Name: spontaneous change in matter and energy
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 2 min 21 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Chemistry

CH.7

CH.7.d

Spontaneity

A spontaneous reaction is one that occurs without any external intervention under a particular set of conditions. A nonspontaneous process is one that requires external forces to proceed.
Spontaneity is affected by conditions such as temperature and pressure.
For example, the melting of ice at a temperature of 0 &#176;C or below is not spontaneous; however, the melting of ice at room temperature is spontaneous because it does not need any external intervention.
A spontaneous reaction is usually unidirectional, and its reversal requires an external force to make it proceed as a nonspontaneous reaction. For example, ice that melts at room temperature cannot spontaneously freeze back to its original form.
Spontaneous processes occur due to the dispersal of energy and matter. If a flask containing a gas is connected to another empty flask through a valve, opening the valve will lead to the spontaneous transfer of the gas to the empty flask, which will continue until the gas is equally distributed in both flasks.
Similarly, if a hot object is placed in cold water, the heat from the object will be spontaneously transferred to the water until the temperatures of the object and the water reach equilibrium.
Spontaneity in thermodynamics does not indicate the speed of a process. The reaction could be extremely fast, like an acid-base neutralization, or extremely slow, like the conversion of diamonds to graphite.&#160;
Consider the decomposition of hydrogen peroxide into water and oxygen gas. The reaction, although spontaneous, proceeds very slowly at room temperature. Adding a catalyst, such as manganese oxide, speeds up the reaction and therefore increases the decomposition rate of hydrogen peroxide.
Although the catalyst enhances the rate of the spontaneous reaction, it cannot make the nonspontaneous reverse reaction be spontaneous.

spontaneity

A spontaneous process is one that occurs naturally under certain conditions. A nonspontaneous process, on the other hand, will not take place unless it is &#8220;driven&#8221; by the continual input of energy from an external source. Processes have a natural tendency to occur in one direction under a given set of conditions. Water will naturally flow downhill (spontaneous process), but uphill flow (nonspontaneous process) requires outside intervention such as the use of a pump. Iron exposed to the earth&#8217;s atmosphere will corrode (spontaneous process), but rust is not converted to iron (nonspontaneous process) without intentional chemical treatment. A process that is spontaneous in one direction under a particular set of conditions is nonspontaneous in the reverse direction. At room temperature and typical atmospheric pressure, for example, ice will spontaneously melt, but water will not spontaneously freeze.
Spontaneity is Independent of Rate of Reaction
The spontaneity of a process is not correlated to the speed of the process. While a catalyst may be used to speed up or slow down a process, its presence does not influence spontaneity: nonspontaneous reactions cannot be made spontaneous using a catalyst. A spontaneous change may be so rapid that it is essentially instantaneous or so slow that it cannot be observed over any practical period of time. To illustrate this concept, consider the decay of radioactive isotopes. Radioactive decay is, by definition, a spontaneous process in which the nuclei of unstable isotopes emit radiation as they are converted to more stable nuclei. All the decay processes occur spontaneously, but the rates at which different isotopes decay vary widely. Technetium-99m is a popular radioisotope for medical imaging studies that undergoes relatively rapid decay and exhibits a half-life of about six hours. Uranium-238 is the most abundant isotope of uranium, and its decay occurs much more slowly, exhibiting a half-life of more than four billion years.
Dispersal of Matter and Energy
Consider an isolated system consisting of two flasks connected with a closed valve. Initially, there is an ideal gas in one flask, and the other flask is empty. When the valve is opened, the gas spontaneously expands to fill both flasks equally. Since the system is isolated, no heat has been exchanged with the surroundings. The spontaneity of this process is, therefore, not a consequence of any change in energy that accompanies the process. Instead, the driving force appears to be related to the greater, more uniform dispersal of matter that results when the gas is allowed to expand.
Now consider two objects at different temperatures: object X at temperature TX and object Y at temperature TY, with TX &#62; TY. When these objects come into contact, heat spontaneously flows from the hotter object (X) to the colder one (Y). This corresponds to a loss of thermal energy by X and a gain of thermal energy by Y. From the perspective of this two-object system, there was no net gain or loss of thermal energy; rather the available thermal energy was redistributed among the two objects. This spontaneous process resulted in a more uniform dispersal of energy.
As demonstrated by the two processes, an important factor in determining the spontaneity of a process is the extent to which it changes the dispersal or distribution of matter and/or energy. In each case, a spontaneous process took place that resulted in a more uniform distribution of matter or energy.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/16-1-spontaneity">Openstax, Chemistry 2e, Chapter 16.1: Spontaneity</a>.

A spontaneous process is one that occurs naturally under certain conditions. A nonspontaneous process, on the other hand, will not take place unless it is ...

Education

JoVE Core

Thermodynamics

KEY TERMS AND CONCEPTS

spontaneous change in matter and energy

Spontaneous change in matter and energy happens naturally under certain conditions. A nonspontaneous process will not occur unless it is driven by a continual input of energy from an outside source. Under a given set of conditions, processes also tend to move in one direction rather than the other.

Water flows downhill on its own, so that is a spontaneous process. Water flowing uphill needs a pump, so that is nonspontaneous. Iron in the earth’s atmosphere will corrode, but rust does not turn back into iron unless it is given intentional chemical treatment. A process that is spontaneous in one direction is nonspontaneous in reverse. At room temperature and typical atmospheric pressure, ice melts spontaneously, but water does not spontaneously freeze.

Spontaneity is not the same as speed. A catalyst can speed up or slow down a process, but it does not change whether the process is spontaneous. Some spontaneous changes happen almost instantly. Others are so slow that they are hard to observe over any practical time.

