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From Wikipedia

Latent heat

Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature, meaning a phase transition such as the melting of ice or the boiling of water. The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden.

In meteorology, latent heat flux is the flux of heat from the Earth's surface to the atmosphere that is associated with evaporation or transpiration of water at the surface and subsequent condensation of water vapor in the troposphere. It is an important component of Earth's surface energy budget. Latent heat flux is commonly measured with the Bowen ratio technique, or by eddy covariance.

Usage

Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting) and latent heat of vaporization (boiling). These names describe the direction of energy flow when changing from one phase to the next: from solid to liquid, and to gas.

In both cases, the change is endothermic, meaning that the system absorbs energy on going from solid to liquid to gas. The change is exothermic (the process releases energy) for the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains less water vapor than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

The terms sensible heat and latent heat are not special forms of energy, instead they characterize the same form of energy, heat, in terms of their effect on a material or a thermodynamic system. Heat is thermal energy in the process of transfer between a system and its surroundings or between two systems with a different temperature.

Both sensible and latent heats are observed in many processes while transporting energy in nature. Latent heat is associated with the phase changes of atmospheric water vapor, mostly vaporization and condensation, whereas sensible heat is energy transferred that affects the temperature of the atmosphere.

History

The term latent heat was introduced around 1750 by Joseph Black, and is derived from the Latin latere, meaning to lie hidden. In 1847, James Prescott Joule characterized latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy, and the sensible heat as an energy that was indicated by the thermometer, relating the latter to thermal energy.

Specific latent heat

A specific latent heat (L) expresses the amount of energy in form of heat (Q) required to completely effect a phase change of a unit of mass (m), usually , of a substance as an intensive property:

L = \frac {Q}{m}

Intensive properties are material characteristics and are not dependent on the size or extend of the sample. Commonly quoted and tabulated in the literature are the specific latent heat of fusion and the specific latent heat of vaporization for many substances.

From this definition, the latent heat for a given mass of a substance is calculated by

Q = {m} {L}

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),
m is the mass of the substance (in kg or in lb), and
L is the specific latent heat for a particular substance (kJ-kgm−1 or in BTU-lbm−1), either Lf for fusion, or Lv for vaporization.

Table of latent heats

The following table shows the latent heats and change of phase temperatures of some common fluids and gases.

Latent heat for water

The latent heat of condensation of water in the temperature range from −40 °C to 40 °C is approximated by the following empirical cubic function:

L_{water}(T)=-0.0000614342 T^3+0.00158927 T^2-2.36418 T+2500.79

with a determination coefficient of R^2=0.999988, where T is in °C.



From Yahoo Answers

Question:1. A cube of ice is taken from the freezer at -8.1 C and placed in a 91-g aluminum calorimeter filled with 2.8E2 g of water at room temperature of 22.0 C. The final situation is observed to be all water at 17.5 C. What was the mass of the ice cube? hint: The heat lost by the aluminum and 2.8E2 g of liquid water must be equal to the heat gained by the ice in warming in the solid state, melting, and warming in the liquid state. 2.An iron boiler of mass 2.1E2 kg contains 8.0E2 kg of water at 22 C. A heater supplies energy at the rate of 5.2E4 kJ/h. a) How long does it take for the water to reach the boiling point? b) How long does it take for the water to all have changed to steam? 3. What mass of steam at a temperature of 100.0 C, must be added to 4.1 kg of ice at a temperature of 0.0 C to yield liquid water having a temperature of 15.0 C?

Answers:Your questions here arenot impossible, but very simple. You just need to use the following equation: Q = mc(ice) T + mL(fusion) + mc(water) T + mL(vapor) + mc(vapor) T + Mc(container) T where Q is the change in heat energy, m is the mass of water, M is the mass of the container, L is the latent heat (this can be latent heat of fusion or latent heat of vaporization depending on the phase transition), c is the specific heat capacity, and T is the change in temperature. So here you must calculate the net Q by adding contributions from (1) changing temperature of ice, (2) transition from ice to water, (3) changing temperature of water, and (4) changing temperature of container. Note that some terms in the equation is not relevant. It depends on the question.

