Chemical Reactions and Equations

TL;DR

Chemical reactions are processes where atoms rearrange to form new substances. We use chemical equations to represent these changes, showing the reactants that combine and the products that are formed. Balancing equations ensures that the law of conservation of mass is upheld.

1. The Mental Model

Imagine LEGO bricks. A chemical reaction is like taking apart some existing LEGO models (reactants) and using the same bricks to build completely new models (products). The bricks themselves don't disappear or appear out of nowhere; they just change how they're connected.

2. The Core Material

Chemical reactions are fundamental to everything around us, from burning wood to cooking food to the processes in your body. They involve breaking existing chemical bonds and forming new ones.

What is a Chemical Reaction?

At its heart, a chemical reaction is a process that involves the rearrangement of the atomic structure of substances. This means atoms are not created or destroyed; they just get new partners. You start with one or more substances (called reactants) and end up with one or more different substances (called products).

You can often tell a chemical reaction has occurred by observing certain signs, such as:
* Color change: For example, rust forming on iron.
* Temperature change: Reactions can release heat (exothermic) or absorb heat (endothermic).
* Gas production: Bubbles forming.
* Formation of a precipitate: A solid forming in a liquid solution.
* Odor change: A new smell appearing.

Chemical Equations: The Shorthand

We use chemical equations to describe chemical reactions in a concise way. They show the formulas of the reactants on the left side, an arrow indicating the direction of the reaction, and the formulas of the products on the right side.

A basic structure looks like this:
Reactant(s) $\rightarrow$ Product(s)

For example, when hydrogen gas ($\text{H}_2$) reacts with oxygen gas ($\text{O}_2$) to form water ($\text{H}_2\text{O}$), the unbalanced equation is:
$\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$

The Law of Conservation of Mass

A crucial principle in chemistry is the Law of Conservation of Mass. This law states that matter cannot be created or destroyed in an isolated chemical system. In simpler terms, the total mass of the reactants must equal the total mass of the products. This means the number of atoms of each element must be the same on both sides of a chemical equation.

Balancing Chemical Equations

Since atoms are conserved, we need to make sure our chemical equations reflect this. This process is called balancing chemical equations. When you balance an equation, you adjust the coefficients (the numbers in front of the chemical formulas) so that the number of atoms of each element is identical on both the reactant and product sides. You never change the subscripts within a chemical formula, as that would change the substance itself.

Let's balance the water formation equation:
Unbalanced: $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$

1. Count atoms:

   • Left side: H = 2, O = 2
   • Right side: H = 2, O = 1

2. Balance oxygen: You have 2 oxygen atoms on the left and only 1 on the right. Put a coefficient of '2' in front of $\text{H}_2\text{O}$:
   $\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$

3. Recount atoms:

   • Left side: H = 2, O = 2
   • Right side: H = 4 (because $2 \times 2$), O = 2 (because $2 \times 1$)

4. Balance hydrogen: Now you have 2 hydrogen atoms on the left and 4 on the right. Put a coefficient of '2' in front of $\text{H}_2$:
   $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$

5. Final count (check):

   • Left side: H = 4, O = 2
   • Right side: H = 4, O = 2
     The equation is balanced!

3. Worked Example

Let's balance the reaction of methane ($\text{CH}_4$) burning in oxygen ($\text{O}_2$) to produce carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$). This is a combustion reaction.

Unbalanced Equation: $\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$

1. Count atoms for each element on both sides:

   • Reactants (Left):
     • C: 1
     • H: 4
     • O: 2
   • Products (Right):
     • C: 1
     • H: 2
     • O: 3 (2 from $\text{CO}_2$ + 1 from $\text{H}_2\text{O}$)

2. Balance Carbon (C): Carbon is already balanced (1 on each side).

3. Balance Hydrogen (H): You have 4 H atoms on the left and 2 H atoms on the right. To balance, put a '2' coefficient in front of $\text{H}_2\text{O}$:
   $\text{CH}_4 + \text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

4. Recount atoms (especially oxygen, which was affected):

   • Reactants (Left):
     • C: 1
     • H: 4
     • O: 2
   • Products (Right):
     • C: 1
     • H: 4 (from $2 \times \text{H}_2$)
     • O: 4 (2 from $\text{CO}_2$ + 2 from $2\text{H}_2\text{O}$)

5. Balance Oxygen (O): Now you have 2 O atoms on the left and 4 O atoms on the right. To balance, put a '2' coefficient in front of $\text{O}_2$:
   $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

6. Final Check:

   • Reactants (Left):
     • C: 1
     • H: 4
     • O: 4 (from $2 \times \text{O}_2$)
   • Products (Right):
     • C: 1
     • H: 4
     • O: 4
       All atoms are balanced.

4. Key Takeaways

• A chemical reaction is a process where substances change into new ones by rearranging atoms.
• Reactants are the starting substances, and products are the new substances formed.
• Chemical equations are symbolic representations of chemical reactions.
• The Law of Conservation of Mass dictates that atoms are neither created nor destroyed in a chemical reaction.
• Balancing chemical equations ensures that the number of atoms for each element is equal on both sides of the equation.
• You adjust coefficients (numbers in front of formulas) to balance equations, never subscripts within formulas.

Common mistakes to avoid:
* Changing subscripts: Don't alter the chemical formulas themselves to balance an equation.
* Ignoring polyatomic ions: If a polyatomic ion (like $\text{SO}_4^{2-}$ or $\text{NO}_3^-$) stays intact on both sides, balance it as a whole unit.
* Not recounting after each step: Always recount atoms after adding a coefficient, especially for elements that appear in multiple compounds.
* Assuming a coefficient of 1: If there's no number, it implicitly means "1" of that molecule.

5. Now Try It

Balance the following chemical equation:
$\text{N}_2 + \text{H}_2 \rightarrow \text{NH}_3$ (This is the Haber process for producing ammonia.)

What to do:
1. Write down the unbalanced equation.
2. List the number of atoms for each element on both the reactant and product sides.
3. Choose one element to start balancing (often, it's helpful to leave oxygen and hydrogen for last).
4. Add coefficients as needed to balance that element, then recount all atoms.
5. Repeat until all elements are balanced on both sides.

What success looks like:
You should arrive at an equation where the count of nitrogen atoms on the left equals nitrogen atoms on the right, and the same for hydrogen atoms, using the smallest whole-number coefficients possible.