Predicting products

AP Chemistry Rapid Learning Series - 09

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Predicting Products

AP Ch i t R id Learning Series
Chemistry Rapid L
i
S i
Wayne Huang, PhD
Kelly Deters, PhD
Russell Dahl, PhD
Elizabeth James, PhD
Debbie Bilyen, M.A.

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© Rapid Learning Inc. All rights reserved.

1


Learning Objectives
By viewing this tutorial you will learn…
How to answer the AP Exam
question #4 by:
Predicting Products
Writing Net Ionic
Reactions

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Concept Map
Previous content
Chemistry

New content
Studies

Matter
Undergoes

Chemical
Equations

Shown in

Chemical
Changes

Different
categories of

Written in

Chemical
Reactions
Different categories
can be used to

Predict Products
of a Reaction
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2


What is the AP Free
Response Question #4?

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AP Predicting Products
The AP exam has a question that is unique to the
exam.
It’s always q
y question #4 of the Free Response section.
p
The question has 3 required reactions—there is no choice

Part I gives reactants in word form
It asks for the balanced net ionic reaction given those
reactants
1 point is earned for the correct reactant formulas.
Up to 2 points are earned for the correct product formulas.
1 point for correct balancing
balancing.

Part II asks a question about the reaction
Questions may ask about the type of reaction,
stoichiometry, physical observations during the reaction,
etc.
Earn 1 point for correct answer to this question.
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3


Predicting Products is Difficult!
A difficult task – This is a very difficult question for
most AP students, but you can be successful.
This tutorial will systematically introduce
you to each type of reaction included in
the question and give tips!
However, much of a students’ ability to
answer these questions relates to
experience—the more reactions you see
and work with throughout your course,
the better you’ll be able to predict
products!
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Net Ionic Equations

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4


Definition: Net Ionic Equation
When an ionic compound dissolves in water, the
ions dissociate (separate) and move freely
throughout the solution.
Ionic Equation – Any soluble ionic
compound is dissociated into ions.
Net Ionic Equation – Only ions
necessary for the chemical reaction
y
are shown.

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When to Dissociate a Compound
How do you know what to dissociate and what not
to dissociate?
DO dissociate

DO NOT EVER dissociate

Soluble IONIC
compounds when
aqueous

COVALENT compounds
(soluble or not)

Strong acids and bases
in water

Weak acids and bases

The AP instructions say to
assume reactants are aqueous
solutions unless otherwise
stated!

Solids, liquids or gases
(even ionic and strong
acids/bases!)

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5


Ionic Solubility Rules
Look up the anion in the left column to see which
cations it forms insoluble compounds with.
Forms insoluble compounds with
NO3CH3COO-

Ag+

Cl-, Br-, I-

Ag+, Pb2+, Hg22+, Ti+

SO42-

Ag+, Pb2+, Ba2+, Sr2+, Ca2+

CrO42-

Ag+, Pb2+, Ba2+, Sr2+

S2-

All cations except NH4+, column 1 & 2

OH-

All cations except NH4+, Column 1, Ba2+ and Sr2+

CO32-, PO43-

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No common cations

All cations except NH4+, Column 1 (except Li+)

NH4+, Na+ and K+ are soluble with all common anions.

Using Ionic Solubility Rules
Will each of the following compounds be soluble?
NaNO3
NO3- is insoluble with no common cations

Soluble

AgCl
Cl- is insoluble with Ag+, Pb2+, Hg22+, Ti+

Insoluble

Mg(OH)2
OH- is insoluble except with NH4+, cations of column 1,
Ba2+, Sr2+

Insoluble

Na2Cr2O7
There is no rule for Cr2O72-, but Na+ is soluble with all
anions

Soluble

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6


Common Strong Acids and Bases
There are only a few common strong acids and bases to
remember—the rest will most likely be weak!

Strong Acids

HCl, HBr, HI, HNO3, HClO3, HClO4

Strong Bases

NaOH, KOH, Ca(OH)2, Ba(OH)2, Sr(OH)2

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Writing Net Ionic Equations - 1
1

Dissociate all soluble ionic compounds
Only dissociate soluble, ionic compounds in water (“aq”)
Only subscripts within a polyatomic ion remain as subscripts
All other subscripts are moved to coefficients.

