Lecture VI

Physics 367

Chemical Energy

Chemical Energy is the root source of energy used for

heat,
transportation and
industry

The electric energy we use comes in large part from chemical processes. Many of the chemical compounds used to produce energy are burned and involve chemical changes:

gasoline

powers cars and trucks

kerosene

jet fuel

natural gas

heats homes

coal and fuel

generate electricity

A basic understanding of chemical change is thus central to any discussion of

energy

energy production.

Atoms

The atom is the building block of all molecules and complex chemicals.

Atoms are composed of:

protons

neutrons

electrons.

An atom is about the same size as a molecule:

10^-10 m in diameter.

The nucleus of an atom contains

protons

neutrons.

The protons and neutrons in an atom are concentrated in the atom's center.

The atom is mostly empty space.

The chemical behavior of atoms is determined by the number and the configuration of the electrons in the atom.

Why should one bother to discuss the nucleus?

The number of protons in the nucleus determines the number of electrons in the atom.

More on Atoms

Most atoms in the world around us are electrically neutral. They have no net charge.

protons are positively charged

electrons are negatively charged

charge of a proton = -charge of electron

The presence of 10 protons in a nucleus the presence of 10 electrons to balance the charge.

The same is true of any nucleus.

The simplest atom, hydrogen, has just one proton in the nucleus.

The next atom has two protons in the nucleus...

Atoms are designated by symbols [see periodic table].

Hydrogen's symbol is H. Other important atomic symbols are carbon, C, nitrogen, N, and oxygen, O.

A bit of the Periodic Table (!!!)

H							He
Li	Be	B	C	N	O	F	Ne

Atoms can also be designated by the number of protons in them (the proton number or atomic number is given the symbol Z).

The number of neutrons--or, actually, protons plus neutrons--is also important. The mass number, A, is the total number of protons and neutrons in an atom's nucleus.

The type of atom is then determined by giving its Z and A. Chemical notation is redundant.

Example:

ordinary hydrogen is designated

ordinary carbon is designated

Z = 1 already tells us that it's hydrogen;
Z = 6, that it's carbon.

The properties of atoms are determined by Quantum Mechanics.

First three Hydrogen orbits

Quantum Theory of the Atom

Why do we see atomic spectra?
Why are there stationary states?
Why is there quantum mechanics?

electrons have wave properties
deBroglie:

for electron to be in a stable orbit
Bohr:

Bohr:

or
Bohr: angular momentum is Quantized

what is the energy of the stationary state
Balance Forces:

Coulomb:
Newton:

Radius:

or:

Energy:

Bohr: radius and energy are Quantized

Types of Chemical Reactions

The formation of a compound occurs when the compound is more stable than the constituents were when they were alone. There are two different sorts of chemical reactions

those that take in energy - endothermic

those that give off energy - exothermic

Exothermic reactions result when the chemical bonds between atoms in molecules are reformed or when the overall result of a chemical reaction is the making of bonds.

The reactants fall into their most stable available state by giving up energy. These bonds all involve atomic electrons. The nuclei of the constituents remain unchanged.

Problem of the day

What is the residence time of H₂O in the Earth's atmosphere?

Once again we will use a steady-state model....so what do we expect?

And what do we need?
The amount of water M in the atmosphere is: 1.3x10¹³m³
The global precipitation rate F_w is: 5.2x10¹⁴m³/yr

Then the residence time is:

T = M/F_w

T = 1.3x10¹³m³/5.2x10¹⁴m³/yr

T= 0.025 yr

T = 9 days

Does this agree with what we expect?