Method for solving chemical reaction enthalpy temperature and chemical reaction enthalpy

Abstract

The invention relates to a method for solving chemical reaction enthalpy temperature and chemical reaction enthalpy. The method comprises the following steps: obtaining an initial temperature / enthalpy temperature TH of a sample reaction from a thermal analysis curve; under the condition of linear temperature rise of thermal analysis TA, obtaining an initial temperature point Tsr when the temperature delta T, the heat q / mv or the weight loss ratio Ws of the sample cell deviates from a base line of a reference cell, and enabling Tsr to be equal to TH which is the enthalpy temperature of the reaction; a direct method of thermal analysis or an indirect method of thermodynamics can solve the chemical reaction enthalpy of the reaction; according to the invention, only a unique chemical reaction enthalpy of a reaction is determined from DSC or DTA isothermal temperature and linear heating and thermodynamic data consistency, and the chemical reaction enthalpy does not change along with temperature.

Description

Technical fields:

[0001] This invention provides a method for determining the enthalpy temperature and enthalpy of a chemical reaction, belonging to the fields of thermodynamics, chemical thermodynamics, enthalpy, and reaction enthalpy. Background technology:

[0002] The thermodynamic cycle of a chemical reaction in a closed system can be described as follows: Figure 1 The enthalpy diagram shown:

[0003] The mathematical expression for this enthalpy diagram is the famous Kirchhoff (German chemist, 1824-1877) law:

[0004]

[0005] in:

[0006]

[0007] ΔC p =ΣΔC p / p (product)-ΣΔC p / r (Reactants) (4)

[0008] Δ trs H=ΣΔ trs H p (product)-ΣΔ trs H r (Reactants) (5)

[0009] In equation (1), and These are the enthalpies of the chemical reaction at T2 and T1, respectively, while ΔH 21 It is the physical enthalpy of reactants and products as temperature changes. Therefore, it can be said that... yes and ΔH 21 "Mixed enthalpy" or yes and ΔH 21 The "enthalpy of mixing". Unable to distinguish. or Who is a real chemical reaction enthalpy?

[0010] For example, consider the classic experiment on the heat of combustion of benzoic acid and oxygen. First, benzoic acid and oxygen need to be ignited / heated with a hot wire of approximately 50J, causing the temperature of the reactants to rise from 298.15℃ to T (T ≥ T). H T H (The lowest temperature at which benzoic acid and oxygen react to form a combustion reaction). When T ≥ T HAt this point, benzoic acid and oxygen undergo a combustion reaction. The heat of reaction generated by the combustion reaction exchanges heat with the surrounding water, and the reaction products cool down to a certain temperature, TK. The heat of reaction for the combustion reaction of benzoic acid and oxygen at this point is:

[0011]

[0012] Where: ΔT=T-298.15, A×C is a thermal constant of the instrument. Therefore, when T H At >298.15, according to equation (1), the standard enthalpy of a reaction is:

[0013]

[0014] Because the test of the combustion reaction of benzoic acid and oxygen includes the physical heat effects of the reactant heating and product cooling, as well as the heat of reaction generated during the combustion of benzoic acid and oxygen, a "reaction enthalpy" can be obtained through the oxygen bomb experiment. It is actually a "mixed enthalpy" that contains both physical and chemical heat effects. Secondly, we do not know the actual temperature T at which the combustion reaction occurs. H Therefore, we cannot calculate the true enthalpy of the reaction. It is also impossible to obtain ΔH 21 (T H →298.15) values.

[0015] Differential thermal analysis (DTA / 1887) and differential scanning calorimetry (DSC / 1955) are simple and rapid methods for testing the heat effect of a reaction, and have been used for nearly a century. In DTA and DSC, the heat effect (Q) of the reaction can be calculated according to the Speil equation by integrating the area of ​​the curve in the DSC or DTA thermogram:

[0016]

[0017] (T / temperature, ΔT=Ts-Tr / temperature difference between reference cell and sample cell, mv / milliwatt, K / thermal coefficient, A / integral area).

