Download Balancing ANY chemical Equation

Balancing ANY chemical
Equation
Elena Man
By the end of this workshop you will be able to:
1) How to write chemical equations based on written
descriptions
2) How to consistently and efficiently balance chemical
equations
3) How to identify different types of reactions
4) How to predict products of chemical reactions
5) How to write balanced net ionic equations
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Chemical Reactions:
A new substance is created. Bonds that hold the atoms together are broken and
new bonds form.
Indications of a Chemical Reaction:
1. Production of Heat and/or Light
2. Production of a Gas ( CO2 , H2 )
3. Precipitate (solid in a liquid) forms.
We use balanced chemical equations to REPRESENT a chemical reaction.
Balanced: EQUAL NUMBERS of atoms for each element involved in the reaction is
represented on the reactant and product sides. This is a requirement the equation
must satisfy to be consistent with the law of conservation of matter.
Law of Conservation of Matter : Matter (atoms) is neither created nor destroyed.
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How to write chemical equations based on
written descriptions
1- Read each statement
2- Write the correct formula for the substance indicated
3- Replace each word “and”, “combine” or “plus” with the
symbol “+”
4- Replace each phrase indicating change (make, produces,
yield, etc.) with and arrow pointing toward the right.
5- Balance each equation
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Additional information in Chemical Equations
• The physical states of reactants and products in chemical equations
very often are indicated with a parenthetical abbreviation following
the formulas. Common abbreviations include s for solids, l for liquids,
g for gases, and aq for substances dissolved in water.
2Na(s) + 2H2O(l)⟶2NaOH(aq) + H2(g)
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Examples of writing equations from
descriptions
Lead (II) nitrate and potassium carbonate produce lead (II) carbonate and
potassium nitrate:
Pb(NO3)2 + K2CO3 → PbCO3 + KNO3 We need to balance this equation now.
Pb(NO3)2 + K2CO3 → PbCO3 + 2KNO3
Potassium chlorate decomposes into potassium chloride and oxygen
KClO3 → KCl + O2 We need to balance this equation now.
2KClO3 → 2KCl + 3O2
Gas ammonia is produced when nitrogen gas combines with hydrogen gas.
N2 + H2 → NH3
We need to balance this equation now.
N2 + 3H2 → 2NH3
Let’s look at our workshop outline and check our learning….(1)
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How to balance ANY chemical equation:
𝐶𝑢(𝑠) + 𝐻𝑁𝑂3(𝑎𝑞) → 𝐶𝑢(𝑁𝑂3 )2 𝑎𝑞 + 𝑁𝑂2
𝑔
+ 𝐻2 𝑂(𝑙)
• Step 1: List out the elements present on BOTH the PRODUCT and
REACTANT sides of the equation. Then, count how many of each
atom are present initially.
• Step 2: Determine which element to balance first. You want to
balance the easiest elements first. The easiest elements to balance
are the ones that appear in the fewest substances in the equation.
Therefore, to determine balancing order, count
how many
substances each element appears in.
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Remember, if at any point one of the easier elements to balance becomes
unbalanced, rebalance that element before continuing to the harder
elements.
• Step 3: Only change coefficients (the numbers in front of the
substance), NEVER CHANGE SUBSCRIPTS.
• Step 4: When trying to balance an odd number of atoms on one side
with an even number of atoms on the other side DOUBLE the
coefficient of the compound that is causing the number of atoms to
be ODD and rebalance.
𝐶𝑢(𝑠) + 𝟐𝐻𝑁𝑂3(𝑎𝑞) → 𝐶𝑢(𝑁𝑂3 )2 𝑎𝑞 + 𝟐𝑁𝑂2 𝑔 + 𝐻2 𝑂(𝑙)
NO2 was causing the number of N’s to be odd on the product side,
therefore I doubled the coefficient of NO2 from 1 to 2.
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Balancing Nitrogen Continued
I rebalanced N by changing the coefficient of HNO3 to 4. However, now
I have unbalanced H again, therefore I must rebalance H before trying
to balance O.
• And now we have a balanced equation!
𝐶𝑢(𝑠) + 𝟒𝐻𝑁𝑂3(𝑎𝑞) → 𝐶𝑢(𝑁𝑂3 )2 𝑎𝑞 + 𝟐𝑁𝑂2
𝑔
+ 𝟐𝐻2 𝑂(𝑙)
Let’s look again at our workshop outline and check our learning….(2)
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How to identify 5 types of reactions
•Synthesis Reactions
•Decomposition Reactions
•Combustion Reactions
•Single Replacement (Displacement):
•Double Replacement (Displacement)
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Synthesis Reactions
Easiest to recognize – 1 product is formed:
Formula: A + B  AB
Ex: 4Al + 3O2  2Al2O3
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Decomposition Reactions
ONE substance breaks down into simpler substances.
Opposite of synthesis – 1 reactant.
Formula: AB  A + B
Ex: 2CaCO3 2CaO + 2CO2
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Combustion Reactions
Hydrocarbons (contain C & H) combine with O2 and produce
CO2 and H2O.
Release energy in form of light and heat
2 C6H14 + 19 O2  12 CO2 + 14 H2O
2 C2H6 + 7 O2  4 CO2 + 6 H2O
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Single Replacement (Displacement)
One substance replaces another in a compound. Creates a
“free” element B. These are redox reactions in which one or
more elements change its oxidation number.
Formula: A + BC  AC + B
Ex1: Zn + CuSO4  ZnSO4 + Cu
Ex2: Cl2 + 2KBr --> 2KCl + Br2
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Double Replacement (Displacement)
Metathesis reactions
Cations of two compounds exchange places to form two new compounds.
