CHEMICAL REACTIONS
- Writing Chemical Equations
o A chemical reaction is a process in which substances interact to form 1 or more different
substances
o The reactants (or starting substances) interact and form different substances called
products
o A chemical equation can be a word equation, a skeleton equation, or a balanced
chemical equation
 1. Word equations
 Reactants and products are identified by their names only
o Ex. Iron + oxygen iron (III) oxide
 An arrow shows the direction of a chemical change
 Some reactions are reversible, so products can change back into
reactants
o We use a double arrow, ↔, to represent reversible reactions
 Ex. hypochlorus acid + hydrochloric acid → chlorine gas
+ dihydrogen oxide
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2. Skeleton Equation
 Chemical formulas are used to represent the substances
 Show the number of atoms of each element and the physical state of
each substance
o Ex. Fe (s) + O2 (g) → Fe2O3 (s)
o HClO (aq) + HCl (aq) → Cl2 (g) + H2O (l)
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3. Balanced chemical reactions
 Use coefficients (positive number in front of a formula) to have the
same number of atoms on reactants side, as you would have on product
side
o Ex. 2Al (s) + 3Br2 (g) 2AlBr3
 Some tips:
o Start with substances with the largest number of atoms
o Balance H, O and any element that appears in more than 2
substances later
o Balance polyatomic ions as single items
 Ex. Mg (s) + AlCl3 (aq) →
 C3H8 + O2 →
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Synthesis Reactions and Decomposition Reactions
o Synthesis Reactions
 Two or more substances react to produce a single compound
 A + B → AB
 Can be any combination of elements and compounds, however, product is
always a single compound
 3 types:
 2 elements forming a binary compound
o A univalent metal (metal that can form ions with only 1 charge)
reacting with a non-metal to form an ionic compound
 Ex. 2 Na (s) + Cl2 (g) → 2 NaCl (s)
o A multivalent metal reacting with a non-metal to form various
compounds
 Ex. 2 Cu (s) + Cl2 (g) → 2 CuCl (s)
 Cu (s) + Cl2 (g) → CuCl2 (s)
 Copper can have a charge +1, or +2
o 2 non-metals combining to form a molecular compound
 Because they do not form ions, there are no charges to
help determine chemical formulas
 Ex. C (s) + O2 (g) → CO2 (g)
 C (s) + ½ O2 (g) → CO (g)
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Element and compound forming a new compound
o Ex. 2SO2 (g) + O2 (g) → 2SO3 (g)
o Ex. phosphorus trichloride + chlorine gas in a reversible
reaction to produce phosphorus pentachloride.
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2 compounds forming a new compound
o Non-metal oxide reacting with water
 Ex. CO2 (g) + H2O (l) → H2CO3 (aq)
 SO3 (g) + H2O (l) → H2SO4 (aq)
o Metal oxide reacting with water
 Ex. Na2O (s) + H2O (l) → 2NaOH (aq)
Decomposition Reactions
o A reaction in which a compound breaks down into 2 or more elements or simpler
compounds
 It is often the reverse of a synthesis reaction
 Ex. AB → A + B
o A single reactant (which is always a compound) is changed into more than 1 product
o 4 types:
 A binary compound decomposing into its elements
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Heat or electric current is used to break the compound
o Ex. An electrolysis reaction uses electrical energy to split water
molecules
 2H2O (l) → 2H2 (g) + O2 (g)
o Ex. 2NaC l (l) → 2Na (l) + Cl2(g)
o 2HgO (s) → 2Hg (l) + O2 (g)
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Metal nitrate decomposing into a metal nitrite and oxygen gas
 Ternary compounds do not usually decompose into their individual
elements
o Ex. 2NaNO3 (s) → 2NaNO2 (s) + O2 (g)
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Metal carbonate decomposing into metal oxide and carbon dioxide
 Ex. CaCO3 (s) → CaO (s) + CO2 (g)
