Foundations of Chemical Change: Equations and Stoichiometry

TL;DR

This topic covers how to write and interpret chemical equations, focusing on organic reactions like elimination, addition, and substitution, while ensuring atoms and mass are conserved. You'll also learn about different organic compound structures, functional groups, and how to define key terms like molecular formula and homologous series. We'll also touch on catalysts and acid-base reactions.

1. The Mental Model

Think of chemistry as building with LEGOs. Chemical equations are your instruction manuals, showing exactly which pieces (atoms) combine, rearrange, or break apart to make new things, always making sure you don't lose any pieces along the way.

2. The Core Material

Organic Molecular Structures and Definitions

Let's start with the building blocks of organic chemistry.

• Molecular formula: This tells you exactly which elements and how many of each atom are in a molecule. E.g., C$_2$H$_6$ for ethane.
• Homologous series: These are families of organic compounds that share a general formula or where one member differs from the next by a CH$_2$ group. They have similar chemical properties.
• Saturated compounds: These have no multiple bonds between carbon atoms in their hydrocarbon chains. Think of them as "full" with hydrogens—only single C-C bonds.
• Unsaturated compounds: These have one or more multiple bonds (double or triple) between carbon atoms in their hydrocarbon chains. They are "not full" and can add more atoms.
• Functional group: This is the specific atom or group of atoms that largely determines a compound's physical and chemical properties. It's the "active" part of the molecule.
• Structural isomer: Organic molecules that have the same molecular formula but different structural formulae. They are made of the same atoms but are put together differently.

Here's a breakdown of common functional groups and homologous series:

You should be able to write condensed structural formulae, structural formulae, molecular formulae, and IUPAC names (up to 8 carbon atoms) for alkanes, alkenes, alkynes, haloalkanes (primary, secondary, tertiary), alcohols (primary, secondary, tertiary), carboxylic acids, esters, aldehydes, and ketones.

For example, 2-butanol is an alcohol. You should be able to draw its structural formula:

graph TD
    A("CH3") --- B("CH(OH)")
    B --- C("CH2")
    C --- D("CH3")

Chemical Equations and Stoichiometry

Chemical equations are shorthand for chemical reactions. They must always be balanced to show:

• Conservation of atoms: The number of atoms of each element on the reactant side (left) must equal the number of atoms of each element on the product side (right).
• Conservation of mass: Based on the conservation of atoms, the total mass of the reactants must equal the total mass of the products. You can verify this using relative atomic masses.

Types of Organic Reactions

1. Elimination Reactions: Products lose atoms from adjacent carbons, forming a double or triple bond.

• Dehydrohalogenation of haloalkanes: Elimination of hydrogen (H) and a halogen (X) from a haloalkane to form an alkene.

  • Conditions: Strong base (e.g., KOH or NaOH) in hot alcoholic solution.
  • Equation (condensed structural formulae):
    CH$_3$CHBrCH$_3$ + KOH (hot, alc) → CH$_3$CH=CH$_2$ + KBr + H$_2$O
    (2-bromopropane reacting to form propene)

• Dehydration of alcohols: Elimination of water (H$_2$O) from an alcohol to form an alkene.

  • Conditions: Concentrated sulfuric acid (H$_2$SO$_4$) or phosphoric acid (H$_3$PO$_4$) as a catalyst, and heat.
  • Equation (condensed structural formulae):
    CH$_3$CH$_2$CH$_2$OH $\xrightarrow{conc. H_2SO_4, \text{heat}}$ CH$_3$CH=CH$_2$ + H$_2$O
    (Propan-1-ol dehydrating to form propene)

• Cracking of alkanes: Longer chain hydrocarbons are broken down into shorter, more useful molecules (alkanes and alkenes).

  • Conditions: High temperature and/or catalyst (e.g., Al$_2$O$_3$/SiO$_2$).
  • Equation (condensed structural formulae, example):
    C${10}$H${22}$ $\xrightarrow{heat/catalyst}$ C$8$H${18}$ + C$_2$H$_4$
    (Decane cracking into octane and ethene)

2. Addition Reactions of Alkenes: Unsaturated compounds (alkenes) gain atoms across the double bond, becoming more saturated. You'll need to identify major and minor products, often following Markovnikov's rule (hydrogen adds to the carbon with more hydrogens already attached).

