Transcript ch 4 notes

All This STOICH has got me really
burned up!!!
Important Terms!!!
Solute – The substance that dissolves.
Solvent – The substance that something dissolves in.
Solution – The two together.
Concentration – Amount of solute per given amount of solvent.
Water is considered by chemists to be
the Universal Solvent.
• This is great when trying to mix up a solution
but not so great when trying to keep our rivers
and oceans clean.
• The bond angle of water
is 105°
• The oxygen end of water
is very negative making
the H end slightly
positive
The greek letter delta (δ) indicates a
partial charge.
• Water is a POLAR molecule. A polar molecule
is a molecule having an unequal charge
distribution. One side is + and the other -.
• The polarity of water gives H2O it great ability
to dissolve compounds.
• The δ+ end of H2O are attracted to the –
charged anion AND the δ- of H2O are
attracted to the + charged cations (unlikes
attract!!)
• Hydration: the interaction
between solute particles and
water molecules.
• Water molecules surround
ions or molecules due to
electrostatic attraction
between opposite charges.
Draw a molecule of sodium chloride showing
the electron arrangement around the atoms.
Simulations
http://phet.colorado.edu/en/simulations/category/chemistry
1. Salts and Solubility
2. Sugar and Salt Solution
Draw a diagram showing the hydration
of water with NaCl
Hydration Number
• Hydration number is the number of water
molecules associated with a particular ion.
(Usually, it requires 4 – 6 water molecules for
a salt to “fall apart” or dissolve or dissociate)
• When salts (ionic compounds) dissolve, they
break into their cations and anions.
• NH4NO3 → ____________ + _____________
If the molecule is polar, the water
molecule will dissolve it.
• If nonpolar, water will NOT dissolve it!
• Ethyl alcohol and water (show how water is attracted to ethyl
alcohol)
• Many substances do not dissolve in
water.
• Water will NOT dissolve animal fats, oils,
etc. because they are Nonpolar. This
means that this molecule does NOT
have slight positive and negative
charges and thus do not interact with
H2O molecules.
A useful rule for predicting solubility…
• Like Dissolves Like
• In other words, polar dissolves polar or
nonpolar dissolves nonpolar.
• In general, polar and ionic substances are
soluble in water.
Electrolytes
• A solution is a homogeneous mixture
(meaning it has the same composition
throughout) with dissolved solute in a
solvent.
• Some solutions will conduct electrical
currents – this is called electrical
conductivity .
• A solution that conducts an electrical current
is called an ELECTROLYTE. There are three
types of electrolytes.
Three Types of Electrolytes
1. Strong Electrolytes – conducts an
electrical current efficiently
2. Weak Electrolyte – conducts a small
electrical current.
3. NonElectrolyte – conducts NO
electrical current.
The extent to which a solution
conducts an electrical current
depends on the NUMBER of dissolved
ions in that solution!!
• In other words…if there are LOTS of
dissolved ions floating around in the
solution, then the solution will conduct
electricity very well!!! (strong electrolyte)
• Remember that an electrical current
is flowing electrons. The electrons
can be transported by ions so that
the greater the number of ions, the
greater the amount of current!
1. Strong Electrolytes
• Depends on the solubility – the amount of a
substance that dissolves in a given volume of
solvent at a given temperature.
• Three Types of Strong Electrolytes
1. Soluble salts
2. Strong acids
3. Strong bases
A. Soluble Salts
• When salts dissolve in water,
they dissociate completely so
that large numbers of ions are
present to carry the current.
• The more ions, the greater the
conductivity.
B. Strong Acids…
• This work was done by the Swedish chemist
Arrhenius – a Nobel prize winner for his work
with acids.
• HCl →
+
+1
-1
• HNO3 → H + NO3
• H2SO4 → 2H+1 + SO4 -2
+1
H
-1
Cl
All of these – HCl, HNO3 and H2SO4 –
have 100% dissociation.
• This means that for 100 molecules of HCl that
are put into a solution, 100 H+1 and 100 Cl-1
will be produced.
• So, Arrhenius proposed that an ACID is a
substance that produces H+1 ions (protons) in
aqueous solutions.
• HCl, HNO3 , and H2 SO4 are all strong acids and
therefore strong electrolytes.
C. Strong Bases
• Strong bases contain OH-1 (hydroxide) and
completely dissociate (like strong acids)
• The 2 most common strong bases:
NaOH →
KOH →
• Weak acids are weak electrolytes
• Acetic acid (vinegar)
HC2H3O2 → __________ + ___________
• With weak acids, only a relatively small percentage of
molecules dissociate (compared to 100% for strong acids).
So, for a weak acid (HX) that dissociates 1%, if 100
molecules of weak acid were dissolved in water,
approximately 99 molecules remain intact and one will
dissociate into H+1 and X-1 . So…weak acids
dissociate only slightly in aqueous
solutions!
Three examples of weak acids…
1. Benzoic acid
HC7H5O2 →
2. Carbonic acid
H2CO3 →
3. Hydrosulfuric acid
H2 S →
B. Weak Bases
• Weak bases also dissociate to small extents.
The most common weak base is ammonia
(NH3)
• NH3 + H2O → _________ + __________
Note: Ammonium ion – NH4+
Ammonia – NH3 a polar compound.
Concentrations of Solutions
• There are three common types of concentrations:
1. MOLARITY =
2. MOLALITY =
3. NORMALITY = omit this!!!! Equivalents / liter
Similar to M but, specific to a given
reaction!
Molarity is the most commonly used
concentration in the happy world of
chemistry.
• A 1-Molar solution (or 1 M) of NaCl would
contain 1 mole of NaCl in 1 Liter of water.
• A 2-Molar solution would contain 2 moles of
solute in 1 Liter of water.
