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Chapter 7
Chemistry in Action
Chemical Reactions
• A chemical reaction is a chemical change
resulting from a collision of atoms or molecules.
• The original substances are reactants
• The substances produced by the reaction are
called products
for example: carbon can collide with
oxygen and make carbon dioxide
Chemical Equation:
C + 2 O = CO2
Conservation of Mass
In any chemical reaction, atoms are conserved…
That is, the same number of atoms used in the
reaction is the same number of atoms in the
products.
This is conservation of mass (or matter)…Matter is
not created or destroyed, it just changes form.
Energy in Chemical Reactions
• Energy Changes:
Some reactions “need” energy to occur and
therefore absorb heat during a reaction – these
are endothermic reactions
Reactants + energy → products
Some reactions “give” energy in a reaction and
therefore give off heat – these are exothermic
reactions
Reactant → products + energy
Energy in Chemical Reactions
Heat and other natural processes in a system always tend
toward less usable energy and greater disorder…
This is known as the
second law of thermodynamics
When you eat something, only about 55% of energy is
actually used…the rest is converted into heat and “lost”
to your body
This energy cannot be used again….
Energy in Chemical Reactions
Entropy:
The amount of decay or disorder in a
system
According to the 2nd law of
thermodynamics, entropy always tends to
increase; a decrease in entropy in one
place requires a greater increase of
entropy somewhere else.
Factors Affecting Rates of
Reactions
• Much of practical chemistry involves
identifying the reaction characteristics of
different substances so that reactions
between them may be conducted safely
and efficiently in the laboratory and
industry!
Factors Affecting Rates of
Reactions
1. Temperature – an increase in
temperature will usually speed up a
chemical reaction.
This is because the rising
temperature causes the atoms to move
more quickly.
Factors Affecting Rates of
Reactions
2. Concentration – Increased concentration
increases the rate of reaction.
This is because, the more
atoms that are squeezed into a limited
space, the more likely they are to react
with each other.
Factors Affecting Rates of
Reactions
3. Surface Area – increasing the surface area of
the reactants increases the speed of the
reaction.
This is because there is more area
over which the molecules can collide and react.
Example: Grinding a substance into a
powder allows it to burn more easily!
Factors Affecting Rates of
Reactions
4. Catalysts – A catalyst is a substance that alters
the rate of a chemical reaction without being
permanently changed in the reaction.
Catalysts change the activation energy
of the reaction!!!
Your body has thousands of catalysts
called enzymes that God created to keep your
body running smoothly!
Types of Chemical Reactions
There are four basic categories of chemical
reactions:
1. Combination Reactions
2. Decomposition Reactions
3. Single-replacement Reactions
4. Double-replacement Reactions
Combination Reactions
• Chemical reactions which combine two or more
substances to form a third substance.
• Also called synthesis reactions.
A + B → AB
Example:
C + O2 → CO2
Decomposition Reactions
• Complex compounds break down into
different substances.
AB → A + B
Example:
CaCO3 → CaO + CO2
Single-Replacement Reactions
• One element is replaced by another
element.
A + BC → AC + B
Example:
2 NaBr + Cl2 → 2 NaCl + Br2
Double-Replacement Reactions
• Two compounds swap ingredients.
• Two compounds react to form two new
compounds.
AC + BD → AD + BC
Example:
AgNO3 + NaCl → NaNO3 + AgCl
Reversing a Reaction
• Some chemical reactions are reversible.
• These reactions can be made to go back
the other way.
Salts
• A salt is a general term that refers to an
ionic compound.
• When salts dissolve in water, they
“dissociate” (physically separate) in
component ions.
• So in water, NaCl is really Na+ ions and
Cl- ions.
Acids and Bases
Acids produce hydrogen ions (H+) in solution…
A acid has a pH of 1 to 6
Bases produce hyroxide ions (OH-) in solution
A base has a pH of 8 to 14
Something neutral has a pH of 7…examples are milk and
salts
Human blood is a weak base…it has a pH of 7.3
pH scale
The pH Scale
Organic Chemistry
• Organic chemistry has to do with any
compound containing carbon.
• Organic compounds include: fuels,
foodstuff, paper products, cosmetics,
plastics, soaps, fabrics, and paints.
Carbon is Special!
•
More compounds are formed with carbon
than all other elements combined!
• Why?
1. Carbon forms four covalent bonds.
2. Carbon can bond to form several
different shapes.
1. Rings
2. Chains
3. Three dimensional shapes
Carbon is Special!
3. Carbon atoms may form single, double, or triple
bonds.
4. Carbon may form single and double bonds with
the atoms of many other elements.
5. Carbon may form compounds that contain
different structural arrangements and
combinations with the same molecular formula.
isomers: carbon compounds having
the same molecular formula but different
structural formulas.
Hydrocarbons
• Compounds that contain only hydrogen
and carbon are called hydrocarbons.
• Most industrial compounds are
hydrocarbons.
Naming Hydrocarbons
• Hydrocarbons are named according to the
number of carbons in the molecules and
the arrangement.
• The first part is the prefix…it is based on
the number of carbon atoms.
Naming Hydrocarbons
• Alkanes: Hydrocarbons with one bond are
called alkanes.
– Example: propane, petroleum jelly
• Alkenes: Hydrocarbons with one or more
double bond are called alkenes.
– Example: beta carotene (vitamin A), olive oil
Naming Hydrocarbons
• Alkynes: Hydrocarbons with one or more
triple bond are classified as alkynes.
– Example: used in manufacture of plastics
• Cyclic hydrocarbons: ring shaped carbon
molecules.
– Example: cyclohexane
• Aromatic hydrocarbons: ring shaped, but
there are no double or triple bonds.
– Example: benzene, TNT, mothballs
Substituted Hydrocarbons
• Hydrocarbons that have one or more
hydrogen atoms replaced by different
atoms or groups of atoms are called
substituted hydrocarbons.
• The substituted group is called a functional
group.
• Substituted hydrocarbons include:
haloalkanes, alcohols, caboxylic acid, and
esters.
Haloalkanes
• The hydrogen atoms are replaced by one
of the halogens (chlorine, fluorine,
bromine, iodine, or astatine)
• Example: freon and methane
Alcohols
• One or more of the
hydrogens are
replaced by an OH
group.
• Example: methanol
made from methane.
Carboxylic Acids
• One or more hydrogen atom replaced by a
carboxyl group (COOH)
• Example: acetic acid
Esters
• The insertion of an ester group.
• An ester group is –COO• Most have pleasant odors and are used
for perfumes and flavor enhancers.
Soaps
• Soaps are a useful organic compound in
that it can dissolve non-polar substances.
Polymers
• Polymers are synthetic materials and have
been useful in plastics and synthetic
fibers.
• They are long molecular chains that link
many smaller molecules together.
• Common polymers are polyesters,
polystyrene, and teflon.