intermolecular forces

# Eg consider n pentane in a cylinder with a movable

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E.g. consider n-pentane in a cylinder with a movable piston at 25˙C and 510 mmHg. No matter how the piston moves the n-pentane will undergo more vaporization if there is an increase in volume and therefore a decrease in pressure and vice versa: when a system in dynamic equilibrium is disturbed, the system responds so as to minimize the disturbance and return to a state of equilibrium (this principle of retaining the same dynamic equilibrium is known as Le Chatelier's Principle Le Chatelier's principle - the retention of equilibrium - is applicable to any system in equilibrium Temperature Dependence of Vapor Pressure and Boiling Point an increase in the temperature of a liquid increases the vapor pressure because the number of molecules that can vaporize increases the boiling point of a liquid is the temperature at which its vapor pressure equals external pressure. The temperature is just high enough to allow interior molecules of a liquid, including surface molecules of a liquid, to break free from a liquid form into a gas form the bubbles in boiling water are pockets of gaseous water that have formed within the liquid water the normal boiling point of a liquid is the temperature at which its vapor pressure equals 1 atm which is 100˙C for water. However, at lower pressures water boils at a lower point

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e.g. in Denver, Colorado the pressure is 83% of sea level pressure and water boils at approximately 94˙C once boiling point is reached, additional temperature only increases the rate of boiling, not the temperature of the liquid therefore boiling water at 1 atm will always have a temperature of 100˙C, as long as liquid water is present it cannot raise its' temperature above 100˙C: notice, only once all water is converted to steam can its temp go above 100˙C The Clausius- Clapeyron Equation
the vapor pressure of a liquid increases with increasing temperature, but the relationship isn't linear; a doubling of temperature = a more than doubling of the pressure. The relationship between vapor pressure and temperature is exponential and can be expressed as follows: where which can be rearranged to: so, the Clausius-Clapeyron equation reveals that the natural log of vapor pressure is proportional to the inverse of temperature.

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The Clausius-Clapeyron equation allows us an easy way to measure the heat of vaporization in the laboratory 1. measure vapor pressure of a liquid as a function of temperature 2. create a plot of natural log of vapor pressure versus the inverse of temperature 3. determine slope of line to determine heat of vaporization There's also a two-point version if we known the enthalpy of vaporization and the normal boiling point or vapor pressure at some other temperature: pressure is measured in torr, and temperature is measured in kelvin Critical Point: Transition of an Unusual State of Matter
as n-pentane is heated the pressure and temperature increases, which increases vapor pressure, this increases gas density, while lowering the density of the liquid until the meniscus between gas and liquid disappears. The temperature at which the gas and liquid states commingle to become supercritical fluid

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