Question: What makes something exothermic?

Exothermic process



In thermodynamics, the term exothermic process (exo-: "outside") describes a process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen). Its etymology stems from the Greek prefix έξω (exō, which means "outwards") and the Greek word θερμικός (thermikόs, which means "thermal").[1] The term exothermic was first coined by Marcellin Berthelot. The opposite of an exothermic process is an endothermic process, one that absorbs energy in the form of heat.
The concept is frequently applied in the physical sciences to chemical reactions, where as in chemical bond energy that will be converted to thermal energy (heat).
Exothermic (and endothermic) describe two types of chemical reactions or systems found in nature, as follows.
Simply stated, after an exothermic reaction, more energy has been released to the surroundings than was absorbed to initiate and maintain the reaction. An example would be the burning of a candle, wherein the sum of calories produced by combustion (found by looking at radiant heating of the surroundings and visible light produced, including increase in temperature of the fuel (wax) itself, which with oxygen, have become hot CO2 and water vapor,) exceeds the number of calories absorbed initially in lighting the flame and in the flame maintaining itself. (i.e. some energy produced by combustion is reabsorbed and used in melting, then vaporizing the wax, etc. but is (far) outstripped by the energy produced in breaking carbon-hydrogen bonds and combination of oxygen with the resulting carbon and hydrogen).
On the other hand, in an endothermic reaction or system, energy is taken from the surroundings in the course of the reaction. An example of an endothermic reaction is a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from the surroundings, and the reaction cools the pouch and surroundings by absorbing heat from them. An endothermic system is seen in the production of wood: trees absorb radiant energy, from the sun, use it in endothermic reactions such as taking apart CO2 and H2O and combining the carbon and hydrogen generated to produce cellulose and other organic chemicals. These products, in the form of wood, say, may later be burned in a fireplace, exothermically, producing CO2 and water, and releasing energy in the form of heat and light to their surroundings, e.g., to a home's interior and chimney gasses.
Overview
Exothermic refers to a transformation in which a system releases energy (heat) to the surroundings, expressed by
Q < 0.
When the transformation occurs at constant pressure, one has for the enthalpy
∆H < 0,
and constant volume, one has for the internal energy
∆U < 0.
In an adiabatic system (i.e. a system that does not exchange heat with the surroundings), an exothermic process results in an increase in temperature of the system.[2]
In exothermic chemical reactions, the heat that is released by the reaction takes the form of electromagnetic energy. The transition of electrons from one quantum energy level to another causes light to be released. This light is equivalent in energy to the stabilization energy of the energy for the chemical reaction, i.e. the bond energy. This light that is released can be absorbed by other molecules in solution to give rise to molecular vibrations or rotytions, which gives rise to the classical understanding of heat. In contrast, when endothermic reactions occur, energy is absorbed to place an electron in a higher energy state, such that the electron can associate with another atom to form a chemical complex. Net energy is absorbed by an endothermic reaction. In an exothermic reaction, the energy needed to start the reaction is less than el energy that is subsequently released, so there is a net release of energy. This is the physical understanding of exothermic and endothermic reactions.
Examples

Some examples of exothermic processes are:[3]
Combustion of fuels such as wood, coal and oil petroleum
Thermite reaction
Reaction of alkali metals and other highly electropositive metals with water
Condensation of rain from water vapor
Mixing water and strong acids or strong bases
Mixing acids and bases
Dehydration of carbohydrates by sulfuric acid
The setting of cement and concrete
Some polymerisation reactions such as the setting of epoxy resin
Reaction of most metals with halogens or oxygen
Nuclear fusion in hydrogen bombs and in stellar cores (to iron)
Nuclear fission of heavy elements
Implications for chemical reactions

Chemical exothermic reactions are generally more spontaneous than their counterparts, endothermic reactions. 
In a thermochemical reaction that is exothermic, the heat may be listed among the products of the reaction.
Contrast between thermodynamic and biological terminology
Because of historical accident, students encounter a source of possible confusion between the terminology of physics and biology. Whereas the thermodynamic terms "exothermic" and "endothermic" respectively refer to processes that give out heat energy and processes that absorb heat energy, in biology the sense is effectively inverted. The metabolic terms "ectothermic" and "endothermic" respectively refer to organisms that rely largely on external heat to achieve a full working temperature, and to organisms that produce heat from within as a major factor in controlling their bodily temperature.
See also

Calorimetry
Chemical thermodynamics
Differential scanning calorimetry
Endergonic
Endergonic reaction
Exergonic
Exergonic reaction
Endothermic reaction





 Observe exothermic reactions in a simple experiment