Wondering Why?

Why do Endothermic reactions occur?

Why do Endothermic reactions occur?

Chemical reactions give out energy in the form of sound, light and heat. These types of reactions are called as exothermic reactions. Exothermic reactions are generally spontaneous and increase in entropy of the system. The heat flow in the system will be towards the surroundings and the change in enthalpy will be negative in this system. Other types of chemical reactions are endothermic which are not spontaneous and external work should be performed in order to make these reactions to take place.
The endothermic reactions take in energy and drop in temperature will occur in this reaction. Endothermic reactions are defined as those which encourage heat flow into the system and the change in enthalpy will be positive. An important example for endothermic reaction is photosynthesis. Plants make use of energy from the sun to transfer it from carbon-di-oxide and water to glucose and oxygen. This reaction utilizes 15 mega joules of energy for every kilogram of glucose that has resulted.
If exothermic reactions occur spontaneously and do not need excess energy, why do endothermic reactions occur by making use of energy? If exothermic reactions do not need more energy why can’t all the reactions be exothermic? Why do some endothermic reactions also take place? For example in an endothermic reaction, A+B gives rise to C+D. The products C+D have more energy than the reactants. This does not indicate always that C+D will give rise to A+B.
Any reaction will occur, if enthalpy is favorable to it and even if entropy is not favorable to it. But, those reactions that are not favorable in terms of enthalpy might continue if they are favored by entropy. This is determined properly by an equation ∆G= ∆H – T∆S. A reaction occurs successfully if the ∆G is negative (for spontaneous reaction). If the change in entropy at a particular temperature is able to carry out the reaction successfully than the unfavorable change in enthalpy, then the endothermic reactions can occur. The same reaction may not act as exothermic in reverse process as the change in entropy is not favorable to make ∆G positive, which is non-spontaneous.

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