Cross out any ion that is dissociated on both sides

2

These are called spectator ions

3

Re-write the final net ionic equation

Examples: Write the net ionic equation for:
2 NaOH (aq) + Mg(NO3)2 (aq)
Mg(OH)2 (s) + 2 NaNO3 (aq)

2

1
2 Na+ + 2 OH- + Mg+2 + 2 NO3Mg+2 + 2 OH-

Mg(OH)2 (s) + 2 Na+ + 2 NO3Mg(OH)2 (s)

3

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7


Writing Net Ionic Equations - 2
1

Dissociate all soluble ionic compounds
Only dissociate soluble, ionic compounds in water (“aq”)
Only subscripts within a polyatomic ion remain as subscripts
All other subscripts are moved to coefficients.

2

Cross out any ion that is dissociated on both sides

3

Re-write the final net ionic equation

These are called spectator ions.

Examples: Write the net ionic equation for:
AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

2

1
Ag+ + NO3- + Na+ + ClAg+ + Cl-

AgCl (s) + Na+ + NO3AgCl (s)

3

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Double Displacement
- Precipitation

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8


Definition - Precipitation Reaction

Precipitation Reaction – One type of
Double Displacement reaction: Two
soluble compounds react to form an
insoluble compound (the precipitate).

Recognizing this type – 2 ionic
compounds in solution.

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Designating a Precipitate

If an insoluble compound is formed from aqueous
solutions of two soluble compounds, it is a
l ti
ft
l bl
d
i
precipitate and should be designated with a “(s)”.

Example:
p
AgNO3 (aq) + NaCl (aq)

AgCl (s) + NaNO3 (aq)
Precipitate

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9


Double Displacement – Precipitation 1
1

Determine it’s Double Displacement (two compounds,
each with a cation and anion).

CaCl2 (aq) + AgNO3 (aq)

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2

Combine the cation of the first reactant with the
anion of the second reactant.


CaCl2 (aq) +

AgNO3 (aq)

CaNO3

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10


3

Combine the cation of the second reactant with the
anion of the first reactant.


CaCl2 (aq) +

AgNO3 (aq)

CaNO3

+

AgCl

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4

Remember to write cations first …
& balance charges with subscripts when writing
formulas


CaCl2 (aq) +

AgNO3 (aq)

Ca(NO3)2

+

AgCl

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11


5

Determine the precipitate using solubility rules.


CaCl2 (aq) +

AgNO3 (aq)

Ca(NO3)2 (aq) +

AgCl (s)

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6

Write the net ionic equation & check to be sure it’s
balanced.
Don’t balance until the end as it’s often easier to
balance the net ionic than the original equation!


CaCl2 (aq) +

AgNO3 (aq)

Ca+2 + Cl-1 + Ag+1 + NO3-1
Cl-1 + Ag+1

Ca(NO3)2 (aq) +

AgCl (s)

Ca+2 + 2 NO3-1 + AgCl
AgCl

It’s balanced.

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12


Double Displacement
- Acid Base

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Definition - Acid/Base Reaction
Acid-Base reaction – One type of
Double Displacement reaction: An
acid and a base react to form salt and
water.
Recognizing this type – Acid (begins
with “H”) and a base (ends with “OH”
or is NH3).

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13


Double Displacement - Acid/Base 1
1

Determine it’s Double Displacement: acid-base
(contains an acid and base for reactants)

HNO3 (aq) + Na OH (aq)

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2



H NO3 (aq) +

Na OH (aq)

H2O

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14


3

Combine the cation of the second reactant with the
anion of the first reactant.


H NO3 (aq) +

Na OH (aq)

HOH

+

NaNO3

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4

& balance charges with subscripts when writing
formulas
f
l


H NO3 (aq) +

Na OH (aq)

H2O

+

NaNO3

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15


5

Write the net ionic equation & be sure it’s balanced.
DO NOT dissociate weak acids and bases - only
strong ones that are dissolved in water (Aqueous)!
DO NOT dissociate water!


H NO3 (aq) +

Na OH (aq)

H+ + NO3-1 + Na+1 + OH-1
H+ + OH-

H2O

+

NaNO3

H2O + Na+ + NO3-1
H2O

It’s balanced.

