ESSENTIAL REVIEW
LIMITING REACTANT
Limiting Reagent – reactant which limits the amount of product that can be formed from a reaction
* during reaction, sufficient quantities of all reactants are required:
A chemical equation is a chemist’s shorthand expression for
describing a chemical change.
Fe + S → FeS
1 mole Fe 1 mole S → 1 mole FeS
As an example, consider what takes place when iron rusts.
The equation for this change is:
* if the quantity of one reactant is insufficient to completely consume the other reactant during the reaction, the insufficient
reactant (limiting reactant) will determine the quantity of product formed:
Fe + O2 → Fe2O3
In this expression, the symbols and formulas of the reacting
substances, called the reactants, are written on the left
side of the arrow and the products of the reaction are
written on the right side. The arrow is read as “gives”,
“yields”, or “forms” and the plus (+) sign is read as “and”.
When the plus (+) sign appears between the formulas for
two reactants, it can be read as “reacts with”. (The + sign
does not imply mathematical addition)
3 moles Fe
1 mole S
(excess)
(limiting reagent)
→
1 mole FeS
2 moles Fe
3 mole S
(limiting reagent)
→
2 mole FeS
(excess)
*example 1: for the reaction of Fe with H2O, how many moles of Fe3O4 are formed from 4.36 moles of Fe and 5.50 moles
of H2O?
3 Fe + 4 H2O -----> Fe3O4 + 4 H2
STEPS
1. Make sure the equation is balanced
2. Since it is not obvious which reactant is limiting, solve for the quantity of Fe 3O4 produced by making an assumption
that either Fe or H2O is limiting. JUST DO IT!!!:
a) assuming Fe is limiting: (that is, all 4.36 moles of Fe will be used up and converted into Fe 3O4, i.e., the starting
quantity of 4.36 moles Fe will determine the quantity of Fe3O4 formed):
BALANCING
Recall this is a trial and error process
a) Place coefficients to balance #moles of react. and
prod. “Make left match right”
b) In order balance:
-Polyatomics (Balance as one whole thing)
-Metals
-Nonmetals
-Oxygen
-Hydrogen (Except when combustion, do H 1st)
c) NEVER change the subscripts.
d)Ensure coefficients in smallest whole # ratio and
check work.
4.36 mol Fe
1 molFe3O4
3 mol Fe
1.45 mol Fe3O4
b) assuming H2O is limiting: (that is, all 5.50 moles of H2O will be used up and converted into Fe 3O4, i.e., the
starting quantity of 5.50 moles H2O will determine the quantity of Fe3O4 formed):
4.36 mol Fe
1 molFe3O4
3 mol Fe
1.45 mol Fe3O4
3. compare the two quantities of Fe3O4: the smallest of these quantities will be produced,
quantity of Fe3O4 produced: 1.38 mol Fe3O4
limiting reactant: H2O
excess reactant: Fe
PERCENT YIELD
Percent Yield – ratio of the actual or experimental yield over the theoretical or calculated yield multiplied by a
hundred.
actual yield
% yield = ----------------------theoretical yield x 100
MOLAR MASS
The average atomic mass given for an element on the PT
may be (AND SHOULD BE) interpreted as the
Example: what is the percent yield for the reaction if a student reacting 1.5 grams of Cu(NO3)2 with enough
measurement in grams equal to one mole of that atom.
Na2CO3 collects 0.875 grams of CuCO3 after the reaction?
FOR AN ATOM/ION
Cu(NO3)2 + Na2CO3 → CuCO3 + 2 NaNO3
Use PT and round average atomic mass to the nearest
HUNDRETH of a g/mol.
actual yield: 0.875 grams CuCO3
theoretical yield: ?
FOR A COMPOUND
1.5 g Na2CO3
1 mol Na2CO3
2 mol CuCO3
123.56 g CuCO3
0.99 g CuCO3
Find the molar mass in grams of each component
187.57 g Na2CO3
1 mol Na2CO3
1 mol CuCO3
element of the formula. Multiply each elements MM by
0.875 g CuCO3
actual yield
the times it occurs in the formula and then add the sum.
percent yield:
% yield = ----------------------x 100
% yield = ----------------------- x 100= 88% Yield
theoretical yield
0.99 g CuCO3
WATCH SIG FIGS!!!!!!!!!!!!!
i.e. MM of H2O
O’s 15.999 = 16.00 g/mol
Molar
Liters
Solution
Volume of Gas A @
Volume of Gas B @
H’s of
1.0079=
1.01 g/mol
Ratio
A
STP
STP
MMH20 = 2 x 1.01 g/mol
+ 1 x 16.00 g/mol
18.02 g/mol
Molarity
# moles
1L
Molar Mass
Mass A
# grams
1 mole
STP
MOLE MAP
22.4 L
1 mole
Mole A
Molar
Ratio
Mole B
Molar Mass
# grams
1 mole
Avogadro’s #
Avogadro’s #
Molarity
6.022 x 1023
6.022 x 1023
# moles
1L
1 mole
The MOLE
STP
22.4 L
1 mole
Particles A
(atoms, FU, molecules)
1 mole
Molar
Ratio
Particles B
(atoms, FU, molecules)
Mass A
Liters of Solution
B
MOLE MAP TO APPLICATION
-REMEBER that units are your guide.
-REMEMBER that if you do not know where to start go to the mole.
~To switch between different types moles use the mole ratio.
