Chemistry

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Chemistry Society

• Chapter 5 Energy

Virginia State University Fall 2005 Dr. Victor
Vilchiz
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What is energy?

• Energy is the ability of a system to perform
  work.
• What is work?
• The cost of moving an object a certain distance
  against a force.
• Units of energy
• Joules (SI)
• Erg
• calorie
• Calorie (nutritional)
• BTU

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Types of energy

• Energy can be broken down into two main types of
  energy.
• Kinetic Energy (movement)
• Potential Energy (position)
• Forms of energy
• Heat
• Electrical
• Chemical
• Nuclear
• Electromagnetic

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Sources of Energy

• Sun
• Windmills
• Waves
• Hydroelectric plants
• Food
• Fuels
• Nuclear
• Alpha Decay
• Beta Decay
• Gamma Radiation

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Where does energy we use go?

• First Law of Thermodynamics
• Also known as the Law of Conservation of Energy
• The energy of the universe cannot be created or
  destroyed. It can only be moved from one place to
  another.
• Second Law of Thermodynamics
• The entropy of the universe is always increasing.
• Third Law of Thermodynamics
• Only a perfect crystal at absolute zero has an
  entropy of ZERO.

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Energy Efficiency

• It is impossible to transfer 100 of the energy
  from one place to another.
• We always end up losing energy due to friction,
  collisions.
• The energy still transformed (moved) just not to
  where we want it.

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Thermodynamics

• Every process has a certain amount of energy
  change associated with it.
• If a process has no change in energy then that
  process is the act of doing NOTHING.
• Thermodynamics is the branch of science that
  studies the movement of energy as heat.
• Thermochemistry is the branch of chemistry that
  studies the movement of energy in chemical
  reactions.

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Chemical Energy

• The energy stored in fuels which is released
  during the chemical reaction (combustion) is a
  form of potential energy (chemical potential).
• Energy is not a thing but a property of a given
  substance or object.
• The property of where the object is located, how
  fast it is traveling or how it is made up.
• In a chemical compound the energy is stored in
  the bonds that hold the compound together.

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Heat vs Temperature

• We have previously discussed this to some extent.
• Heat is energy and it is manifested in how fast
  molecules travel.
• The faster the molecules travel the more likely
  they are to collide with one another or the walls
  of a container.
• The more collisions there are with the wall, the
  more energy that is transfered to the containers
  walls.

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Heat vs Temperature

• Temperature is just a relative measurement of the
  object we are probing and a reference point.
• There are several temperature scales
• Centigrade/Celsius
• Fahrenheit
• Kelvin/Absolute

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Temperature Scales

• Fahrenheit Scale
• The scale was originally based on 4 values (now
  only 3).
• 0F temp of a water/ice/salt mixture
• 30F Water freezing
• 90F Body Temperature
• 240F Water boiling
• Since the first is no longer used, the
  temperature depends on the composition of the
  mixture. The other 3 have been adjusted to 32,
  98.6 and 212.

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Temperature Scales

• Celsius/Centigrade
• Based on the boiling and freezing points of water
  and using the traditional decimal system.
• Kelvin Scale
• Based on experimental Pressure vs Temperature
  results involving gases at different volumes. It
  clearly demonstrated the existence of an absolute
  temperature (0 K).
• Current low temperature record is 0.15K in 2000

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Electromagnetic Radiation

• Light
• The human eye can detect EM radiation with
  wavelengths in the range of 400-800 nm.
• The longer the wavelength the smaller the energy
  of the radiation
• The human eye is most sensitive to green light
  (500nm).
• The human eye can see yellow light (550nm) the
  farthest.

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Light

• Light is made up of particles that contain energy
  but have no mass, photons
• Light contains two characteristics
• Wavelength distance between the same point in
  adjacent waves.
• Symbolized l in m. Visible light in nm.
• Frequency number of waves passing a given point
  in a second.
• Symbolized n in (1/s of Hz).

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Water wave (ripple).
Amplitude is not a characteristic of a
wave. Amplitude determines how bright or dimmed
the light is
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Photoelectric effect

• In the experiment electrons are ejected from a
  metal sheet when light shines on it.
• However, the electrons are ejected only if the
  light exceeds a certain threshold frequency.
• Violet light, for example, will cause potassium
  to eject electrons, but no amount of red light
  (which has a lower frequency) will lead to
  ejected electrons.

