Chemical reaction refers tomoleculeRupture intoatomThe process by which atoms rearrange and combine to form new molecules.The reaction is often accompanied by luminescence, heating, discoloration, precipitation, etc. The basis for judging whether a reaction is a chemical reaction is whether the reaction generates new molecules.
nuclear reactionIt is not a chemical reaction.Ionic reaction is a chemical reaction.
The essence of chemical reaction is oldChemical bondFracture and the formation of new chemical bonds.
The reaction is often accompanied by luminescence, heating, discoloration, precipitation, etc.Whether a reaction is a chemical reaction depends on whether the reaction generates new substances.according toChemical bond theoryIt can also be judged whether it is a chemical reaction according to whether there are old bonds breaking and new bonds forming in a change process.
Isomerization: (A → B): Compounds form structural reorganization without changing chemical composition.
Chemical synthesis: chemical reaction
Abbreviated as: A+B=C: two or more elements or compounds synthesize a complex product.(That is, the reaction of forming a new substance from two or more substances.)
Chemical decomposition: decomposition reaction
Abbreviated as: A=B+C: compounds are decomposed into constituent elements or small molecules.(i.e., chemical reactionReverse reaction。It refers to the reaction that a compound decomposes into two or more simple simple substances or compounds under specific conditions.)
Abbreviated as: A+BC=B+AC: it means that an additional reaction element replaces one element in the compound.(It refers to the reaction between one simple substance and one compound to form another simple substance and another compound.)
(Displacement relation means that some element of the compound is replaced by the element of the simple substance. The displacement reaction must beRedox reactionHowever, the redox reaction is not necessarily a displacement reaction.)
according toreactantAnd productsSimple substanceThe displacement reaction has the following four conditions:
① The more active metal replaces the less active metal or hydrogen
Abbreviated as: AB+CD=AD+CB:aqueous solutionTwo compounds exchange elements or ions to form different compounds.(That is, the reaction in which two compounds exchange components with each other to form the other two compounds.)
The essence of metathesis reaction is that it is difficult for ions in solution to combineionizationSubstances (such as water), insoluble substances orvolatilityGas, so that the metathesis reaction tends to be completed.The reaction between acid, base and salt solutions is generally formed by the exchange of components between two compounds, that is, the compounds participating in the reaction are ionized and dissociated into free moving ions in the aqueous solution, and the ions are reassembled into new compounds. Therefore, the reaction between acid, base and salt solutions is generally a metathesis reaction.Metathesis reactionIt is the recombination of ions or ion clusters, because theValenceThere is no change, so the double decomposition reaction is notRedox reaction。
Of course, there are more complex situations, but they can still be simplified gradually and regarded as theContinuous reaction。It is difficult to establish a simple classification standard due to the variety of chemical reactions.However, some similar chemical reactions can still be classified, such as:
It refers to the oxidation-reduction reaction between the same element of the same valence in the molecule of the same substance.During the oxidation-reduction reaction of elements with the same valence, "differences on the change of valence" occur, some increase, some decrease.The elements with disproportionation reaction must have corresponding high valence and low valence compounds, and disproportionation reaction only occurs on the elements with intermediate valence.Fluorine (Ftwo）No disproportionation becauseFluorine elementThe electronegativity is the largest, with no positive valence but only negative valence.
Homing reactionAnd disproportionation reaction are oxidation-reduction reactions between the same substance, and disproportionation reaction isAutoredox reactionHowever, its own redox reaction is not necessarily a disproportionation reaction.
Centering reaction (disproportionation reaction):
It refers to the oxidation-reduction reaction between the same element with different valence states in the substance.That is, the valence state of the same element changes from the high and low valence before the reaction to the intermediate valence state after the reaction. There is a rule in the change of the valence state of the element in the chemical reaction: only close, not cross.Therefore, the high and low prices of elements can only move closer to the middle.The homing reaction and disproportionation reaction are two 'opposite' processes, and these two reactions must beRedox reaction。
Redox reaction: refers to the electron transfer between two compounds (e.gSubstitution reactionAnd combustion reaction)
Combustion reaction(also called oxidation reaction in junior high school chemistry books): refers to the relationship between substances andoxygenReaction.
