I am not sure if I understand the question. Normally reactions are exothermic as they release heat. Endothermic reactions means they take heat. An example of endothermic reaction could be shebert, they make bubbles. Exothermic reactions could be combustion. Maybe another form of endothermic reaction could be sublimation. The combination you mean, Haldane and the intermediates of reaction?

Yes, what is the pattern that some combination reactions release heat and why some combination reactions take heat.

A combination reaction is a reaction where two or more elements or compounds (reactants) combine to form a single compound (product). Such reactions are represented as X + Y → XY. According to thermodynamics, a change of the free Gibbs energy is dG=dH-TdS for a reaction must be negative. Most of the combination reactions result in reduction of the entropy change, dS X+Y+Z+.....

Hence, dH can be positive, dH>0, but still making dG 0 means that the reaction is endothermic.

The bottom line: the relationship of the dH and -TdS defines the type of the reaction and its thermodynamics making it either exothermic or endothermic.

Yes, so I can rephrase the question, what is the pattern that some combination reactions are negative entropy and why some combination reactions are positive entropy.

Standard molar entropies for a variety of substances and pure elements are listed in Thermodynamic tables for a reference temperature (like 298 K) and 1 atm pressure (i.e. the entropy of a pure substance at 298 K and 1 atm pressure). When comparing standard molar entropies for a substance that is either a solid, liquid or gas at 298 K and 1 atm pressure, the gas will have more entropy than the liquid, and the liquid will have more entropy than the solid. (Unlike enthalpies of formation, standard molar entropies of elements are not 0.) The entropy change in a chemical reaction is given by the sum of the entropies of the products minus the sum of the entropies of the reactants.

For example, for the combination reaction N2(g)+3H2(g)⇌2NH3(g)

at 298K as a standard temperature: S0(NH3) = 192.5 J/mol K ;S0(H2) = 130.6 J/mol K; S0(N2) = 191.5 J/mol K. Hence, ΔS0 = 2*192.5 - [191.5 + (3*130.6)]=-198.3 J/mol K. This reaction results in a decrease in entropy, i.e. a decrease in disorder because the number of gas molecules in the reaction is decreased. However, some other combination reactions could result in increase of the entropy. It all depends on the relationships between the molar entropies of the reactants and products, as well as the stoichiometric coefficients of each product and each reactant and their phase states.

I think the answer relies on physicla chemistry. why would you sometimes have reactions in the two different forms.

Most reactions (exothermic) are as we would describe them normally, just like combustion. combustion is one of the reactions that take place normally. We need to give heat fro some reactions to occur, but maybe there are not at the minimum possible energy when we do. This means they release heat, and they describe most reactions. However, sometimes, the reaction takes heat for it to occur. you might as well notice the flask is cold. This means the reaction has taken heat. When this happens, the reaction has been given heat, but not enough to accomply the reaction, the energy given then drives the reaction to take more heat and the reaction finishes. Otherwise the substances will be just hotter. No chemical change. It would be just a physical change. This is why reactions are sometimes like this, they normally give away heat. Combinatorial chemistry and maybe quantum mechanics. Why would you have a reaction which takes you to a certain result? The one which will be closest. The easiest and closest to obtain. This is why there are results which are more likely and achievable. The easiest path for any. We normally say it is easier to give heat o achieve a stable state. Unlike in biology we are constantly throwing energy to keep life. Perhaps if not by this, life would not exist. Steady state or the state in movement. why would you sometimes have different reactions occuring? The easiest to be taken to the formation of products, but the restof the reactions are impeded, and just counting with the usual elements, this is the result. Can you try anothr combination to obtrain the same kind of products and not the reaction? you could. maybe say there is a discrepancy in calling them heat or combustion, but this is all. Why would you burn yourself if it feels cold? cold can damage, and even confuse, like a burn. you take it to a different extreme. cold is the absence of heat, above all. this does not mean you acnnot feal heat in different ways. as such the ideal temperature. of course, it will be the darkest theories and answers of combinatorial chemistry. why the combination of atoms, is as it is and not in a different way? you could try to reach the same kind of result ina different way, or you culd stll use the same elements of reaction to obtain the result. However, we need now to make clear on the names we are using. is it still combustion or heat or ashes? at least we can say we are trying to obtain ashes...

Alexander Kolker and whoever else, I think I found the answer. Gibbs free energy, but it is "it is impossible to predict S and H a priori without any computer quantum calculations."

This is the same question like asking "why does some things dissolve better in colder water, and hotter water" (exo and endo), so again, asking why is something exo or endo when dissolved in water.

Now I'm curious to know, if there's a 3rd category of questions, where the ultimate answer is due to the inpredictibility of Gibbs free energy equation...

(Besides combination/decomposition reactions, being exo or endo, and dissolving in water, being exo or endo.). Or dS > 0, dS < 0.

It looks like that you totally missed my answer posted earlier on Aug 28. Yes, ultimately S and H for all substances can be calculated from the first principles using quantum calculations but it is very difficult and time consuming. Therefore S and H are collected for many substances (but, of course, not all) in Tables of Standard Thermodynamic values. You have to learn how to use these data for practical calculations...

Alexander yes, you said something about the coefficients too. So Gibbs calculations alone doesn't answer, I need to find an example where the same equation but different coefficients change the outcome of H or S.