Download UNIT 7 Lecture Notes

Unit 7: Chemical Reactions
Part 1: Chemical Equations and Balancing
Part 2: Types of Chemical Reactions
Unit Synapsis
In previous unit we looked at the some basics about substances.
We addressed differences between molecular and ionic compounds and
how to name these compounds. In this unit we will look at how these
molecules and compounds react with one another and also introduce the
dissolving process. Previous concepts vital to this unit include identifying
parent ions as well as naming.
Part 1: Chemical Equations and Balancing
Corresponds to pages 282 to 288 in your textbook.
Part 1: Chemical Equations & Balancing / Learning Targets
“After this lesson I can…
• …identify the parts of a chemical equation.”
• …recite the law of conservation of mass.”
• …balance a chemical equation.”
• …translate a balanced chemical equation into words and vice
versa.”
Chemical Equations
• A chemical equation is a model for a chemical change.
• Here are three examples of chemical equations:
• CH4(g) + 2 O2(g)  2 H2O(l) + CO2(g)
• 3 H2(g) + N2(g)  2 NH3(g)
• KCl(s)  K+(aq) + Cl-(aq)
• A chemical change is when chemical bonds are broken, atoms are
rearranged, and new bonds are formed to create entirely new
molecules or compounds
• This is different than a nuclear change, which is when the atoms
that make up the molecules change.
• This is also different than a physical change, which is when the
attractions between molecules or compounds are overcome or
commence because of changes in temperature or pressure.
• The parts of a chemical equation are defined in detail in a few
slides but before we get to that let’s look at a major law all
chemical equations must follow.
The Law of Conservation of Matter
• The law of conservation of mass states that in any type of change,
matter cannot be created or destroyed. Another way of saying this is
that the total amount of matter in the universe is constant.
• One of the things this means is that chemical equations must be
balanced.
• A balanced chemical equation has the same number of atoms going
into and out of the equation.
• The reactants are the substances to the left of the arrow in a chemical
equation, & the products are the substances to the right of the arrow.
• If an equation is presented in it’s unbalanced form, it can be balanced
by inserting coefficients in front of the chemical formulas. If a
chemical formula does not have a coefficient in front of it, the
coefficient is 1.
• The coefficients technically represent the number of moles but
thinking of them as just the number of molecules makes things easier.
• When balancing, you must NEVER alter the chemical formulas. You
can only change or add coefficients in front of the chemical formulas.
Parts of a Chemical Equation
Physical States
• Note that on the previous slide there were exactly:
• 7 carbon atoms in the products & reactants
• 16 hydrogen atoms in the products & reactants
• 22 oxygen atoms in the products & reactants
• Another important part of a chemical equation is that the physical
state is sometimes noted next to the chemical formulas.
• When it comes to balancing the physical state is not important and
sometimes it is left out. However, you should know what each
letter means when you see it.
• There is a letter for each physical state, and a 4th notation is used
to describe something dissolved in water.
• (l): liquid
• (g): gas
• (s): solid
• (aq): aqueous (dissolved in water)
Balanced & Unbalanced Chemical Equation Example #1
Balanced & Unbalanced Chemical Equation Example #2
Balanced & Unbalanced Chemical Equation Example #3
Balanced & Unbalanced Chemical Equation Example #4
Balanced & Unbalanced Chemical Equation Example #5
Balancing Chemical Equations Tips
1) Balance elements that appear twice on one side of the equation
last. So if Oxygen is in two of the reactants and/or two of the
products, save it for last. (This is a lot of equations so as a
general rule leave oxygen for last)
2) Only whole numbers are allowed, but if you end up with half a
number, you can get yourself out of that situation by doubling
ALL the coefficients in the equation. (if you end up with .25 you
can get yourself out of that situation by quadrupling all
coefficients)
3) Take your time. Balancing is a skill that takes practice but with
enough patience an answer will be found.
