Labels: | at 9:55 PM

   










Energy Relations in Chemistry: Thermochemistry

Sugar you eat is "combusted" by your body to produce CO2 and H2O. During this process energy is also released.

This energy is used (among other things) to:
  • Operate your muscles
  • Maintain your body temperature
Chemical reactions involve changes in energy:
  • Some reactions produce energy
  • Some reactions require energy
Our society as an "organism" requires energy: 90% of our energy comes from chemical reactions involving the combustion of petroleum products.
The study of energy and its transformations is known as thermodynamics
The relationship between chemical reactions and energy changes is known as thermochemistry.

The Nature of Energy
A Force is any kind of push or pull exerted on an object.
  • Gravity is a force which keeps us stuck to the earth.
  • The Electrostatic force attracts electrons to protons in an atom.
If you move an object against some force, work is being done.
The amount of work (w) being done is relative to the distance (d) the object is moved and the strength of the force (F) against the object:
w = F * d
Energy, in the form of work, must be used to move an object against a force.
When we do work, our body temperature increases (and we sweat to cool us down). Our bodies are generating Heat energy.
Heat is an energy which is transferred from one object to another depending on the relative temperature:
  • Heat energy flows from an object towards other objects of lower temperature


Energy is the capacity to do work or to transfer heat



Objects can possess energy due to their motions and positions, as kinetic energy and potential energy.
Kinetic and Potential Energy
Kinetic energy is the energy of motion. The magnitude of the kinetic energy (Ek) of an object depends upon its mass (m) and velocity (v):


Potential energy is the result of the attractions and repulsion between objects. An electron has potential energy when located near a proton due to the attractive electrostatic force between them.

  • Chemical energy is the potential energy stored in the arrangement of electrons and protons.
  • Thermal energy reflects the kinetic energy of the molecules of a substance.
Energy Units

The SI unit for energy is the joule ("J"). In honor of James Prescot Joule (1818-1889) a British Scientist who investigated work and heat. (Note: SI is short for the French term Systeme International d'Unites. Which defines metric standards).
Kinetic energy for example is defined as:



Thus, the joule must have units of:

kg*(meters/second)2

and, in fact, 1 joule is defined as:



Traditionally, energy changes accompanying chemical reactions have been expressed in calories, which is a non-SI unit (though still widely used).

1 calorie = 4.184 J

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The Nature of Energy II

Labels: | at 6:10 AM

Energy is a scalar quantity that can perform work in various forms. It has many forms including heat, motion, and light. 


  • The law of conservation of energy states that “energy can never be created or destroyed” but it can be transformed into other types of energy. 
However, Einstein’s equation:
e=mc^2 
is a formula where e is energy, m is mass, and c is the speed of light. Mass can be converted to energy, and this equation shows how. The energy produce will be equal to the mass of the object (usually a neutron) times the square of the speed of light. 



Mechanical energy has 2 forms, kinetic and potential. These kinds of energy are what cause motion in physical objects. 
  • Potential energy can be defined as stored energy, and kinetic energy as the energy possessed by an object’s motion. In potential energy there are further divisions, such as gravitational potential energy, elastic potential energy, and electric potential energy. 
  • Thermal energy, or heat, could also be considered mechanical, although the motion it causes is mostly on the atomic level. The total amount of thermal energy, or heat, in an object is simply a measure of the movement of its atomic components. The hotter an object, the faster the atoms are moving. Thermal energy can cause motion on a larger scale as well, such as when water is heated to steam, which then rises due to its lower density. Steam can be used to turn turbines and do work.

  • Waves are a way to transport energy. Mechanical waves need a medium in which to move, and electromagnetic waves do not. Sound waves, ocean waves, and shock waves are examples of mechanical waves. In mechanical waves, energy is moved through a medium, but returning the medium to its original resting position once it has left. A mechanical wave moving through a solid medium, such a rope will cause the whole rope to move at one time or another, but once the wave has passed through, the rope returns to its original position. When one end of the rope is fixed, the energy doesn’t transmit, it only reflects. The loss of energy and seizing of the wave is only caused by the air around the rope that gets moved during the motion of the wave, unless some energy is lost to the motion of the fixed end, such as a loose ring stand.
 Sound waves are longitudinal, and the wave vibrates parallel to the direction of the waves motion, whereas the wave in the rope or on the ocean is transverse and vibrates perpendicular to the motion of the wave.



  • Electromagnetic waves do not need a medium in which to transfer energy. This is because they are self propagating waves, which means that the electric and magnetic forces produce the other. If the electric energy propelled the wave along a vertical plane, it would produce a magnetic wave that propelled it along a horizontal plane. 


  • Sound waves are generated by the compression and expansion of air as the energy moves through it. Sound waves spread from a central point in a spherical motion, that is, every direction. The speed of a sound wave is determined only by the medium in which it is moving through. The more elastic the medium, the faster the sound wave can move, and also the longer distance it can travel.


  • Light is an electromagnetic wave, as well as a mass less particle known as a photon. Many chemical reactions produce light as a product, like the burning of tungsten in an incandescent light bulb. Although the photon is considered mass less, it is however affected by gravitational forces.


  • Visible light makes up a very small part of the entire electromagnetic spectrum, only extending from 400 to 700 nanometers. The longer wavelengths have less energy than the shorter ones, because their speeds are the same so more energy can be moved in a shorter time when the wavelengths are shorter.


  • Color is simply different wavelengths of light. Red is the longest and least energetic, and violet is the shortest and most energetic wavelength of light. Pure light is white light, which contains all the wavelengths of light. A Venn diagram of color contains 3 circles, red, green, and blue. Where they meet in the middle they produce white light. 