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The photoelectric effect.
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Excitation and Relaxation

• If an electron is excited it absorbs energy.
  (heat, current, light)
• Then it MUST relax, as it relaxes it emits energy
  (heat, light).
• This is how fireworks work.
• They absorb energy from the powder explosion and
  then as electrons relax they emit light.
• Different metals emit different colors.

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Energy

• Some reactions need energy to take place and some
  release energy.
• In order to boil water we must provide energy.
• When we burn methane we obtain energy.
• A reaction that needs energy it is said to be
  ENDERGONIC.
• A reaction that releases energy as it takes place
  it is said to be EXERGONIC.

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Thermicity

• Thermicity
• Themicity is a bit more specific it deals with
  just heat and not energy in general.
• Explains the direction of heat flow in a process.
• A reaction that requires heat to proceed is
  Endothermic.
• A process that releases energy as it takes place
  is Exothermic.
• Changes in heat are measured by changes in
  Enthalpy (DH).

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Enthalpy

• In the case of exothermic reactions DH is (-)
  since the final state has less heat than the
  initial state.
• Endothermic reactions have DH that is () since
  energy went into the system.
• DH is the change in heat at standard
  conditions.
• Standard conditions are
• 25C, 1atm pressure, 1M, pure substances.

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Why do we care about energy?

• Every process involves energy.
• Energy costs money.
• Production of energy depletes natural resources.
• I have a molecule where is the energy found?
• It can be said that energy is stored in the bonds
  holding the molecule together.
• This implies that when bonds break they release
  energy that is not the case.
• It costs energy to break a bond.

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Energy in Chemicals

• Energy is released when bonds are formed.
• While it may cost energy to break a bond, when a
  bond is formed after that we hope we obtain more
  energy in return.
• The energy transferred in a chemical reaction
  depends on three factors
• Starting materials.
• Products made.
• Amounts of chemicals.

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Spontaneity

• Reactions that occur without the help of humans
  are said to be spontaneous.
• Some reactions need help to get going but no help
  after that.
• Combustion of gas requires us to provide the
  initial spark but after that continues without
  our help.
• Reactions that must continuously need human
  intervention are non-spontaneous.
• Water will not boil if we do not continuously add
  heat.

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Kinetics

• Why do we have to provide energy for spontaneous
  reactions at the beginning?
• Thermodynamic is only one important part of the
  process.
• Kinetics explains the reason for the initial
  energy.
• Imagine you want to slide a heavy desk
• It takes more energy to start the moving than to
  continue the process.
• The initial energy is the activation energy
• Activation energy is the minimum energy required
  to start a reaction.

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Kinetics

• Kinetics studies how fast reactions take place,
  rate of reaction.
• There are some factors that affect this rate
• Temperature
• Pressure
• Concentration
• Catalyst

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Temperature Kinetics

• As we said before Temperature is a direct
  representation of molecular energy.
• High temperature means a high rate of energy
  transfer to the thermometer or vessels walls.
• This in turns means that many collisions are
  taking place.
• Which means that particles are traveling fast.
• When molecules travel fast and collide they will
  have enough energy to overcome the activation
  energy

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Pressure Kinetics

• Pressure is a measurement of force in a given
  area.
• If the pressure increases that means there is a
  higher force being applied to the same area.
• This in turn means the molecules are pushing
  harder.
• Only way for molecules to push harder is to
  collide with the wall harder.
• This means molecules are traveling faster see
  previous slide to see how this affect reactions.

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Concentration Kinetics

• As the number of molecules in a reaction vessel
  increases the chance of collisions increases.
• If there are no cars on the road other than your
  own the chance you will run into a car is rather
  low if there are 300 cars on the road the chance
  of running into one increases
• More collisions means a higher probability of
  having a collision with the correct amount of
  energy to over come Ea.

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Catalysts Kinetics

• Catalysts are chemical substances that help speed
  up a reaction but are not used during the
  chemical process.
• They are used in one of the steps but are
  salvaged in a later step.
• Amount of catalyst at start and end must be the
  same.
• Amount of catalyst during the reaction may be
  lower than at start.
• Catalysts work by lowering the activation energy.
• Lower activation energy means less energy
  required to start the actual reaction.