Thermodynamics Freezing - Chapter Summary

Freezing From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Freezing (disambiguation). In physical science, freezing or solidification is the process in which a liquid turns into a solid when cold enough. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is almost the exact opposite of freezing. All known liquids undergo freezing when the temperature is lowered enough, with the sole exception of liquid helium, which remains liquid at absolute zero and can only be solidified under pressure. For most substances, the melting and freezing points are the same temperature, however, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F); this process is known as hysteresis. The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a liquid's flash point requires an ignition source. At the flash point, the vapor may cease to burn when the source of ignition is removed. The flash point is not to be confused with the autoignition temperature, which does not require an ignition source. The fire point, a slightly higher temperature, is defined as the temperature at which the vapor continues to burn after being ignited. Neither the flash point nor the fire point is related to the temperature of the ignition source or of the burning liquid, which are much higher. The flash point is often used as a descriptive characteristic of liquid fuel, and it is also used to describe liquids that are not normally used as fuels. “Flash point” refers to both flammable liquids and combustible liquids. There are various international standards for defining each, but most agree that liquids with a flash point less than 43°C are flammable, while those having a flash point above this temperature are combustible. Examples of flash points Fuel Flash point Autoignition temperature Ethanol 12.8 °C (55 °F) 365 °C (689 °F) Gasoline (petrol) <−40 °C (−40 °F) 246 °C (475 °F) Diesel >62 °C (143 °F) 210 °C (410 °F) Jet fuel >60 °C (140 °F) 210 °C (410 °F) Kerosene (paraffin oil) >38–72 °C (100–162 °F) 220 °C (428 °F) Vegetable oil (canola) 327 °C (620 °F)[1] Biodiesel >130 °C (266 °F) Gasoline (petrol) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature. Diesel fuel flash points vary between 52 °C and 96 °C (126 °F to 204 °F). Diesel is designed for use in a high-compression engine. Air is compressed until it has been heated above the autoignition temperature of diesel; then the fuel is injected as a high-pressure spray, keeping the fuel-air mix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have a high flash point and a low autoignition temperature. Jet fuel flash points also vary greatly. Both Jet A and Jet A-1 have flash points between 38 °C and 66 °C (100 °F to 150 °F), close to that of off-the-shelf kerosene. Yet both Jet B and FP-4 have flash points between -23 °C and -1 °C (-10 °F to +30 °F ). The temperature at which the vapour above a volatile liquid forms a combustible mixture with air. At the flash point the application of a naked flame gives a momentary flash rather than sustained combustion, for which the temperature is too low. (chemistry) The lowest temperature at which a volatile combustible substance vaporizes rapidly enough to form above its surface an air-vapor mixture which burns continuously when ignited by a small flame. The fire point of a fuel is the temperature at which it will continue to burn for at least 5 seconds after ignition by an open flame. At the flash point, a lower temperature, a substance will ignite briefly, but vapor might not be produced at a rate to sustain the fire. Most tables of material properties will only list material flash points, but in general the fire points can be assumed to be about 10ºC higher than the flash points. However, this is no substitute for testing if the fire point is safety critical. [1] (chemistry) The temperature at which a material (solid, liquid, or gas) will self-ignite and sustain combustion in air without an external spark or flame. The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical will ignite decreases as the pressure increases or oxygen concentration increases. It is usually applied to a combustible fuel mixture. The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid. Fuels and their boiling points at atmospheric pressure can be found in the table below.  T(oC) = 5/9[T(oF) - 32] Fuel Acetaldehyde Acetone Acetylene Benzene Butylene Ethyl Alcohol Ethan Ethylene Fuels Oil No.1 Gasoline IsoButane Isobutene Isopropyl alcohol Iso-Octane IsoPentane Kerosine Methane (Natural Gas) Methyl Alcohol n-Butane n-Heptane n-Hexane n-Octane n-Pentane n-Pentene Naphthalene NeoPentane NeoHexane Propane Propylene Triptane Toluene Boiling Point (oF) 70 134 -119.2 176.2 21.2 172 -127.5 -154.7 304 - 574 100 - 400 10.9 19.6 181 243.9 82.2 304 - 574 -258.7 149 31.1 209.1 155.7 258.3 97.0 86.0 424.4 49.1 121.5 -43.8 -53.9 177.6 321.1 Xylene 281.1 Factors to consider when choosing a fuel Energy Value Energy Value is the heat of combustion of a fuel given per gram of fuel. The higher the energy value, the more energy is released, the better the fuel. Heat of combustion of hydrogen is 285kJ/mole 1 mole of hydrogen gas (H2) has a mass equal to its molecular mass (molecular weight) = 2 x 1.008 = 2.016 The heat produced per gram of hydrogen gas = 285 ÷ 2.016 = 141.4kJ/g The energy value for hydrogen gas is 141.4kJ/g Ignition Temperature Ignition Temperature is the minimum temperature to which the fuel-oxidiser mixture (or a portion of it) must be heated in order for the combustion reaction to occur. High ignition temperature means the fuel is difficult to ignite, low ignition temperature means the fuel ignites easily making the fuel potentially hazardous. The greater the activation energy of a reaction, the higher the ignition temperature will be. A match and its striking surface contain a fuel and its oxidiser with a low activation energy and therefore low ignition temperature, so low that the friction of striking the match generates enough heat to raise the temperature sufficiently for ignition to occur. Petrol and oxygen in a car engine have a higher activation energy and therefore a higher ignition temperature. A spark is needed to raise the temperature of the mixture sufficiently near the