Examples of spontaneous change in the following topics:
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- ΔG determines the direction and extent of chemical change.
- In a spontaneous change, Gibbs energy always decreases and never increases.
- $\Delta G > 0$: The reaction will occur spontaneously to the left.
- where ΔG = change in Gibbs free energy, ΔH = change in enthalpy, T = absolute temperature, and ΔS = change in entropy
- In particular, notice that in the above equation the sign of the entropy change determines whether the reaction becomes more or less spontaneous as the temperature is raised.
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- There are two types of processes (or reactions): spontaneous and non-spontaneous.
- Spontaneous changes, also called natural processes, proceed when left to themselves, and in the absence of any attempt to drive them in reverse.
- This means a release of free energy from the system corresponds to a negative change in free energy, but to a positive change for the surroundings.
- The second law of thermodynamics states that for any spontaneous process, the overall ΔS must be greater than or equal to zero; yet, spontaneous chemical reactions can result in a negative change in entropy.
- Spontaneity does not imply that the reaction proceeds with great speed.
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- The concept of entropy evolved in order to explain why some processes (permitted by conservation laws) occur spontaneously while their time reversals (also permitted by conservation laws) do not; systems tend to progress in the direction of increasing entropy.
- Increases in entropy correspond to irreversible changes in a system.
- Thus, when the "universe" of the room and ice water system has reached a temperature equilibrium, the entropy change from the initial state is at a maximum.
- Physical chemist Peter Atkins, for example, who previously wrote of dispersal leading to a disordered state, now writes that "spontaneous changes are always accompanied by a dispersal of energy".
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- In a galvanic cell, where a spontaneous redox reaction drives the cell to produce an electric potential, the change in Gibbs free energy must be negative.
- In a galvanic cell, where a spontaneous redox reaction drives the cell to produce an electric potential, the change in Gibbs free energy must be negative.
- If E°cell > 0, then the process is spontaneous (galvanic cell)
- Because the change in Gibbs free energy is negative, the redox process is spontaneous.
- Calculate the change in Gibbs free energy of an electrochemical cell, and discuss its implications for whether a redox reaction will be spontaneous
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- In the late 19th century, Josiah Willard Gibbs formulated a theory to predict whether a chemical reaction would be spontaneous based on free energy:
- Here, ΔG is the change in Gibbs free energy, T is absolute temperature, R is the gas constant, and Q is the reaction quotient.
- Gibbs' key contribution was to formalize the understanding of the effect of reactant concentration on spontaneity.
- The change in Gibbs free energy for an electrochemical cell can be related to the cell potential.
- When a change in the concentration or activity of reactants occurs, or the temperature or pressure changes, the output voltage changes.
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- Turning the reaction around doesn't change the relative strengths of the oxidizing or reducing agents.
- However, turning the equation around changes the sign of the standard electrode potential, and can therefore turn an unfavorable reaction into one that is spontaneous, or vice versa.
- In order to predict if two reactants will take part in a spontaneous redox reaction, it is important to know how they rank in an electrochemical series.
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- All chemical processes are accompanied by energy changes.
- Due to this relation, the change in enthalpy, $\Delta H$, is often referred to as the "heat of reaction."
- They are also generally non-spontaneous, since endothermic reactions yield products that are higher in energy than the reactants.
- As such, the change in enthalpy for an endothermic reaction is always positive.
- Therefore, the change in enthalpy is negative, and heat is released to the surroundings.
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- Temporary dipoles are created when electrons, which are in constant movement around the nucleus, spontaneously come into close proximity.
- Although charges are usually distributed evenly between atoms in non-polar molecules, spontaneous dipoles can still occur.
- How does changing the Van der Waals attraction or charging the atoms affect the melting and boiling point of the substance?
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- When a system changes from an initial state to a final state, the Gibbs free energy (ΔG) equals the work exchanged by the system with its surroundings, minus the work of the pressure force.
- As such, it is a convenient criterion of spontaneity for processes with constant pressure and temperature.
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- The text box commentary will change to suit the examples.
- For example, a chloroform solution of the diaxial 2-bromo-3-chlorosteroid, shown on the left below, spontaneously rearranges to the more stable diequatorial 2-chloro-3-bromo isomer drawn on the right.