[0018] In linear heating experiments of DSC or DTA (e.g.) Figure 2 The heat effect Q in the reaction can be obtained from the integral value from t2 to t1 according to equation (8) or (9). However, since the corresponding temperatures at t1 and t2 are T1 and T2 respectively, it is impossible to determine whether the integral value (Q) from T2 to T1 is Q(T1) or Q(T2). In the thermodynamic definition, equation (1) or enthalpy diagram ( Figure 1),Q(T) is a constant at a certain temperature. This phenomenon has brought a very embarrassing situation to the temperature annotation Q(T) of the reaction enthalpy in thermal analysis. In experiments with different linear heating rates in DSC, a series of Q1(β1), Q2(β2), ……, Q n (β n ) DSC curves can be obtained. For each heating rate β n there will be a Q n , and for each Q n there will correspond a T1 and T2 as shown in Figure 2 . This makes it more difficult for us to perfectly define a Q(T).

[0019] In isothermal experiments of DSC or DTA, a set of Q1(T1), Q2(T2), ……, Q n values at different temperatures (T n ) can be obtained. Whether these Q n (T n ) are the reaction enthalpies of a reaction at different temperatures is also a very thorny problem.

[0020] In DSC and DTA experiments ( Figure 2 ), T1 is the starting temperature when the reaction begins to occur, and T2 is the ending temperature when the reaction ends. The area of the curve between T1→T2 is the thermal effect of the reaction. This area does not include the thermal effect during the heating stage of the reactants (T < T1), nor does it include the thermal effect during the cooling stage of the products (T > T2). Therefore, the thermal effect measured by DSC or DTA is a pure reaction thermal effect and does not include the physical thermal effects during the heating of the reactants and the cooling of the products. The DSC or DTA method is called the "direct method" for measuring the reaction thermal effect. The bomb calorimetry uses a thermal cycle measurement method, and the temperature change during its test process is 298.15→T→298.15. The principles of these two methods are different, so that the thermal effect data measured by the bomb calorimeter and DSC technology cannot be unified, hindering the further development of these two fields. Summary of the Invention:

[0021] The present invention aims to solve the problems of finding the true reaction enthalpy of a chemical reaction and unifying the consistency of enthalpy data in thermodynamic methods and thermal analysis methods.

[0022] The present invention proposes a 111 - enthalpy theory, aiming to provide a method for calculating and determining the reaction enthalpy.

[0023] (1) Enthalpy temperature

[0024] ​​Linear heating experiments in thermal analysis (TA) can yield the W-T curve for thermogravimetric analysis (TGA), the ΔT-T curve for differential thermal analysis (DTA), and the q / mv-T curve for differential scanning calorimetry (DSC). The temperature T at which W, ΔT, or q / mv deviates from the baseline is determined by the following parameters: W-T curve for TGA, ΔT-T curve for DTA, and q / mv-T curve for DSC. H This is called the enthalpy temperature of a reaction.

[0025] (2) Enthalpy of chemical reaction in DSC or DTA isotherm experiments

[0026] In an isothermal experiment, when measuring the heat effect of a reaction, both the initial reactants and the final products of the reaction system are in an isothermal state, meaning that the reactants and products do not undergo temperature changes. Therefore, ΔH 21 =0. Rewrite formula (1):

[0027]

[0028] Therefore, in a series of isothermal experiments at different temperatures,

[0029]

[0030] Based on the isothermal experimental curves from DSC or DTA, the heat effect of the reaction can be obtained by integrating equation (8 or 9). The temperature of the enthalpy is denoted as T. H At that time, the chemical enthalpy of a chemical reaction can be obtained.

[0031] (3) Enthalpy of chemical reaction in linear heating experiments of DSC or DTA

[0032] In the linear heating experiment of DSC, any point in the linear heating process is regarded as a tiny isothermal process, and the thermal effect at each temperature point can be considered as the thermal effect at an isothermal point (ΔH). 21 =0), and the total heat effect will be the sum of a series of isothermal heat effects. Therefore, the enthalpy of a chemical reaction is:

[0033]

[0034] Based on the linear heating experimental curves of DSC or DTA, the heat effect of the reaction can also be obtained by integrating equation (8 or 9), and the temperature of the enthalpy is labeled as T. H At that time, the chemical enthalpy of a chemical reaction can be obtained.

