Unit 8: Reactions
* refer to page 1 of Reactions notes for extra tips *
* equations must be net ionic *
* write the formulas first, balance last *
Synthesis Reactions



A + B  AB
“NO acid, MO base”: nonmetal oxides form acids, metal oxides form bases
Remember to DISSOCIATE if one of the 8 STRONG bases or 7 STRONG acids is formed
Be careful if an unstable compound is formed – they decompose into more stable products. These are
Ammonium hydroxide…
NH4OH  H2O + NH3
Carbonic acid…
H2CO3  H2O + CO2
Sulfurous acid…
H2SO3  H2O + SO2
*If there is at least one single element by itself on the reactants side, it is also a redox reaction*
1. Metal + Nonmetal  ionic compound (salt)
[redox]
(a) Lithium metal is strongly heated in nitrogen gas.
(i) Balanced equation: 6Li + N2  2 Li3N
(ii) Predict the algebraic sign of entropy change for the reaction. ∆S would be negative because a
solid is formed from two reactants, one of which is a gas.
(b) A strip of magnesium metal is heated
strongly in pure nitrogen gas.
3 Mg + N2  Mg3N2
*Whenever a transition metal reacts with a gas like O2, F2, Cl2 with the addition of heat, the metal will
become ionized to its maximum positive charge
(c) Excess chlorine gas is passed over hot
Cl2 + Cu → CuCl2
copper turnings.
3 O2 + 4 Fe  2 Fe2O3
(d) Excess oxygen gas is passed over hot
iron filings.
2. Metal + oxgyen  metal oxide
(a) Solid calcium metal burns in air.
(b) Magnesium ribbon is burned in oxgyen.
3. Metal oxide + water  base
(a) Solid potassium oxide is added to water.
(b) Solid barium oxide is added to distilled
water.
(c) Solid cesium oxide is added to water.
[also redox AND combustion]
2 Ca + O2  2 CaO
2 Mg + O2  2 MgO
(might be helpful to write water as HOH)
K2O + HOH  2 K+ + 2 OHBaO + HOH  Ba2+ + 2 OH–
Cs2O + HOH  2 Cs+ + 2 OH-
4. Metal oxide + carbon dioxide  Metal carbonate
(a) Solid calcium oxide is exposed to a
stream of carbon dioxide gas.
CaO + CO2 → CaCO3
5. Nonmetal + oxygen  nonmetal oxide
[also redox AND combustion]
N2 + 4 O2  2 NO2
(a) Nitrogen gas is heated in the presence of
oxygen gas.
6. Nonmetal oxide + water  acid
(a) Solid dinitrogen pentoxide is added to
water.
(b) Dinitrogen trioxide gas is bubbled
through water.
N2O5 + H2O → 2H+ + 2NO3–
N2O3 + H2O  2 HNO2
(c) Sulfur trioxide gas is bubbled into water.
SO3 + H2O  H+ + HSO4SO2 + H2O  H2SO3
(d) Sulfur dioxide gas is bubbled through
water.
7. Nonmetal + Nonmetal  Molecular compound
P4 +5 O2  P4O10
(a) Pure, solid phosphorus (white form) is
burned in air.
(b) Solid white phophorus is heated in the
presence of excess chlorine gas.
P4 + 10 Cl2  4 PCl5
8. Metal oxide + nonmetal oxide  Metal w/ polyatomic
(a) A mixture of solid calcium oxide and solid tetraphosphorus decaoxide is heated.
(i) Balanced equation:
6 CaO + P4O10 → 2 Ca3(PO4)2
(ii) Is the product compound(s) soluble in water?
No. Phosphates other than those of group 1 elements and NH4 + are insoluble.
(b) Sulfur dioxide gas is passed over solid calcium oxide.
(i) Balanced equation:
SO2 + CaO → CaSO3
(ii) Name the product compound(s).
calcium sulfite
(c) Solid magnesium oxide is heated with sulfur trioxide gas.
(i) Balanced equation:
MgO + SO3  MgSO4
(ii) What state of matter is the product? aqueous
Decomposition Reactions:
The driving force is usually the production of H2O or a gas (know the common gases on p. 2 of Reactions notes)
 A binary ionic compound breaks apart into metal + nonmetal (electrolysis)
 Metal chlorates break apart into metal chlorides + oxygen
 Hydrogen peroxide breaks apart into water + oxygen
 Acids break apart into nonmetal oxides + water
 Ammonium compounds containing O break apart into ammonia gas + water
 Metal carbonates break apart into metal oxides + carbon dioxide
 Metal bicarbonates (hydrogen carbonate) break apart into metal oxides + carbon dioxide + water
 Metal nitrates break apart into the metal oxide + nitrogen dioxide (brown fumes) + oxygen
 Metal hydroxides break apart into the metal oxide + water
 Metal sulfites break apart into the metal oxide + sulfur dioxide
1. Liquid sodium chloride is electrolyzed.
2. Solid potassium chlorate is strongly
heated.
3. A solution of hydrogen peroxide is
catalytically decomposed.
2 NaCl  2 Na + Cl2
KClO3  KCl + O2
2 H2O2  2 H2O + O2
4. A sample of carbonic acid is heated.
H2CO3  H2O + CO2
5. A sample of sulfurous acid is heated.
H2SO3  H2O + SO2
MgCO3  MgO + CO2
6. Powdered magnesium carbonate is heated
strongly.
7. Solid sodium carbonate is strongly
heated.
8. Solid sodium hydrogen carbonate is
strongly heated.
9. Solid ammonium carbonate decomposes
as it is heated.
