Name: temperature and spontaneous processes
Uploaded: 2020-09-24T15:06:52.000Z
Duration: 2 min 11 s
Channel: JoVE

K12 Standards

new-york-state-science-learning-standards

New York State Science Learning Standards

High School

Physical Science

HS. Chemical Reactions

HS-PS1-12.

Effects of Temperature on Free Energy

For a reaction to be spontaneous at constant temperature and pressure, the change in Gibbs free energy, &#916;G, must be less than zero. 
The sign of &#916;G depends on the signs and the relative values of enthalpy, entropy, and temperature.
Enthalpy favors spontaneity when the reaction releases heat to the surroundings, while entropy favors spontaneity when there is more disorder in the system. 
If &#916;H is negative and &#916;S is positive, as in the reaction between sodium hydroxide and hydrochloric acid, &#916;G is negative at all temperatures. Thus, exothermic reactions&#8212;where the entropy of the system increases&#8212;are always spontaneous. 
If both &#916;H and &#916;S are negative, &#916;G depends on the temperature. Consider the freezing of water into ice, an exothermic reaction where the entropy of the system decreases. 
At temperatures below the freezing point of water, the water will freeze spontaneously, releasing heat and becoming more ordered. Thus, reactions with negative enthalpy and entropy changes are spontaneous only at low temperatures. 
&#916;G is also dependent on temperature if both &#916;H and &#916;S are positive. 
A common example is a chemical cold pack, where solid ammonium nitrate dissolves in water, that absorbs heat from the surroundings. This endothermic reaction proceeds spontaneously at room temperature due to the increase in disorder of the system. Thus, reactions with positive enthalpy and entropy changes are spontaneous only at higher temperatures.
If the temperature were lowered such that the T&#916;S becomes smaller than &#916;H, &#916;G would be positive, and the reaction would become nonspontaneous.
When &#916;H is positive and &#916;S is negative, &#916;G is always positive, and the reaction is nonspontaneous at all temperatures.

effects of temperature on free energy

The spontaneity of a process depends upon the temperature of the system. Phase transitions, for example, will proceed spontaneously in one direction or the other depending upon the temperature of the substance in question. Likewise, some chemical reactions can also exhibit temperature-dependent spontaneities. To illustrate this concept, the equation relating free energy change to the enthalpy and entropy changes for the process is considered:
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The spontaneity of a process, as reflected in the arithmetic sign of its free energy change, is then determined by the signs of the enthalpy and entropy changes and, in some cases, the absolute temperature. Since T is the absolute (kelvin) temperature, it can only have positive values. Four possibilities, therefore, exist with regard to the signs of the enthalpy and entropy changes:
<ol>
	<li>Both &#916;H and &#916;S are positive. This condition describes an endothermic process that involves an increase in system entropy. In this case, &#916;G will be negative if the magnitude of the T&#916;S term is greater than &#916;H. If the T&#916;S term is less than &#916;H, the free energy change will be positive. Such a process is spontaneous at high temperatures and nonspontaneous at low temperatures.</li>
	<li>Both &#916;H and &#916;S are negative. This condition describes an exothermic process that involves a decrease in system entropy. In this case, &#916;G will be negative if the magnitude of the T&#916;S term is less than &#916;H. If the T&#916;S term&#8217;s magnitude is greater than &#916;H, the free energy change will be positive. Such a process is spontaneous at low temperatures and nonspontaneous at high temperatures.</li>
	<li>&#916;H is positive, and &#916;S is negative. This condition describes an endothermic process that involves a decrease in system entropy. In this case, &#916;G will be positive regardless of the temperature. Such a process is nonspontaneous at all temperatures.</li>
	<li>&#916;H is negative, and &#916;S is positive. This condition describes an exothermic process that involves an increase in system entropy. In this case, &#916;G will be negative regardless of the temperature. Such a process is spontaneous at all temperatures.</li>
</ol>
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Openstax, Chemistry 2e, Section 16.4: Free Energy.</a>

The spontaneity of a process depends upon the temperature of the system. Phase transitions, for example, will proceed spontaneously in one direction or ...

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KEY TERMS AND CONCEPTS

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 some cases on absolute temperature, T. Because T is measured in kelvin, it is always positive.

Four sign combinations can occur. If both ΔH and ΔS are positive, the process is endothermic and the system entropy increases. In this case, ΔG is negative when TΔS is larger than ΔH, so the process is spontaneous at high temperature and nonspontaneous at low temperature.

If both ΔH and ΔS are negative, the process is exothermic and the system entropy decreases. Here, ΔG is negative when the size of TΔS is less than ΔH. That means the process is spontaneous at low temperature and nonspontaneous at high temperature.

If ΔH is positive and ΔS is negative, the process is endothermic and the system entropy decreases. In this case, ΔG stays positive no matter the temperature, so the process is nonspontaneous at all temperatures. If ΔH is negative and ΔS is positive, the process is exothermic and the system entropy increases. Then ΔG is negative at all temperatures, so the process is spontaneous at all temperatures.

Temperature and Spontaneous Processes

spontaneity

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

Entropy is a state function, so the standard entropy change of a chemical reaction can be found from standard molar entropy values in a table. The reaction entropy, written as ΔS°rxn, is calculated by subtracting the total entropy of the reactants from the total entropy of the products. The balanced equation matters because the stoichiometric coefficients set how many moles of each substance are included. A table of standard entropies gives values in J/mol·K for substances such as graphite,...

gibbs free energy

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...

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

Learn how temperature affects spontaneity using free energy, enthalpy, and entropy. See when a process is spontaneous at high, low, or all temperatures.

Temperature and Spontaneous Processes | JoVE K12

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Temperature and Spontaneous Processes

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