Under what situation will reaction occur ga?

呢個係一個哲學問題, 你問左chemistry入而最根底既一個問題! 要完全明白既話, 就要不斷讀上去. 冇捷徑.

但雖如此, 卻萬變不離其宗. 就是要追求穩定(stable), 而穩定既其中一個重要指標就係要勢能,(potential energy) 低. 點變低? 咪放出黎囉. 可以用兩個form 放, 一個係熱能(heat energy), 一個係熵(entropy), entropy可以暫時唔理. 但熱能, 非常易理解, 就係你可以摸到既熱.

好多reaction 發生時, 佢都可以放出熱, 例如中和作用(neutralization), 沈澱作用(precipitation), 氧化還原反應(redox reactions), 佢地都係由高potential energy既野變低potential energy既野. 例如precipitation:
Ca2+ + (SO4)2- --> CaSO4

佢之所以要咁行, 係因為CaSO4既晶格(crystal lattice)非常stable, 所以形成呢個lattice會比之前的能量低, 所以可以放出heat energy.

neutralization都係類似: H+ + OH- --> H2O
之所以會形成鍵(form bond), 係因為個bond會令佢地更穩定.

所以多觀察, 睇下咩野溝埋會變熱, 多數是有reaction了.


至於吸熱反應(endothermic reactions), 大可以不用理, 當佢係fact背左佢就算 (fact就係佢會吸熱, 同放熱反應相反). 因為佢背後有entropy呢個極奇難明的概念, 等你有興趣再談.

usually two soluble ionic compounds will undergo double decomposition,
that is, exchanging anions/cations
however, if both sides of the equation contain solid
there is no reaction
for example
Ca(s) + H2SO4 → CaSO4(s) + H2
since both sides of the equation have solid
so no reaction

if the salts are neutral in nature, basically they are stable and have no reaction but precipitation reaction. for some ion pair just like Mg2+ and CO32-, as they have a greater attraction and tends to bind together releasing more energy(exotermic) to gain extra stability. thus Mg2+ and CO32- are more likely to precipitate. in other words, when two ions are to be insoluble in water, they precipitate in aqueous solution.
unfortunately, in CE, all insoluble salt are to be remembered in order to determine whether precipitation occurs when two kinds of solution of soluble salts are mixed.

meanwhile, some other salt may not be essentially neutral, then neutralisation is possible when an acid salt meets an basic salt. for instance when ammonium sulphate is dissolved into sodium carbonate solution
NH4+(aq) + CO32-(aq) <--> NH3(aq) + HCO3-(aq)
where NH4+ and CO32- act as acid and base respectively.