Why should the heat of combustion specify temperature and pressure

You first need to understand the concept of heat of reaction.

The heat of reaction usually refers to the heat of reaction when a chemical reaction occurs under constant pressure and without non-expansion work, if the temperature of the product returns to the starting temperature of the reactant, then the heat released or absorbed by the system is called the heat of reaction. That is, the heat of reaction usually refers to:

The heat emitted or absorbed by the system when physical or chemical changes occur during isothermal and isobaric processes. There are many forms of heat of chemical reaction, such as: heat of formation, heat of combustion, heat of neutralization, etc.

The heat of combustion is a type of heat of reaction, and the heat energy stored in the reactants at different temperatures is different, which will affect the calculation of the heat difference before and after combustion, so the temperature should be specified. There is also pressure, if the pressure is not specified, then there will be a pressure difference before and after the reaction, and the change of pressure must have a process of doing work, which can only be converted from the heat of combustion, if you want not to affect the calculation of the heat of combustion, of course, you must also specify the pressure!

The heat of combustion at different temperatures and pressures is different.

At 101 kPa, the heat emitted when 1 mol of pure substance is completely burned to form a stable compound is called the heat of combustion of the substance in kj mol

The pressure of 101 kpa is specified to measure the heat emitted by the reaction, because the pressure is uncertain, and the value of the heat of reaction is different

For example, the heat of combustion of hydrogen is akj mol. However, under other pressures, the calorific value of its combustion is not equal to AKJ mol.

Note: When it is not the standard pressure, it cannot be the heat of combustion, but can only be called the calorific value of combustion.

The heat of combustion refers to the amount of heat emitted when a substance undergoes a complete combustion reaction with oxygen.

It is generally measured in terms of the amount per unit of matter, the amount per unit mass, or the energy emitted per unit volume of fuel when burned. The combustion reaction is usually a reaction in which hydrocarbons are burned in oxygen to produce carbon dioxide, water, and exothermic. The heat of combustion can be measured with a bullet calorimeter, or it can be obtained by subtracting the enthalpy of the reactants and products by directly preparing the orange contact table.

At 25, 100 kPa, (the old standard state pressure was 1 atm = 101 kPa, i.e. 1 standard atmosphere, the new standard state pressure was changed to 1 bar = 100 kPa.) Peking University Press, Principles of General Chemistry (Fourth Edition), page 85, note 1) The heat emitted when 1 mol of pure substance is completely burned to form a stable imitation of a well-defined compound is called the heat of combustion of the substance, and the unit is kj mol.

The heat of combustion is measured by 1 mol of combustibles, so when calculating the heat of combustion, the stoichiometric numbers of other substances in the thermochemical equation often appear as fractions: e.g., H2(G)+1 2O2(G)==H2O(L); δH = 285 8 kJ·mol 1 The fraction represents the number of moles (i.e., the amount of substances participating in the reaction) rather than the number of molecules, so it is reasonable.

Size comparison:

Among the alkane isomers with the same number of carbon atoms, the combustion heat of straight-chain alkanes is the largest, and the more branched chains, the smaller the combustion heat.

The most basic point is that in organic matter, the higher the saturation of the compound (in other words, the greater the number of hydrogen atoms), the greater the corresponding heat of combustion. For example, alkanes have a greater heat of combustion than alkenes and alkynes or aromatic hydrocarbons with the same number of carbons. Of course, the emphasis here is limited to comparing organic matter with the same number of carbon atoms.

For hydrocarbon derivatives, the same is true for the amount of acetaldehyde in the heat of combustion of ethanol and other substances. The heat of combustion of ethane is greater than that of ethyl bromide.

If it is a combustion heat of isomers, then it depends on the specific situation, usually the more unstable the isomer, the greater the combustion heat.

Of course, the greater the molecular weight of organic matter, the greater its heat of combustion. With the same degree of saturation, the greater the carbon content, the greater the heat of combustion of organic matter. When the carbon content is the same, the higher the hydrogen content, the greater the combustion heat of the organic matter.

The definition mentions 25 but does not specify it, and the definition is as follows:

kpa pressure.

