then the change in any of its parameters, which is a function of the state, is equal to zero, hence Δ H= 0, or
Enthalpy differential expressed in eigenvariables - through entropy S and pressure p:
Since in quasi-equilibrium processes it is the amount of heat supplied to the system, this implies the physical meaning of introducing the concept of enthalpy: its change is the heat supplied to the system in an isobaric process (at constant pressure). The practical application of this function is based on the fact that many chemical processes in real or laboratory conditions are carried out precisely at constant (atmospheric) pressure when the tank is open. Thus, the enthalpy of formation is the amount of energy that is released or absorbed during the formation of a complex substance from simple substances.
All chemical reactions are accompanied by the release (exothermic) or absorption (endothermic) of heat. A measure of the heat of reaction is the change in enthalpy ΔH, which corresponds to heat transfer at constant pressure. In the case of exothermic reactions, the system loses heat and ΔH is a negative value. In the case of endothermic reactions, the system absorbs heat and ΔH is a positive value.
In fact, this is a consequence of the first law of thermodynamics, but it was formulated earlier than the first law. The thermal effect of an isobaric (or isochoric) process depends only on the initial and final states of the system and does not depend on intermediate stages. It has been proven experimentally, but now it can also be deduced from the fact that the thermal effect is the difference in state functions (H or U). Let's say we carry out the process in two ways:
In the first case, Q = Q 21 + Q 32 + Q 43 = (U 2 –U 1) + (U 3 –U 2) + (U 4 –U 3) = U 4 –U 1.
In the second, immediately Q = U 4 –U 1.
Thanks to Hess's law, it is possible to calculate the heat of processes that are inconvenient to carry out experimentally.
Corollary 1. The thermal effect of the reaction is equal to the difference between the sum of the heats of formation of the products and the sum of the heats of formation of the starting substances, taking into account the coefficients in the reaction equation.
Standard heat of formation substance is the thermal effect of the formation of one mole of this substance from the corresponding simple substances , taken in their stable (standard) states. Education is denoted by the index f (formation) with the letter D. Example. We obtain CO in three ways:
1) 2C (graphite) + O 2 = 2CO; D1H;
2) 2C (diamond) + O 2 = 2CO; D2H;
3) C (graphite) + CO 2 = 2CO; D3H.
Which of the following thermal effects is the heat of formation of CO?
No. The second reaction involves an unstable form of carbon, and the third involves the production of something other than simple substances. 1 is best, but it turns out not one mole, but two. Finally: D f H(CO) = D 1 H/2.
Thermal effects are usually defined for standard states substances (see § 7.3), and the corresponding thermodynamic functions are marked with a zero at the top, for example, D f H° 298 (CO). When asking a student what the standard state is, the answer usually starts with a temperature of 298 K (25°C). This is just not the main thing. Standard status and standard functions can be determined for any temperature (subscript), although in reference books it is most often given specifically for 298 K.
Heats of formation of ions in solutions. Many inorganic substances are strong electrolytes and exist in solutions as ions. Therefore, I would like to know the D f H° of the ions. Knowing them for 20 cations and 30 anions, we will know D f H ° 600 electrolytes in dilute solutions. But we cannot synthesize a solution containing only one type of ion (electroneutrality requirement). The heat of formation of two (or more) types of ions at once is always determined, but it is unknown how to divide it between them. That's why conditionally we took the standard heat of formation of a hydrated hydrogen ion D f H°(H + aq) to be zero at all temperatures. Then you can determine D f H° of any strong acid anion, for example: D f H° (Сl - aq) = D f H° (HCl aq). Knowing them, you can determine D f H ° of other cations, for example:
D f H°(Ca 2+ aq) = D f H°(CaCl 2 aq) – 2D f H°(Сl – aq).
A note on terminology and symbolism. In colloquial speech, and sometimes in literature, the expressions “enthalpy of formation”, “enthalpy of dissolution”, etc. are used. This is not accurate. Enthalpy H is a function state , and formation, dissolution, etc. - This processes , i.e. state changes , which is denoted by the letter D. Therefore, the index indicating the type of process (for example, f) is placed not under the letter H, but under the letter D. Instead of “enthalpy of formation” one should say “change in enthalpy during formation” or, in short, “heat of formation” . The same applies to other processes and other functions discussed below.
12.4. Three more consequences of Hess's law
2. The thermal effect of the process is equal to the difference between the sums of the heats of combustion of the starting substances and products, taking into account the coefficients in the reaction equation. (provided that the combustion products are the same).
3. The thermal effect of the process is equal to the difference between the sums of the heats of dissolution of the starting substances and products, taking into account the coefficients in the reaction equation. (provided that when dissolved they give the same products, for example, ions).
4. The thermal effect of the process is equal to the difference between the sums of the heats of atomization of the starting substances and products, taking into account the coefficients in the reaction equation.
Why are the starting materials and products rearranged compared to the first consequence? When we write down the equation for the formation of a substance, it is on the right side, and in the equations of combustion, dissolution, atomization, it is on the left.
The second consequence is especially important in organic chemistry. Most organic reactions are difficult to complete and strictly according to one equation, without byproducts. But all organic substances burn and produce the same products.
Example: determine and compare the heats of formation of butane and isobutane.
The reaction 4C (graphite) + 5H 2 = n-C 4 H 10 (or iso-C 4 H 10) is not feasible experimentally. No matter how we heat graphite with hydrogen, at best we will get methane with an admixture of other hydrocarbons, but pure n-butane (or isobutane) will not work out that way. This means that you need to get it in some other way, burn it and measure the heat of combustion. Similarly, measure the heat of combustion of graphite and hydrogen and then algebraically combine the equations and thermal effects:
| C + O 2 = CO 2; D 1 H H 2 + 0.5 O 2 = H 2 O; D 2 H C 4 H 10 + 6.5 O 2 = 4 CO 2 + 5 H 2 O; D 3 H | –1 |
| 4C + 5H 2 = C 4 H 10; DH= | 4D 1 H + 5D 2 H – D 3 H |
This is the sum of the heats of combustion of the starting substances minus the sum of the heats of combustion of the products.
Corollary 3 is especially important in inorganic chemistry. Most salts, oxides, bases, all metals are non-molecular substances and retain their individuality only in the solid state, and when dissolved in water, acid or molten salt they turn into a set of the same ions. The heats of solid-phase reactions are difficult to measure, while the heats of solution are easier.
Example: determine the thermal effect of a reaction
CaO(solid) + Fe 2 O 3 (solid) = CaFe 2 O 4 (solid); DH = ?
This reaction can be carried out almost completely. But prolonged heating at high temperatures is required, and the DH reaction is imperceptible against the background of huge heat flows in the furnace. Therefore, we synthesize the product, dissolve it, for example, in hydrochloric acid (or in a suitable molten salt), dissolve the starting oxides under the same conditions and measure the heat of solution.
| CaO + 2HCl(aq) = CaCl2(aq) + H2O; D 1 H Fe 2 O 3 + 6HCl(aq) = 2FeCl 3 (aq) + 3H 2 O; D2H | |
| CaFe 2 O 4 + 8HCl (aq) = CaCl 2 (aq) + 2FeCl 3 (aq) + 4H 2 O; D 3 H | –1 |
CaO + Fe 2 O 3 = CaFe 2 O 4; DH = D 1 H + D 2 H – D 3 H
Corollary 4 allows, knowing the heats of formation of complex substances, to calculate their heats of atomization, since the heats of atomization of simple substances have already been measured.
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