Unit 9: Reactions
A chemical equation shows what happens in a chemical reaction:
β’ Reactants: substances that START the reaction (left side of arrow)
β’ Products: new substances FORMED by the reaction (right side of arrow)
β’ Arrow (β): reads as "yields" or "produces"
Word equation: uses names β "hydrogen + oxygen β water"
Formula equation: uses chemical formulas β "2Hβ + Oβ β 2HβO"
State symbols written in subscript after the formula:
β’ (s) = solid
β’ (l) = liquid
β’ (g) = gas
β’ (aq) = aqueous (dissolved in water)
Conditions written above/below the arrow:
β’ Ξ (delta) = heat applied
β’ Temperature, pressure conditions
β’ Catalysts
Balancing equations (Law of Conservation of Mass):
Atoms are neither created nor destroyed β the same number of each type of atom must appear on both sides.
Method: adjust COEFFICIENTS (numbers in front of formulas), NEVER subscripts.
There are five main reaction types:
1. SYNTHESIS (Combination): Two or more reactants combine to form ONE product.
General: A + B β AB
Action words: "combine," "react to form," "unite"
Example: 2Hβ + Oβ β 2HβO
2. DECOMPOSITION: ONE compound breaks down into two or more simpler substances.
General: AB β A + B
Action words: "decomposes," "breaks down," "splits"
Example: 2HβO β 2Hβ + Oβ (with electricity)
3. COMBUSTION: A hydrocarbon (C and H compound) reacts with Oβ to produce COβ and HβO.
Complete combustion (excess Oβ): β COβ + HβO
Example: CHβ + 2Oβ β COβ + 2HβO
4. SINGLE DISPLACEMENT: One element replaces another in a compound.
General: A + BC β AC + B (element A replaces element B)
Example: Zn + 2HCl β ZnClβ + Hβ (Zn replaces H)
β Only occurs if A is MORE reactive than B (see reactivity series)
5. DOUBLE DISPLACEMENT: Two compounds swap partners.
General: AB + CD β AD + CB
Action: identify ionic pairs and swap the cations/anions
Example: NaCl + AgNOβ β NaNOβ + AgClβ (β = precipitate)
The reactivity series (activity series) ranks metals and halogens by how reactive they are. More reactive elements can displace less reactive ones from compounds.
Metal reactivity series (most reactive β least reactive):
Li > K > Ba > Ca > Na > Mg > Al > Zn > Fe > Ni > Sn > Pb > (H) > Cu > Ag > Au > Pt
Halogen reactivity series:
F > Cl > Br > I
Using the series for single displacement:
β’ If Metal A is ABOVE Metal B in the series: A will displace B from solution. Reaction OCCURS.
β’ If Metal A is BELOW Metal B: Reaction does NOT occur.
β’ Metals above H will displace hydrogen from acids; metals below H will not.
Examples:
β’ Mg + FeSOβ β MgSOβ + Fe (Mg is above Fe β OCCURS)
β’ Cu + ZnSOβ β No reaction (Cu is below Zn β DOES NOT OCCUR)
Synthesis: identify the product formed when the two reactants combine.
β’ Metal + oxygen β metal oxide
β’ Metal + nonmetal β ionic compound
Decomposition: identify what simpler substances the compound breaks into.
β’ Metal carbonate β metal oxide + COβ
β’ Metal oxide β metal + oxygen (sometimes)
Double Displacement products: swap the ionic partners.
β’ Identify the cation and anion from each compound
β’ Pair them with the opposite compound's ion
β’ Identify if a precipitate (insoluble solid) forms (marked β)
β’ Identify if a gas is produced (marked β or (g))
Precipitate identification:
β’ A precipitate forms when ions combine to form an INSOLUBLE compound
β’ A "cloudy" or "solid forms in solution" observation = precipitate product
Combustion products: always COβ and HβO (complete combustion)
CβHβ + 5Oβ β 3COβ + 4HβO