1.2 Combustion and Alkanes

Combustion: Overview and Representations

Combustion is introduced as a topic to connect three ways of representing chemical change: macro, micro, and symbolic.
It typically involves a fuel reacting with an oxidizer (often oxygen) to release energy in the form of heat and light.

Representations of Chemical Reactions (Three Representations)

Macro (lab observations): what we can see in the experiment (flame, color change, heat, gas production).
Micro (particle level): atoms, molecules, electrons, bonds rearranging during the reaction.
Symbolic (letters and numbers): chemical formulas and chemical equations that encode the reaction.
Goal: connect these three representations fluidly so the macro observations correspond to micro-level particle events and to the symbolic equation.

Think-Pair-Share and Observational Tools

Prompt used: What do you already know about combustion?
Activities include: Think, pair, share; demonstrations to illustrate concepts.

Video Demonstrations (resource references)

Burning methane bubbles: https://www.youtube.com/watch?v=J6Xn_ycdhEo
Burning hydrogen balloons: https://www.youtube.com/watch?v=nLuOM9aOWvk
Striking a match in a bag full of hydrogen gas: https://www.youtube.com/watch?v=6lK8vi3FGRY
Burning magnesium: https://www.youtube.com/watch?v=IqOrCiOquRI

Dalton’s Atomic Theory (1808)

Elements are made of tiny, indivisible particles called atoms.
The atoms of each element are unique (distinct for each element).
Atoms can join together in whole-number ratios to form compounds.
Atoms rearrange in chemical reactions but are not created or destroyed (law of conservation of mass).

Matter: Atoms, Molecules, Formula Units

Matter is composed of fundamental units: atoms, molecules, or formula units.
These units underpin how substances are represented in formulas and reactions.

Combustion of Methane: Reaction and Notation

Reactants: methane (CH$4$) and oxygen (O$2$)
Products: carbon dioxide (CO$2$) and water (H$2$O)
Unbalanced representation (as shown on the slide): CH$4$ + 2 O$2$ → CO$2$ + 2 H$2$O
Balanced chemical equation (corrected, standard notation): CH4 + 2 O2 \rightarrow CO2 + 2 H2O
Key idea: the equation must reflect the rearrangement of atoms without creating or destroying them.

Balancing Equations and Conservation of Mass

The balanced equation ensures the same number of each type of atom on both sides of the equation.
Law of conservation of mass: matter cannot be created or destroyed in a chemical reaction; atoms are simply rearranged.
Analogy: disassembling and reassembling Lego bricks to form a new structure without adding or removing bricks.
In the methane combustion example, balancing ensures the carbon, hydrogen, and oxygen atoms are accounted for on both sides.

Writing Formulas: Subscripts and Coefficients

Subscripts describe how many atoms of each element are in a single molecule.
Coefficients describe how many molecules are present in the reaction.
The order of elements in a formula typically lists the elements in a conventional sequence; the note in the slide mentions that the element in the middle is usually listed first in the formula (a convention used in some teaching materials).
Example concepts:
- For methane, CH$_4$ means 1 carbon atom and 4 hydrogen atoms per molecule.
- For water, H$_2$O means 2 hydrogen atoms and 1 oxygen atom per molecule.

Practice: Writing Formulas (Elements to Represent as Molecules)

Write formulas for these diatomic elements commonly encountered:
- Oxygen → O$_2$
- Hydrogen → H$_2$
- Nitrogen → N$_2$
- Chlorine → Cl$_2$

Combustion: General Pattern

Combustion pattern (complete combustion) for a hydrocarbon fuel:
- Fuel + O$2$ → CO$2$ + H$_2$O
Generalized hydrocarbon combustion formula:
- ext{C}x ext{H}y + \left( x + \frac{y}{4} \right) \text{O}2 \rightarrow x \text{CO}2 + \frac{y}{2} \text{H}_2 ext{O}
Note: Incomplete combustion can yield CO or C (soot) as products, but the slide focuses on complete combustion products (CO$2$ and H$2$O).