Chlorination reaction;fingerchlorineReact with organic matter;
Nuclear reactions are not chemical reactions, nuclear reaction belongs toNuclear physical change。Atoms areChemical changeThe chemical reaction can also be seen as the recombination of atoms. The type of elements and the number of atoms remain unchanged before and after the reaction.andnuclear reactionThe atom in has become another atom, so it is notChemical change。
Reaction energy
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Net change in energy
according toThe second law of thermodynamics, any isothermal isobaricClosed systemReduced propensityGibbs free energy。Without the influence of external force, the same is true for any reaction mixture.For example, the analysis of enthalpy in the system can obtain thethermodynamicscalculation.The calculation of enthalpy in reaction adopts the standard enthalpy of reaction and the law of additivity of reaction heat（Hess law）。
Take the combustion reaction of methane and oxygen as an example:
The energy difference between reactants and products can only be calculated by interrupting all bonds on the left and right sides of the reaction to obtain energy data.With ΔHIndicates the energy difference.Δ (Delta) means difference,HEnthalpy is equal to heat conduction energy under fixed pressure.Δ H is expressed in kilojoules or kcal.
Reaction judgment
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Suppose there is a system.The enthalpy of the system before and after the reaction is calculated as Δ H.
If ΔHIf the calculation is negative, the reaction must beExothermic reaction。For example, combustion means that the fuel reacts violently with oxygen in the air to release heat.
If ΔHIf the calculation is positive, the reaction must be endothermic.For example, limestone reacts and decomposes under high temperaturecalcium oxide(quicklime) andcarbon dioxide。
If ΔHIf the calculation is equal to zero, the reaction is neither endothermic nor exothermic;The outside world does not work on the system, and the system does not work on the outside world.
Whether the reaction is endothermic or exothermic depends entirely on the ΔHdecision.However, whether the reaction proceeds or not depends entirely on the Gibbs free energy Δ of the systemGTo represent.The specific calculation formula is as follows:
ΔG= ΔH-TΔS
ΔGIs the change of free energy, Δ H is the change of enthalpy, and ΔSIs entropy change, and T isKelvin temperature。
Reaction conditions
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It is necessary or can be improved by chemical reactionreaction rateSuch as heating (△), ignition, high temperature, electrification (electrolysis), ultraviolet ray or catalyst.
reaction rate
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The reaction rate of a chemical reaction is a measure of the change of the relevant acceptor concentration with time.The analysis of reaction rate has many important applications, such asChemical Engineeringorchemical equilibriumResearch.The reaction rate is affected by the following factors:
Concentration of reactants: If it is increased, the reaction will usually be accelerated.
The higher the activation energy, the more difficult the reaction is to start, and therefore the slower the reaction rate.
Reaction temperature: temperature rise will accelerate the reaction, because the higher the temperature is, the more energy there is, which makes the reaction easy to occur.
Catalyst: A catalyst is a substance that changes the reaction rate by changing the activation energy.Moreover, the catalyst will not be damaged or changed during the reaction, so it canRepeated action。
The reaction rate is related to the concentration of substances involved in the reaction.The substance concentration can penetrateLaw of mass actionQuantitative.
chemical equilibrium
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according toGibbs free energy criterion , when ΔrGm=At 0, the reaction reaches the maximum, atEquilibrium state。The chemical balance is established byReversible reactionIs the premise.Reversible reaction refers to the reaction that can both go forward and backward under the same conditions.Most chemical reactions are reversible and can reach equilibrium to varying degrees.
