Steps for writing thermochemical equations, precautions

1) The heat of reaction is related to the measurement conditions such as temperature and pressure, so the temperature and pressure kpa at the time of reaction must be indicated when writing, which can be omitted).

2) The state of the substance is indicated by a parenthesed () on the right side of the chemical formula of each substance. G, L, and S can be used to represent gaseous, liquid, and solid states, respectively. When the solid has different crystal states, the crystal states should also be indicated, such as S (orthorhombic), S (monoclinical), C (graphite), C (diamond), etc.

The concentration of the reactive substance in the solution must be indicated, with aq representing aqueous solution and (aq, ) representing infinitely diluted aqueous solution.

3) The stoichiometric number in the thermochemical equation only indicates the amount of the substance, not the number of molecules or atoms of the substance, therefore, it can be an integer or a fraction.

4) H can only be written on the right side of the chemical equation, if it is an exothermic reaction, then h is "-", if it is an endothermic reaction, then h is "+", and its unit is generally kj mol. For the same reaction, the number of measurements is different, and its h is different.

5) The thermochemical equation is the quantity that indicates the completion of the reaction. Since h is related to the amount of material in which the reaction is completed, the stoichiometric number before the chemical formula in the equation must correspond to h, and when the reaction is carried out in the opposite direction, its heat of reaction is equal to the value of the heat of reaction of the positive reaction, and the sign is opposite.

6) Do not mark "" or " ".

7) Reaction conditions are not indicated, e.g. (heating).

8) The organic thermochemical equation does not use "=", but "".

The following points must be taken into account when writing and applying thermochemical equations:

The heat of reaction is related to the measurement conditions such as temperature and pressure, so the temperature and pressure at the time of the reaction are specified when writing, and if the temperature is 25 degrees Celsius and the air pressure is 101 kPa in the standard state, it can be omitted. The right side of the chemical formula of each substance is indicated by a parenthetical bracket indicating the aggregation state of the substance. In the thermochemical equation, the macro stoichiometric number only represents the amount of the substance, not the number of molecules or atoms of the substance, so it can be an integer or a fraction.

When the reaction is carried out in reverse, the heat of reaction is equal to the heat of reaction of the positive reaction, and the sign is opposite. There is no need to mark the gas rise symbol and the precipitation symbol. Thermochemical equations do not require the writing of the reaction conditions of the heating and catalyst.

Even organic thermochemical equations or reversible reactions are cleared with equal signs, not arrows and invertible symbols.

The precautions for writing thermochemical equations are as follows:

The attention to the thermochemical equation is to mark the state of each molecular formula, (g for gaseous, l for liquid, s for solid), and secondly, the energy unit must be clearly marked. If it is an exothermic reaction, a negative sign must be filled in before the value.

Thermochemical equations are a compulsory knowledge point in the college entrance examination, sometimes it is difficult, sometimes it is very simple. It often depends on what the teacher has in mind, but there are only a few types of questions. The first is to plot the entire chemical reaction that takes place, and ask you to find the most exothermic or endothermic value.

Generally, students who are careless will often forget to fill in the negative sign or forget to write the unit, so they miss two points.

The second is to give you four or five thermochemical equations, and then based on the relationship between these thermochemical equations, find another thermochemical equation and find the heat of the reaction. Errors that occur in this process are often caused by the uneven matching of equations, the forgetting of the symbols of the state of matter, and the forgetting of the values of exothermic or endothermic values.

Thermochemical equations are often time-consuming, and the cost performance is very low, often only two points. If the candidate cannot take this type of question within two minutes, then he must give up decisively. Because you can get more grades by spending a lot of time elsewhere, and if you spend time here, the answer can be wrong.

The heat of reaction h is related to the determination conditions (temperature, pressure, etc.). When writing thermochemical equations, the measurement conditions (temperature, pressure) of h should be indicated. The unit of h is generally kj mol.

h can only be written on the right (back) side of the chemical equation labeled with the state of reactants and products, and used in conjunction with "; "Spaced. If it is an exothermic reaction, h is " and if it is an endothermic reaction, h is " ".

The energy possessed by the matter itself is related to the state of aggregation of the matter. Depending on the aggregation state of the reactants and the product, the value and symbol of the heat h may be different. Therefore, it is necessary to indicate the state of aggregation of substances (reactants and products) (gas:

g Liquid: l Solid: S dilute solution:

aq), in order to fully reflect the significance of the thermochemical equation. In thermochemical equations, " " and " " " are not used

Under the same conditions (temperature, pressure), chemical reactions of the same substance (reciprocal reactions, not necessarily reversible reactions), reactions carried out in the forward direction and reactions carried out in the opposite direction, the heat h values of the reaction are equal and the signs are opposite.

In the ordinary chemical equation, the stoichiometric number before the chemical formula of each substance can represent the number of molecules of the substance, the amount of the substance, etc. However, in the thermochemical equation, only the amount of the substance of the substance is expressed, so it can be an integer, a fraction, or a decimal. For the same chemical reaction, the stoichiometric number is different, and the heat h of the reaction is also different.

The "mol" in the unit of heat of reaction h kj mol refers to the entire system of chemical reaction (i.e., "chemical reaction per mole"), not specifically to which substance is in the reaction. For example, it refers to the energy emitted per friction reaction, not to the amount of matter of each substance in the reaction.

Regardless of whether the chemical reaction is reversible or not, the heat h of the reaction in the thermochemical equation represents the change in energy at the end of the reaction (complete transformation).

The precautions for writing the combustion thermochemical equation are as follows:

1. Note that the symbol and unit of 4h 4h can only be written on the right side of the chemical equation marked with the state of reactants and products. If it is an exothermic reaction, h is "——if it is an endothermic reaction, h is "+" and the unit of h is generally kj moj.

2. Pay attention to the reaction conditions and the measurement conditions (temperature, pressure, etc.). Therefore, thermochemical equations should be noted when writing. Determination conditions for h.

The vast majority of 4 hours are measured at kPa, and the reaction performed under this condition can be performed without temperature and pressure indicated.

3. Note that the aggregation state of the substance, the aggregation state of the reactant and the product are different, and the heat of reaction is different. Therefore, the aggregation state of the matter must be noted in order to fully reflect the significance of the thermochemical equation. "G" for gases and :: for liquids

l", "S" for solids, and "AQ" for solutions.

Calorific value:

Calorific valueSometimes the heat of combustion of a fuel can be expressed as hhv (high calorific value), lhv (low calorific value), or ghv (gross calorific value). The low calorific value is associated with water that is discharged in a gaseous form, so the energy used to vaporize the water cannot be considered heat. The total calorific value is related to the water that is discharged in the gaseous form and includes the water that is present in the fuel before combustion.

This value is important for fuels such as wood or coal, which usually contain a certain amount of water before the combustion stares are on.

A high calorific value equals the heat of combustion, because the enthalpy change in the reaction assumes that the compound is kept at room temperature before and after combustion, in which case the water produced by combustion is liquid. 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.