High School Chemistry: Organic Chemistry Basics - Hydrocarbons - Only Carbon and Hydrogen, Examples Methane, Propane, Octane

Hydrocarbons: The Building Blocks of Life

1. What This Is (In Plain English)

Hydrocarbons are molecules made up of only two elements: carbon (C) and hydrogen (H). They're like LEGO blocks, but instead of plastic bricks, we use carbon and hydrogen atoms to build all sorts of cool things.

In real life, hydrocarbons matter because they're the main ingredients in many things we use every day, like gasoline for cars, natural gas for heating, and even the plastic in your favorite toy. Without hydrocarbons, we wouldn't have many of the modern conveniences we enjoy.

2. Key Ideas & Definitions

• Hydrocarbon: A molecule made up of only carbon (C) and hydrogen (H) atoms.
  • Definition: A type of molecule that's like a LEGO block, but made of atoms.
  • Example: Think of a LEGO brick as a hydrocarbon molecule – it's a single unit that can be connected to other bricks to build something bigger.
• Carbon (C): A chemical element that's like a master builder, able to connect to many other atoms to form different molecules.
  • Definition: A chemical element that's great at forming bonds with other atoms.
  • Example: Imagine a carbon atom as a superhero with many arms, each one able to connect to another atom.
• Hydrogen (H): A chemical element that's like a tiny fuel tank, providing energy to the molecule.
  • Definition: A chemical element that's great at releasing energy when it bonds with other atoms.
  • Example: Think of a hydrogen atom as a tiny can of gasoline, providing energy to the molecule.
• Alkane: A type of hydrocarbon that's like a straight line, with only single bonds between the carbon atoms.
  • Definition: A type of hydrocarbon that's like a straight line, with no branches.
  • Example: Imagine an alkane molecule as a straight road, with no turns or branches.
• Alkene: A type of hydrocarbon that's like a bent line, with at least one double bond between the carbon atoms.
  • Definition: A type of hydrocarbon that's like a bent line, with at least one double bond.
  • Example: Think of an alkene molecule as a road with a sharp turn, where the carbon atoms are connected by a double bond.
• Alkyne: A type of hydrocarbon that's like a wavy line, with at least one triple bond between the carbon atoms.
  • Definition: A type of hydrocarbon that's like a wavy line, with at least one triple bond.
  • Example: Imagine an alkyne molecule as a road with a big hill, where the carbon atoms are connected by a triple bond.
• Isomer: A molecule that's like a twin, but with a different arrangement of atoms.
  • Definition: A molecule that's identical to another molecule, but with a different arrangement of atoms.
  • Example: Think of two isomers as identical twins, but with different hairstyles.
• Saturated: A molecule that's like a full tank, with no extra space for more atoms.
  • Definition: A molecule that's fully bonded, with no extra space for more atoms.
  • Example: Imagine a saturated molecule as a full gas tank, with no room for more fuel.
• Unsaturated: A molecule that's like a half-empty tank, with space for more atoms.
  • Definition: A molecule that's not fully bonded, with space for more atoms.
  • Example: Think of an unsaturated molecule as a half-empty gas tank, with room for more fuel.

3. How To Do It (Step-by-Step)

Let's draw a Lewis structure for the molecule methane (CH4). This is like building a LEGO model, but with atoms instead of bricks.

1. Start by drawing a carbon atom (C) in the center of the page.
2. Draw four hydrogen atoms (H) around the carbon atom, one on each side.
3. Connect each hydrogen atom to the carbon atom with a single bond ().
4. Make sure to count the number of valence electrons (electrons in the outer shell) for each atom:
   • Carbon has 4 valence electrons.
   • Hydrogen has 1 valence electron.
5. Add the valence electrons to the Lewis structure:
   • Carbon has 4 valence electrons, so it needs 4 more to be happy.
   • Hydrogen has 1 valence electron, so it needs 1 more to be happy.
6. Since each hydrogen atom is connected to the carbon atom with a single bond, each hydrogen atom gets 1 more valence electron from the carbon atom. This makes each hydrogen atom happy!

Here's the final Lewis structure for methane (CH4):

H - C - H
| / |
H H

4. Watch Out! (Common Mistakes)

• Mistake: Forgetting to count the valence electrons for each atom.
  • Fix: Make sure to count the valence electrons for each atom before drawing the Lewis structure.
  • Analogy: Think of valence electrons as money in a bank account. You need to count how much money each atom has before you can make any transactions.
• Mistake: Drawing a molecule with too many or too few bonds.
  • Fix: Make sure to follow the octet rule (each atom needs 8 valence electrons to be happy).
  • Analogy: Think of bonds as roads connecting atoms. You need to make sure there are enough roads (bonds) to connect all the atoms.
• Mistake: Not considering the shape of the molecule.
  • Fix: Use the VSEPR (Valence Shell Electron Pair Repulsion) theory to determine the shape of the molecule.
  • Analogy: Think of the molecule as a building. You need to consider the shape of the building to make sure it's stable and safe.

5. Practice Problems

Problem 1: Draw a Lewis structure for the molecule propane (C3H8).

Solution:

1. Start by drawing three carbon atoms (C) in a row.
2. Draw eight hydrogen atoms (H) around the carbon atoms, one on each side.
3. Connect each hydrogen atom to the carbon atom with a single bond ().
4. Make sure to count the number of valence electrons for each atom:
   • Carbon has 4 valence electrons.
   • Hydrogen has 1 valence electron.
5. Add the valence electrons to the Lewis structure:
   • Carbon has 4 valence electrons, so it needs 4 more to be happy.
   • Hydrogen has 1 valence electron, so it needs 1 more to be happy.
6. Since each hydrogen atom is connected to the carbon atom with a single bond, each hydrogen atom gets 1 more valence electron from the carbon atom. This makes each hydrogen atom happy!

Here's the final Lewis structure for propane (C3H8):

H - C - C - H
| / | \ |
H H H H

Takeaway: When drawing a Lewis structure, make sure to count the valence electrons for each atom and follow the octet rule.

Problem 2: What is the molecular formula for the molecule butane (C4H10)?

Solution: Since butane has 4 carbon atoms and 10 hydrogen atoms, the molecular formula is C4H10.

Takeaway: The molecular formula is a way to describe the number of atoms in a molecule.

6. Cram Sheet

• Hydrocarbons are molecules made up of only carbon (C) and hydrogen (H) atoms.
• Carbon (C) is like a master builder, able to connect to many other atoms to form different molecules.
• Hydrogen (H) is like a tiny fuel tank, providing energy to the molecule.
• Alkanes are a type of hydrocarbon with only single bonds between the carbon atoms.
• Alkenes are a type of hydrocarbon with at least one double bond between the carbon atoms.
• Alkynes are a type of hydrocarbon with at least one triple bond between the carbon atoms.
• Isomers are molecules that are identical to another molecule, but with a different arrangement of atoms.
• Saturated molecules are like full tanks, with no extra space for more atoms.
• Unsaturated molecules are like half-empty tanks, with space for more atoms. Remember to count the valence electrons for each atom when drawing a Lewis structure. Make sure to follow the octet rule when drawing a Lewis structure. Consider the shape of the molecule using the VSEPR theory.

7. Where to Learn More

• YouTube: Crash Course Chemistry (hosted by Hank Green) has a great video on hydrocarbons.
• PhET Simulation: The University of Colorado Boulder's PhET Simulation has a great interactive simulation on hydrocarbons.
• School-friendly website: The Royal Society of Chemistry's website has a great article on hydrocarbons, including examples and practice problems.