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An Example With a Weak Acid - 1
1

Determine it’s Double Displacement: acid-base
(contains an acid and base for reactants).

H2CO3 (aq) + Na OH (aq)

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16


2



H2CO3 (aq) +

Na OH (aq)

HOH

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3

Combine the cation of the second reactant with the anion of the
first reactant


H2CO3 (aq) +

Na OH (aq)

H2O

+

NaCO3

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17


4

& balance charges with subscripts when
writing formulas.


H2CO3 (aq) +

Na OH (aq)

H2O

+

Na2CO3

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5

Write the net ionic equation & be sure it’s balanced.
DO NOT dissociate weak acids and bases - only
strong ones that are dissolved in water (Aqueous)!
DO NOT dissociate water!


H2CO3 (aq) +

Na OH (aq)

H2CO3 + Na+1 + OH-1
H2CO3 + 2 OH-

H2O

+

Na2CO3

H2O + 2 Na+ + CO3-2
It’s
-2
balanced.
2 H2O + CO3

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18


Decomposition

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Definition - Decomposition
Decomposition – One molecule decomposes into
more than one thing.
Recognizing this type – Only one reactant. Often
uses “heated” in description.

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19


Common Decompositions
Metal carbonates – Decompose to form metal
oxide and carbon dioxide.
Ammonium salts – Decompose to form ammonia
gas.

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Decomposition - Examples
(NH4)2CO3 (s)

2 NH3 + CO2 + H2O

Hint: Carbonates d
Hi t C b
t decompose t f
to form CO2 as one of the products!
f th
d t !
MgCO3 (s)

6 NH4NO3 (s)

MgO + CO2

8 NH3 + 2 N2 + 9 O2

Hint: All of these are solids or gases - none dissociate!

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20


Combination

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Definition - Combination
Combination – More than one
molecule combine to form one
molecule.

Recognizing this type – Something
being added to water OR two
compounds or elements being
“heated” together.

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21


Common Combination Reactions

Metal or metal oxide added to water
– Produce a base.
Non-metal or non-metal oxide
added to water
– Produce an acid.

Strong acids and bases produced MUST be
dissociated. Weak ones MUST be left together.
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Combination Examples
BaO + H2O
BaO + H2O

Ba(OH)2
Ba+2 + 2 OH-1

Hint: Remember to dissociate soluble ionic compounds,
strong acids and strong bases when something is in
water!
SO2 + H2O

3 Mg + N2

H2SO3

Mg3N2

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22


Complex Ion
Formation

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Definition - Complex Ion Formation
Complex Ion – A species with several
electron donating polyatomic ions
bonded to a single metal atom or ion.

reacted with “excess” hydroxide,
cyanide or ammonia.
ammonia

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23


Common Complex Ions
Cyanide complex ions – Ag(CN)2-1, Fe(SCN)+2

Ammonia complex ions – Cu(NH3)4+2, Zn(NH3)4+2,
Ag(NH3)2+1, Ni(NH3)6+2

Hydroxide complex ions – Al(OH)4-1, Zn(OH)4-2

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Complex Ion Examples
NaCN (aq) + AgNO3 (aq)
2 CN-1 + Ag+1
g

Ag(CN)2-1 + NaNO3
Ag(CN)2-1
g( )

Hint: Remember to cancel out ions that dissociate on both
sides of the reaction when writing the net ionic reaction.

2 NH3 (aq) + AgCl (s)

NH3 (aq) + NiSO4 (aq)
6 NH3 + Ni+2

Ag(NH3)2+1 + Cl-1

Ni(NH3)6+2 + SO4-2
NI(NH3)6+2

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24


Combustion

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Definition - Combustion

Combustion – Combination with O2.

reacted with O2 .

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25


Combustion Examples
2 CH3CH2OH (g) + 7 O2 (g)

4 CO2 + 6 H2O

Hint: You’ll learn how to write the simple organic molecules
You ll
from words in a future tutorial (Descriptive and Organic
Chemistry).
C2H4 (g) + 3 O2 (g)

2 CO2 + 2 H2O

Hint: If hydrocarbons (hydrogen, carbon and oxygen containing)
are added to O2, the products are ALWAYS CO2 & H2O.
p

CS2 + 4 O2

CO2 + 2 SO2

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Redox

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26


Definition - Redox
Redox – Reduction/Oxidation. Charges are
changed - one increases and one decreases.
Recognizing this type – Single replacement
reaction or one with a common oxidizer (NO3-1,
MnO4-1, Cr2O7-2, H2O2).