2. The amount of combustible substances is 1 mol

3. Stable oxides must be generated.

4. The heat emitted by the complete combustion of combustibles to produce stable compounds is the standard.

In the thermochemical equation, the heat of combustion δh represents the difference between the total enthalpy of the product and the total enthalpy of the reactant. It can be seen that it has nothing to do with temperature, so some books or sources have 25 statements, but most sources do not mention this temperature. This temperature is also not emphasized in textbooks.

Only the pressure is emphasized to be 101kpaBecause pressure has an effect on the heat of combustion.

The prescribed determination is at 25 degrees.

The definition mentions 25 liquids, but does not prescribe it, and the definition is as follows:

kpa pressure.

2. The amount of combustible substances is 1 mol

3. Stable oxides must be generated.

4. The heat emitted by the complete combustion of combustibles to produce stable compounds is the standard.

After the gas is compressed by the compression stroke, the internal energy increases, the temperature rises, and when combustion, a chemical reaction occurs, and a large amount of heat is released at the same time, a large amount of gas is generated, and when the volume is constant, a high-temperature and high-pressure gas is formed, so as to promote the piston to do external work. After that, the internal energy decreases and the internal energy is converted into mechanical energy.

Generally speaking, the greater the pressure of the gas, the smaller the molecular spacing, so the molecular potential energy increases, which can affect the internal energy.

Factors influencing internal energy: temperature (mainly affecting the kinetic energy of the molecule), mass (number of molecules), state (molecular spacing, and thus the potential energy of the molecule).

at 25 degrees Celsius, 101

kpa, 1

The heat emitted when mol combustibles are completely burned to form a stable compound is called the heat of combustion of the substance

Definition Points: Specified in 101

KPA pressure, the heat emitted by the reaction is measured at room temperature of 25 degrees, because the pressure and temperature are uncertain, and the heat of reaction value is not the same

The amount of combustible substances specified is 1

MOLs stipulate that the heat emitted by the complete combustion of combustibles to form stable compounds is the standard (e.g., H2S(G)+1 2O2(G)===H20(L)+S;

H1, because the generated S is not completely combusted, so the heat released by this reaction δH1 cannot be used as the heat of combustion of H2S, when H2S(g)+3 2O2(G)===H20(L)+SO2(G); ΔH2, the state of water is a stable liquid state, and a stable oxide SO2 is also generated, so ΔH2 is the heat of combustion of H2S. ）

Note: The heat of combustion is 1

Mol combustibles are determined as a standard, so when calculating the heat of combustion, the stoichiometric number of other substances in the thermochemical equation often appears as a fraction: e.g., H2(g).

1/2o2(g)====h2o(l)；δh=－285．8

kj·mol－1

In this case, the fraction represents the number of moles rather than the number of molecules, so it is reasonable.

Hello! Not only that, the product must be a stable oxide, for example, C can be burned to form CO or CO2, and the heat of combustion must be CO2. The water must be produced in a liquid state, not a gaseous state.

The maximum combustion temperature of any combustible material is the theoretical combustion temperature, such as CO combustion 2CO + O2 = 2CO2, according to the heat of CO, the temperature at which CO2 can be heated after complete combustion.

Note: 1. If the oxidant is air, the nitrogen in the air should also be heated; 2. The theoretical combustion temperature refers to the fact that the heat is not absorbed by other objects after combustion, that is, in the adiabatic state.

Generally, the maximum temperature of the furnace of pulverized coal boiler is about 1500 degrees.

The heat of combustion refers to the heat emitted when 1 mol of combustible material is completely burned to form a stable oxide under certain conditions. Known thermochemical equation: Co ( G ) 1 2 O2 ( G ) CO2 ( G ) δh = kj · mol-1 H2 ( g ).

At 25 degrees Celsius and 101 kpa, the heat emitted when 1 mol of combustibles is completely burned to form a stable compound is called the heat of combustion of the substance Key points of definition: It is specified that the heat is measured at a pressure of 101 kPa and a normal temperature of 25 degrees, because the pressure and temperature are uncertain, and the value of the heat of reaction is not the same The amount of combustible substances is specified to be 1 mol It is specified that the heat emitted by the complete combustion of combustibles to form a stable compound is the standard Note: The heat of combustion is 1 Mol combustibles are determined as a standard, so when calculating the heat of combustion, the stoichiometry of other substances in the thermochemical equation often appears as fractions

e.g. h2(g) 1 2o2(g)====h2o(l);δh=－285．8 kj·mol－1