Wednesday, December 28, 2011

Physical Changes; Heat of Fusion, Heat of Vaporization, Specific Heat; Heat - Chemistry HW


PHYSICAL CHANGES

Melting or fusion is a process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased as heat goes into the solids as it melts, resulting in a rise of its temperature to the melting point, at which the rigid ordering of molecular entities in the solid breaks down to a less-ordered state and the solid liquefies.

Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. Heat leaves the liquid as it freezes.

Vaporization is the change of state from liquid to gas. Heat goes into the liquid as it vaporizes.

Condensation is the change of state from gas to liquid. Heat leaves the gas as it condenses.

Sublimation is the conversion of a solid directly into a gas without passing through an intermediate liquid phase. Heat goes into the solid as it sublimes.

Deposition or desublimation is a process in which gas transforms into solid. Heat leaves the gas as it solidifies.

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Heat of fusion is the amount of necessary heat absorbed by one mole of a substance to convert a given amount of a solid into a liquid.  This energy is required to separate the solid particles. It results in the higher potential energy of the liquid as compared with that of the solid.

Heat of Vaporization is the amount of necessary heat absorbed by one mole of a substance to change a given amount of a liquid at its boiling point into a gas.

Specific Heat is the heat capacity of 1 gram of a substance. It is a physical property of a substance. It is the amount of heat that must be absorbed or lost per unit mass required to change the temperature by one degree Celsius.

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The molar heat of fusion equation looks like this:
q = ΔHfus (mass/molar mass)
The meanings are as follows:
1) q is the total amount of heat involved
2) ΔHfus is the symbol for the molar heat of fusion. This value is a constant for a given substance.
3) (mass/molar mass) is the division to get the number of moles of substance

The molar heat of vaporization equation looks like this:
q = ΔHvap (mass/molar mass)
The meanings are as follows:
1) q is the total amount of heat involved
2) ΔHvap is the symbol for the molar heat of vaporization. This value is a constant for a given substance.
3) (mass/molar mass) is the division to get the number of moles of substance

Q=cpmΔT
Q is the amount of heat needed to change the temperature of a substance
m is the mass of the heated substance
cp is the specific heat capacity
ΔT (pronounced delta T) is the temperature difference; the difference in temperature before and after you applied the heat

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MOST COMMON UNITS FOR HEAT

BTU - British Thermal Unit

The unit of heat in the imperial system - the BTU - is
  • the amount of heat required to raise the temperature of one pound of water through 1oF (58.5oF - 59.5oF) at sea level (30 inches of mercury).
  • 1 Btu (British thermal unit) = 1055.06 J = 107.6 kpm = 2.931 10-4 kWh = 0.252 kcal = 778.16 ft.lbf = 1.0551010 ergs = 252 cal = 0.293 watt-hours
An item using one kilowatt-hour of electricity generates 3412 Btu.

Calorie

A calorie is commonly defined as
  • the amount of heat required to raise the temperature of one gram of water 1oC
  • the kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius
  • 1 kcal = 4186.8 J = 426.9 kp.m = 1.163 10-3 kWh = 3.088 ft.lbf = 3.9683 Btu = 1000 cal
Be aware that alternative definitions exists - in short: 
  • Thermochemical calorie  
  • 4 °C calorie
  • 15 °C calorie
  • 20 °C calorie
  • Mean calorie
  • International Steam Table calorie (1929)
  • International Steam Table calorie (1956)
  • IUNS calorie (Committee on Nomenclature of the International Union of Nutritional Sciences)
The calorie is outdated and commonly replaced by the SI-unit Joule.

Joule

The unit of heat in the SI-system the Joule is
  • a unit of energy equal to the work done when a force of one newton acts through a distance of one meter
  • 4.184 joule of heat energy (or one calorie) is required to raise the temperature of a unit weight (1 g) of water from 0oC to 1oC, or from 32oF to 33.8oF
  • 1 J (Joule) = 0.1020 kpm = 2.778 10-7 kWh = 2.389 10-4 kcal = 0.7376 ft.lbf = 1 kg.m2/s2 = 1 watt second = 1 Nm = 1 ft.lb = 9.478 10-4 Btu

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