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Video : nature of energy

Labels: | at 4:15 AM

 

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Labels: | at 10:15 AM


  



Mechanical energy

It manifest in many forms,but can be broadly classified into elastic potential energy and kinetic energy.
Mechanical energy is converted
intoby
Mechanical energyLever
Thermal energyBrakes
Electric energyDynamo
Electromagnetic radiationSynchrotron
Chemical energyMatches
Nuclear energyParticle accelerator
  • Potential energy refers to the energy any object gets due to its position in a force field.The name "potential" energy originally signified the idea that the energy could readily be transferred as work—at least in an idealized system (reversible process, see below). 
Potential energy, symbols Ep, V or Φ, is defined as the work done against a given force (= work of given force with minus sign) in changing the position of an object with respect to a reference position (often taken to be infinite separation). If F is the force and s is the displacement,
E_{\rm p} = -\int \mathbf{F}\cdot{\rm d}\mathbf{s}
with the dot representing the scalar product of the two vectors.
  • Elastic potential energy
     is defined as a work needed to compress (or expand) a spring. The force,F, in a spring or any other system which obeys Hooke's law is proportional to the extension or compression, x,

F = − kx
where k is the force constant of the particular spring (or system). In this case, the calculated work becomes
E_{\rm p,e} = {1\over 2}kx^2
only when k is constant. Hooke's law is a good approximation for behaviour of chemical bonds under normal conditions, i.e. when they are not being broken or formed.
  • Kinetic energy, symbols Ek, T or K, is the work required to accelerate an object to a given speed. 
As a ball falls freely under the influence of gravity, it accelerates downward, its initial potential energy converting into kinetic energy. On impact with a hard surface the ball deforms, converting the kinetic energy into elastic potential energy. As the ball springs back, the energy converts back firstly to kinetic energy and then as the ball re-gains height into potential energy. Energy conversion to heat due to inelastic deformation and air resistance cause each successive bounce to be lower than the last.
Surface energy
A minimal surface, for example, represents the smallest possible energy that a surface can have if its energy is proportional to the area of the surface. For this reason, (open) soap films of small size are minimal surfaces (small size reduces gravity effects, and openness prevents pressure from building up. Note that a bubble is a minimum energy surface but not a minimal surface by definition).

Sound energy

Sound is a form of mechanical vibration which propagates through any mechanical medium. 

Gravitational energy

The gravitational force near the Earth's surface varies very little with the height, h, and is equal to the mass, m, multiplied by the gravitational acceleration, g = 9.81 m/s². In these cases, the gravitational potential energy is given by
Ep,g = mgh
A more general expression for the potential energy due to Newtonian gravitation between two bodies of masses m1 and m2, useful in astronomy, is
E_{\rm p,g} = -G{{m_1m_2}\over{r}},
where r is the separation between the two bodies and G is the gravitational constant, 6.6742(10)×10−11 m3kg−1s−2. In this case, the reference point is the infinite separation of the two bodies.

Thermal energy


It is the energy associated with the microscopical random motion of particles constituting the media.
A heat is defined as a transfer (flow) of thermal energy across certain boundary (for example, from a hot body to cold via the area of their contact. A practical definition for small transfers of heat is
\Delta q = \int C_{\rm v}{\rm d}T
where Cv is the heat capacity of the system. This definition will fail if the system undergoes a phase transition—e.g. if ice is melting to water—as in these cases the system can absorb heat without increasing its temperature. In more complex systems, it is preferable to use the concept of internal energy rather than that of thermal energy.
Electric energy
Electrostatic energy
The electric potential energy of given configuration of charges is defined as the work which must be done against the Coulomb force to rearrange charges from infinite separation to this configuration (or the work done by the Coulomb force separating the charges from this configuration to infinity). 
Electric energy
If an electric current passes through a resistor, electric energy is converted to heat; if the current passes through an electric appliance, some of the electric energy will be converted into other forms of energy (although some will always be lost as heat). 
Magnetic energy
There is no fundamental difference between magnetic energy and electric energy: the two phenomena are related by Maxwell's equations. The potential energy of a magnet of magnetic moment m in a magnetic field B is defined as the work of magnetic force (actually of magnetictorque) on re-alignment of the vector of the magnetic dipole moment.
Chemical energy
It is the energy due to associations of atoms in molecules and various other kinds of aggregates of matter. If the chemical energy of a system decreases during a chemical reaction, the difference is transferred to the surroundings in some form (often heat or light); on the other hand if the chemical energy of a system increases as a result of a chemical reaction - the difference then is supplied by the surroundings (usually again in form of heat or light).
Nuclear energy
Nuclear potential energy, along with electric potential energy, provides the energy released from nuclear fission and nuclear fusion processes. The result of both these processes are nuclei in which the more-optimal size of the nucleus allows the nuclear force (which is opposed by the electromagnetic force) to bind nuclear particles more tightly together than before the reaction.
The Weak nuclear force (different from the strong force) provides the potential energy for certain kinds of radioactive decay, such as beta decay.
The energy released in nuclear processes is so large that the relativistic change in mass (after the energy has been removed) can be as much as several parts per thousand.
Nuclear particles (nucleons) like protons and neutrons are not destroyed (law of conservation of baryon number) in fission and fusion processes. A few lighter particles may be created or destroyed (example: beta minus and beta plus decay, or electron capture decay), but these minor processes are not important to the immediate energy release in fission and fusion. Rather, fission and fusion release energy when collections of baryons become more tightly bound, and it is the energy associated with a fraction of the mass of the nucleons (but not the whole particles) which appears as the heat and electromagnetic radiation generated by nuclear reactions. This heat and radiation retains the "missing" mass, but the mass is missing only because it escapes in the form of heat or light, which retain the mass and conduct it out of the system where it is not measured.

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Video:Types of Energy

Labels: | at 5:46 AM


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