spark for the mixture to ignite. The heat of reaction generated heats up more of the mixture so the reaction becomes self-sustaining. Volatility Fuels function by releasing combustible gases (vapours) Boiling Point is an indicator of volatility: the higher the boiling point, the less volatile the fuel. Vapour pressure is an indicator of volatility: the higher the vapour pressure, the more volatile the fuel. Vapour pressure increases with temperature, so the volatility of a fuel can be increased by raising the temperature. A highly volatile fuel is more likely to form a flammable or explosive mixture with air than a non-volatile fuel. By definition, gases are volatile. Liquid fuels are either sufficiently volatile at room temperature to produce combustible vapour (ethanol, petrol) or produce sufficient combustible vapours when heated (kerosene). Solid fuels decompose above the vapourisation temperature to produce combustible vapours. Solid fuels will have a higher ignition temperature than liquid or gaseous fuels. Flashpoint Flashpoint is the minimum temperature to which the pure liquid fuel must be heated so that the vapour pressure is sufficiently high for an explosive mixture to be formed with air when then the liquid is allowed to evaporate and is brought into contact with a flame, spark or hot filament. Flashpoints are lower than Ignition temperatures. A fuel with a flashpoint well above room temperature (kerosene) means that it can safely be handled at room temperature since exposure to flames, sparks or hot filaments will not cause an explosion. A fuel with a flashpoint below room temperature (petrol, alcohol) is a safety hazard since exposure to flames, sparks or hot filaments will cause an explosion. These fuels need to be stored in a cool place to prevent the increased temperature raising the vapour pressure of the fuel and in a well-ventilated place so that any vapours that escape do not accumulate, and preferably in robust metal containers with narrow mouths and tightly sealing lids to prevent vapours escaping. Ease of liquefaction Gases occupy large volumes, whereas liquids of the same mass occupy much less volume making them easier to transport. Critical temperature is the temperature below which a gas can be liquified (condensed) by increasing the pressure. Liquid Petroleum Gas (LPG) is made up of propane (critical temperature 97oC) and butane (critical temperature 152oC) both of which are gases at room temperature and pressure but can be easily condensed to the liquid at room temperature by increasing the pressure since their critical temperatures are above room temperature. Products of Combustion  Complete combustion of carbon-based fuels produce carbon dioxide and water vapour. Carbon dioxide gas is the main contributor to the greenhouse effect.      Incomplete combustion of carbon-based fuels produces toxic carbon monoxide and solid carbon(soot). Sulfur and nitrogen are present in fossil fuels. The sulfur burns to produce oxides which contribute to acid rain, while the nitrogen burns to produce oxides that contribute not only to acid rain but also to photochemical smog. Solid fuels such as coal contain incombusitible minerals leading to ash. The ash can damage machinery and can cause lung disease. Uncombusted fuel can also be released. Unburnt hydrocarbons from cars contribute to photochemical smog and some are carcinogenic. Some fuels contain additives (such as the lead in leaded petrol) which can be harmful The viscosity of a fluid is its resistance to shear or flow, and is a measure of the fluids adhesive/cohesive or frictional properties. The viscosity will arise due to internal molecular friction within a fluid producing the frictional drag effect. There are two related measures of fluid viscosity which are known as dynamic (or absolute) and kinematic viscosity. Resistance of a fluid to a change in shape, or movement of neighbouring portions relative to one another. Viscosity denotes opposition to flow. It may also be thought of as internal friction between the molecules. Viscosity is a major factor in determining the forces that must be overcome when fluids are used in lubrication or transported in pipelines. It also determines the liquid flow in spraying, injection molding, and surface coating. The viscosity of liquids decreases rapidly with an increase in temperature, while that of gases increases with an increase in temperature. The SI unit for viscosity is the newtonsecond per square metre (N-s/m2). viscosity, resistance of a fluid to flow. This resistance acts against the motion of any solid object through the fluid and also against motion of the fluid itself past stationary obstacles. Viscosity also acts internally on the fluid between slower and faster moving adjacent layers. All fluids, i.e., all liquids and gases, exhibit viscosity to some degree. Viscosity may be thought of as fluid friction, just as the friction between two solids resists the motion of one over the other but also makes possible the acceleration of one relative to the other (e.g., the friction between the wheels of an automobile and a highway), so viscosity resists the motion of a solid through a fluid but also makes it possible for a propeller or other device to accelerate the solid through the fluid. Use over time The first use of fuel was the combustion of wood or sticks by Homo erectus near 2 million years ago.[6][page needed] Throughout the majority of human history fuels derived from plants or animal fat were the only ones available for human use. Charcoal, a wood derivative, has been used since at least 6,000 BCE for smelting metals. It was only supplanted by coke, derived from coal, as the forests started to become depleted around the 18th century. Charcoal briquettes are now commonly used as a fuel for barbecue cooking.[7] Coal was first used as a fuel around 1000 BCE in China. With the development of the steam engine in 1769, coal came into more common use as a power source. Coal was later used to drive ships and locomotives. By the 19th century, gas extracted from coal was being used for street lighting in London. In the 20th century, the primary use of coal is for the generation of electricity, providing 40% of the world's electrical power supply in 2005.[8]