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Chemical Equations
Chemical Reactions
Signs of a Chemical Reaction/Change Precipitation (a solid forms from two liquids or a liquid
and a gas)
Energy Change
Releases heat - exothermic
Absorbs heat - endothermic
Odor Changes
Gas release
Sounds
(but not the most reliable)
Chemical Equations
Forms of Equations:
Word equations – show the names of each
component in a reaction
Chemical equation – show the chemical
formulas and amounts of each component in a
chemical reaction
2H2(g) + O2(g) 2H2O(l)
Hydrogen and oxygen reacted to form water.
Writing Chemical Equations Reactant = substances that undergo chemical
change
Product = substances that are produced by a chemical change
Arrow defines chemical change – “yields”, “produces”, “makes”
Chemical equation:
Think of reactant as “before”, product as “after” change
Can write both word equations and symbol equations
Reactant Product
Types of Reactions
1. Synthesis Reactions
A + B AB
2. Decomposition Reactions
AB A + B
3. Combustion Reactions
CXHY + O2 CO2 + H2O
4. Single Replacement Reactions
A + BC AC + B
5. Double Replacement Reactions
AB + CD AD + CB
Synthesis Reactions
(Also called “combination”)
A compound is formed by the combining
two or more simpler substances
A + B AB
Example---
Magnesium + oxygen → magnesium oxide
2Mg(s) + O2(g) → 2MgO(s)
Synthesis Reactions
Practice
Write the chemical equations for the following:
1. carbon + oxygen → carbon dioxide
C(s) + O2(g) → CO2(g)
2. sodium oxide + water → sodium hydroxide
Na2O(s) + H2O(l) → 2NaOH(aq)
Decomposition Reactions
A compound breaks down to form 2 or
more simpler substances
AB A + B
Example--
Calcium carbonate, when heated, forms calcium
oxide and CO2(g).
CaCO3(s) → CaO(s) + CO2(g)
Mercury (II) oxide breaks down upon heating
to form elemental mercury and oxygen.
HgO (s) Hg (l) + O2(g)
Decomposition Reactions
Decomposition Reactions
Practice
Write the chemical equations for the following:
1. Potassium chlorate, when heated, decomposes into
potassium chloride and oxygen.
2KClO3(s) → 2KCl(s) + 3O2(g)
2. Sulfuric acid, when heated, decompose into sulfur
trioxide and water.
H2SO4 → H2O(l) + SO3(g)
D
D
Combustion Reactions
A hydrocarbon is burned with sufficient
oxygen supply. The products are always
carbon dioxide and water vapor.
Hydrocarbon
(CxHy) + O2(g) → CO2(g) + H2O(g)
Combustion Reactions
Practice
Write the chemical equations for the
following:
1. Methane (CH4) is burned in air.
CH4(g) + O2(g) → CO2(g) + H2O(g)
2. Butane (C4H10) is burned in air.
C4H10(g) + O2(g) → CO2(g) + H2O(g)
2 2
10 8 13 2
Single Replacement
Reactions
A lone element replaces another atom in an
ionic compound.
Cation is replaced
A + XC → AC + X
Or
Anion is replaced
A + BY → BA + Y
A Single Replacement Reaction
Single Replacement
Examples
Replacement of a copper in copper sulfate
by iron (II).
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Replacement of bromine in sodium bromide
by chlorine.
Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(l)
Single Replacement
Practice
Replacement of hydrogen in water by
sodium.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Reminder---
Don’t forget the diatomic molecules. “HBrONClIF” Instead of lone elements,
these like to be in pairs!!!
The Activity Series
Not ALL Single Replacement Reactions will
occur.
We can predict if a reaction will occur, however,
by using the activity series
Activity Series of Metals
Metal higher in
activity series will
displace lower
metal
Examples:
Mg + HgCl2 MgCl2 + Hg
Cu + HgCl2 CuCl2 + Hg
Ag + HgCl2 No Reaction!
The Activity Series
Single
replacement
reactions will
occur ONLY if
the lone
element is
above the
element it
replaces on
this list.
Example
Iron is
above
hydrogen
on the
activity
series, so
it WILL
replace
hydrogen
in this
reaction.
Double Replacement
Reactions
A pair of ions come together. Both cations switch
places or both anions.
AX + BY → AY + BX
Example---
Sodium chloride reacts with silver nitrate to form
sodium nitrate and silver chloride.
NaCl (aq) + AgNO3(aq) → NaNO3(aq) + AgCl(s)
A Double Replacement Reaction
Double Replacement
Practice
Write the chemical equations for the following:
1. Hydrochloric acid reacts with iron(II) sulfide to form iron(II) chloride and dihydrogen sulfide.
HCl(aq) + FeS(s) → FeCl2(aq) + H2S(g)
2. Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.
