The Chemistry of Energy

Energy from the Atom

Energy from the Atom

Energy can be released from the atom in three ways:

1. Radioactivity

2. Chemical Reactions

3. The Excitement of Electrons

Radioactivity

Radioactivity

• In the late 19th century, Becquerel and Marie and Pierre Curie discovered that certain elements emit radiation

• Radioactivity is the property of elements that emit radiation

• Radiation is the stream of particles that are actually emitted from a radioactive substance

Types of Radiation

• Unstable nuclei emit three common types of radiation:– Alpha Rays () rays:

heavy positive charge– Beta () rays: light

negative charge– Gamma () rays:

neutral particles

β

What Causes Radiation?• Certain isotopes have unstable nuclei• In order to become stable, they must emit a

particle (decay)• The ratio of neutrons to protons is largely

determinant of the stability of the nucleus and the tendency for radioactive decay to occur– The higher the ratio of neutrons to protons, the more

likely the isotope is radioactive

• Radioactive decay is spontaneous – we cannot predict when an atom will undergo decay

Alpha Radiation

• Some nuclei decay by emitting an alpha particle• Occurs when atomic nuclei have too many

protons and neutrons (i.e., Are heavy)• Most radioactive nuclei more massive than Pb

will emit Alpha particles during decay.• An alpha particle consists of two protons and

two neutrons (i.e. a helium nucleus)• When an alpha particle is emitted, the atomic

number of the element is reduced by two while the atomic mass is reduced by four

Americium-241 (241 = atomic mass) becomes Neptunium-237 after emitting an alpha particle

Alpha Radiation

Beta Radiation

• Some nuclei decay be emitting a beta particle • Occurs when atoms have too many neutrons

(i.e., Are “neutron-rich”) and decay by emitting a negative beta particle (ß-)

• When a neutron emits a beta particle, the neutron loss its neutrality and become a proton

• The atomic number will increase by one but the atomic mass will remain about the same.

Beta Radiation

When tritium decays, a neutron becomes a proton and an electron is emitted

Gamma Radiation• Some nuclei decay by emitting a gamma

particle• A gamma particle is just pure energy (a photon)• Helium-3 has too much energy, so it emits a

gamma-ray photon• Note that the atomic mass and atomic number

do not change with Gamma Radiation

Transmutation• When an atom decays by alpha or beta

radiation, it transmutates or changes into a different element

Half-life

• The time it takes half of the atoms in a radioactive sample to decay is the half-life.

Half Life Problem

• Question: Actinium (Ac) 227 has a half-life of 13.5 years. If an object contains 120mg of this radioactive substance, how much will be left after 54 years?

• Answer: 7.5 mg

Half Life Problem

The amount of Ac-227 is cut in half every 13.5 years (the half life). Since it takes four half lives to get to 54 years (54 / 13.5 =4), the original 120 mg amount would be cut in half four times:

120mg/2 = 60mg (1 half life)60mg/2 = 30mg (2 half lives)30mg/2 = 15mg (3 half lives)

15mg/2 = 7.5mg (4 half lives)

Activity of Radioactive Substances• Activity: The number of atoms that

disintegrate per second in a radioactive substance.

• Measured in Curies (Ci) which is equal to 37 billion atoms decay in one second

• The activity depends on the number of nuclei present and the half-life of the substance.– Activity increase with a shorter half life

and or a larger amount of the radioactive substance.

Binding Energy

• The binding energy is a measure of the force holding the nucleus of an atom together.– The nucleus is made up of protons that are

positive and neutrons that have no charge. – Because similar charges repel each other,

binding energy holds the positive protons together in the nucleus

• The higher the binding energy, the more tightly bound is the nucleus, the more potential energy available during nuclear reactions.

Energy in Nuclear Reactions

• There is a tremendous amount of binding energy stored in nuclei.

• Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.

• During radioactivity, a small amount of this potential binding energy is converted to kinetic nuclear energy when a particle is released from the nucleus.

Energy in Nuclear Reactions• When uranium-238 undergoes decay it emits

0.0046 g of mass.

• The change in energy, E, is then

E = (m)c2

E = (4.6 10−6 kg)(3.00 108 m/s)2

E = 4.1 1011 J• This is equivalent to the energy of 20 tons of coal

from one emission!• There can be BILLIONS of emissions per

second!!!!

Chemical Reactions

Chemical Reactions

• A chemical reaction is a change in matter in which different chemical substances are created by forming or breaking chemical bonds.– Chemical bonds are formed when atoms of

elements bind together to form compounds.– Chemical bonds are broken when a

compound is decomposed into simpler substances.

Chemical Reactions

• Every chemical bond contains a definite amount of chemical energy, which can be large or small depending on the types of atoms in the bond.

• When you burn wood in a campfire, high-energy chemical bonds in the wood are broken and low-energy chemical bonds with oxygen (from the air) are formed.

