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From Wikipedia

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid. Evaporation is also part of the water cycle.

Evaporation is a type of phase transition; it is the process by which molecules in a liquidstate (e.g., water) spontaneously become gaseous (e.g., water vapor). In general, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed to a significant volume of gas. Vaporization and evaporation however, are not entirely the same processes.

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid. With sufficient heat, the liquid would turn into vapor quickly (see boiling point). When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to escape.

Liquids that do not evaporate visibly at a given temperature in a given gas (e.g., cooking oil at room temperature) have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the heat energy necessary to turn into vapor. However, these liquids are evaporating. It is just that the process is much slower and thus significantly less visible.

Evaporation is an essential part of the water cycle. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plantstomata) are collectively termed evapotranspiration. Evaporation is caused when water is exposed to air and the liquid molecules turn into water vapor, which rises up and forms clouds.

Theory

For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces. Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid, thus, decreases. This phenomenon is also called evaporative cooling. This is why evaporating sweat cools the human body. Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with higher vapor pressure. For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day. Three key parts to evaporation are heat, humidity, and air movement.

On a molecular level, there is no strict boundary between the liquid state and the vapor state. Instead, there is a Knudsen layer, where the phase is undetermined. Because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface can be seen.

Evaporative equilibrium

If evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium, the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur. For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by the Clausius-Clapeyron relation:

\ln \left( \frac{ P_2 }{ P_1 } \right) = - \frac{ \Delta H_{ vap } }{ R } \left( \frac{ 1 }{ T_2 } - \frac{ 1 }{ T_1 } \right)

where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the enthalpy of vaporization, and R is the universal gas constant. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid will boil.

The ability for a molecule of a liquid to evaporate is based largely on the amount of kinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization.

Factors influencing the rate of evaporation

Concentration of the substance evaporating in the air:
If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
Concentration of other s

Vapor-liquid equilibrium

Vapor-liquid equilibrium (sometimes abbreviated as VLE) is a condition where a liquid and its vapor (gas phase) are in equilibrium with each other, a condition or state where the rate of evaporation (liquid changing to vapor) equals the rate of condensation (vapor changing to liquid) on a molecular level such that there is no net (overall) vapor-liquid interconversion. Although in theory equilibrium takes forever to reach, such an equilibrium is practically reached in a relatively closed location if a liquid and its vapor are allowed to stand in contact with each other long enough with no interference or only gradual interference from the outside.

VLE data introduction

The concentration of a vapor in contact with its liquid, especially at equilibrium, is often in terms of vapor pressure, which could be a partial pressure (part of the total gas pressure) if any other gas(es) are present with the vapor. The equilibrium vapor pressure of a liquid is usually very dependent on temperature. At vapor-liquid equilibrium, a liquid with individual components (compounds) in certain concentrations will have an equilibrium vapor in which the concentrations or partial pressures of the vapor components will have certain set values depending on all of the liquid component concentrations and the temperature. This fact is true in reverse also; if a vapor with components at certain concentrations or partial pressures is in vapor-liquid equilibrium with its liquid, then the component concentrations in the liquid will be set dependent on the vapor concentrations, again also depending on the temperature. The equilibrium concentration of each component in the liquid phase is often different from its concentration (or vapor pressure) in the vapor phase, but there is a correlation. Such VLE concentration data is often known or can be determined experimentally for vapor-liquid mixtures with various components. In certain cases such VLE data can be determined or approximated with the help of certain theories such as Raoult's Law, Dalton's Law, and/or Henry's Law.

Such VLE information is useful in designing columns for distillation, especially fractional distillation, which is a particular specialty of chemical engineers. Distillation is a process used to separate or partially separate components in a mixture by boiling (vaporization) followed by condensation. Distillation takes advantage of differences in concentrations of components in the liquid and vapor phases.

In mixtures containing two or more components where their concentrations are compared in the vapor and liquid phases, concentrations of each component are often expressed as mole fractions. A mole fraction is number of moles of a given component in an amount of mixture in a phase (either vapor or liquid phase) divided by the total number of moles of all components in that amount of mixture in that phase.

Binary mixtures are those having two components. Three-component mixtures could be called ternary mixtures. There can be VLE data for mixtures with even more components, but such data becomes copious and is often hard to show graphically. VLE data is often shown at a certain overall pressure, such as 1 atm or whatever pressure a process of interest is conducted at. When at a certain temperature, the total of partial pressures of all the components becomes equal to the overall pressure of the system such that vapors generated from the liquid displace any air or other gas which maintained the overall pressure, the mixture is said to boil and the corresponding temperature is the boiling point (This assumes excess pressure is relieved by letting out gases to maintain a desired total pressure). A boiling point at an overall pressure of 1 atm is called the normal boiling point.

Thermodynamic description of vapor-liquid equilibrium

The field of thermodynamics describes when vapor-liquid equilibrium is possible, and its properties. Much of the analysis depends on whether the vapor and liquid consist of a single component, or if they are mixtures.

Pure (single-component) systems

If the liquid and vapor are pure, in that they consist of only one molecular component and no impurities, then the equilibrium state between the two phases is described by the following equations:

P^{liq} = P^{vap}\,;
T^{liq} = T^{vap}\,; and
\tilde{G}^{liq} = \tilde{G}^{vap}

where P^{liq}\, and P^{vap}\, are the pressures within the liquid and vapor, T^{liq}\, and T^{vap}\, are the temperatures within the liquid and vapor, and \tilde{G}^{liq} and \tilde{G}^{vap} are the molar Gibbs free energies (units of energy per amount of substance) within the liquid and vapor, respectively. In other words, the temperature, pressure and molar Gibbs free energy are the same between the two phases when they are at equilibrium.

