May I ask why H2I2 2HI is an endothermic reaction while H2Cl22HCl is an exothermic reaction?

t 。2008-08.

Satisfied with the answer. I guess you haven't figured out the relationship between bond energy and the internal energy of matter. Internal Energy Angle:

The total energy of the reactant --- the total energy of the endothermic reaction, which is produced by endothermic from the environment. Internal energy cannot be accurately measured, which has qualitative interpretive significance. Key Energy Angle:

Endothermic reactions --- the energy (endothermy) that breaks chemical bonds (reactants) and the energy (exothermic) that forms chemical bonds (products).The heat of reaction can be calculated using bond energy, but generally the heat of reaction is an experimental measurement. In fact, when understanding the heat of reaction, the introduction of the concept of a "reaction system" can reduce the difficulty.

In the process of reaction, the energy of the system decreases when it is exothermic from the outside, and h is "-" when the energy is absorbed from the outside, then the energy of the system is increased h is "the total energy of the reactants" is actually the total energy of the reaction system before the reaction, and the "total energy of the product" is the total energy of the system after the reaction. It is impossible to determine what the total energy value is, but the amount of change can be measured. 1.

All combustion reactions are exothermic reactions 2The reaction of reactive metals with acids is an exothermic reaction 3Neutralization reactions are both exothermic reactions.

1.The reaction that requires continuous heating to take place is the endothermic reaction. Such as CaCO3 decomposition, KCLO3 decomposition, water gas reaction; c+H2O=Co+H2(High temperature) 2

Salt hydrolysis is an endothermic reaction Note: A reaction that requires heating to be carried out is not necessarily an endothermic reaction. For example, the combustion of most fuels needs to be heated to the ignition point before the reaction can be carried out.

Only must"Keep heating"The reaction that can be carried out, that is to say, does not heat, the reaction stops very quickly, and such a reaction is an endothermic reaction. In addition, non-metals react with hydrogen to form gaseous hydrides, and most of these reactions are exothermic reactions, and the more active the non-metals, the more exothermic the reaction. But some are endothermic reactions, such as the reaction of I2 with H2.

h2+i2《=》2hi

h=-9kj/mol

Exothermic. Halogens react with H2 and are exothermic.

Chemistry - Compulsory II - Endothermic reactions.

Is the reduction reaction of CO2+H2 to CH4 endothermic or exothermic?

CO2 reacts with H2 under the condition of a catalyst and heating to produce methane and water under the condition of heating, and this process is an endothermic reaction.

2Na + 2H2O = 2NaOH + H2 is a redox reaction.

Oxidation-reduction reaction is a type of reaction in which the oxidation number of an element changes before and after a chemical reaction.

The essence of a redox reaction is the gain or loss of electrons or the shift of shared electron pairs. The redox reaction is one of the three basic reactions in chemical reactions (the other two are the (Lewis) acid-base reaction and the free radical reaction).

Combustion, respiration, photosynthesis, chemical batteries in production and life, metal smelting, rocket launches, etc. are all closely related to redox reactions in nature. The study of redox reactions is of great significance to the progress of mankind.

At the end of the 18th century, chemists summarized the reactions of many substances with oxygen, found that such reactions had some similar characteristics, and put forward the concept of redox reactions: reactions with oxidation are called oxidation reactions; The reaction of seizing oxygen from the collision of oxygenated compounds is called a reduction reaction.

With the development of chemistry, it has been found that many reactions have similar characteristics to redox reactions in the classical definition, and after the development of the concept of valency in the 19th century, a class of reactions with increased valency was incorporated into oxidation reactions, and a class of reactions with reduced valency was incorporated into reduction reactions.

At the beginning of the 20th century, the electron theory of bonding was established, so the half-reaction of losing electrons was called oxidation reaction, and the half-reaction of gaining electrons was called reduction reaction.

In 1948, on the basis of valence bond theory and electronegativity, the general idea of oxidation number was proposed, and in 1970, IUPAC made a strict definition of oxidation number, and redox reaction was also formally defined: before and after a chemical reaction, a class of reactions in which the oxidation number of an element changes is called a redox reaction.

Summary. H2C=CH2+HL reaction.

Hello, glad to answer for you.

The heat of reaction can be calculated from the heat of formation of each species in the reaction formula.

I looked it up.

co =h2o = -286kj/mol

co2 =h2 =0kj/mol

h=δfh（co2）+δfh（h2）-δfh（co）-δfh（h2o）

0 is an endothermic reaction.

1. Endothermy: the reactant is smaller than the heat roller town that generates tung trapped matter.

Exothermic: The reactant is greater than the amount of heat that produces the large coarse matter.

It can also be judged by bita h, 2, Ba(OH)2+NH4Cl=BaCl2+

According to the formula: h=sum of the total bond energies of the reactants The sum of the total bond energies of the products, h=436+151-299x2=-11<0, so it should.

Because, the chemical bond energy index and 25 degrees Celsius (at room temperature and pressure), the energy required to split 1mol of ideal gas molecule AB into neutral gaseous atoms A and B (the unit is that the greater the bond energy, the stronger the chemical bond, and the more stable the molecule containing the bond.

Here the HH bond and I-I to be broken are 1mol each, that is, +(436+151)kj, and (299x2)kj is required to form a 2molh-i bond, and the addition is negative, that is, the reaction is exothermic, but the equation ignores the marked state! The definition of bond energy is that iodine is a solid state under normal conditions, and the relative energy required is more, more than 11kJ, so it is endothermic.

2. The correct of the following statements about the heat of reaction and enthalpy change is ( ).

a. The enthalpy change is completely equal to the heat of reaction.

b. The unit of heat of reaction is the same as the unit of enthalpy change.

c. The heat of reaction of all chemical reactions can be directly measured experimentally.

d. The enthalpy change of all chemical reactions can be directly measured and resolved by experiments: only the heat of reaction is equal to the enthalpy change when the chemical reaction occurs under constant pressure conditions;

The units of heat of reaction and enthalpy change are both kj mol; The heat of reaction and enthalpy change of some chemical reactions cannot be directly measured experimentally.