Na2CO3  Na2O + CO2
2 NaHCO3  Na2O + 2 CO2 + H2O
(NH4)2CO3  2 NH3 + H2O + CO2
10. Solid strontium nitrate is heated.
2 Sr(NO3)2  2 SrO + 4 NO2 + O2
11. Solid lithium nitrate is heated.
4 LiNO3 2 Li2O + 4 NO2 + O2
12. Solid sodium hydroxide is heated.
2 NaOH  Na2O + H2O
13. Solid ammonium hydroxide is heated.
14. Powdered sodium sulfite is heated.
15. Solid magnesium sulfite decomposes
when heated.
NH4OH  NH3 + H2O
Na2SO3  Na2O + SO2
MgSO3  MgO + SO2
Combustion Reactions:
[these are also redox reactions because there is a single element by itself]
***O2 is always a reactant!
***Balance O last – make sure coefficients are smallest WHOLE numbers***
 If metals are combusted, these are synthesis reactions – combine the metal and the oxygen to
make a metal oxide.
 If nonmetal compounds are combusted, split up the nonmetals by pairing each with oxygen
(these usually produce CO2 and H2O).
Hydrocarbon formulas:
# of C’s
# of H’s
=n
-ane = 2n + 2
-ene = 2n
-yne = 2n – 2
4 Li + O2  2 Li2O
1. Lithium metal is burned in air.
3 O2 + 4 Fe  2 Fe2O3
5
Excess oxygen gas is passed over hot iron
filings.
6
Carbon disulfide vapor is burned in
excess oxygen.
7
Methane gas is burned in air.
8
Propane is burned in air.
C3H8 + 5 O2  3 CO2 + 4 H2O
9
Propene is combusted.
2 C3H6 + 9 O2 6 CO2 + 6 H2O
CS2 + 3O2  CO2 + 2 SO2
CH4 + 2 O2  CO2 + 2 H2O
10 Propyne is burned in air.
C3H4 + 4 O2  3 CO2 + 2 H2O
11 Butane gas is burned in air.
2 C4H10 + 9 O2  8 CO2 + 10 H2O
12 Octane fuel is burned.
2 C8H18 + 25 O2  16 CO2 + 18 H2O
13 Ethyne (acetylene) is burned in air.
2 C2H2 + 5 O2  4 CO2 + 2 H2O
Other Double Replacement Reactions forming gases, water, or weak electrolytes

Some common ones:
1. Metal hydride + water  base + H2 gas
(*same products as if just metal is added to water)
(a) Solid sodium hydride is placed in a beaker of distilled water.
(i) Balanced equation:
NaH + HOH  Na+ + OH- + H2
(ii) The reaction is exothermic, and sometimes small flames are observed as the sodium reacts with
the water. Identify the product of the reaction that burns to produce the flames.
hydrogen gas
(b) Solid lithium hydride is mixed with water.
(i) Balanced equation:
LiH + HOH  Li+ + OH- + H2
(ii) Describe a test to positively identify the gaseous product.
Hydrogen gas will make a popping noise when ignited with a burning splint.
(c) Solid potassium hydride is added to water.
(i) Balanced equation: KH + HOH  K+ + OH- + H2
2. Phosphorus halide + water  hydrohalic acid + phosphoric acid (H3PO4)
(a) Solid phosphorus pentachloride is added to distilled water.
(i) Balanced equation: PCl5 + 4 H2O  5 H+ + 5 Cl- + H3PO4
(b) Solid phosphorus tribromide is placed in water.
(i) Balanced equation: PBr3 + 4 H2O  5 H+ + 3 Br- + H3PO4
Others:
1. A solution of sodium hydroxide is added to a solution of ammonium chloride.
(i) Balanced equation:
NH4+ + OH-  NH3 + H2O
(ii) Name the gas that evolves in the reaction.
ammonia
2. solutions of ammonium chloride and sodium hydroxide are combined
(i) Balanced equation:
+
-
NH4 + OH → NH3 + H2O
(ii) Identify the spectator ion(s) in this reaction.
Na+ and Cl-
3. A solution of sodium hydroxide is added to crystals of ammonium chloride
-
(i) Balanced equation: NH4Cl + OH → NH3 + H2O + Cl
(ii) Identify the spectator ion(s) in this reaction.
-
Na+ only
4. Aqueous potassium hydroxide is added to crystals of ammonium bromide.
(i) Balanced equation:
OH- + NH4Br  NH3 + H2O + Br-
5. Dilute hydrochloric acid is added to a solution of potassium sulfite.
(i) Balanced equation:
2 H+ + SO32–  H2O + SO2
(ii) Name the gas that evolves in the reaction.
sulfur dioxide
6. Solutions of potassium fluoride and hydrochloric acid are mixed.
(i) Balanced equation:
H+ + F–  HF
(ii) Would the resulting solution conduct electricity? Explain.
Even though HF is a molecular compound, K+ and Cl- spectator ions are present. The solution
would conduct electricity.
7. Hydrochloric acid is added to a potassium carbonate solution.
(i) Balanced equation:
2 H+ + CO32-  H2O + CO2
(ii) Describe a test to positively identify the gaseous product.
A glowing splint will be extinguished by the carbon dioxide.
8. Dilute sulfuric acid is added to a solution of barium acetate.
(i) Balanced equation:
H+ + SO42- + Ba2+ + C2H3O2-  BaSO4 + HC2H3O2
(ii) Identify the spectator ions in this reaction. Explain. since both products are poor electrolytes
there are essentially “no” spectator ions
9. Hydrobromic acid is added to a solution of potassium hydrogen carbonate.
(i) Balanced equation:
H+ + HCO3-  CO2 + H2O
(ii) When a gas produced by the reaction is bubbled through limewater (Ca(OH)2 (aq)), what visible
change is expected?
limewater becomes cloudy when carbon dioxide contacts it due to the formation of CaCO3