• The net difference in chemical energy is the reason you built the campfire in the first place--it comes out as heat and light.

Endothermic Reactions

• Reactions, which take in energy, are called endothermic.

• More energy is required to break bonds than is released by forming bonds

A + energy B• This results in an decrease in the temperature of

the surroundings.• Cooking an egg or baking bread are examples of

endothermic reactions

Endothermic Reactions

• Think of the endothermic reaction that takes place in "cold-packs." 

• A seal is broken that separates two containers with the plastic bag.  As the contents from the separate containers begin to react, energy is absorbed from the surroundings. 

• If you place the cold-pack on your body, your body begins to supply some of the energy that is required to get the reaction going.  What you experience as "cold" has to do with the temperature of that area of your body changing as heat flows to the cold-pack.

Photosynthesis• Photosynthesis is an example of an endothermic

chemical reaction. • In this process, plants use the radiant energy from

the sun to convert carbon dioxide and water into glucose and oxygen.

Energy + 6CO2 + H2O = C6H12O6 + 6O2

• The radiant energy is stored in the glucose in the form of chemical energy

Exothermic Reactions

• Reactions, which release energy, are called endothermic.

• More energy is released by forming bonds than is required to break bonds

A B + energy

• Results in an increase in the temperature of the surroundings.

Exothermic Reactions

• When you stand next to a barbecue grill, you feel the heat being released by the combustion reaction that is taking place around the burners. 

• The reaction of the propane gas found in grills is shown below:

• C3H8 + 5O2 ---> 4H2O + 3CO2 + energypropane + oxygen  yields water + carbon dioxide + energy

Respiration

• When you consume glucose from plants, you body can obtain energy through respiration.

• When you breathe, oxygen reacts with the glucose in the following exothermic reaction:

C6H12O6 + 6O2 = 6CO2 + H2O + Energy

• The stored chemical energy in glucose is released by this reaction with oxygen (oxidation).

Activation Energy

• Reactions that proceed immediately when two substances are mixed together are called spontaneous reactions.

• Not all reactions proceed spontaneously. Some require activation energy to precede

A match won't light spontaneously. You first need to input activation energy (thermal energy via friction when striking the match). After the activation energy is absorbed and the reaction begins, the reaction continues until you either extinguish the flame or you run out of material to react.

Activation Energy

• A chemical reaction will not begin until the reactants have enough energy to break the chemical bonds of the reactants.

• Once the bonds of the reactants are broken, the atoms then begin to form new chemical bonds as the products are formed.

• Activation energy is the minimum amount of energy needed to start a chemical reaction.

Combustion of Fossil Fuels

• Combustion releases chemical potential energy stored in fossil fuels.

• The combustion of all fossil fuels follows a very similar exothermic reaction:

Fuel (any hydrocarbon source) plus oxygen plus activation energy yields carbon dioxide and water and energy.

Fuel + O2 + activation energy = CO2 + H2O + Energy

Combustion of Methane

CH4 + 2 O2 --> CO2 + 2 H2O + energyActivation energy is required to start this EXOTHERMIC reaction

Activation Energy

The Excitement of Electrons

The Excitement of Electrons• Recall that electrons orbit the nucleus in “shells.”

Each electron shell is an energy level.• When energy is added to an atom (if it is heated

for example) the electrons can jump to a higher energy level.

• An electron gains or loses energy only by moving from one energy level to another.

Quantum Leap

• After the electron “leaps” to a new level, radiant energy is released and the electron drops back into its original shell.

• Each time an electron makes a "quantum leap," moving from the temporary higher energy shell to a lower energy shell, it emits a photon of radiant energy at a specific wavelength or frequency.

Atomic Spectra• The atomic spectra refers to the colors of light

emitted from various elements where a quantum leap has been initiated by heat or electric discharges.

• Every element emits a characteristic spectrum of radiant energy when excited.

Neon Lights

• Inside a glass tube there is a gas like neon, argon or krypton at low pressure. At both ends of the tube there are metal electrodes.

• When you apply a high voltage to the electrodes, the neon gas ionizes, and electrons flow through the gas.

• These electrons excite the neon atoms and cause the electrons to leap to a higher shell. This releases radiant energy in the form of red light.

• Only the red “n” in the sign results from from neon.

• The other colors are emitted by other gases in the tubes.

Neon Lights

A tube with neon gas excited by means of a 5000 volt transformer.

Other Gasses Used in “Neon” Lights

Mercury

Nitrogen

Argon

Fluorescent Light

• A fluorescent light works on a similar idea to a neon light but it has an extra step.

• Inside a fluorescent light is low-pressure mercury vapor. When ionized, mercury vapor emits ultraviolet light.

• The inside of a fluorescent light is coated with a phosphor. A phosphor is a substance that can accept energy in one form and emit the energy in the form of visible light.

• In a fluorescent lamp, the phosphor accepts the energy of ultraviolet photons and emits visible light.

Fluorescent Light