Combustion of Magnesium ( Mg + O$_2$ )

Magnesium reacts with oxygen to form magnesium oxide (MgO).
Balanced equation: 2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}

Applying the Pattern to Magnesium and Oxygen

Using the general combustion pattern for a metal reacting with oxygen, magnesium combines to form the oxide MgO.
The balanced equation shows a 2:2 ratio of Mg to MgO with oxygen consumption consistent with forming two MgO units.

Why Magnesium Oxide, Water, and Carbon Dioxide Have Different Properties

Question to ponder: Why do MgO, H$2$O, and CO$2$ have different properties?
Answer (from slide): Because their structures are different at the macro and micro levels.
MgO forms an ionic crystal lattice (extended 3D structure), while H$2$O and CO$2$ are discrete molecules with different inter-molecular interactions.
Structural differences lead to differences in properties such as melting point, hardness, solubility, and phase behavior.

Types of Bonding: Ionic vs Covalent

Ionic bonding:
- Electrons are transferred from a metal to a nonmetal, forming positively and negatively charged ions.
- Ions are held together by electrostatic attraction (ionic bonds).
- Results in extended 3D crystal structures and formula units; typically solids.
Covalent bonding:
- Electrons are shared between two nonmetal atoms.
- Atoms are held together by the attraction between nuclei and shared electrons.
- Results in molecules; substances can be solids, liquids, or gases.

Periodic Table of the Elements: Overview

The periodic table organizes elements by periodic trends and groups/periods.
Key groupings and features:
- Metals, nonmetals, metalloids
- Alkali metals, alkaline earth metals (alkaline earths)
- Transition metals
- Chalcogens (Group 16), Halogens (Group 17)
- Lanthanide series and Actinide series (f-block elements)
The slide provides a stylized map of elements with representative symbols and approximate positions (e.g., H, He, Li, Be, B, C, N, O, F, Ne, etc.).

Mark on Your Periodic Table

Categories to identify on the periodic table:
- Metals
- Nonmetals
- Metalloids
- Alkali Metals
- Alkaline Earth Metals (Alkaline Earths)
- Transition Metals
- Chalcogens
- Halogens

Practice: Metal, Nonmetal or Metalloid? and Family

Elements to classify: S, Te, Po, Fe, Li, F, Ba, C
Likely classifications (based on common teaching conventions):
- S: Nonmetal; Chalcogen family (Group 16)
- Te: Metalloid; Chalcogen family
- Po: Metalloid (sometimes treated as a metalloid or post-transition metal depending on chart)
- Fe: Metal; Transition metal
- Li: Metal; Alkali metal (Group 1)
- F: Nonmetal; Halogen family
- Ba: Metal; Alkaline earth metal (Group 2)
- C: Nonmetal; Carbon family (Group 14)

Practice: Bonding Type Between Pairs

Determine the bonding type for each pair:
- C and O: Covalent (polar covalent, due to electronegativity difference)
- Li and Cl: Ionic
- Cl and Cl: Covalent (nonpolar)
- Fe and S: Typically Ionic in simple ionic compounds (e.g., FeS), though some covalent character can exist depending on compound
- C and F: Covalent (highly polar)

Summary: Core Concepts to Remember

Representations: Macro (observations), Micro (particle interactions), Symbolic (formulas/equations) are interconnected.
Dalton’s atomic theory provides the foundation for understanding atoms, compounds, and reactions.
In chemical reactions, atoms are conserved; balancing equations enforces the conservation of mass.
Combustion patterns are predictable for complete combustion of hydrocarbons: fuel + O$2$ → CO$2$ + H$2$O, with generalized form for C$x$H$_y$ fuels.
Magnesium combustion demonstrates metal-oxide formation and the same balancing principles apply.
The nature of bonding (ionic vs covalent) explains macroscopic properties and crystal vs molecular structures.
The periodic table organizes elements into families and groups that guide predictions about bonding, reactivity, and properties.
Practice sections help classify elements by metal/nonmetal/metalloid identities and family groups, and determine bonding type for simple pairs.