From the kinetic point of view, at the beginning of the reaction, the concentration of reactants is larger and the concentration of products is smaller, so the positive reaction rate is greater than the reverse reaction rate.As the reaction proceeds, the concentration of reactants decreases and the concentration of products increases, so the positive reaction rate decreases and the reverse reaction rate increases.When the forward and reverse reaction rates are equal, the concentration of each substance in the system will not change and the reaction will reach equilibrium.
In the 1950s and 1960s, the basic laws of thermodynamics had been clarified, but some thermodynamic concepts were still vague, and digital processing was tedious, which could not be used to solve slightly complicated problems, such as the direction of chemical reactions.At that time, most chemists were engaged in the research of organic chemistry, and some people tried to solve the direction of chemical reaction.In addition to the law of mass action, there are other people who try to explore the direction of reaction from other angles. Some empirical laws have been proposed.
Study of chemical changes
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During this period, the Danes Thomson and Bertro triedheat effectTo explain the directionality of chemical reactions.They believe that reaction heat is reactant chemistryAffinityEvery simple or complex pure chemical action is accompanied by the generation of heat.Betero more clearly expounded the same point of view, and called it "the principle of maximum work". He believed that any pure chemical change without external energy influence was carried out in the direction of producing the substance that released the maximum energy.Although he found someEndothermic reactionIt can also be carried out spontaneously, but he subjectively assumes that there is an exothermic physical process.This wrong conclusion was finally admitted by him in the 1930s, when he limited the application scope of the "maximum work principle" to the reactions between solids, and proposed that in fact“Free enthalpy”The concept of chemical heat.In the 1960s and 1980s, HorstmannLe ChatelierAnd Van Hoff have also made certain contributions in this regard.First, Horstman is studyingammonium chlorideDuring the sublimation ofClausius ClapeyronEquation: dP/dt=Q/T(V'-V）, whereQrepresentativeDecomposition heat，V、V'represents the total volume before and after decomposition.Fan Hoff derived the following formula based on an equation:
lnK=-(Q/RT)c
This formula can be applied to any reaction process, in whichQRepresents the heat absorbed by the system (i.eSublimation heat）。Van HoffThe above equation is called the dynamic balance principle, and he explained it. He said that any balance between two different states of matter moves towards the balance direction of the two systems generating heat due to the temperature drop.In 1874 and 1879, Moodier and Robin also put forward such principles.Moodier proposed that the increase of pressure is conducive to the reaction of corresponding reduction of volume.After that,Le ChatelierIt further explains this principle generally.He said that any system in chemical equilibrium will lead to a transformation in one direction due to the change of one of the multiple factors in the equilibrium. If this transformation is unique, it will lead to a change opposite to the change sign of the factor.
However, Gibbs has made outstanding contributions in this respect, and his position in the history of thermal chemistry is extremely important.Gibbs' contribution to thermochemistry can be summarized in four aspects.First, on the basis of the second law established by Claudius et al., Gibbs derived the judgment basis of balance and correctly limited the judgment basis of entropy to the scope of isolated systems.It makes it possible to deal with general practical problems generally.Second, internal energy, entropy and volume are used to describe the system state instead of temperature, pressure and volume as variables.It is pointed out that Thomson's description of system state by temperature, pressure and volume is incomplete.He advocatedstate equation , and in the internal energy, entropy and volume3D coordinatesIn the figure, the complete description systemThermodynamic propertiesThe surface of.Thirdly, Gibbs introduced the variable "concentration" in thermodynamics, and defined the derivative of the concentration of the component to the internal energy as“Thermodynamic potential”。
In this way, thermodynamics can be used to deal with multi-componentPolyphase systemThe problem of chemical balance also has the conditions to deal with.Fourth, he further discussed the system'selectric、magnetismAnd surface under the influence of balance problems.Moreover, he derived the simplest, most essential and most abstract thermodynamic relationship in thermodynamics, namelyPhase rule, on, andEquilibrium stateIt is indicated by the phase lawfreedomThe state of being zero.