An equivalent, more common way to express the vapor-liquid equilibrium condition in a pure system is by using the concept of fugacity. Under this view, equilibrium is described by the following equation:

f^{\,liq}(T_s,P_s) = f^{\,vap}(T_s,P_s)

where f^{\,liq}(T_s,P_s) and f^{\,vap}(T_s,P_s) are the fugacities of the liquid and vapor, respectively, at the system temperature T_s\, and pressure P_s\,. Using fugacity is often more convenient for calculation, given that the fugacity of the liquid is, to a good approximation, pressure-independent, and it is often convenient to use the quantity \phi=f/P\,, the dimensionless fugacity coefficient, which is 1


From Yahoo Answers

Question:1. Alcohol 2. Peroxide 3. oil 4. liquid soap I found that it takes about 600 calories of heat energy to change 1 gram of liquid water into a gas. Does anyone know the amount of heat energy per gram of liquid that it takes to evaporate the others? 1. Alcohol 2. Peroxide 3. vegetable oil 4. liquid dish soap I found that it takes about 600 calories of heat energy to change 1 gram of liquid water into a gas. Does anyone know the amount of heat energy per gram of liquid that it takes to evaporate the others?

Answers:You cannot determine some of this directly because they can only be found through experimental analysis. "Oil" could be relating to vegetable oil, crude oil, baby oil but i am going to assume you are talking about crude oil in which case the purity would affect the rate of evaporation. liquid soap is mostly made of animal fat and would this depends on what else is used in the soap. this does not have enough information to give a direct answer and you also cannot figure this out without experimental analysis of at least the last two. the first two you can find on sigma-aldrich. a chemical manufacturer that we at the university of oregon

Question:THE QUESTION IS HERE!!! the vapor phases of liquids such as acetone & alcohol are more flammable than their liquid phases. for flammable liquids, what is the relationship between evaporation rate and the likelihood that the liquid will burn? this question is so f'd up!! helppp pleaz!

Answers:Flammable liquids evaporate and their vapours form part of the surrounding atmosphere. As the concentration of vapours increase the air vapour mixture can get to a concentration called the Lower Explosive Limit abbreviated "LEL". Once the vapour concentration is above the "LEL" any energy discharge in the are can trigger an explosion. However if no explosion was to occur the concentration could increase to go past the "UEL" or upper explosive limit and no explosion will occur, simply because there is insufficient oxygen to burn the vapours. Read the link provided. The rate of evaporation depends on the partial pressure of the material in the surrounding atmosphere. Also evaporation causes cooling, so as the liquid vaporises it drops temperature and slows down the rate of evaporation. When the partial pressure of a compound is equal to the atmospheric pressure in the vicinity, the compound has reached its boiling point. The greater the evaporation rate the more likely you will attain the LEL and cause a small explosion which will result in the burning of the liquid phase of the compound as well. If however you do not provide the initial energy you can get to the UEL and the compound will not burn since insufficient oxygen will be present. An external energy source is required to start most combustion reactions. Sodium and phosphorus can spontaneously oxidise at room temperatures and cause fires.

Question:A 1.4-L container of liquid nitrogen is kept in a closet measuring 1.2 m by 1.2 m by 2.0 m. Assuming that the container is completely full, that the temperature is 29.3 C, and that the atmospheric pressure is 1.2 atm, calculate the percent (by volume) of air that would be displaced if all of the liquid nitrogen evaporated. (Liquid nitrogen has a density of 0.807 G/mL.) Total %?

Answers:use perhaps the Riemann's dzeta function

Question:A 1.4L container of liquid nitrogen is kept in a closet measuring 1.2 m by 1.1 m by 1.8 m. Assuming that the container is completely full, that the temperature is 22.8C, and that the atmospheric pressure is 1.1atm, calculate the percent (by volume) of air that would be displaced if all of the liquid nitrogen evaporated. (Liquid nitrogen has a density of 0.807g/mL

Answers:1.4litres with density 0.807g/ml = 1,129.8g N2 Molar mass N2 = 28.014g/mol 1,129.8g = 1129.8/28.014 = 40.330mol N2 Volume of N2 at 22.8 C and 1.1atm pressure PV = nRT 1.1*V = 40.330*0.082057*295.8 V = 889.9 litres = 0.8899m Volume of closet = 1.2*1.1*1.8 = 2.376m % displced = 0.8899/2.376*100 = 37.45%

From Youtube

Liquid Nitrogen Fun :poaring Liquid Nitrogen at about -190 C in glass of warm water 40 C causes fast evaporation of nitrogen and cooling water to the point of freezing forming an ice crust at the top of the glass...hear the ticking of ice?

Liquid Nitrogen Water Cannon.wmv :A liquid nitrogen water cannon is fired off at Green River Community College on Jan 15, 2010. A 20 oz. soda bottle is filled about 1/3 of the way with liquid nitrogen, then closed with a screw-on cap. The weighted bottle is then dropped into a tall tube filled with water, where it immediately sinks to the bottom. As the liquid nitrogen (boiling point -196 degrees Celsius) evaporates, the pressure in the soda bottle rises. The pressure quickly reaches the bursting point of the bottle, approximately 10 atmospheres or 150 lbs. per square inch. When the soda bottle bursts, the extreme pressure of the expanding nitrogen gas blows the water up out of the tube in a towering water eruption.