• Hydrohalogenation: Addition of a hydrogen halide (HX, e.g., HBr) to an alkene.

  • Conditions: No specific catalyst usually, sometimes heat for faster reaction.
  • Equation (structural formulae):
    H$_2$C=CH$_2$ + H-Br → H$_3$C-CH$_2$-Br
    (Ethene + HBr → Bromoethane)

• Halogenation: Reaction of a halogen (X$_2$, e.g., Br$_2$, Cl$_2$) with an alkene.

  • Conditions: Room temperature, often in a non-polar solvent like CCl$_4$. This reaction is used to distinguish saturated from unsaturated compounds using bromine water (bromine water turns from reddish-brown to colorless in the presence of an alkene/alkyne).
  • Equation (structural formulae):
    H$_2$C=CH$_2$ + Br-Br → H$_2$CBr-CH$_2$Br
    (Ethene + Br$_2$ → 1,2-Dibromoethane)

• Hydration: Addition of water (H$_2$O) to an alkene.

  • Conditions: Acid catalyst (e.g., H$_2$SO$_4$), steam.
  • Equation (condensed structural formulae):
    CH$_3$CH=CH$_2$ + H$_2$O $\xrightarrow{H_2SO_4, \text{steam}}$ CH$_3$CH(OH)CH$_3$
    (Propene + H$_2$O → Propan-2-ol, major product)

• Hydrogenation: Addition of hydrogen (H$_2$) to an alkene.

  • Conditions: Metal catalyst (e.g., Ni, Pt, Pd), heat, and pressure.
  • Equation (condensed structural formulae):
    CH$_3$CH=CH$_2$ + H$_2$ $\xrightarrow{Ni, \text{heat}}$ CH$_3$CH$_2$CH$_3$
    (Propene + H$_2$ → Propane)

3. Substitution Reactions: An atom or group of atoms is replaced by another atom or group.

• Hydrolysis of haloalkanes: Reaction of a compound (haloalkane) with water (often in the presence of a base) to replace the halogen with a hydroxyl group (-OH).

  • Conditions: Aqueous base (e.g., NaOH(aq)), heat.
  • Equation (condensed structural formulae):
    CH$_3$CH$_2$Br + NaOH(aq) $\xrightarrow{heat}$ CH$_3$CH$_2$OH + NaBr
    (Bromoethane + aqueous NaOH → Ethanol + Sodium bromide)

• Reactions of HX (X = Cl, Br) with alcohols: Alcohols react with hydrogen halides to produce haloalkanes.

  • Conditions: Concentrated HX, sometimes heat, or a Lewis acid catalyst (e.g., ZnCl$_2$).
  • Equation (condensed structural formulae):
    CH$_3$CH$_2$OH + HBr $\xrightarrow{heat}$ CH$_3$CH$_2$Br + H$_2$O
    (Ethanol + HBr → Bromoethane + Water)

• Halogenation of alkanes: Reaction of a halogen (Br$_2$, Cl$_2$) with an alkane.

  • Conditions: UV light. This is a free radical substitution reaction.
  • Equation (condensed structural formulae):
    CH$_4$ + Cl$_2$ $\xrightarrow{UV \text{ light}}$ CH$_3$Cl + HCl
    (Methane + Chlorine → Chloromethane + Hydrogen chloride)

Mechanism of Reaction and Catalysis

• Catalyst: A substance that increases the rate of a chemical reaction without itself undergoing a permanent chemical change. It participates in the reaction but is regenerated at the end.

Acid-Base Reactions

You need to know how to write reaction equations for aqueous solutions of acids and bases. Typically, this refers to neutralization reactions, forming a salt and water.

• Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H$_2$O(l)

3. Worked Example

Let's look at the dehydration of butan-2-ol.

1. Identify the reactant: Butan-2-ol (an alcohol)
   • Molecular formula: C$4$H${10}$O
   • Condensed structural formula: CH$_3$CH(OH)CH$_2$CH$_3$
2. Identify the reaction type: Dehydration (elimination of water).
3. Identify conditions: Concentrated sulfuric acid (H$_2$SO$_4$), heat.
4. Determine products: Water is eliminated. The -OH group is removed from C2, and a H atom is removed from an adjacent carbon. This can be carbon 1