Chemical Equations as Sentences
• A chemical equation can be stated as a sentence and you should
be able to translate back and forth between a sentence and an
equation. Start by writing the equation down and then balance it
after. Here are 3 examples:
• “Magnesium reacts with oxygen gas (O2) to form Magnesium Oxide”
• 2 Mg(s) + O2(g)  2 MgO(s)
• “Sodium Metal reacts with Iron(III) Oxide to produce Sodium Oxide and
Iron metal”
• 6 Na(s) + 2 Fe2O3(s)  3 Na2O(s) + 4 Fe(s)
• “Tetraphosphorus trisulfide reacts with oxygen gas (O2) to produce
tetraphosphorus hexaoxide and Sulfur dioxide”
• P4S3(s) + 6 O2(g)  P4O6(g) + 3 SO2(g)
Chemical Equations as Sentences
• 2 Na3PO4 + 3 CaCl2  Ca3(PO4)2 + 6 NaCl
• “Sodium phosphate reacts with calcium chloride to produce
calcium phosphate and sodium chloride”
• 2 N2O5  4 NO2 + O2
• “dinitrogen pentaoxide breaks down to produce nitrogen dioxide
and oxygen gas”
• 2 KClO3(s)  2 KCl(s) + 3 O2(g)
• “Potassium Chlorate breaks down to form potassium chloride
and oxygen gas”
Part 1 Additional Resources
• WikiHow on balancing chemical equations.
• Khan Academy on Balancing Chemical Equation.
Part 2: Types of Chemical Reactions
Corresponds to pages: 276 to 284 in your textbook.
Part 2: Types of Chemical Reactions / Learning Targets
“After this lesson I can…
• …Identify a chemical equation as representing a
combustion reaction, single replacement reaction,
double replacement reaction synthesis reaction,
decomposition reaction, or an ionic compound
dissolving in water.
• …Define combustion reactions, single replacement
reactions, double replacement reactions, synthesis
reactions, and decomposition reactions.
• …Write the equation for an ionic compound
dissolving in water (simplified and complete).
#1: Ionic Compounds Dissolving in Water
• While it’s not exactly a chemical reaction, when an ionic
compound dissolves in water it splits apart into it’s parent ions.
• When the ionic compound splits apart it’s called ionization or
dissociation.
• For these equations you will have to use what you learned about
parent ions in UNIT 6 and what you learned about balancing in
part 1 of this unit.
• Like physical states, charges do NOT matter for balancing.
• Sample equations for ionic compounds dissolving in water:
• NaCl(s)  Na+(aq) + Cl-(aq)
• CaBr2(s)  Ca2+(aq) + 2 Br-(aq)
• KNO3(s)  K+(aq) + NO3-(aq)
• Fe(NO3)3(s)  Fe3+(aq) + 3 NO3-(aq)
• How to Identify?: Look for 1 reactant and 2 products that are both ions
and in the aqueous physical state.
Ionic Compounds Dissolving in water Sample Problems
Write the equation for the following compounds dissolving in water
1) FeBr3
2) Cu(NO3)2
3) LiBr
4) Na3PO4
5) MgSO4
6) BaCl2
Ionic Compounds Dissolving in Water
• Sometimes chemists get lazy and don’t want to take the time to
write out longer equations so they simplify using the “aq” symbol
to indicate an ionic compound that’s been dissolved in water.
• Sample Equations for Ionic compounds dissolving in water:
• NaCl(s)  Na+(aq) + Cl-(aq)
could be written simply as:
• NaCl(s)  NaCl(aq)
• CaBr2(s)  Ca2+(aq) + 2 Br-(aq)
could be written simply as:
• CaBr2(s)  CaBr2(aq)
• Fe(NO3)3(s)  Fe3+(aq) + 3 NO3-(aq)
could be written simply as:
• Fe(NO3)3(s)  Fe(NO3)3(aq)
Ionic Compounds Dissolving in Water
• When dealing with ionic compounds dissolved in water it is
important to start paying closer attention to the physical state
symbols.
• You must remember that:
• “CuSO4(aq)”really means “Cu2+(aq) + SO42-(aq)”
• “AlCl3(aq)” really means “Al3+(aq) + 3 Cl-(aq)”
• “Na2CO3(aq)” really means “2 Na+(aq) + CO32-(aq)”
#2: Synthesis Reactions (a.k.a. combination reactions)
• In a synthesis reaction, two or more elements or compounds
combine to form a larger, more complex compound.
• Sample Chemical Equations:
• 3 H2 + N2  2 NH3
• 2 H2 + O2  2 H2O
• Mg + O2  MgO
• CaO + H2O  Ca(OH)2
• Generic Chemical Equation:
• A+BC
• How to identify?: Look for 1 product
#3: Decomposition Reactions
• In a decomposition reaction, A compound breaks down into two
or more elements or compounds. This often occurs through
heating but not always.