Radioactive decay shows this idea clearly. Radioactive decay is a spontaneous process because unstable nuclei emit radiation as they change into more stable nuclei. The decay of different isotopes can happen at very different rates. Technetium-99m decays relatively quickly and has a half-life of about six hours. Uranium-238 decays much more slowly and has a half-life of more than four billion years.

The spread of matter and energy also helps explain spontaneity. In an isolated system with two flasks, one holding an ideal gas and the other empty, the gas spontaneously expands when the valve opens and fills both flasks equally. No heat is exchanged with the surroundings, so the process is not driven by a change in energy. Instead, it leads to a more uniform dispersal of matter.

A similar pattern appears with heat flow. If object X is hotter than object Y, heat flows spontaneously from X to Y when they touch. X loses thermal energy, and Y gains it. For the two-object system, there is no net gain or loss of thermal energy. The energy is simply redistributed, and the result is a more uniform dispersal of energy. In both examples, a spontaneous process makes matter or energy more evenly spread out.

Spontaneous Change in Matter and Energy

Spontaneous change in matter and energy happens naturally under certain conditions. A nonspontaneous process will not occur unless it is driven by a continual input of energy from an outside source. Under a given set of conditions, processes also tend to move in one direction rather than the other. Water flows downhill on its own, so that is a spontaneous process. Water flowing uphill needs a pump, so that is nonspontaneous. Iron in the earth’s atmosphere will corrode, but rust does not turn...

entropy

Entropy in Matter and Energy Changes

Entropy describes how energy and matter are spread through a system. It is a thermodynamic state function, so it can change when a physical or chemical change occurs. When entropy increases, disorder increases. When entropy decreases, disorder decreases. A change in entropy is written as ΔS = Sf - Si. The concept is also linked to microstates, which are the specific arrangements of the locations and energies of all atoms or molecules in a system. Boltzmann’s relation, S = k ln W, connects...

second law of thermodynamics

Entropy and Spontaneous Change

Entropy and spontaneous change are linked through the entropy of the universe. Entropy is a thermodynamic property that helps predict whether a process will happen on its own. Processes with an increase in system entropy, or ΔS &gt; 0, are often spontaneous, but there are many exceptions. To make a better prediction, entropy changes must include both the system and the surroundings. In thermodynamics, the system plus the surroundings make up the universe. When heat moves between two objects,...

third law of thermodynamics

Entropy at Absolute Zero

The third law of thermodynamics links entropy to absolute zero. A pure, perfectly crystalline solid with no kinetic energy at 0 K has only one microstate. Because there is only one possible arrangement for its identical atoms or molecules, its entropy is zero. This limiting case sets the entropy scale for real substances. Careful calorimetric measurements can be used to find how entropy changes with temperature and to calculate absolute entropy values under specific conditions. Standard...

standard entropy change for a reaction

Calculating Entropy Change from Tables

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Predicting Spontaneity with Gibbs Energy

Gibbs free energy helps predict whether a process is spontaneous under constant temperature and pressure. It is a thermodynamic property based on the system’s enthalpy and entropy. Chemists also call it free energy, and it is a state function. The idea was introduced in the late nineteenth century by American mathematician Josiah Willard Gibbs. Unlike the second law approach, it uses system properties only. That makes it a useful shortcut when tracking both the system and the surroundings is...

effects of temperature on free energy

Temperature and Spontaneous Processes

Temperature can change whether a process is spontaneous. In chemistry, spontaneity means a process can happen on its own under a given set of conditions. Phase changes are one clear example, and some chemical reactions also change spontaneity as temperature changes. The relationship uses free energy change, enthalpy change, and entropy change. Entropy is a measure of disorder in the system. The key equation shows that the sign of free energy depends on the signs of enthalpy and entropy, and in...

calculating standard free energy changes

Using Formation Data to Find Free Energy

Standard free energy change is the free energy change for a reaction under standard conditions of 1 bar pressure and 298 K. Free energy is a state function, so its value depends only on the starting and ending states of the system. That makes it possible to calculate standard free energy changes from tabulated thermodynamic data. One common method uses standard enthalpy and entropy values to find standard free energy change, ΔG°. Another method uses standard free energies of formation, ΔGf°,...

free energy changes for nonstandard states

free-energy-changes-for-nonstandard-states

Predicting Spontaneity with Reaction Quotient Q

Free energy under nonstandard conditions depends on the reaction quotient, Q, and the standard free energy change. Nonstandard conditions mean that concentrations are not 1 M or pressures are not 1 bar. The relation links the gas constant R, the absolute temperature T in kelvin, and Q. It can be used to predict whether a process is spontaneous under a given set of conditions. The reaction quotient, Q, helps show the status of a reversible reaction. It is based directly on the balanced chemical...

free energy and equilibrium

Predicting Reaction Direction with Free Energy

Free energy and equilibrium help predict which way a chemical reaction will move. A negative value of ΔG means the forward reaction is favored. A positive value means the reverse reaction is favored. When ΔGrxn is zero, the forward and reverse driving forces are equal, and the system is at equilibrium. The reaction quotient, Q, helps show whether a reaction must move forward or backward to reach equilibrium. Q is the numerical value of the mass action expression for the system. If Q is less...

key terms and concepts

Explore spontaneous and nonspontaneous processes, from rusting and melting ice to radioactive decay and heat flow, and see how matter and energy spread out.

Spontaneous Change in Matter and Energy | JoVE K12

chemistry

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CH.7.g

Texas Essential Knowledge and Skills

High School

CHEM.13

CHEM.13.A

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CH.7.a

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CH.7.c

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CH.7.f

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Spontaneous Change in Matter and Energy

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