[0035] (4) Deriving chemical reaction enthalpy from thermodynamic enthalpy

[0036] The enthalpy temperature T of the reaction can be obtained through a series of linear heating experiments using DSC. H By querying the standard enthalpy, specific heat, and heat of phase transition of reactants and products from a thermodynamic database, and substituting the standard enthalpy of reactants and products into equation (2), we can obtain... The standard enthalpy, specific heat, and heat of phase transition of the reactants and products, as well as T... H Substituting into equations (3, 4, 5) allows us to solve for ΔH. 21 In T H The value at time. The reaction and ΔH 21 Substituting into equation (1) yields the enthalpy of a chemical reaction.

[0037] The present invention has significant advantages over the prior art as follows:

[0038] 1. At 298.15 K, many exothermic or endothermic reactions cannot occur; they can only be induced by heating the reactants to raise their temperature. To date, there is no clear definition or method for determining the critical temperature at which a reaction occurs. This invention proposes a definition of enthalpy temperature (T0). H It clearly indicates the critical temperature at which the reaction begins; when T <T H When T ≥ T, the reaction cannot occur. H The reaction can occur at any time; the temperature point at which the TGA, DTA, or DSC curve deviates from the baseline can be determined by thermogravimetric analysis (TGA), differential thermal analysis (DTA), or differential scanning calorimetry (DSC). This temperature point is the enthalpy temperature of the reaction.

[0039] 2. Because the classic standard enthalpy or combustion enthalpy is a mixed enthalpy, it contains not only the reaction enthalpy of the chemical reaction but also the physical enthalpy of a test process, and cannot reflect the true reaction enthalpy in a chemical reaction; this invention provides a method for solving the chemical reaction enthalpy (absolute value) through thermal analysis (TA) and thermodynamic data; (1) derive the chemical reaction enthalpy from the thermodynamic enthalpy, (2) solve the chemical reaction enthalpy from the DSC or DTA curves in the DSC or DTA isothermal or linear heating experiment;

[0040] 3. Classical enthalpy theory holds that the enthalpy of a reaction varies with temperature, and there is a reaction enthalpy at any temperature point; this invention proves from the consistency of DSC or DTA isothermal and linear heating thermodynamic data that a reaction has only one unique chemical reaction enthalpy, and it does not change with temperature. Attached image description:

[0041] Figure 1 A thermodynamic cycle (enthalpy diagram) for a chemical reaction in a closed system.

[0042] Figure 2 Linear heating curves for DSC or DTA: temperature / T or time / t and integral area / Q; T = temperature, t = time, T H T is the enthalpy temperature. onset A = KQ, which is the starting temperature in typical thermal analysis. p This is the area under the integral of the DSC or DTA curve.

[0043] Figure 3 This is a DSC curve of the dehydration reaction of calcium oxalate monohydrate (COM) under linear heating.

[0044] Figure 4 The DSC curves are for the COM dehydration reaction at 120 and 140 °C.

[0045] Figure 5 Thermodynamic calculation diagram for the dehydration reaction of COM.

[0046] Figure 6 This is a triangular thermal cycle diagram for an endothermic reaction. Detailed implementation method:

[0047] The present invention will be further described in conjunction with examples and accompanying drawings of chemical reaction enthalpy:

[0048] This invention provides a method for determining the enthalpy temperature and enthalpy of a chemical reaction, the specific steps of which are as follows:

