The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. The total entropy of a system and its surroundings can remain constant in ideal cases where the system is in thermodynamic equilibrium , or is undergoing a fictive reversible process. In all processes that occur, including spontaneous processes ,  the total entropy of the system and its surroundings increases and the process is irreversible in the thermodynamic sense. The increase in entropy accounts for the irreversibility of natural processes, and the asymmetry between future and past. Historically, the second law was an empirical finding that was accepted as an axiom of thermodynamic theory.
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The Second Law of Thermodynamics states that the state of entropy of the entire universe, as an isolated system , will always increase over time. The second law also states that the changes in the entropy in the universe can never be negative. Why is it that when you leave an ice cube at room temperature, it begins to melt? Why do we get older and never younger? And, why is it whenever rooms are cleaned, they become messy again in the future?
Certain things happen in one direction and not the other, this is called the "arrow of time" and it encompasses every area of science. The thermodynamic arrow of time entropy is the measurement of disorder within a system. Assume a box filled with jigsaw pieces were jumbled in its box, the probability that a jigsaw piece will land randomly, away from where it fits perfectly, is very high.
Almost every jigsaw piece will land somewhere away from its ideal position. The probability of a jigsaw piece landing correctly in its position, is very low, as it can only happened one way. Thus, the misplaced jigsaw pieces have a much higher multiplicity than the correctly placed jigsaw piece, and we can correctly assume the misplaced jigsaw pieces represent a higher entropy.
To understand why entropy increases and decreases, it is important to recognize that two changes in entropy have to considered at all times.
The entropy change of the surroundings and the entropy change of the system itself. Given the entropy change of the universe is equivalent to the sums of the changes in entropy of the system and surroundings:.
In an isothermal reversible expansion, the heat q absorbed by the system from the surroundings is. Therefore, for a truly reversible process, the entropy change is. In reality, however, truly reversible processes never happen or will take an infinitely long time to happen , so it is safe to say all thermodynamic processes we encounter everyday are irreversible in the direction they occur.
The second law of thermodynamics can also be stated that "all spontaneous processes produce an increase in the entropy of the universe". If this equation is replaced in the previous formula, and the equation is then multiplied by T and by -1 it results in the following formula. Now it is much simpler to conclude whether a system is spontaneous, non-spontaneous, or at equilibrium. Temperature comes into play when the entropy and enthalpy both increase or both decrease.
The reaction is not spontaneous when both entropy and enthalpy are positive and at low temperatures, and the reaction is spontaneous when both entropy and enthalpy are positive and at high temperatures.
The reactions are spontaneous when the entropy and enthalpy are negative at low temperatures, and the reaction is not spontaneous when the entropy and enthalpy are negative at high temperatures.
The first step is to convert the temperature to Kelvin, so add One must work backwards somewhat using the same equation from Example 1 for the free energy is given. By simply viewing the reaction one can determine that the reaction increases in the number of moles, so the entropy increases. Now all one has to do is to figure out the enthalpy of the reaction.
The enthalpy is positive, because covalent bonds are broken. When covalent bonds are broken energy is absorbed, which means that the enthalpy of the reaction is positive. Another way to determine if enthalpy is positive is to to use the formation data and subtract the enthalpy of the reactants from the enthalpy of the products to calculate the total enthalpy.
The enthalpy of the reaction is kJ. One may have to calculate the enthalpy of the reaction, but in this case it is given. If the enthalpy is negative then the reaction is exothermic. Now one must find if the entropy is greater than zero to answer the question. Using the entropy of formation data and the enthalpy of formation data, one can determine that the entropy of the reaction is Because both enthalpy and entropy are negative, the spontaneous nature varies with the temperature of the reaction.
The temperature would also determine the spontaneous nature of a reaction if both enthalpy and entropy were positive. When the reaction occurs at a low temperature the free energy change is also negative, which means the reaction is spontaneous. However, if the reaction occurs at high temperature the reaction becomes nonspontaneous, for the free energy change becomes positive when the high temperature is multiplied with a negative entropy as the enthalpy is not as large as the product.
Only after calculating the enthalpy and entropy of the reaction is it possible for one can answer the question. The enthalpy of the reaction is calculated to be Unlike the previous two examples, the temperature has no affect on the spontaneous nature of the reaction. Looking at the formula for spontaneous change one can easily come to the same conclusion, for there is no possible way for the free energy change to be positive. Hence, the reaction is spontaneous at all temperatures.
The second law occurs all around us all of the time, existing as the biggest, most powerful, general idea in all of science. When scientists were trying to determine the age of the Earth during s they failed to even come close to the value accepted today. They also were incapable of understanding how the earth transformed. Lord Kelvin, who was mentioned earlier, first hypothesized that the earth's surface was extremely hot, similar to the surface of the sun.
He believed that the earth was cooling at a slow pace. Using this information, Kelvin used thermodynamics to come to the conclusion that the earth was at least twenty million years, for it would take about that long for the earth to cool to its current state.
Twenty million years was not even close to the actual age of the Earth, but this is because scientists during Kelvin's time were not aware of radioactivity. Even though Kelvin was incorrect about the age of the planet, his use of the second law allowed him to predict a more accurate value than the other scientists at the time.
Some critics claim that evolution violates the Second Law of Thermodynamics, because organization and complexity increases in evolution. However, this law is referring to isolated systems only, and the earth is not an isolated system or closed system. This is evident for constant energy increases on earth due to the heat coming from the sun. So, order may be becoming more organized, the universe as a whole becomes more disorganized for the sun releases energy and becomes disordered.
This connects to how the second law and cosmology are related, which is explained well in the video below. Introduction Why is it that when you leave an ice cube at room temperature, it begins to melt? The current form of the second law uses entropy rather than caloric, which is what Sadi Carnot used to describe the law.
Caloric relates to heat and Sadi Carnot came to realize that some caloric is always lost in the motion cycle. Thus, the thermodynamic reversibility concept was proven wrong, proving that irreversibility is the result of every system involving work. Rudolf Clausius was a German physicist, and he developed the Clausius statement, which says "Heat generally cannot flow spontaneously from a material at a lower temperature to a material at a higher temperature.
Derivation and Explanation To understand why entropy increases and decreases, it is important to recognize that two changes in entropy have to considered at all times. Application of the Second Law The second law occurs all around us all of the time, existing as the biggest, most powerful, general idea in all of science.
Explanation of Earth's Age When scientists were trying to determine the age of the Earth during s they failed to even come close to the value accepted today. Evolution and the Second Law Some critics claim that evolution violates the Second Law of Thermodynamics, because organization and complexity increases in evolution.
References Chang, Raymond. Physical Chemistry for the Biosciences. Sausalito, California: University Science Books, How the Earth Was Made. Peter Chin. Petrucci, Ralph H. Harwood, F. Herring, and Jeffry D. Conditional content Pro member.
Second law of thermodynamics
Thermodynamics is a branch of physics which deals with the energy and work of a system. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. In aerodynamics, the thermodynamics of a gas obviously plays an important role in the analysis of propulsion systems but also in the understanding of high speed flows. The first law of thermodynamics defines the relationship between the various forms of energy present in a system kinetic and potential , the work which the system performs and the transfer of heat. The first law states that energy is conserved in all thermodynamic processes. We can imagine thermodynamic processes which conserve energy but which never occur in nature.
What is the Second Law of Thermodynamics?
In order to avoid confusion, scientists discuss thermodynamic values in reference to a system and its surroundings. Everything that is not a part of the system constitutes its surroundings. The system and surroundings are separated by a boundary. For example, if the system is one mole of a gas in a container, then the boundary is simply the inner wall of the container itself.