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Metal hydroxide decomposing into a metal oxide and water
 Ex. Ca(OH)2 (s) → CaO (s) + H2O (g)
Other examples of decomposition reactions:
 2C7H5N3O6 (s) → 3N2 (g) + 5H2O (g) + 7CO (g) + 7C (s)
 (NH4)2Cr2O7 (s) → N2 (g) + Cr2O3 (s) + 4H2O (g)
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Combustion Reactions
o Oxygen combines with another substance and releases energy in the form of heat and
light
 Ex. C4H10 (g) + 13/2 O2 (g) → 4CO2 (g) + 5H2O (g)
o Complete combustion reactions always produce carbon dioxide gas and water vapour
o Incomplete combustion reactions occur due to small amounts of oxygen being present
 Ex. Yellow flame produced by a lit candle
 Ex. 2C3H8 (g) + 7O2 (g) → 2C (s) + 2CO (g) + 2CO2 (g) + 8H2O (g)
o Other possible reactions, aside from hydrocarbons and oxygen, are other
elements/compounds mixed with oxygen to produce oxides
 Ex. 2H2 (g) + O2 (g) → 2H2O (g)
 C6H12O6 (s) + 6O2 (g) → 6CO2 (g) + 6H2O (g)
 Combustion of glucose
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Single Displacement Reactions
o A reaction in which 1 element takes the place of another element in a compound
o Types:
 Metal displaces another metal from an ionic compound
o A + BX → AX + B
o Ex. Cu (s) + 2AgNO3 (aq) → Cu(NO3)2 (aq) + 2Ag (s)
 We use an activity series to determine id a reaction will proceed or if
the reaction can reverse
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The more reactive metal will always be responsible for the reaction
occurring
o The metal displacing the other metal in the ionic compound
must be higher on the activity series
 Ex. Zn (s) + Pb(SO4)2 (aq) → ZnSO4 (aq) + Pb (s)
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Metal displacing H from acid or water
 Although H is not a metal, its single valence electron allows it to form a
H ion
o When it is replaced by a metal it produces H gas
 Ex. Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
 2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
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Non-metal displacing another non-metal from an ionic compound
 AX + Y → AY + X
 When 1 reactant is a diatomic and the other is an ionic compound that
contains a halogen
o The halogens replace each other
 Activity series:
 Most reactive: F, Cl, Br, I: Least reactive
o Ex. Cl2 (g) + 2NaBr (aq) → 2NaCl (aq) +
Br2 (l)
Double Displacement Reactions
o Involves the exchange of positive ions between two ionic compounds to form New Ionic
compounds
 Generally occurs between compounds that are in aqueous solutions
 AX + BY → AY + BX
o Types:
 Reaction that forms a solid
 A common observation of many double displacement reactions is the
formation of a solid precipitate
o A precipitate is an insoluble solid that is formed by a chemical
reaction between 2 soluble compounds
 Ex. AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
o Refer to data table on solubility
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Reaction that forms a gas
 Many of these reactions are 2 reactions that occur in rapid succession
o But then 1 of the products quickly decomposes into water and a
gas
 Generally:
o Acid + compound containing CO3-2 → ionic compound +
water + carbon dioxide
o Compound containing NH4+ + compound containing OH- →
ionic compound + water + ammonia
 Ex. CH3COOH (aq) + NaHCO3 (s) → NaCH3COO (aq) +
H2CO3 (aq)
 H2CO3 decomposes
 Therefore CH3COOH (aq) + NaHCO3 (s) → NaCH3COO
(aq) + H2O (l) + CO2 (g)
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Ex. Potassium sulfate + calcium chloride →
Aluminum sulfate + lead (II) nitrate →
FeCl3 + NaOH →
ZnBr2 + CuCH3COOH →
Ex. NH4Cl (aq) + NaOH (aq) → NaCl (aq) + NH4OH (aq)
 NH4OH decomposes
NH4Cl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) + NH3
(g)
Reaction that forms water
 Are neutralization reactions (acid + base → water + salt)
o ex. H2SO4 (aq) + Ca(OH)2 (aq) → CaSO4 (s) + 2H2O (l)