To
study for this exam, you should:
a. Go over
this review sheet.
b. Be
familar with every experiment. See textbook or web handouts.
c.
Go over all the problems in the book, consulting your notes if
necessary.
d. Practice
balancing equations.
The
experiments can be found in the book. You did every book problem already in class,
and should have notes about each. There are no redos for Chapter
6 exams so study hard!
The main subject in this chapter is the ideas of molecules and atoms,
and the principle of constant proportions.
This chapter deals with chemical reactions
as opposed to physical
reactions. A physical reaction changes a substance's state from a
gas to a liquid or a solid. A physical reaction can easily be
reversed. Dissolving is another example of a physical
reaction. But a
chemical reaction changes the substance itself. It is this fact
that leads to the theory of atoms and molecules.
A substance is made of molecules, the smallest little pieces of the
substance. If you separate molecules one from
another, that is a physcial reaction, the more they move around the
more you go from solid to liquid to gas.
Each molecule is made of a fixed proportion of atoms.
For example every water molecule is made up of two hydrogen atoms and
one oxygen atom. But if you bust up the molecules of a substance
so that the atoms are no longer connected, you have a chemical reaction
that changes the substance. For example, busting up water
molecules (H2O) makes Hydrogen and Oxygen.
The Law of Constant Proportions
Throughout all the decomposition and synthesis experiments, we learned
that fixed ratios of mass are used in building new substances, and the
same is true when breaking those substances down. For example, in
the case of water, 1 gram of Hydrogen and 8 grams of Oxygen are used to
make 9 grams of water. The fixed ratio is true for volume as
well, but because the density of most substances are different, the
actual ratio of volume is different. For example, for water the
volume ratio of hydrogen to oxygen is 2 to 1, and the mass ratio is 1
to 8.
Chemical reactions sometimes require energy and sometimes give off
energy. For example, decomposing water requires energy, and
synthezing water gives off energy. In this chapter we separated
and synthesized (put together or build a
substance) a number of different substances to explore these
ideas. Sometimes (like for water) decomposition absorbs energy
and
synthesis gives off energy, and sometimes it works the other way.
Balancing chemical equations is a tricky and challenging skill that
makes sure that the total number of atoms in any reaction does not
change. Make sure you understand that 4H2O means 4
water molecules each of which has two hydrogens and one oxygen.
That makes 8 Hydrogen atoms and 4 Oxygen atoms. Get used to that
notation and you will have an easier time balancing equations.
Practice your skills here.
Some terminology and ideas you should know:
- molecule
- the smallest unit of a pure substance.
- atom
- the components that make up molecules. (For example, Oxygen,
Carbon, Sodium, Hydrogen, and many more). Atoms have no molecules
and cannot be broken down further via chemical reactions. They
can only be busted up into smaller pieces by nuclear reactions.
- law
of constant
proportions - the principle that every molecule of a given
substance is made up of a fixed proportion of atoms. This was
discovered by noticing the fixed ratios of mass used in chemical
reactions.
- compound
- a substance made from at least two different atoms.
(e.g., salt or NaCl - made from sodium-Na and chlorine-Cl). The
atoms must combine in fixed proportions. For example 2 Hydrogens
and 1 Oxygen make water, 1 Sodium and 1 Chlorine make salt.
- mixture
- a combination of two substances. The proportions are not
fixed. For
example, I can mix any amount water with ispropanol to get a
water-isopropanol mixture. But to make each molecule of
isopropanol (C3H7OH) I need 3 Carbons, 8
Hydrogens, and 1 Oxygen. To make each molecule of water, I need 2
Hydrogens and 1 Oxygen.
- element
- a substance that contains only one kind of
atom (e.g. H2 - hydrogen gas, or O2 - oxygen
gas). An element can be made of atoms, or molecules (if each
molecule contains more than one atom). Another way to look at it
is: if you cannot separate a substance via chemical reaction into
another substance, then it is an element. Elements may or may not
be compounds. For example, single atoms of Hydrogen or Oxygen are
elements, but they are not compounds.
- decomposition
- the chemical reaction of breaking a substance apart into other
substances.
- synthesis
- the chemical reaction of building a
substance from other substances.
- chemical
reaction - a change in a substance that splits up individual molecules,
thereby destroying the characteristic properties of the substance.
- physical
reaction - a minor change in a substance that moves molecules further
or closer apart, but does not break individual molecules.
- reactant
- a substance or substances that is/are about to undergo a chemical
reaction.
- complete
reaction - a chemical reaction where at least one of the reactants is
all used up. For example, Zinc and Hydrochloric Acid: The
reaction will continue until of the two substances is gone.
- incomplete
reaction - a chemical reaction in which none of the reactants are
all used up. For example, Copper and Oxygen: neither
reactant will be completely used up.