Chapter 8: Chemical Reactions - Notes

Chapter 8: Chemical Reactions

Section 1: Describing Chemical Reactions

• Chemical Reaction: Substances change into different substances.

• Reactants: Original substances.

• Products: Resulting substances.

• Law of Conservation of Mass: Mass of reactants equals mass of products.

• Chemical Equation: Uses symbols and formulas to represent reactants and products.

  • Example: $(NH<em>4)</em>2Cr<em>2O</em>7(s) \rightarrow N<em>2(g) + Cr</em>2O<em>3(s) + 4H</em>2O(g)$

• Indications of a Chemical Reaction: Heat/light, gas production, precipitate formation, color change.

• Characteristics of Chemical Equations: Known facts, correct formulas, law of conservation of mass.

  • Coefficient: Number in front of a formula.

• Diatomic Molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

• Word Equation: Uses words.

  • Example: methane + oxygen → carbon dioxide + water

• Formula Equation: Uses symbols and formulas.

  • Example: $CH<em>4(g) + O</em>2(g) \rightarrow CO<em>2(g) + H</em>2O(g)$

• Balancing Equations: Ensuring atoms are the same on both sides.

• Methane Combustion: $CH<em>4 + 2O</em>2 \rightarrow CO<em>2 + 2H</em>2O$

• Sample Problem A: $Na<em>2O(s) + H</em>2O(l) \rightarrow 2NaOH(aq)$

• Significance: Coefficients show relative amounts.

• Limitations: Doesn't show if a reaction occurs, reaction speed, or bonding changes.

• Balancing Equations by Inspection: Balance atoms of elements combined and appearing only once on each side.

• Sample Problem C: $Zn(s) + 2HCl(aq) \rightarrow ZnCl<em>2(aq) + H</em>2(g)$

• Sample Problem D: $Al<em>4C</em>3(s) + 12H<em>2O(l) \rightarrow 3CH</em>4(g) + 4Al(OH)_3(s)$

Section 2: Types of Chemical Reactions

• Five Basic Types of Reactions: Synthesis, Decomposition, Single-displacement, Double-displacement, Combustion

• Synthesis Reactions: Two or more substances combine to form a new compound.

  • General equation: $A + X \rightarrow AX$

• Reactions of Elements with Oxygen and Sulfur: Element combines with oxygen to produce an oxide.

  • Example: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$

• Reactions of Metals with Halogens: Metals react with Group 17 elements (halogens) to form ionic or covalent compounds.

  • Example: $2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$

• Synthesis Reactions with Oxides: Oxides of active metals react with water to produce metal hydroxides.

  • Example: $CaO(s) + H<em>2O(l) \rightarrow Ca(OH)</em>2(s)$

• Decomposition Reactions: A single compound breaks down into two or more simpler substances.

  • General equation: $AX \rightarrow A + X$

• Single-Displacement Reactions: One element replaces a similar element in a compound.

  • General equations: $A + BX \rightarrow AX + B$ or $Y + BX \rightarrow BY + X$

• Displacement of a Metal: $2Al(s) + 3Pb(NO<em>3)</em>2(aq) \rightarrow 3Pb(s) + 2Al(NO<em>3)</em>3(aq)$

• Displacement of Hydrogen: $2Na(s) + 2H<em>2O(l) \rightarrow 2NaOH(aq) + H</em>2(g)$

• Displacement of Halogens: $F<em>2(g) + 2NaCl(aq) \rightarrow 2NaF(aq) + Cl</em>2(g)$

• Double-Displacement Reactions: Ions of two compounds exchange places.

  • General equation: $AX + BY \rightarrow AY + BX$

• Combustion Reactions: A substance combines with oxygen, releasing energy.

  • Example: $2H<em>2(g) + O</em>2(g) \rightarrow 2H_2O(g)$

Section 3: Activity Series of the Elements

• Activity: The ability of an element to react.

• Activity Series: Elements organized by ease of reaction.

• Activity Series and Single-Displacement Reactions: Most-active element replaces elements below it.

• Activity of Metals: (Examples) Li, K, Na, Mg, Al…