Lecture14221

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Published on June 6, 2008

Source: slideshare.net

Description

a supplemental resource for students

Thermochemistry: Energy, Heat and Work Lecture 14

Whenever matters changes, whether physically or chemically, the energy content of the matter changes also.

Energy is absorbed and released.

Energy is absorbed and released.

Some industries manufacture products that release energy (common fuels - natural gas, oil, wood, coal); absorb energy (fertilizers); limit the flow of energy (insulators - plastic, fiberglass, ceramic and other materials).

release energy (common fuels - natural gas, oil, wood, coal);

absorb energy (fertilizers);

limit the flow of energy (insulators - plastic, fiberglass, ceramic and other materials).

Energy is interconverted.

Solar, electrical, nuclear, chemical, etc. forms of energy are all examples of potential and kinetic energy on the atomic and molecular scales.

When energy is transformed from one object to another, it appears as work and/or as heat.

A system is a part of the universe that is under consideration.

The surroundings is the rest of the universe.

Boundary (or wall), real or imaginary, separates the system from the surroundings.

The system and the surroundings.

The internal energy of the system is the total of the kinetic energy due to the motion of particles and the potential energy associated with their arrangement.

The internal energy of the system includes the energy in all the chemical bonds, and the energy of the free, conduction electrons in metals.

The internal energy of the system changes as its contents change from reactants to products.

The change in the internal energy is determined as a difference between the system’s internal energy after the change and before the change: ΔE = E final – E initial = E products – E reactants

ΔE (delta ee:) refers to the final state of the system minus the initial state of the system: ΔE = E final – E initial

A change in the energy of the system is always accompanied by an opposite change in the energy of the surroundings.

A system can change its internal energy in of two ways: By losing some energy to the surroundings: E final < E initial , ΔE < 0 By gaining some energy from the surroundings: E final > E initial , ΔE > 0 The change in energy is always a transfer of energy from system to surroundings, or vice versa.

By losing some energy to the surroundings: E final < E initial , ΔE < 0

By gaining some energy from the surroundings: E final > E initial , ΔE > 0

The change in energy is always a transfer of energy from system to surroundings, or vice versa.

Energy transfer diagrams

Energy transfer outward from the system or inward from the surroundings can appear in two forms: heat and work .

Heat is the energy transferred between a system and its surroundings as a result of difference in their temperatures only.

Work is the energy transferred between a system and its surroundings as a result of moving object by a force.

The total change in a system’s internal energy is the sum of the energy transferred as heat and/or work: ΔE = q + w

The sign of the energy transfer is defined from the system’s prospective. Energy coming into the system is positive . Energy going out from the system is negative .

If energy is transferred as heat only and heat is flowing out from a system (blue arrow), its energy decreases until its temperature equals to that of the surroundings: E final < E initial , q is negative, ΔE < 0.

and heat is flowing out from a system (blue arrow), its energy decreases until its temperature equals to that of the surroundings: E final < E initial , q is negative, ΔE < 0.

If energy is transferred as heat only and heat is flowing into a system (red arrow), its energy increases until its temperature equals to that of the surroundings: E final > E initial , q is positive, ΔE > 0.

and heat is flowing into a system (red arrow), its energy increases until its temperature equals to that of the surroundings: E final > E initial , q is positive, ΔE > 0.

If energy is transferred as work only and work is done by a system (blue arrow), its energy decreases: E final < E initial because w is negative, ΔE < 0 .

and work is done by a system (blue arrow), its energy decreases: E final < E initial because w is negative, ΔE < 0 .

If energy is transferred as work only and work is done on a system (red arrow), its energy increases: E final > E initial because w is positive, ΔE > 0 .

and work is done on a system (red arrow), its energy increases: E final > E initial because w is positive, ΔE > 0 .

The sign conventions for q, w, and ΔE: Q + w = ΔE ------------------------------------------------------------ + + + + - depends on sizes of q and w + - depends on sizes of q and w - - -

Q + w = ΔE

------------------------------------------------------------

+ + +

+ - depends on sizes of q and w

+ - depends on sizes of q and w

- - -

THE END

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