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Redox Examples
Zn (s) + CuSO4 (aq)

ZnSO4 + Cu
Zn+2 + Cu

Zn + Cu+2

Pb (s) + AgNO3 (aq)
Pb + 2 Ag+

Ag + Pb(NO3)2
2 Ag + Pb+2

Don’t forget you must balance charge as well as number of
atoms!
This one isn’t balanced as it first appears!
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27


Going from “Word”
Word
Reactants to
“Formula” Reactants

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Writing the Reactants
The question will give you – The reactants in
word form with word clues about state of matter.
Writing the reactants – 1 point is awarded for each
of the 3 required sets of reactants that you write
correctly in formula form.
(they must be dissociated correctly to get the 1
point).
point)

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28


Know What Terms Mean
They use several terms that mean the similar things.
Solid, powdered, metal, filings, strip = solid (don’t
dissociate)
Solution = dissociate any soluble ionic compounds
or strong acid/base, including in saturated solutions.
Suspension = a solid precipitate spread throughout
water; doesn’t dissociate.
If there is a solution or water in the reactants
reactants—
assume that the water (solution) is still there in the
products and break up any soluble ionic compounds
or strong acids or bases in the products.
Acidified solution = make H+ one of the reactants
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Part II Questions

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29


What are the Part II Questions?
There is a question after each of the 3 reactions that
focuses on the meaning of the reaction. It might ask:
Type of reaction
To identify a participant in the
reaction (species being oxidized,
etc.)
About the stoichiometric ratio of
the equation
About physical observations during
the reaction

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Using Part II Questions as Hints
You might be able to gain hints about the reaction in
Part I from the Part II question.
A strip of magnesium is added to a solution of
silver nitrate.
What is the change in oxidation number of the
magnesium?

The P t
Th Part II question tells you that
ti t ll
th t
the reaction in Part I is a Redox
reaction!

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30


Example 1

A strip of magnesium is added to a solution of
silver nitrate.
Mg (s) + AgNO3 (aq)
Mg + 2 Ag+1

Mg(NO3)2 (aq) + Ag (s)
Mg+2 + 2 Ag

What is the change in oxidation number of the
magnesium?
Mg is going from 0 to +2

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Example 2

Zinc metal is placed in a solution of copper (II) nitrate.
Zn (s) + Cu(NO3)2 (aq)
Zn + Cu+2

Zn(NO3)2 (aq) + Cu (s)
Zn+2 + Cu

Describe the color change during the process.
Cu+2 is blue. As it reacts to form Cu solid and Zn+2 the
blue color will fade as Zn+2 is colorless.

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31


Tips for AP Free
Response
Question #4

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Helpful Hints for Question #4
1

2

Pay attention to the states of matter of the
reactants—don’t dissociate solids, liquids or
gases…only aqueous solutions!

3

Be sure to cross out spectator ions!

4

There is always a reaction…if you think everything
will cancel out in the ionic equation, go back and
find your mistake!
o r

5

Be sure to balance the final net ionic equation
(don’t do the initial equation…the final one is
easier)

6
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Know the solubility rules & list of strong
acids/bases so you’ll know what to dissociate!

Use the Part II questions as hints for what the
reaction might be in Part I


32


Learning Summary
Be sure to
balance the final
net ionic
reaction.

Question 4
requires you to
write balanced
net ionic
reactions after
predicting
products.

Know the solubility
rules and how to
write net ionic
reactions.

Use the Part II
question as a
hint for the
products on
Part I.

Know what common
phrases are used to
describe states of
matter.

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Congratulations
You have successfully completed
the core tutorial

Predicting Products


33


Chemistry :: Biology :: Physics :: Math

What’s N t
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Step 1: Concepts – Core Tutorial (Just Completed)
Step 2: Practice – Interactive Problem Drill
Step 3: Recap – Super Review Cheat Sheet

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