What Causes a Reaction to Be Spontaneous Provide an Example in Your Explanation
11.5: Spontaneous Reactions and Energy
- Page ID
- 155700
Learning Outcomes
- Describe the meaning of a spontaneous reaction in terms of enthalpy and entropy changes.
- Ascertain energy.
- Determine the spontaneity of a reaction based on the value of its change in free free energy at high and low temperatures.
The change in enthalpy and modify in entropy of a reaction are the driving forces behind all chemical reactions. In this lesson, we volition examine a new function chosen free energy, which combines enthalpy and entropy and can exist used to determine whether or not a given reaction will occur spontaneously.
Spontaneous Reactions
A spontaneous reaction is a reaction that favors the germination of products at the weather condition under which the reaction is occurring. A roaring blaze (run across figure below) is an example of a spontaneous reaction. A burn is exothermic, which means a decrease in the energy of the system every bit energy is released to the surroundings as oestrus. The products of a fire are equanimous mostly of gases such as carbon dioxide and water vapor, so the entropy of the system increases during most combustion reactions. This combination of a subtract in energy and an increase in entropy means that combustion reactions occur spontaneously.
A nonspontaneous reaction is a reaction that does not favor the formation of products at the given fix of atmospheric condition. In order for a reaction to be nonspontaneous, one or both of the driving forces must favor the reactants over the products. In other words, the reaction is endothermic, is accompanied by a decrease in entropy, or both. Out 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.
\[\ce{N_2} \left( grand \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{NO} \left( g \right)\]
Fortunately, this reaction is nonspontaneous at normal temperatures and pressures. It is a highly endothermic reaction with a slightly positive entropy change \(\left( \Delta South \right)\). However, nitrogen monoxide is capable of being produced at very high temperatures, and this reaction has been observed to occur 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 irksome. Yous exercise not have to worry about a piece of paper on your desk-bound 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, information technology would begin to burn, at which signal the reaction would go on 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.
\[\ce{H_2CO_3} \left( aq \right) \rightleftharpoons \ce{CO_2} \left( thousand \right) + \ce{H_2O} \left( 50 \right)\]
If you were to showtime with pure carbonic acid in water and let the system to come to equilibrium, more than \(99\%\) of the carbonic acid would be converted into carbon dioxide and water. The forrad reaction is spontaneous because the products of the forward reaction are favored at equilibrium. In the reverse reaction, carbon dioxide and h2o are the reactants, and carbonic acid is the product. When carbon dioxide is bubbled into water (see figure below), less than \(one\%\) is converted to carbonic acid when the reaction reaches equilibrium. The opposite of the above reaction is not spontaneous. This illustrates some other important point about spontaneity. But considering a reaction is not spontaneous does not mean that information technology does not occur at all. Rather, it ways that the reactants will be favored over the products at equilibrium, even though some products may indeed form.
Gibbs Gratuitous Energy
Many chemical reactions and concrete processes release energy that can be used to do other things. When the fuel in a car is burned, some of the released energy is used to ability the vehicle. Free energy is energy that is available to do work. Spontaneous reactions release free energy every bit they go on. Think that the determining factors for spontaneity of a reaction are the enthalpy and entropy changes that occur for the organization. The costless energy change of a reaction is a mathematical combination of the enthalpy change and the entropy change.
\[\Delta G^\text{o} = \Delta H^\text{o} - T \Delta S^\text{o}\]
The symbol for free free energy is \(G\), in accolade of American scientist Josiah Gibbs (1839 - 1903), who made many contributions to thermodynamics. The change in Gibbs costless energy is equal to the change in enthalpy minus the mathematical product of the change in entropy multiplied by the Kelvin temperature. Each thermodynamic quantity in the equation is for substances in their standard states, as indicated past the \(^\text{o}\) superscripts.
A spontaneous reaction is one that releases complimentary energy, and then the sign of \(\Delta One thousand\) must exist negative. Since both \(\Delta H\) and \(\Delta S\) tin can be either positive or negative, depending on the characteristics of the particular reaction, at that place are four different possible combinations. The outcomes for \(\Delta M\) based on the signs of \(\Delta H\) and \(\Delta S\) are outlined in the table below. Recall that \(- \Delta \text{H}\) indicates that the reaction is exothermic and a \(+ \Delta \text{H}\) means the reaction is endothermic. For entropy, \(+ \Delta \text{Due south}\) means the entropy is increasing and the system is becoming more disordered. A \(- \Delta \text{South}\) means that entropy is decreasing and the organisation is becoming less disordered (more ordered).
| \(\Delta H\) | \(\Delta S\) | \(\Delta G\) |
|---|---|---|
| negative | positive | always negative |
| positive | positive | negative at higher temperatures, positive at lower temperatures |
| negative | negative | negative at lower temperatures, positive at higher temperatures |
| positive | negative | ever positive |
Go on in listen that the temperature in the Gibbs free energy equation is the Kelvin temperature, so it can only have a positive value. When \(\Delta H\) is negative and \(\Delta S\) is positive, the sign of \(\Delta G\) will always exist negative, and the reaction volition be spontaneous at all temperatures. This corresponds to both driving forces being in favor of product formation. When \(\Delta H\) is positive and \(\Delta S\) is negative, the sign of \(\Delta G\) will always be positive, and the reaction tin never be spontaneous. This corresponds to both driving forces working against product germination.
When one driving forcefulness favors the reaction, only the other does non, it is the temperature that determines the sign of \(\Delta G\). Consider first an endothermic reaction (positive \(\Delta H\)) that likewise displays an increment in entropy (positive \(\Delta S\)). It is the entropy term that favors the reaction. Therefore, equally the temperature increases, the \(T \Delta Due south\) term in the Gibbs free energy equation will begin to predominate and \(\Delta G\) volition become negative. A common instance of a process which falls into this category is the melting of ice (see effigy below). At a relatively low temperature (below \(273 \: \text{G}\)), the melting is not spontaneous because the positive \(\Delta H\) term "outweighs" the \(T \Delta South\) term. When the temperature rises above \(273 \: \text{K}\), the process becomes spontaneous because the larger \(T\) value has tipped the sign of \(\Delta Yard\) over to existence negative.
When the reaction is exothermic (negative \(\Delta H\)) but undergoes a subtract in entropy (negative \(\Delta S\)), it is the enthalpy term which favors the reaction. In this case, a spontaneous reaction is dependent upon the \(T \Delta S\) term being small relative to the \(\Delta H\) term, so that \(\Delta G\) is negative. The freezing of h2o is an case of this blazon of process. It is spontaneous only at a relatively depression temperature. In a higher place \(273. \: \text{K}\), the larger \(T \Delta S\) value causes the sign of \(\Delta G\) to be positive, and freezing does not occur.
Contributors and Attributions
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Allison Soult, Ph.D. (Department of Chemistry, University of Kentucky)
Source: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book:_Chemistry_for_Allied_Health_%28Soult%29/11:_Properties_of_Reactions/11.05:_Spontaneous_Reactions_and_Free_Energy
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