Other things

Quiz 1 answers - average 6.0 / 9

Proofs of atoms...

• Brownian motion *alone* is not evidence for atoms...
• Atoms are indivisible?? what about electrons, protons....
• Why are atom ratios evidence?

Atoms and molecules

Molecule: The smallest unit into which a pure substance can be divided and still retain the chemical properties of that substance. [Britannica] Examples $H_2O, CO_2, N_2$

Element: Most pure substances can be decomposed into other pure substances. But a handful (just over 100) pure substances cannot be further decomposed. These non-de-composable substances are the elements.

Atom: The smallest unit into which an element can be divided and still retain the chemical properties of that element. Examples $N, O, He$.

Also, an atom consists of a single central core containing protons and neutrons, and surrounded by a 'cloud' of lighter weight electrons.

Chemical bonds: Atoms can share electrons with other atoms to form a chemical bond. Sometimes ($H-H$, and $O-O$) this results in a weak bond which is easy to break apart. Sometimes ($H-O$) the bond is strong, and harder to break apart.

Chemical reaction: Bonds of one type of molecule are broken, and then the atoms have a chance to re-arrange into other molecules. Breaking takes place by electricity, or by heating, or light, or a catalyst or by putting a chemical in solution. $$2H_2O \to 2 H_2 + O_2$$ in water... $$NaOH + HCl \to NaCl +H_2O$$ If the new bonds are tighter ones, there is some left-over energy that is released as heat. If the new bonds are looser ones, then energy needs to be added to make the reaction happen.

Combustion (burning!): Many elements make strong bonds with oxygen. An "explosion" is a chain reaction in which...

1. Some heat breaks bonds,
2. One or more elements re-combine with oxygen,
3. Which gives off *more* heat,
4. Causing more bonds to break....
$$H_2+2O_2\to 2H_2O$$

See Krulwich's essay on breaking carbon bonds.

Hydrocarbons and carbon-dioxide

"Hydrocarbons" contain hydrogen and/or carbon.

Burning hydrocarbons re-combine with oxygen to give a mix of $CO_2$ and $H_2O$.

Where does the oxygen come from? Why do they combine with oxygen instead of nitrogen?

We should figure out which reactions give the most/least carbon dioxide from...

model hydrocarbon Energy per gram
hydrogen gas, $H_2$ 26 Calories
natural gas:
methane
$CH_4$
13
mixture of things like:
gasoline:
molecules with 5-12 carbons,
~octane $C_8H_{18}$
10
mixture of things like...
coal, ummmm 6

What patterns do you notice?

Which leaves behind least/most $CO_2$ when burning?

Why do we use gasoline for cars?

Complete the Combustion comparison page.

APS water