Transcript Ch. 4 Chemical Reactions Homework: 4.17, 4.18, 4.30, 4.33, 4.43, 4.45, 4.49, 4.55, 4.60, 4.62, 4.63, 4.73, 4.78, 4.100

Ch. 4 Chemical Reactions
Homework: 4.17, 4.18, 4.30, 4.33,
4.43, 4.45, 4.49, 4.55, 4.60, 4.62,
4.63, 4.73, 4.78, 4.100
Chemical Reactions
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In a chemical change, also known as a
chemical reaction, one or more of the
starting materials, called reactants, are
converted into one or more products
There are 3 aspects of a chemical reaction:
1) Mass relationship
2) Types of Reactions
3) Heat gains and loses
Background for Mass relationships
• Formula Weight- the sum of the atomic
weights in amu of all atoms in the
compound’s formula
• Formula weight applies to ionic
compounds, and covalently bonded
molecules
• Molecular Weight- means the same thing
but only applies to molecules
The mole
• Mole- the amount of substance that
contains as many atoms, molecules, or
ions as there are atoms in 12 grams of
carbon 12.
• A mole always contains the same number
of formula units
• That number, known as Avogadro’s
Number, is 6.02 x 1023
Molar Mass
• Molar Mass- the mass of one mole of any
substance is the formula weight expressed
in grams.
• To convert between grams and moles, we
use the Molar Mass as the conversion
factor:
Writing Chemical Equations
• We use the formulas for the reactants and
products
• We use an arrow to show the direction of
the reaction
• We also designate the state of all
reactants and products!!
– (s)=solid, (l)=liquid, (g)=gas, (aq)=aqueous
– (aqueous means the substance is dissolved in
water)
• Example:
• Chemical Equations must be balanced!!!!
• This means there must be the same
number of atoms on the left as on the right
• To balance equations, we put coefficients
in front of the formulas until we have the
same number of each type of atom on
both sides.
Guidelines for balancing Eq.
1) Begin with atoms that appear in only one
compound on the left and right
2) If an atom occurs as a free element,
balance this element last
3) You can only change coefficients, NOT
FORMULAS!!!!!
4) A balanced equation is always written
with the lowest possible set of
coefficients.
Examples
Stoichiometry
• Stoichiometry- the study of mass
relationships in chemical reactions
• The balanced equation gives us the molar
ratios of the reactants and products.
• We can use this information to calculate
the mass of starting material needed to
produce a certain amount of product or
vice versa.
• Basic Process=
• Example=
Limiting Reagent
• The limiting reagent is the reactant that will
be used up first.
• Example-
Percent Yield
• Actual Yield- the mass of a product
formed in a chemical reaction that is
physical carried out
• Theoretical Yield- the mass of product
that SHOULD form according to the
stoichiometry of the balanced equation
• Percent Yield- The actual yield divided by
the theoretical yield time 100%
Combination Example
• A student combusted 6.0 grams of C3H8 in
the presence of 30 grams of O2. 7.2
grams of water was captured. What was
the percent yield for the reaction?
Ionic Reaction in Aqueous
Solutions
• When positive and negative ions are
placed in water, the ions are separated
• This is called dissociation
• Ions in water react with each other in
solution only when one of the following
can happen:
Types of Reactions
1) Two ions form a solid that is insoluble in
water
2) 2 ions form a gas that escapes from the
reaction mixture as bubbles
3) An acid neutralizes a base
4) One of the ions can oxidize the other
Example
• Say we mix a solution of NaCl with a
solution of AgNO3
Ion Solubility Rules
1) All compounds containing Na+, K+, or NH4+
are soluble in water
2) All nitrates(NO3-) and acetates(CH3COO-) are
soluble in water
3) Most Chlorides(Cl-), and Sulfates(SO42-) are
soluble in water. EXCEPTIONS: AgCl,
BaSO4, PbSO4
4) Most Carbonates(CO32-), phosphates(PO43-),
sulfides(S2-), and hydroxides(OH-) are
insoluble in water. EXCEPTIONS: LiOH,
NaOH, KOH, NH4OH
Example
• Will a reaction occur if a solution of
Na2CO3 and a solution of LiCl are mixed
together?
Oxidation Reduction Reactions
• Oxidation- is the loss of electrons
• Reduction- is the gain of electrons
• And Oxidation/Reduction reaction, aka
Redox reaction, involves the transfer of
electrons from one species to another.
Example
• These reactions are not independent of
one another
• Something can’t gain electrons from
nowhere!!!
Organic Definitions
• In organic Chemistry, it is very hard to figure out
what lost electrons and what gained them, so
we use different definitions:
• Oxidation- a reaction in which there is an
increase in the number of bonds to Oxygen
and/or a decrease in the number of bonds to
Hydrogen.
• Reduction- a reaction in which there is an
increase in the number of bonds to Hydrogen
and/or a decrease in the number of bonds to
Oxygen.
Categories of Redox Reactions
1) Combustion
-What ever is burned is oxidized by O2
2) Respiration
-O2 oxidizes carbon containing molecules to produce
CO2 and water, much like a combustion but
much slower and lower temperature
3) Rusting
-Iron is oxidized to Iron Oxide
4) Bleaching
-Most bleaching involves oxidation
5) Batteries
- The reaction in the battery is a redox
reaction.
Heats of Reactions
• Heat of Reaction- the heat given off or
absorbed by a chemical reaction
• If a reaction gives off heat, it is
exothermic
• If a reaction absorbs heat, it is
endothermic
• Heat given off in a combustion reaction is
called heat of combustion.