An exothermic reaction whose entropy increases will be spontaneous at all temperatures. The freezing of a liquid or the condensation of a gas are the most common examples of this condition.
Think of melting and boiling. Substance A always has a greater number of accessible energy states, and is therefore always the preferred form.
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:.
Skip to main content. 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.
There is a tendency in nature for systems to proceed toward a state of greater disorder or randomness. Entropy is a measure of the degree of randomness or disorder of a system. Entropy is an easy concept to understand when thinking about everyday situations. An easy way to remember the difference between these two reaction types is by their prefixes: endo- means to draw in, and exo- means to give off. We will explore these concepts in more detail after introducing the concept of enthalpy.
Enthalpy signified as H is a measure of the total energy of a system and often expresses and simplifies energy transfer between systems. Since the total enthalpy of a system cannot be measured directly, we most often refer to the change in enthalpy for a particular chemical reaction. At constant pressure, the change in enthalpy is equal to the heat given off, or the heat absorbed, in a given chemical reaction:.
Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.
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.
You do not have to worry about a piece of paper on your desk suddenly bursting into flames, although its combustion is a spontaneous reaction. What is missing is the required activation energy to get the reaction started.
If the paper were to be heated to a high enough temperature, it would begin to burn, at which point the reaction would proceed spontaneously until completion. In a reversible reaction, one reaction direction may be favored over the other. Carbonic acid is present in carbonated beverages.
It decomposes spontaneously to carbon dioxide and water according to the following reaction. The forward reaction is spontaneous because the products of the forward reaction are favored at equilibrium.
In the reverse reaction, carbon dioxide and water are the reactants, and carbonic acid is the product. The reverse of the above reaction is not spontaneous. This illustrates another important point about spontaneity. Just because a reaction is not spontaneous does not mean that it does not occur at all.
0コメント