Chemical Reactions & Equations

A chemical reaction happens when substances transform into new substances with different properties. When you light a candle, cook rice, or even digest food -- chemical reactions are at work. Writing and balancing equations is the language we use to describe them.

Writing Chemical Equations

A chemical equation shows reactants on the left and products on the right, connected by an arrow.

Word equation: Hydrogen + Oxygen -> Water

Symbol equation: H2 + O2 -> H2O (unbalanced)

Balancing Equations

Law of Conservation of Mass: Atoms cannot be created or destroyed. The number of each type of atom must be equal on both sides.

Steps to balance:

1. Write the unbalanced equation
2. Count atoms of each element on both sides
3. Add coefficients (numbers in front) to balance
4. Check that all elements are balanced

Example: Balance H2 + O2 -> H2O

• Unbalanced: Left has 2H, 2O. Right has 2H, 1O.
• Add coefficient 2 to H2O: H2 + O2 -> 2H2O
• Now: Left has 2H, 2O. Right has 4H, 2O. Hydrogen is unbalanced.
• Add coefficient 2 to H2: 2H2 + O2 -> 2H2O
• Check: Left 4H, 2O. Right 4H, 2O. Balanced!

Types of Chemical Reactions

1. Combination (Synthesis): Two or more substances combine into one. A + B -> AB. Example: 2Mg + O2 -> 2MgO

2. Decomposition: One substance breaks into two or more. AB -> A + B. Example: 2H2O -> 2H2 + O2 (electrolysis)

3. Displacement (Single): A more reactive element replaces a less reactive one. A + BC -> AC + B. Example: Zn + CuSO4 -> ZnSO4 + Cu

4. Double Displacement: Two compounds exchange ions. AB + CD -> AD + CB. Example: NaCl + AgNO3 -> AgCl + NaNO3

5. Combustion: A substance reacts with oxygen, releasing heat. Example: CH4 + 2O2 -> CO2 + 2H2O

Oxidation-Reduction (Redox) Basics

Oxidation = loss of electrons (or gain of oxygen)

Reduction = gain of electrons (or loss of oxygen)

Remember: OIL RIG -- Oxidation Is Loss, Reduction Is Gain (of electrons).

In the reaction 2Mg + O2 -> 2MgO: Magnesium is oxidized (loses electrons), Oxygen is reduced (gains electrons).

Nepal Connection: The rusting of iron bridges and roofs is an oxidation reaction. In Nepal's humid climate, especially during monsoon, iron structures corrode faster, which is why proper coating and maintenance are important.

Key Takeaways

• Chemical equations must be balanced (same atoms on both sides)
• There are five main types of reactions: combination, decomposition, displacement, double displacement, and combustion
• Oxidation means losing electrons; reduction means gaining electrons
• The law of conservation of mass governs all chemical reactions