Exothermic reaction spontaneous nature

Recall that usually, although not always, exothermic reactions occur naturally (spontaneously). The temperature can also have an impact on whether or not a particular process occurs naturally. This temperature effect is related to the concept of entropy. In fact, it is the entropy change, AS, which must be considered in helping to determine whether or not a process occurs naturally. 

Regeneration with air can be done with continuous or periodic addition of small amounts of air. Both must be done carefully because of exothermic reaction. Regeneration is never complete, so the beds must be eventually changed out. This must be done carefully because of the pyrophoric (spontaneously combustible) nature of the iron sulfide. The entire bed is wetted first. 

The combustion of natural gas (mainly CH4) in air is a spontaneous, exothermic reaction. 

However, pursuing this idea further, this situation would further mean that only exothermic reactions (for which AH° < 0 and enthalpy is lost from the system) would be expected to proceed naturally (i.e. be spontaneous). 

Scientists have observed two tendencies in nature that explain why chemical reactions occur. The first tendency is for systems to go from a state of high energy to a state of low energy. The state of low energy is more stable. Thus, for example, exothermic reactions are more likely to occur than endothermic ones, all other things being equal. The second tendency is for systems to become more disordered. Spontaneous reactions tend to occur if energy decreases and if disorder increases. 

Oxidation is an exothermic reaction and, since the rate of a chemical reaction increases for each 10°C (18°F), the reaction will generate heat at a faster rate than can be dissipated or expelled from the stockpile by natural ventilation. Hence, the temperatnre will rise to appoint where spontaneous ignition occnrs and combustion ensues. 

Since some spontaneous reactions are exothermic and others are endothermic, enthalpy alone can t account for the direction of spontaneous change a second factor must be involved. This second thermodynamic driving force is nature s tendency to move to a condition of maximum randomness or disorder (Section 8.13). 

If the mixture is too cold, the quaternization reaction will be delayed. On the other hand, if no cooling is provided, the reaction mixture may boil over spontaneously because of the exothermic nature of the reaction. Heat cracks the quaternary compound to 2-vinylthiophene which, if not removed by distillation, may undergo thermal polymerization. 

Some materials are naturally unstable and can spontaneously decompose, polymerize, or undergo other reactions. These reactions can be initiated or accelerated by promoters such as light, heat, sources of free radicals or ions, or catalysts such as metal surfaces. These reactions can sometimes be retarded by adding inhibitors or diluents. Reactions of this kind are usually exothermic and if allowed to proceed will lead to a runaway reaction with serious consequences. 

The subject of catalytic oxidation of alkanes goes back to the early 19th century when Sir Humphrey Davy and Michael Faraday found that a Pt wire would glow spontaneously in air when exposed to fuels such as H2, NH3, and natural gas. They correctly identified this as a surface catalytic reaction in which the heat of the exothermic oxidation reaction heated the catalyst to high temperatures where rates are extremely high. 

These combustion reactions are the basis for the use of hydrocarbons for heat (natural gas and heating oil) and for power (gasoline). An initiation step is required—usually ignition by a spark or flame. Once initiated, the reaction proceeds spontaneously and exothermically. 

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