[0049] 1) Determine the thermochemical equation for a chemical reaction; under the linear heating condition of thermal analysis (TA), test the thermogravimetric analysis (TGA), differential thermal analysis (DTA), or differential scanning calorimetry (DSC) curves of a sample; the linear heating of TA is the linear heating experiment of TA, specifically heating the sample cell at different linear heating rates, with heating rates ranging from 1 to 30 K / min, and intervals of 1 K / min, 2 K / min, 4 K / min, 5 K / min, or 10 K / min; through this linear heating experiment, the W-T curve of TGA, the ΔT-T curve of DTA, or the q / mv-T curve of DSC can be obtained;

[0050] 2) Obtain the initial temperature / enthalpy temperature T of the sample reaction from the thermal analysis curve. H Under the linear heating condition of thermal analysis (TA), the temperature ΔT of the sample cell, the heat q / mv, or the weight loss rate W are obtained. s The initial temperature point T at which the reference cell temperature T initially deviates from the baseline. sr T sr =T H T H It is the enthalpy temperature of this reaction;

[0051] 3) Solve for the enthalpy of chemical reactions using direct thermal analysis or indirect thermodynamic methods;

[0052] Integrating the curves of DTA and DSC, the area of ​​integration is the heat effect of the reaction divided by the enthalpy of the chemical reaction.

[0053] Thermodynamic indirect method for solving chemical reaction enthalpy; based on the enthalpy temperature T obtained from thermal analysis experiments. H Alternatively, thermodynamic data of reactants and products can be retrieved from a thermodynamic database and substituted into the enthalpy of the chemical reaction:

[0054] Further solutions are as follows:

[0055] 1) Determine the thermochemical equation for a chemical reaction;

[0056] 2) Obtain the W-T curve for thermogravimetric analysis (TGA), the ΔT-T curve for differential thermal analysis (DTA), or the q / mv-T curve for differential scanning calorimetry (DSC) from linear temperature rise experiments using thermal analysis (TA). Determine the temperature T at which W, ΔT, or q / mv in the TGA W-T curve, DTA ΔT-T curve, or DSC q / mv-T curve deviates from the baseline of the reference cell. H This is called the enthalpy temperature of a reaction;

[0057] 3) Obtain the ΔT-T curve of DTA or the q / mv-T curve of DSC from linear heating or isothermal experiments using differential thermal analysis (DTA) or differential scanning calorimetry (DSC). Based on the ΔT-T curve of DTA or the q / mv-T curve of DSC obtained from the analysis (TA) and the experimental parameters, according to...

[0058]

[0059] (T / temperature, ΔT=Ts-Tr / temperature difference between reference cell and sample cell, A is the integral area, t1 is the starting time of the DTA or DSC curve, t2 is the ending time of the DTA or DSC curve, K is the instrument integration constant, Q) p (For heat). Calculate the heat of reaction Q of the sample. p ;

[0060] 4) Order:

[0061]

[0062] Another way to solve for the enthalpy of a chemical reaction is:

[0063] a. Based on the thermochemical equation of the reaction, look up the standard enthalpy of formation of the reactants and products in the thermodynamic database, and calculate the enthalpy of reaction under standard conditions according to equation (2):

[0064]

[0065] b. Query the specific heat of reactants and products from the thermodynamic database, and calculate the enthalpy of heat capacity of reactants and products at 298.15 according to equation (4):

[0066] ΔC p =∑ΔC p / p (product)-∑ΔC p / r (Reactants) (4)

[0067] c. The enthalpy of reactants and products ΔC p and the enthalpy temperature T determined in step 3). H Substitute:

[0068]

[0069] The reaction can be obtained from 298.15 → T H The change in enthalpy of heat.

[0070] d. Search the thermodynamic database for reactants and products at 298.15→T H The phase transition heat within the interval is calculated as the difference between the phase transition heats of the reactants and products according to equation (5):

[0071] Δ trs H = ∑Δ trs H p (product)-∑Δ trs H r (Reactants) (5)

[0072] e. Calculate ΔC from steps c and d. p / H and Δ trs Substituting H into equation (3):

[0073] ΔH 21 =ΔH2-ΔH1=ΔC p / H +Δ trs H (3)

[0074] f. The steps a) and e) and ΔH 21 Substituting into equation (15), we can obtain the chemical enthalpy of a reaction.