Gibbs's research results on balance are mainly published in his three articles.In 1873, he successively published the first two articles in the journal of Connecticut College, which immediately causedmaxwellAttention.Gibbs's first two articles can be said to be just a preparation. In 1876 and 1878, he published the third article, "On the Balance of Multiphase Matter", in two parts. The article was more than 300 pages long, including more than 700 formulas.Two articles are about single chemical system, and this article is about multi-component multiphase system.Due to the introduction of thermodynamic potentialOne component systemWith a slight change in the equation of state, the problem of multi-component systems can be dealt with.
For Gibbs's work,Le ChatelierIt is considered that this is the opening up of a new field, and its importance can be compared with the law of mass immortality.However, after the publication of Gibbs' three articles, their significance was not recognized by most scientists. It was not until 1891 that they were translated into German by Ostwald. After Le Chatelier's translation into French was published in 1899, the situation suddenly changed.After Gibbs, thermodynamics can only deal with systems in ideal state.At that time, American Lois published articles in 1901 and 1907 respectively, proposing“Fugacity”And“activity”The concept of.Lewis talked about the concept of "escape trend" and pointed out that some thermodynamic quantities, such as temperature, pressure, concentration, thermodynamic potential, etc. are all scales of escape trend measurement.
The concepts of fugacity and activity put forward by Lewis make Gibbs' theory get beneficial supplement and development, so that it is possible to unify the deviation of ideal system, and make the actual system have the same thermodynamic relationship with ideal system in form.in summary,Chemical equilibrium stateIt refers to reversible reaction, positive reaction and reverse reaction under certain conditionsrateEqual, the concentration of each component in the reaction mixture remains unchanged.
The chemical equilibrium state has the characteristics of inverse, equal, dynamic, stationary and variable.
Etc.: when in equilibrium, the positive and reverse reaction rates are equal, that isvjust=vinverse。
Dynamic: when balancing, the reaction is still in progress, yesdynamic equilibriumThe reaction is carried out to the maximum extent.
Determination: when the equilibrium state is reached, the concentration of each component in the reaction mixture remains unchanged, the reaction rate remains unchanged, the conversion rate of reactants remains unchanged, and the content of each component remains unchanged.
oxidation reaction
Change: Like all dynamic equilibrium, chemical equilibrium is conditional, temporary and relative. When conditions change, the equilibrium state will be destroyed, from equilibrium to imbalance, and then new equilibrium will be established under new conditions.There are many factors affecting chemical equilibrium, such as pressure, temperature, concentration, etc.
Note: The catalyst does not affect the chemical balance, only the reaction rate.When other conditions remain unchanged, increasing the concentration of reactants or decreasing the concentration of products can move the equilibrium to the positive reaction direction.
Le Chatelier principle: If changes affectbalanceThe equilibrium moves in the direction that can weaken the change under a condition of (concentration pressure or temperature, etc.).
Reaction phenomenon
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Exothermic, endothermic, luminous, discoloration, precipitation, gas generation
Reversible and spontaneous reactions
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Each chemical reaction is theoretically reversible.A positive reaction defines the conversion of a substance from a reactant to a product.Reverse reactionOn the contrary, the product is converted intoreactant。
Chemical equilibrium means that the positive reaction rate and the reverse reaction rate are equal, so reactants and products will exist.However,Equilibrium stateThe reaction direction can be changed by changing the reaction state, such as temperature or pressure.Le Chatelier's principle is used here to predict the formation of products or reactants.
Although all reactions are reversible in some extent, some reactions can still be classified asIrreversible reaction。"Irreversible reaction" means "complete reaction".It means that almost all reactants form products, and it is difficult to reverse the reaction even under extreme conditions.
Another reaction mechanism is calledSpontaneous reaction, is a thermodynamic tendency, indicating that this reaction causes a net increase in the overall entropy.Spontaneous reaction (as opposed to non spontaneous reaction) will occur without external assistance (such as energy supply).In a chemical equilibrium system, the direction of spontaneous reaction in the reaction process can be expected to form more substances.
