Chapter 2
Chemistry Comes Alive
35
2
atoms are bonded together with carbon to form the methane
molecule, but 4H signifies four unjoined hydrogen atoms.
A chemical equation is like a sentence describing what hap-
pens in a reaction. It contains the following information: the
number and kinds of reacting substances, or
reactants
; the
chemical composition of the
product(s)
; and in balanced equa-
tions, the relative proportion of each reactant and product.
In the equations above, the reactants are atoms, as indicated
by their atomic symbols (H, C). Te product in each case is a
beads up into spheres when it sits on a hard surface and why water
striders can walk on a pond’s surface (Figure 2.10b).
Although hydrogen bonds are too weak to bind atoms to-
gether to form molecules, they are important
intramolecular
bonds
(literally, bonds within molecules), which hold different
parts of a single large molecule in a specific three-dimensional
shape. Some large biological molecules, such as proteins and
DNA, have numerous hydrogen bonds that help maintain and
stabilize their structures.
Check Your Understanding
10.
What kinds of bonds form between water molecules?
11.
Oxygen (
8
O) and argon (
18
A) are both gases. Oxygen
combines readily with other elements, but argon does not.
What accounts for this difference?
12.
Assume imaginary compound XY has a polar covalent bond.
How does its charge distribution differ from that of XX
molecules?
For answers, see Appendix H.
Chemical Reactions
Define the three major types of chemical reactions:
synthesis, decomposition, and exchange. Comment on
the nature of oxidation-reduction reactions and their
importance.
Explain why chemical reactions in the body are often
irreversible.
Describe factors that affect chemical reaction rates.
As we noted earlier, all particles of matter are in constant motion
because of their kinetic energy. Movement of atoms or molecules
in a solid is usually limited to vibration because the particles are
united by fairly rigid bonds. But in liquids or gases, particles dart
about randomly, sometimes colliding with one another and inter-
acting to undergo chemical reactions. A
chemical reaction
occurs
whenever chemical bonds are formed, rearranged, or broken.
Chemical Equations
We can write chemical reactions in symbolic form as chemical
equations. For example, we indicate the joining of two hydrogen
atoms to form hydrogen gas as
H
1
H
S
H
2
(hydrogen gas)
reactants
product
and the combining of four hydrogen atoms and one carbon
atom to form methane as
4H
1
C
S
CH
4
(methane)
reactants
product
Notice that in equations, a number written as a
subscript
indicates that the atoms are joined by chemical bonds. But a
number written as a
prefix
denotes the number of
unjoined
at-
oms or molecules. For example, CH
4
reveals that four hydrogen
O
O
O
O
O
H
H
H
H
H
H
H
H
H
(a) The slightly positive ends (
δ
+
) of the water molecules become
aligned with the slightly negative ends (
δ
-
) of other water
molecules.
(b) A water strider can walk on a pond because of the high surface
tension of water, a result of the combined strength of its
hydrogen bonds.
δ
+
δ
-
δ
-
δ
-
δ
-
δ
-
δ
+
δ
+
δ
+
δ
+
δ
+
Hydrogen bond
(indicated by
dotted line)
H
Figure 2.10
Hydrogen bonding between polar water
molecules.
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