[0075]

[0076] Based on the above method for calculating the enthalpy of chemical reactions, the following scheme is proposed:

[0077] For example: the dehydration reaction of calcium oxalate monohydrate (COM):

[0078] CaC2O4·H2O(S)→CaC2O4(S)+H2O(l) (16)

[0079] 1) One of the technical solutions of this invention is:

[0080] In linear heating experiments (5K, 10K, 15K / min) of DSC, a set of Q and T values ​​for the COM dehydration reaction can be obtained. H value.

[0081] Figure 3 The DSC curves (5 K / min, 10 K / min, 15 K / min) are shown for linear heating during the dehydration reaction of calcium oxalate monohydrate. H =The average values ​​of T1 and Q are 104.71℃ and 454.53J / g, respectively.

[0082] according to Figure 3 The thermal parameters in the equation can be used to obtain the enthalpy temperature of the reaction.

[0083] T H =104.71+273.15=377.86K (17)

[0084] According to T sr =T H From the definition of T, we can obtain H =377.86, therefore:

[0085]

[0086] The advantage of the linear heating method of DSC is that the enthalpy temperature and enthalpy of a reaction can be obtained directly through a set of experiments.

[0087] 2) The second technical solution of the present invention is: in the DSC isothermal (120℃ and 140℃) experiment, a set of chemical reaction enthalpies of the COM dehydration reaction can be obtained:

[0088] like Figure 4 The DSC curves for the COM dehydration reaction at 120 and 140 °C are shown. The average heat effect (Q) of the COM dehydration reaction is 460.98 J / g, and the reaction times are 84 min (120) and 32 min (140), respectively.

[0089] According to T obtained from one of the technical solutions H =377.86, then we have:

[0090]

[0091] The DSC isotherm method can directly obtain the chemical enthalpy of a reaction, but determining the thermal enthalpy temperature of the reaction requires more isothermal experiments to explore.

[0092] 3) The third technical solution of the present invention is: the enthalpy temperature of the sample reaction can be detected by thermogravimetric analysis (TGA), DSC, or DTA. According to the DSC experiment in one of the technical solutions, the enthalpy temperature of COM can be obtained. H =377.86. The C values ​​of reactants and products were found in a thermodynamic database. p The heat of phase transition and the heat of formation can be obtained as follows:

[0093] Table 1. Basic thermodynamic data for the dehydration reaction of COM

[0094]

[0095] According to Kirchhoff's laws, a thermodynamic calculation diagram of the COM dehydration reaction can be drawn, such as... Figure 5 As shown:

[0096] T H =104.71+273.15=377.86K (20)

[0097] According to equation (2):

[0098]

[0099] According to equations (3, 4, 5):

[0100] ΔH 21 =ΔH2+ΔH3+ΔH4+ΔH5-ΔH1=39.78KJ·mol -1 (twenty two)

[0101] Substitute (21) and (22) into equation (15):

[0102]

[0103] in conclusion:

[0104] In the COM dehydration reaction, the relative error between the thermodynamically calculated enthalpy and the DSC-measured chemical enthalpy is:

[0105] γ%=(66.36-68.21) / 68.21×100%=-2.72% (24)

[0106] The present invention demonstrates from the consistency of the isothermal, linear heating and thermodynamic data of the above DSC that a reaction has only one unique chemical enthalpy, and that it does not change with temperature.

[0107] The following is an application example:

[0108] Thermal Cycle Design in Chemical Engineering

[0109] One of the main tasks of thermodynamics is to design rational and economical heat cycles in chemical engineering. For example, it may be necessary to dehydrate calcium oxalate monohydrate to calcium oxalate. The dehydration reaction of COM can be obtained from thermodynamic databases. In fact, at 298.15 K, it is impossible to obtain calcium oxalate, the product of the dehydration of calcium oxalate monohydrate, by providing 28.43 KJ / mol of heat.

