The combination of energy decrease and entropy increase dictates that combustion reactions are spontaneous reactions. A nonspontaneous reaction is a reaction that does not favor the formation of products at the given set of conditions.
In order for a reaction to be nonspontaneous, it must be endothermic, accompanied by a decrease in entropy, or both. Our atmosphere is composed primarily of a mixture of nitrogen and oxygen gases. One could write an equation showing these gases undergoing a chemical reaction to form nitrogen monoxide:. Fortunately, this reaction is nonspontaneous at normal temperatures and pressures.
Nitrogen monoxide is capable of being produced at very high temperatures and has been observed to form as a result of lightning strikes. One must be careful not to confuse the term spontaneous with the notion that a reaction occurs rapidly. A spontaneous reaction is one in which product formation is favored, even if the reaction is extremely slow.
A piece of paper will not suddenly burst into flames, although its combustion is a spontaneous reaction. In this lesson, you will learn about a second driving force for chemical reactions called entropy.
The vast majority of naturally occurring reactions are exothermic. In an exothermic reaction, the reactants have a relatively high quantity of energy compared to the products.
As the reaction proceeds, energy is released into the surroundings. Low energy can be thought of as providing a greater degree of stability to a chemical system. Since the energy of the system decreases during an exothermic reaction, the products of the system are more stable than the reactants. We can say that an exothermic reaction is an energetically favorable reaction.
If the drive toward lower energy were the only consideration for whether a reaction is able to occur, we would expect that endothermic reactions could never occur spontaneously. In an endothermic reaction, energy is absorbed during the reaction, and the products thus have a larger quantity of energy than the reactants.
This means that the products are less stable than the reactants. Therefore, the reaction would not occur without some outside influence such as persistent heating. However, endothermic reactions do occur spontaneously, or naturally. There must be another driving force besides enthalpy change which helps promote spontaneous chemical reaction.
A very simple endothermic process is that of a melting ice cube. Energy is transferred from the room to the ice cube, causing it to change from the solid to the liquid state. The solid state of water, ice, is highly ordered because its molecules are fixed in place. The melting process frees the water molecules from their hydrogen-bonded network and allows them a greater degree of movement.
Water is more disordered than ice. The change from the solid to the liquid state of any substance corresponds to an increase in the disorder of the system. Chemistry Thermochemistry Energy Change in Reactions. Chuck W. Apr 14, Related questions Why do endothermic reactions happen? What causes energy changes in chemical reactions? How do endothermic reactions absorb heat?
How do I relate equilibrium constants to temperature change to find the enthalpy of reaction?
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