Metathesis (Exchange) Reactions
General Form: 𝑨𝑩 + 𝑪𝑫 → 𝑨𝑫 + 𝑪𝑩
- Involve the exchange or “changing out” of certain ions for others.
- Positive ions switch places and get new “partners” or negative ions.
- The new products are formed based on the charges of the ions being put
together
. Ex: BaCl2 + Na2SO4  2NaCl + BaSO4
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Conditions for metathesis reactions to occur
Must be some net change to the system (i.e. the removal of ions from the
system). There are four conditions under which a metathesis reaction will be
“driven forward”:
1. The formation of a gas. (gas evolution reaction)
Example: 𝟐𝑯𝑪𝒍 𝒂𝒒 + 𝑵𝒂𝟐 𝑺 𝒂𝒒 → 𝟐𝑵𝒂𝑪𝒍 𝒂𝒒 + 𝑯𝟐 𝑺(𝒈)
2. The formation of a liquid. (acid-base neutralization rxn)
Example: 𝑯𝑪𝒍 𝒂𝒒 + 𝑵𝒂𝑶𝑯 𝒂𝒒 → 𝑵𝒂𝑪𝒍 𝒂𝒒 + 𝑯𝟐 𝑶 𝒍
3. The formation of a solid. (precipitation reaction)
Example: 𝑲𝟐 𝑪𝑶𝟑 𝒂𝒒 + 𝑴𝒈𝑰𝟐 𝒂𝒒 → 𝟐𝑲𝑰 𝒂𝒒 + 𝑴𝒈𝑪𝑶𝟑 (𝒔)
4. The formation of a weak electrolyte.
Example: 𝑵𝒂𝑭 𝒂𝒒 + 𝑯𝑩𝒓 𝒂𝒒 → 𝑯𝑭 𝒂𝒒 + 𝑵𝒂𝑩𝒓(𝒂𝒒)
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Predicting products of chemical reactions
The solubility guidelines may be used to predict
whether a precipitation reaction will occur.
When solutions of soluble ionic compounds are mixed together we
need to identify all the ions present in the solution and then consider if
possible cation/anion pairing could result in an insoluble compound.
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Solubility Guidelines
Soluble Compounds contain
Exceptions include:
-Group 1 metal cations (Li+, Na+,
K+, Rb+, Cs+) and ammonium ion
NH4+
-Halide ions (Cl−, Br−, and I−)
-Acetate(C2H3O2−),
-Bicarbonate(HCO3-) ion
-Chlorate (ClO3−) ion
-Sulfate (SO42−) ion
-Nitrate (NO3-) ion
-Halides of Ag+, Hg22+ and Pb2+
are insoluble
+, Ba2+ ,Ca2+, Hg 2+
-Sulfates
of
Ag
Pb2+and Sr2+ are insoluble 2
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Solubility table continued…
Insoluble Compounds contain
-
Carbonate ions (CO32−),
Phosphate ions(PO43−),
Chromate ions(CrO42−),
Sulfide ions (S2−)
Hydroxide ion (OH−)
Exceptions
-Compounds of ALL these anions
with group IA and ammonium ion
(NH4+)are soluble.
-Compounds of Chromate ions
(CrO42−), are also soluble with Ca+2
and Mg +2
-Sulfide ions (S2−) are also soluble
with Group IIA
-Hydroxides are also soluble Sr+2 ,
*Ca+2, and Ba2+.
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What is a weak electrolyte?
• Electrolytes: Substances that form ions when dissolved in solution.
Electrolytes can be weak or strong.
• Strong Electrolytes: Substances that completely separate into their
component ions when dissolved. (All soluble ionic compounds and
strong acids are strong electrolytes.)
• Weak Electrolytes: Substances that exist in solution mostly as neutral
molecules, with only a small fraction separating into ions. (These are
mostly weak acids and bases.)
• Nonelectrolytes: Substances that do NOT form ions when dissolved
in solution. These are usually molecular.
Let’s look again at our workshop outline and check our learning…(3,4,5)
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Net Ionic Equations
For metathesis reactions it is useful to express the net change of a
chemical reaction, including only the species involved in the chemical
reaction. These are:
NET IONIC EQUATIONS
How to Write a Net Ionic Equation
1. Write and balance the molecular equation
𝑲𝟐 𝑪𝑶𝟑 + 𝑴𝒈𝑰𝟐 → 𝟐𝑲𝑰 + 𝑴𝒈𝑪𝑶𝟑
2. Determine the phases of each substance in the reaction. For ionic substances,
use the solubility rules to determine if the substance is soluble (aq) or insoluble (s).
𝑲𝟐 𝑪𝑶𝟑 𝒂𝒒 + 𝑴𝒈𝑰𝟐 𝒂𝒒 → 𝟐𝑲𝑰 𝒂𝒒 + 𝑴𝒈𝑪𝑶𝟑(𝒔)
3. Write the complete ionic equation by breaking up any aqueous strong
electrolytes into separated ions. The phases of all separated ions should be
aqueous.
𝟐𝑲+ (𝒂𝒒) + 𝑪𝑶𝟑 𝟐− (𝒂𝒒) + 𝑴𝒈𝟐+ (𝒂𝒒) + 𝟐𝑰− (𝒂𝒒) → 𝟐𝑲+ (𝒂𝒒) + 𝟐𝑰− (𝒂𝒒) + 𝑴𝒈𝑪𝑶𝟑(𝒔)
4. Cancel out any ions that are present in the same form on both the product side
and the reactant side of the equation. These ions are called spectator ions since
they are not involved in the reaction. Rewrite what’s left.
𝑪𝑶𝟑 𝟐− (𝒂𝒒) + 𝑴𝒈𝟐+ (𝒂𝒒) → 𝑴𝒈𝑪𝑶𝟑 (𝒔)
Let’s look again at our workshop outline and check our learning….(6,7)
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