[0110] According to the enthalpy theory of 111, COM needs to be heated to 104.71℃ for the dehydration reaction to occur, and this heating process requires the absorption of 12.12 kJ·mol⁻¹ of heat. -1 = (104.71-25)×0.152. The dehydration reaction requires endothermic reaction at 104.71℃. Therefore, the enthalpy theory of 111 indicates that the minimum temperature for the dehydration reaction of COM is 104.71℃, and the minimum heat supply is 12.12 + 66.36 = 78.48 kJ·mol⁻¹. -1 The enthalpy theory provides scientific and specific thermal parameters for the thermodynamic process of the COM dehydration reaction. In fact, if the enthalpy temperature (T) of the reaction is unknown... H Without this, it would be impossible to design many thermal parameters in chemical engineering based on thermodynamic functions.

Claims

1. A method for determining the enthalpy temperature and enthalpy of a chemical reaction, characterized in that: The method includes the following specific steps: 1) Determine the thermochemical equation for a chemical reaction; Under linear temperature rise conditions in thermal analysis (TA), test the thermal analysis curves of a sample using thermogravimetric analysis (TGA), differential thermal analysis (DTA), or differential scanning calorimetry (DSC). 2) Obtain the initial temperature / enthalpy temperature T of the sample reaction from the thermal analysis curve. H Under the linear heating condition of thermal analysis (TA), the temperature difference ΔT of the sample cell, the heat q / mv, or the weight loss rate W are obtained. s The initial temperature point T at which the reference cell temperature T initially deviates from the baseline. sr T sr =T H T H It is the enthalpy temperature of this reaction; 3) Solve for the enthalpy of chemical reactions using direct thermal analysis or indirect thermodynamic methods; Direct thermal analysis: Integrating the curves of DTA and DSC, the area of ​​integration is the heat effect of the reaction divided by the enthalpy of the chemical reaction. Indirect thermodynamics: based on the enthalpy temperature T obtained from thermal analysis experiments. H Alternatively, thermodynamic data of reactants and products can be retrieved from a thermodynamic database and substituted into the database: It is the chemical enthalpy of this reaction.

2. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 1, characterized in that: The linear heating experiment for thermal analysis (TA) involves heating the sample cell at different linear heating rates, ranging from 1 to 30 K / min, with intervals of 1 K / min, 2 K / min, 4 K / min, 5 K / min, or 10 K / min. This linear heating experiment yields the W-T curve for TGA, the ΔT-T curve for DTA, and the q / mv-T curve for DSC.

3. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 2, characterized in that: The temperature T at which W, ΔT, or q / mv in the W-T curve of TGA, the ΔT-T curve of DTA, and the q / mv-T curve of DSC deviates from the TA baseline. H , which is the enthalpy temperature of a reaction.

4. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 1, characterized in that: The chemical enthalpy of reaction can be obtained by conducting isothermal or linear heating experiments using either DSC or DTA, yielding one or a set of thermograms. The area integral of this integral, calculated using the Speil equation, equals the chemical enthalpy of reaction. Where A is the integration area, t1 is the starting time of the DTA or DSC curve, t2 is the ending time of the DTA or DSC curve, K is the instrument integration constant, and Q is the integration constant. p It is for heat.

5. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 1, characterized in that: The process of obtaining thermodynamic data for chemical reactions is as follows: 1) Determine the thermochemical reaction equation for the chemical reaction; 2) Based on the thermochemical reaction equation, search for the standard enthalpy of formation of the reactants and products in various thermodynamic databases. Specific heat ΔC p and phase change heat Δ trs H p data.

6. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 1, characterized in that: The standard enthalpy of reaction is 7. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 1, characterized in that: The physical heat effect of the reaction ΔH 21 for 8. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 7, characterized in that: enthalpy of reaction ΔC p for 9. The method for solving the enthalpy temperature and enthalpy of a chemical reaction according to claim 7, characterized in that: Phase transition enthalpy Δ of the reaction trs H is Δ trs H = ∑Δ trs H p (product)-∑Δ trs H r (Reactants)

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