Or 1 mole of solute in 0.5 liter
of water
Java - molarity
The Exciting Molarity formula
M = #moles solute / #liters solution
A 0.5M solution of NaCl means:
0.5 mol NaCl
1 L solution
Notice that this is a conversion factor between
moles solute and liters solution!
We can use it to convert a given
volume to number of moles or given
moles to volume!!
Calculate the Molarity of a solution prepared
by dissolving 11.5 g of solid NaOH in enough
water to make 1.5 L of solution.
Calculate the Molarity of a solution prepared
by bubbling 1.56 g of HCl into enough water to
make 26.8 mL of solution.
A studious honors chem student dissolves
63 g of C6H12O6 in 700 mL of water. Find
the M.
How many molecules of glucose are present in
200 mL of a 0.5-Molar solution?
How many grams of potassium chromate are
needed to make a 0.2-M solution in 250 mL of
water?
How much water is needed to produce a 0.4 M solution
of 24 g of NaCl?
Try This One!!
27.5 g of a solute is dissolved in 250 mL of water.
The resulting solution is found to be 0.73 M.
What is the formula weight of the solute?
Calculate the mass of lead (II) iodide formed when 0.5
Liters of a 0.1 M solution of lead (II) nitrate reacts with
excess KI.
1. Write the balanced equation
2. Find the moles of lead II nitrate in solution
3. Determine the mole ratio of PbI2 formed.
4. Determine the grams of ppt formed.
Determine the amount of silver replaced when 100
mL of a 0.2 M solution of silver nitrate reacts with
excess Ca.
Ionic Reactions
• Ionic Reactions are important to
chemists because they actually
show or tell the chemist what is
happening in that reaction or in
that beaker.
Think about what is actually
happening in the beaker!
Na3PO4 → 3Na+ + PO4-3
Mg(NO3)2 → Mg+2 + 2NO3What happens next?
Sodium phosphate and magnesium nitrate
yields magnesium phosphate and aluminum
nitrate
• STEP 1: Write the balanced equation:
• This is called the Molecular equation: it gives
the overall reaction stoichiometry but not
necessarily the actual forms of the reactants
and products in the solution.
STEP 2: Identify the insoluble
product
• Use the solubility rules or the chart on the
back of the Chem Helper
• The insoluble product is _________________
Step 3: Write the complete ionic
reaction
• Complete ionic reaction: represents all
reactants and products that are strong
electrolytes as ions.
Step 4: Identify the spectator ions
• Spectator ions are ions that do not participate
in the reaction and are called spectator ions
(ions that are the same on both sides)
• The spectator ions in this reaction are
___________ and ___________.
Step 5: Write the net ionic
equation
• Net ionic equation includes only those
components undergoing a change. Spectator
ions are never included (only the ions forming
a solid are included!!!)
The exciting World of
Oxidation & Reduction
• Oxidation = Loss of electrons; the element
becomes more positive.
• Reduction = Gain of electrons; the element
becomes more negative.
• OIL RIG
•
or
LEO the lion goes GER
Oxidation
Reduction
Oxidized
Reduced
Reducing Agent
Oxidizing Agent
Fe + O2 → Fe2O3
Iron goes from 0 charge to +3 – so Fe is
oxidized. (And is the reducing agent).
Oxygen goes from 0 charge to -2 – so O2 is
reduced. (And is the oxidizing agent).
• Oxidation numbers are real or apparent
charges on atoms, ions and molecules.
• They are usually used in Redox equations to
keep track of e- lost and gained.
• You have been using these without knowing
it!!! The “II” in Fe(II) and the IV in Ti(IV) are the
oxidation numbers for the ions!
• Charges are “apparent” in covalent
compounds – since we know there are no ions
present.
Rules for Assigning Oxidation
Numbers
1. Oxidation number of a free element = 0.
2. The oxidation number of a monoatomic ion
is equal to its charge.
3. The algebraic sum of the oxidation numbers
of the atoms in a compound = 0.
4. Hydrogen in a compound = +1
5. Oxygen in a compound = -2
6. Algebraic sum of a polyatomic ion is equal to
its charge.
Quick and dirty RULES
1.
2.
3.
4.
5.
6.
Free elements = 0
Monoatomic ions = charge
Compounds = 0
H = +1
O = -2
Polyatomic ions = charge
EXAMPLES
Zn
Zn+2
Cl2
Cl-
P2O5
PO4-3
More fun examples of oxidation numbers…
NH3
NH4+
H2
HBr
O2
NaClO4
CH4 + 2O2 → CO2 + 2H2O
Note that Carbon goes from -4 to +4. Carbon
must lose electrons.
Carbon undergoes oxidation
Carbon is oxidized
Carbon is the reducing agent (e- donor)
Note that Oxygen goes from 0 to -2
(-8 total)
• Oxygen must be gaining electrons
Oxygen undergoes reduction.
Oxygen is reduced.
Oxygen is the oxidizing agent.
Write the REDOX equations…
• Oxidation:
• Reduction:
2Al + 3I2 → 2AlI3
• Oxidation Equation:
• Reduction Equation:
PbO + CO → Pb + CO2
• Oxidation equation:
• Reduction Equation:
End Here or Continue!
Half Reaction Method of Balancing Redox Equations in an acid
FIVE STEPS:
1. Write the oxidation and reduction half reactions.
2. For each half-reaction:
a. balance all elements except H and O
b. Balance O using H2O
c. Balance H using H+1
d. Balance the charge using e3. Multiply to get the number of e- equal.
4. Add the half reactions and cancel identical species.
5. Check that the elements and charges are balanced.
MnO4-1 + Fe+2 → Fe+3 + Mn+2
H+1 + Cr2O7-2 + C2H5OH → Cr+3 + CO2 + H2O