Unit 5H Lecture 2: Types of Reactions and Predicting Products

Driving Forces of Chemical Reaction

• Chemical reactions are propelled by specific driving forces that lead to the transformation of reactants into products. These include:
  • Formation of a gas: The evolution of a gaseous substance during the reaction.
  • Formation of a liquid (molecule): Specifically the formation of stable molecules such as water ($H_2O$), often observed in neutralization reactions.
  • Formation of a solid/precipitate: The creation of an insoluble substance that falls out of solution.
  • Transfer of electrons (redox reactions): These involve the movement of electrons between species.
  • The mnemonic "OIL RIG" is used to remember the process:
    • OIL: Oxidation Is Loss (of electrons, $e^-$).
    • RIG: Reduction Is Gain (of electrons, $e^-$).

The Activity Series

• The Activity Series is a tool used to determine the reactivity of elements and their tendency to form compounds:
  • An element that is more reactive will have a stronger drive to form a compound.
  • Metals: Some metals are more reactive than others, meaning they lose electrons more easily to form compounds.
  • Nonmetals: Some nonmetals are more reactive than others, meaning they gain electrons more easily to form compounds.

Essential Chemical Terminology and Solubility Rules

• Solution: A homogeneous mixture where two or more substances are distributed evenly throughout.
• Insoluble: A substance's inability to dissolve in a specific solvent.
• Soluble: A substance's ability to dissolve in a specific solvent.
  • Aqueous (aq): Refers to a substance that is soluble in water. Ionic compounds in an aqueous state dissociate, or separate into their constituent ions.
  • Example of dissociation: $NaCl(aq) \rightarrow Na^+ + Cl^-$
• Spectator Ions: These ions remain aqueous throughout the reaction and are not directly involved in the driving force of the chemical change.
• NR (No Reaction): Occurs when the products of a potential reaction remain aqueous. If no solid, liquid, or gas is formed, no reaction has taken place.
• Solubility Guarantees: Certain ions are ALWAYS soluble ($aq$) and will not form precipitates. These include:
  • Ammonium ($NH_4^+$)
  • Group 1 metals (Alkali metals)
  • Nitrate ($NO_3^-$)
  • Example of No Reaction: $NaCl(aq) + KNO_3(aq) \rightarrow NR$

Methodology for Predicting Products

• To accurately predict the results of a chemical reaction, follow these sequential steps:
  • Determine ion formation: Identify the charges of the elements or polyatomic ions found in the reactants.
  • Rearrange elements: Use knowledge of chemical compounds to pair reactants.
  • Rule on Subscripts: Do not carry over subscripts from the reactant side to the product side unless that subscript is part of a polyatomic ion.
  • Charge Attraction: A Cation ($+$ charge) is always attracted to an Anion ($-$ charge).
  • Ordering: The Cation is always written first in the chemical formula.
  • Net Zero Charge: Ionic compounds must have no overall charge. Use the "criss-cross" method with the charges of the ions to ensure the compound reaches a net zero charge.
  • Balance the Equation: Once the correct formulas for the products are determined, balance the entire equation by placing coefficients in front of the compounds as necessary.

Synthesis / Combination Reactions

• Definition: A synthesis or combination reaction ($A + X \rightarrow AX$) occurs when a compound is formed from simpler elements or compounds. These reactions start with multiple reactants and result in a single product.
• Predicting Synthesis Products: The two reactants combine based on their ionic charges.
  • Example 1: Magnesium reacting with Oxygen.
  • Reactants: $Mg(s) + O_2(g)$
  • Ion formation: $Mg^{2+}$ and $O^{2-}$ come together in a $1:1$ ratio.
  • Result: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$
• Additional Synthesis Examples:
  • $8Ba(s) + S_8(s) \rightarrow 8BaS(s)$
  • $4Fe(s) + 3O_2(g) \rightarrow 2Fe_2O_3(s)$
  • $2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)$

Decomposition Reactions

• Definition: A decomposition reaction ($AX \rightarrow A + X$) occurs when a single compound breaks down into simpler elements or compounds. It starts with one reactant and ends with multiple products.
• Predicting Decomposition Products: While some decomposition products are difficult to predict, students are expected to predict when a compound breaks down into its constituent elements.
• Specific Decomposition Examples:
  • Hydrogen peroxide: $2H_2O_2 \rightarrow 2H_2O + O_2(g)$
  • Potassium chlorate: $2KClO_3 \rightarrow 2KCl + 3O_2(g)$
  • Sodium azide (the reaction used in vehicle air bags): $2NaN_3(s) \rightarrow 2Na(s) + 3N_2(g)$
  • Water electrolysis: $2H_2O(l) \rightarrow 2H_2(g) + O_2(g)$
  • Calcium carbonate: $CaCO_3(s) \rightarrow CaO(s) + CO_2(g)$
  • Calcium hydroxide: $Ca(OH)_2(s) \rightarrow CaO(s) + H_2O(l)$

Single Replacement Reactions

• Definition: Single replacement reactions ($AB + X \rightarrow XB + A$) occur when one element replaces another similar element within a compound. A metal replaces a metal, or a nonmetal replaces a nonmetal.
• Role of the Activity Series: A single replacement will only occur if the standalone element is more reactive than the element it is attempting to replace.
• Predicting Single Replacement Products: Use ion formation to determine the new compound. The cation will pair with the anion.
• Examples and Practice:
  • Aluminum and Copper (II) Chloride: $2Al(s) + 3CuCl_2(aq) \rightarrow 3Cu(s) + 2AlCl_3(aq)$
  • Magnesium and Hydrochloric Acid: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$
  • Aluminum and Lead (II) Nitrate: $2Al(s) + 3Pb(NO_3)_2(aq) \rightarrow 3Pb(s) + 2Al(NO_3)_3(aq)$
  • Sodium and Water: $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$

Double Replacement and Neutralization Reactions

• Definition: Double replacement reactions ($AX + BY \rightarrow AY + BX$) occur when the metal ions of two compounds exchange places in an aqueous solution to form two entirely new compounds.
• Predicting Double Replacement Products: Switch the metals (cations) between the two compounds and ensure the new pairings are charge-balanced (Cation with Anion).
• Examples:
  • Magnesium hydroxide and Hydrochloric acid: $Mg(OH)_2(aq) + 2HCl(aq) \rightarrow MgCl_2(aq) + 2H_2O(l)$
  • Lead (II) nitrate and Potassium iodide: $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$
  • Iron (II) sulfide and Hydrochloric acid: $FeS(s) + 2HCl(aq) \rightarrow H_2S(g) + FeCl_2(aq)$
• Neutralization Reactions: A specific sub-type of double replacement where an Acid reacts with a Base to produce a Salt and Water.
  • Formula: $\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$
  • Example: $HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$

Combustion Reactions

• Definition: Combustion is the chemical process of burning a substance in the presence of oxygen from the air.
• Types of Combustion:
  • Synthesis-style Combustion: A reaction where one of the reactants is pure oxygen and a single product is formed.
  • Example: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$
  • Hydrocarbon Combustion: A reaction between oxygen and a hydrocarbon ($C_xH_y$). This type of reaction ALWAYS produces carbon dioxide ($CO_2$) and water ($H_2O$).
  • General Formula: $C_xH_y + O_2(g) \rightarrow CO_2(g) + H_2O(l)$
  • Propane Example: $C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g)$
  • Ethanol Example: $C_2H_5OH(g) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$ (Note: Predicted products are $CO_2$ and $H_2O$).