• In decomposition equations the arrow actually means “breaks
down to produce” or “decomposes to produce”
• Sample Chemical Equations:
• 2 H2O2  2 H2O + O2
• CuCO3(s)  CuO(s) + CO2(g)
• C3H5(NO3)3(l)  6 N2(g) + 12 CO2(g) + 10 H2O(g) + O2(g)
• Generic Chemical Equation:
• AB+C
• How to identify?: Look for 1 reactant and 2 or more Products
#4: Single Replacement Reactions
• In a single replacement reaction two elements replace one another.
In most cases, this involves 1 pure metal and 1 metal that is part of
an ionic compound.
• Sample Chemical Equations:
• 2 Al(s) + 3 CuCl2(aq)  3 Cu(s) + 2 AlCl3(aq)
• Fe(s) + CuSO4(aq)  Cu(s) + FeSO4(aq)
• Fe(s) + Pb(NO3)2(aq)  Pb(s) + Fe(NO3)2(aq)
• Generic Chemical Equation:
• A + BC  B + AC
• How to identify?: Look for a pure element and an ionic compound in
the reactants. In the products the pure element should take the place of
the positive ion in the ionic compound.
#5: Double Replacement Reactions
• In a double replacement reaction, two positive ions in an ionic
compound exchange their negative ions.
• Sample Chemical Equations (Solid Precipitation)
• 3 CaCl2(aq) + 2 Na3PO4(aq)  6 NaCl(aq) + Ca3(PO4)2(s)
• 2 NaOH(aq) + CuBr2(aq)  Cu(OH)2(s) + 2 NaBr(aq)
• Li2CO3(aq) + Mg(NO3)2(aq)  MgCO3(s) + 2 LiNO3(aq)
• HCl(g) + NaOH(aq)  H2O + NaCl(aq)
• Generic Chemical Equation
• AB + CD  CB + AD
• How to identify?: Look for two ionic compounds in the reactants. The
products should also contain two ionic compounds but the parent cations
should have switched their parent anions.
#6: Combustion Reactions
• As you might have already realized, combustion reactions are
when carbon compounds burn or explode.
• With one notable exception, which is the combustion of hydrogen
gas, this produces carbon dioxide and water.
• Combustion reactions produce light and heat and occasionally
you will see “heat” written as a product.
• Sample Chemical Equations:
• CH4 + 2 O2  CO2 + 2 H2O
• 2 C8H18 + 25 O2  16 CO2 + 18 H2O
• 2 C2H6O + 7 O2  4 CO2 + 6 H2O
• Generic Chemical Equation:
• Organic Compound + O2  CO2 + H2O
• How to identify?: Look for Carbon Dioxide and Water in the products.
Oxygen should be a reactant.
Other types of reactions
• Not all reactions fit neatly into the six classifications listed above.
Here are some examples of those equations:
• Cu2S + 12 HNO3  Cu(NO3)2 + CuSO4 + 10 NO2 + 6 H2O
• 2 K2MnF6 + 4 SbF5  4 KSbF6 + 2 MnF3 + F2
• It’s not one of our objectives that your able to place every single
chemical reaction into a specific category, just that you are able to
clearly identify the six mentioned on the previous slides.
• There is also reactions that are in a “gray area” between two
different classifications.
• Hydrogen gas, for example, will explode and produce nothing but
water: 2 H2 + O2  2 H2O
• This could be classified as combination. Even though carbon
dioxide is not a product, it could also be classified as
combustion because it produces a flame, has water as a
product, and oxygen gas as a reactant.
Identifying Types of Chemical Reactions Summary
• Combustion: Look for Carbon Dioxide and Water in the
products. Oxygen should be a reactant.
• Ionic compound dissolving in water: Look for 1 reactant and 2
products that are both ions and in the aqueous physical state.
• Single Replacement: Look for a pure element and an ionic
compound in the reactants. In the products the pure element
should take the place of the positive ion in the ionic compound.
• Double Replacement: Look for two ionic compounds in the
reactants. The products should also contain two ionic compounds
but the parent cations should have switched their parent anions.
• Decomposition: Look for 1 reactant and 2 or more Products
• Synthesis/Combination: Look for 1 product
Part 2 Additional Resources
• Quizlet on types of chemical reactions
• Note: this does not cover Ionic compounds dissolving in water