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Chemistry

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This is how fireworks work. ... The energy transferred in a chemical reaction depends on three factors: Starting materials. ... – PowerPoint PPT presentation

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Title: Chemistry


1
Chemistry Society
  • Chapter 5 Energy

Virginia State University Fall 2005 Dr. Victor
Vilchiz
2
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

3
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

4
Sources of Energy
  • Sun
  • Windmills
  • Waves
  • Hydroelectric plants
  • Food
  • Fuels
  • Nuclear
  • Alpha Decay
  • Beta Decay
  • Gamma Radiation

5
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.

6
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.

7
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.

8
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.

9
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.

10
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

11
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.

12
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

13
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14
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.

15
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).

16
Water wave (ripple).
Amplitude is not a characteristic of a
wave. Amplitude determines how bright or dimmed
the light is
17
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.

18
The photoelectric effect.
19
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.

20
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.

21
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).

22
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.

23
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.

24
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.

25
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.

26
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.

27
Kinetics
  • Kinetics studies how fast reactions take place,
    rate of reaction.
  • There are some factors that affect this rate
  • Temperature
  • Pressure
  • Concentration
  • Catalyst

28
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

29
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.

30
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.

31
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.
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