Question:

Answers:Energy to melt = Latent heat * mass = 11 J/g * 15 g = 165 J

Question:Okay so I'm kinda confused with an experiment here and I kinda have to, you know, predict products before the reaction so I need help. Here's how the experiment goes: 1. With the tong, take a small piece of Na metal and press between a folded paper to remove the kerosene. 2. Place a pinch of the solid sample in a dry watch glass and roll the sodium metal in it. Introduce the Na metal now covered with the sample into a small clean dry broken test tube taking care not to leave solid on the sides of the tube. 3. Hold the tube with clean tongs or holder and heat slowly at the start. Gradually increase the flame until the tube is red hot or until sodium vapor bursts into flame 4. Drop the tube into a beaker containing 10 mL of distilled water. Break the tube and allow the part with fused sample to drop into the beaker. 5. Heat to almost boiling and filter. The filtrate, which is colorless, is now ready to be used as a test solution for the elements. Test for S: Acidify 1 mL of the test solution with dilute Acetic Acid and add a few drops of Lead Acetate solution. A black precipitate of PbS indicates presence of S. Test for N: To 1 mL of the test solution add 2 drops of FeSO4. Add enough NaOH (2 drops) to produce a distinct alkanity. Heat to boiling and then filter to remove any FeS. Let stand for 10 minutes. A precipitate of prussian blue indicates presence of N Test for Halogen: To 1 mL of the test solution, add dilute HNO3 to produce a distinct acidity. Boil gently to expel any HCN or H2S that may be present. Add AgNO3 solution. That's the whole procedure. I typed it to give everyone an idea about the experiment. I'm currently struggling on finding out the chemical equations present. And what's supposed to be the duty of the solid sample? It just makes obtaining the equation for the reaction harder. Help please... :) Thanks in advance...

Answers:This experiment is Lassaigne's method for finding the presence of sulfur, nitrogen and/or halogens in an unknown substance. The "solid sample" is the unknown substance, so it's pretty hard to do this without it. Also, we don't know - until we've completed the procedure - what is in it. The idea of the test is to react the unknown substance with the sodium so that any halogens, nitrogen, or sulfur are converted into inorganic sodium salts such as sodium halide (for halides), sodium cyanide (for nitrogen), sodium sulfide (for sulfur), and sodium thiocyanate (for sulfur and nitrogen). When that's done, we test samples of the resulting solution for S, N and halogens in turn. The last test will produce a black precipitate of silver halide(s) if any are present.

Question:a) When molten Cs solidifies to Cs (s) at its normal melting point, is the entropy change S positive or negative? b) Calculate the entropy change S when 15.0 g of Cs(l) solidifies at 28.4 C.

Answers:this is the equation you need... S = Q / T where Q = heat ABSORBED by the system. it's (+) is heat is absorbed, (-) if it is released. for this reaction...(or more precisely "phase change")... Cs(l) ---> Cs(s).... H = - 2.09 kJ/mole so... (a)... S = (-2.09 kJ/mole) x (1 / 298K) x (1000J / 1kJ) = -7.01 J/moleK.. ie S is negative. (b)... S = 15.0g x (1 mole / 132.9g) x (-2090 J/mole) x (1 / 298K) = -0.791 J/K ********** entropy enthalpy

From Youtube

Specific latent heat explained :1) Learn about the states of matter and the names for specific changes of state 2) Learn when to use the latent heat equation

The Heat and Temperature Song - NEW, more singable RECORDING Mr. Edmonds :This is a remake of my original in a better key for singing! I was inspired to do it by a teacher who suggested to make a video parody on "Heat versus Temperature" and I thought it was an excellent idea for students often confuse the two concepts. I want to again thank my science students for their ongoing encouragement and those students and teachers from all over the US and Canada who have offered responses. Temperature is not energy, but it is a measure of the kinetic energy of particles. Heat is measured in joules and may be potential or kinetic energy. If it is used as latent heat of fusion or vaporization, then it is potential. If it is used to raise temperature, then it is kinetic. Word are in DOCS section for dsecms on Teacher Tube