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Tutor profile: Rahul R.

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Rahul R.
Recent Cornell Engineering Graduate
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Questions

Subject: Physics (Thermodynamics)

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Question:

Why is it impossible to reach a temperature of absolute zero?

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Rahul R.
Answer:

There are a few reasons and ways to look at this question, from both classical thermodynamics and a physical chemistry perspective. In classical thermodynamics, the third law states that the entropy approaches a constant value as temperature approaches absolute zero. $$\lim_{T\to 0} \Delta S = 0$$ Thus, the work required to reach absolute zero is infinite, since it approaches that temperature asymptotically. In quantum mechanics, the Heisenberg uncertainty principle states that it is impossible to know both the position and velocity of a particle at a given moment in time. Since temperature is a measure of molecular motion, at absolute zero there would be no motion. Thus, the velocity of the particle would be known. However, since the particle isn't moving, the position would also be known. This paradox prevents us from ever reaching a temperature of absolute zero. In physical chemistry and quantum mechanics, a quantity called zero-point energy also arises. This energy is the bare minimum energy that molecule can have. Thus, the minimum achievable temperature is that at which the zero point energy of the molecule is reached - any lower would also create a paradox and thus absolute zero is impossible to reach.

Subject: Chemistry

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Question:

My textbook says that iron turning to rust is extremely exothermic. Why do iron and rust never feel hot when I touch them?

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Rahul R.
Answer:

You're right! Iron doesn't feel hot when it is turning to rust, even though it is an extremely exothermic reaction. This reaction illustrates the difference between kinetics and thermodynamics. Imagine rolling a ball down a hill. Thermodynamics tells you that the ball will end up at the bottom of the hill. Kinetics, by contrast, can tell you how fast the ball will roll down the hill. In the case of iron turning to rust, iron is the ball at the top of the hill and rust is the ball at the bottom of the hill. Thermodynamics tells us that in the time that the ball spends between the top and bottom of the hill, it will give off a tremendous amount of heat. However, the kinetics of the reaction tell us that the conversion will take place extremely slowly - the ball takes an long time to reach the bottom of the hill. Thus, the reaction does give off tremendous heat - but it does so over such a long period of time that the metal (or rust) never feel hot to the touch.

Subject: Chemical Engineering

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Question:

Why can't binary mixtures with azeotropes be further separated by distillation?

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Rahul R.
Answer:

An azeotrope in a binary mixture occurs when, at equilibrium, both the liquid and vapor forms of the mixture have identical compositions. For example, in water-ethanol mixtures, when the liquid is 95% ethanol, a vapor in equilibrium would also contain 95% ethanol. The principle of distillation involves placing more volatile compounds in the vapor phase while keeping less volatile components in the liquid phase. When both phases have identical composition, there is no driving force for the separation. On a molecular level, imagine the water and ethanol molecules as magnets, with north and south poles. Ideally, these would not interact and distillation would be simple. But, as you can imagine, some of the magnets attract and some repel. These forces make imbalances in the mixture; once the azeotrope is reached, the magnets in the liquid and gas phases attract so strongly (it is also possible they repel strongly), that they no longer want to escape to the other phase.

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