MULTISTEP EXAMPLES
mass A
mass A
mass A
VOL A
mass A
mass A
mass A
mass A
etc.
→ Particle A
→ Solution Vol. A
→ Gas Vol. A
→ Particle A
→ mass B
→ Particle B
→ Solution Vol. B
→ Gas Vol. B
Steps: 2 Steps = MM A / Avogadro’s #
Steps: 2 Steps = MM A / Molarity A
Steps: 2 Steps = MM A / STP Molar Volume
Steps: 2 Steps = Molarity A / Avogadro’s #
Steps: 3 Steps = MM A / Mole Ratio / MM B
Steps: 3 Steps = MM A / Mole Ratio / Avogadro’s #
Steps: 3 Steps = MM A / Mole Ratio / Molarity B
Steps: 3 Steps = MM A / Mole Ratio / MM
SOLUTIONS
A solution is a homogeneous mixture
of two substances: a solute and a
solvent.
-Solute:
substance
being
dissolved; present in lesser
amount.
-Solvent: substance doing the
dissolving; present in larger
amount.
-Solutes and solvents may be of
any phase of matter: solid,
liquid or gas.
FORMING SOLUTIONS
In order for a solution to form, the
solute intermolecular forces
(IMF’s) must be broken as well
as the solvent IMF’s. Then the
solute and solvent form new
intermolecular forces with each
other. If the energy required to
break the IMF’s is much greater
than the energy released when
the new IMF’s are formed, the
solution will not form and thus
the solute is insoluble.
SOLUBILITY RULES
Solubility is a physical property of a pure substance. Many observations over
time have led to some rules (generalizations) about the solubility of
certain salts. These rules are based on the terms soluble, insoluble, and
slightly soluble. Using these rules, we can predict when a particular salt
is likely to be soluble in water, and if we have an unidentified compound
we can design experiments to find out what it is.
Soluble:
• All Nitrates, Acetates, Ammonium,
and Group 1 (IA) salts
• All Chlorides, Bromides, and Iodides,
except Silver, Lead, and Mercury(I)
• All Fluorides except Group 2 (IIA),
Lead(II), and Iron(III)
• All Sulfates except Calcium,
Strontium,
Barium,
Mercury,
Lead(II), and Silver
Insoluble (0.10 M or greater):
• All Carbonates and
Phosphates except Group 1
(IA) and Ammonium
• All Hydroxides except Group 1
(IA), Strontium, Barium, and
Ammonium
• All Sulfides except Group 1
(IA), 2 (IIA), and Ammonium
• All Oxides except Group 1 (IA)
SOLUBILITY
Solutions form between solute and solvent molecules can be predicted due to
similarities between them. “Like dissolves Like,” refers to polar and nonpolar
solvents and solutes.
· Polar solids (this includes ionic solids) dissolve in water because the charged ions
(polar) are attracted to the polar water molecules.
· Nonpolar molecules such as oil and grease dissolve in nonpolar solvents such as
kerosene.
Factors Affecting Solubility
There are three main factors that control solubility of a solute.
(1) Temperature: Generally solubility increases with the rise in temperature and decreases
with the fall of temperature but it is not necessary in all cases. However we must follow
two behaviors: In endothermic process solubility increases with the increase in
temperature and vice versa. In exothermic process, solubility decrease with the
increase in temperature.
(2) Nature of solute or solvent: “Like dissolves Like”
(3) Pressure: The effect of pressure is observed only in the case of gases. An increase in
pressure increases of solubility of a gas in a liquid. For example carbon dioxide is
added to cold carbonated drinks due to pressure.
Thus for gases, as the pressure of the gas above the solution increases, the solubility of the
gas increases. For gases, as the temperature of the solution increases, the solubility of
the gas decreases. For most solids, as temperature increases, the solubility increases.
COLLIGATIVE PROPERTIES
A colligative property is a property that depends on the number of solute particles in the sample.
The vapor pressure of a solution is lower than the pure solvent because the number of solvent
particles on the top layer that can evaporate is lower. Because the vapor pressure is lower, the
boiling point of a solution is always the higher than the pure solvent and the freezing point is
always lower than the pure solvent.
An electrolyte solution, one in which the solute breaks apart into multiple ions which allow electricity
to be conducted, has an even greater change in vapor pressure, boiling point or freezing point
because there are more particles in the solution than molecules added to the solution.
TYNDALL EFFECT
Colloids are mixtures with solute particles large enough
to scatter light. Colloids exhibit the Tyndall Effect,
where light is seen traveling through and spreading
out in the colloid as it travels through it unlike a
solution. The Tyndall Effect can be used as an
indicator to distinguish between a solution and a
colloid
SOLUBILITY CURVES
A solubility curve shows the # of
grams of solute in a saturated
solution containing 100 mL or 100 g
of water at a certain temperature.
Any amount of solute below the line
indicates the solution is unsaturated
at a certain temperature
Any amount of solute above the line in
which all of the solute has dissolved
shows
the
solution
is
supersaturated.
If the amount of solute is above the
line, u, the solution is saturated and
the # grams of solute settled on
the bottom of the container = total
# g in solution – # g of a
saturated solution at that
temperature. (According to the
curve)
Solutes whose curves move upward
w/ increased temperature are
typically solids as the solubility of
solids increases w/ increased
temperature.
Solutes
whose
curves
move
downward
w/
increased
temperature are typically gases as
the solubility of gases decreases
with increased temperature.