Predicting and Classifying Chemical Reactions

Double Replacement and Precipitation Reactions

• Cations combine with anions from opposing compounds to form new ionic products.
• Solubility rules determine phase states:
  • Alkali metals like $K^+$ and $Na^+$ form soluble, aqueous compounds ($(aq)$).
  • Specific cations with sulfate ($SO_4^{2-}$) or hydroxide ($OH^-$) form insoluble precipitates ($(s)$).
• Representative reactions:
  • $BaCl_2(aq) + K_2SO_4(aq) \rightarrow BaSO_4(s) + 2KCl(aq)$
  • $MgCl_2(aq) + 2KOH(aq) \rightarrow Mg(OH)_2(s) + 2KCl(aq)$

Single Replacement Reactions

• A pure element reacts with an aqueous compound, displacing an ion to form a new compound and a liberated element.
• Group 1 metals form $+1$ cations, Group 3A metals form $+3$ cations, and free hydrogen liberates as a diatomic gas ($H_2$).
• Representative reactions:
  • $2HCl(aq) + 2Na(s) \rightarrow 2NaCl(aq) + H_2(g)$
  • $3CuSO_4(aq) + 2Al(s) \rightarrow Al_2(SO_4)_3(aq) + 3Cu(s)$
  • $2Li(s) + H_2SO_4(aq) \rightarrow Li_2SO_4(aq) + H_2(g)$

Acid-Base Neutralization Reactions

• Acids ($H^+$) and bases ($OH^-$) react to yield an aqueous salt and liquid water ($H_2O(l)$).
• Representing water as $HOH$ during balancing simplifies tracking hydrogen and hydroxide ions.
• Representative reactions:
  • $HCl(aq) + KOH(aq) \rightarrow KCl(aq) + H_2O(l)$
  • $Ca(OH)_2(aq) + H_2SO_4(aq) \rightarrow CaSO_4(s) + 2H_2O(l)$
  • $Mg(OH)_2(aq) + 2HNO_3(aq) \rightarrow Mg(NO_3)_2(aq) + 2H_2O(l)$

Classifying Reaction Types

• Synthesis (Combination): Two or more reactants combine to form a single product.
• Decomposition: A single reactant breaks down into two or more products.
• Combustion: A substance reacts with oxygen ($O_2$) gas.