Na(OH)(aq) + HCl(aq) → NaCl(aq) + H2O (l)
Practice Identifying the Type
of Reaction
2NH3(g) N2(g) + 3H2(g) Type: __________
S8(s) + 12O2(g) 8SO3(g) Type: __________
2C5H10+ 15O2(g)10CO2(g) +10H2O Type: __________
2HgO(s) 2Hg(l) + O2(g) Type: __________
2H2O2(l) 2H2O(l) + O2(g) Type: __________
3Zn(s) + 2AlCl3(aq)3ZnCl2(aq)+2Al Type: __________
K2O(s) + H2O(l) 2KOH(aq) Type: __________
2Na(s) + CaO(s) Na2O(s) + Ca(s) Type: __________
Single Replacement
Single Replacement
Synthesis
Synthesis
Decomposition
Decomposition
Decomposition
Combustion
Coefficients
The numbers to the left of the chemical formulas
are called coefficients.
Show how many moles or molecules are used or made
in the reaction.
Always whole numbers
2 H2 + O2 → 2 H2O
Coefficients and
Compounds
More about Coefficients….
Law of conservation of mass
(Lavoisier): matter cannot be gained or lost in
any chemical reactions.
Chapter 3
O
Atoms in a Reaction
Reaction: Methane gas burns in oxygen to form
carbon dioxide and water
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
O=
H=
C=
1 atom of carbon
4 atoms of hydrogen
4 atoms of oxygen
1 atom of carbon
4 atoms of hydrogen
4 atoms of oxygen
Reactants: Products:
Symbols Used in Equations
+ “plus” or “combined”.
Used between each molecule to indicate
they are combined.
“yields” or “produces”. Used to separate the
reactants from the products.
Pt. Indicates a catalyst is used in a reaction.
Indicates heat is used in the reaction.
⇄ Indicates that the reaction is reversible.
N.R. “No reaction” – The given reactants do not
react with each other.
∆
Symbols Used in Equations
(s) “solid”. Indicates the chemical is a solid. “precipitate”
(l) “liquid”. Indicates the chemical is a liquid.
(aq) “aqueous”. Indicates the chemical is
dissolved in water.
Other symbols used…
Indicates a precipitate falls out of solution.
Indicates a gas is formed.
↓
↓
Symbols used to show the state of a component…
Writing Word and Formula
Equations
Write a word equation by :
Write the names of the reactants
Draw an arrow to separate the reactants from the products
Write the names of the product
If there’s more than one product or reactant, plus signs should be
between them
Remember, reactants are on the left, and products are on the right!
Write a formula equation by substituting the chemical
formula for the names
Show any energy changes
If requested, show states
Writing & Balancing
Chemical Equations
Step #1: Write each chemical in an equation form. (Include the state symbols!)
Step #2: Write the chemical formulas for each reactant and each product.
Step #3: Balance using coefficients in front of the reactants and products until each side of the equation has the same number of each atom.
Step #4: Recheck your result to make sure atoms on both sides are equal
Tidbits: 1. Begin by balancing the atom that is used the least in
the reaction.
2. Balance H and O last.
Balancing Chemical
Equations
Law of conservation of matter applies for reactions
- End up with the same number of atomic elements
that you start with = balanced equation
Coefficient = whole numbers that are placed in
front of formulas to balance an equation
NEVER change the subscript
in a formula when balancing
equations!
Method for Balancing
Equations
• Count the number of atoms of each
element on each side of the equation.
• Change the coefficients to make the
number of atoms of each type match on
both sides
• Multiply the subscript by the coefficient
to calculate new numbers
• All elements in a compound must be
multiplied by the new coefficient!
Method for Balancing
Equations
Number of H:
Number of O:
Hydrogen + Oxygen → Water
H2 + O2 → H2O 2
2 = 4
1
2 2
2
2 = 4
2 = 2
2
H H
O O
H
O
H
H
O
H
H H
+
Balanced!!!
An Example from
A Word Equation
Hydrogen gas reacts with oxygen gas to
produce water.
Step 1.
hydrogen(g) + oxygen(g) water(l)
Step 2.
H2(g) + O2 (g) -----> H2O(l)
Step 3.
2 H2(g) + O2(g) -----> 2 H2O(l)
Another Example
Iron(III) oxide reacts with carbon monoxide to produce the iron (II) oxide (FeO) and carbon
dioxide.
iron(III) + carbon iron (II) + carbon
Oxide monoxide(g) oxide dioxide (g)
Fe2O3 + CO (g) FeO + CO2 (g)
Fe2O3 + CO (g) 2 FeO + CO2 (g)
Tricks For Balancing
Combustion Reactions---
In balancing combustion reactions, balance
them in the following order:
If you get stuck, 1. Balance Carbon, then Hydrogen,
2. Multiply all coefficients through by 2.
3. Then balance the oxygen.
Give it a try:
C3H6 + O2(g) → CO2(g) + H2O(g) 3 3 6 6
2 9
CHO
Practice Balancing Reactions
__NH3(g) __N2(g) + __H2(g)
__S8(s) + __O2(g) __SO3(g)
__C5H10+ __O2(g)__CO2(g) +__H2O
__HgO(s) __Hg(l) + __O2(g)
__H2O2(l) __H2O(l) + __O2(g)
__Zn(s) + __AlCl3(aq) __ZnCl2(aq)+ __Al
__K2O(s) + __H2O(l) __KOH(aq)
__Na(s) + __CaO(s) __Na2O(s) + __Ca(s)
2 3
12 8
2
2
15
2
10
3
10
2
2 2
3 2
2
2