Science - Combined (0653) | Stoichiometry

Chemistry C3: Stoichiometry – The Language of Chemical Recipes

Welcome to the chapter on Stoichiometry! Don't let the long name scare you. Stoichiometry is simply the math and language of chemistry.
Think of it as learning how to read and write recipes for chemical reactions. If you know the recipe (the formula and equation), you know exactly how much of each ingredient (atom or molecule) you need and what you will end up with.
This chapter is crucial because it teaches you the fundamental rules for writing chemical formulas and balancing equations—skills you will use throughout your IGCSE Chemistry studies.

1. Writing and Defining Chemical Formulas (Core)

A chemical formula is a quick, easy way to show exactly which atoms are present in a substance and how many of them are bonded together.

What is a Molecular Formula? (C3.1 Core 2)

The molecular formula defines the exact number and type of atoms contained in one single molecule of a compound.

• Example 1: Water is H₂O. This formula tells us that one molecule of water contains two atoms of hydrogen (H) and one atom of oxygen (O).
• Example 2: Carbon Dioxide is CO₂. This molecule contains one atom of carbon (C) and two atoms of oxygen (O).

Identifying Formulas of Simple Elements and Compounds (C3.1 Core 1)

Most elements exist as single atoms (monatomic), but some common non-metal elements exist as molecules made of two atoms (diatomic).

Key Formulas to Know:

• Monatomic (Single Atoms): Sodium (Na), Iron (Fe), Helium (He), Neon (Ne).
• Diatomic (Two Atoms): Hydrogen (H₂), Oxygen (O₂), Nitrogen (N₂), Chlorine (Cl₂).
• Common Compounds: Water (H₂O), Ammonia (NH₃), Methane (CH₄), Carbon Dioxide (CO₂).

Deducing Formulas from Diagrams (C3.1 Core 3)

If you are shown a model or a diagrammatic representation of a simple molecular compound, you just need to count the atoms!

Imagine a compound where:
You see 1 large black sphere (Carbon, C) bonded to 4 small white spheres (Hydrogen, H).
The formula is deduced by counting: 1 Carbon atom and 4 Hydrogen atoms.
The formula is CH₄ (Methane).

2. Deducing Formulas for Ionic Compounds (Supplement)

Ionic compounds are slightly different because they are made of charged particles called ions, not neutral molecules. Ionic compounds are electrically neutral overall.

Understanding Ions and Charges (C3.1 Supplement 6)

Ions are formed when atoms gain or lose electrons.

• Cations are positive ions (metals usually form these, e.g., Na⁺, Ca²⁺).
• Anions are negative ions (non-metals usually form these, e.g., Cl⁻, O²⁻).

To make a compound, the total positive charge must exactly cancel out the total negative charge, resulting in a neutral compound.

Step-by-Step: Deduce Ionic Formulas (The Criss-Cross Trick)

We use the charges of the ions to determine the formula.

Example: Finding the formula for Aluminum Oxide.

1. Write the Ions and Charges:
   Aluminum ion has a +3 charge: \(Al^{3+}\)
   Oxygen ion has a -2 charge: \(O^{2-}\)
2. Criss-Cross the Numbers:
   Take the numerical value of the charge (ignore the + or - sign) and move it down to become the subscript of the *other* ion.
3. Write the Final Formula:
   The 3 from aluminum goes to oxygen. The 2 from oxygen goes to aluminum.
   The formula is \(Al_2O_3\).

Did you know? The term 'Stoichiometry' comes from Greek words meaning 'element' and 'measure'. It's all about measuring the relationship between elements in compounds.

Common Mistake to Avoid: If the charges balance exactly (e.g., \(Na^+\) and \(Cl^-\)), you do not write subscripts. The formula is simply NaCl, not \(Na_1Cl_1\). If you have \(Ca^{2+}\) and \(O^{2-}\), the charges cancel perfectly, so the formula is CaO (not \(Ca_2O_2\)).

3. Chemical Equations

Chemical equations are how we represent chemical reactions, showing what reacts (reactants) and what is made (products).

Word Equations (C3.1 Core 4)

A word equation is the simplest way to show a reaction. It lists the full names of the substances involved.

Reactants \(\rightarrow\) Products

Example:
Methane + Oxygen \(\rightarrow\) Carbon Dioxide + Water

Symbol Equations and State Symbols (C3.1 Core 5)

A symbol equation uses the chemical formulas of the substances. This is much more informative than a word equation.

We also include state symbols to indicate the physical state of each substance:

• (s): solid
• (l): liquid (like water)
• (g): gas (like oxygen)
• (aq): aqueous (dissolved in water/solution)

Example (unbalanced):
\(CH_{4}(g) + O_{2}(g) \rightarrow CO_{2}(g) + H_{2}O(l)\)

Balancing Symbol Equations (C3.1 Core 5)

Equations must obey the Law of Conservation of Mass: atoms are neither created nor destroyed in a chemical reaction.
This means the number of atoms of each element must be the same on the reactant side (left) and the product side (right). We balance equations by putting large numbers (coefficients) in front of the formulas.

Step-by-Step Balancing:

Let’s balance the combustion of methane:
\(CH_{4} + O_{2} \rightarrow CO_{2} + H_{2}O\)

1. Tally the Atoms: Write down the number of atoms for each element on both sides.
   Left (Reactants): C=1, H=4, O=2
   Right (Products): C=1, H=2, O=3
2. Balance Complex Atoms First (C): Carbon is already balanced (1 on each side).
3. Balance Hydrogen (H): We have 4 H on the left, but only 2 H on the right. We need 4 H on the right. Place a coefficient of 2 in front of H₂O.
   \(CH_{4} + O_{2} \rightarrow CO_{2} + 2H_{2}O\)
   (The right side now has 2 x 2 = 4 H atoms.)
4. Balance Oxygen (O) Last: Now recalculate Oxygen atoms on the right.
   Right side O: 2 (in CO₂) + 2 (in 2H₂O) = 4 O total.
   Left side O: We only have 2 O atoms (in O₂). We need 4 O atoms. Place a coefficient of 2 in front of O₂.
   \(CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O\)
5. Final Check:
   Left: C=1, H=4, O=4
   Right: C=1, H=4, O=4
   The equation is balanced!

Constructing Ionic Equations (C3.1 Supplement 7)

This is an Extended concept. Ionic equations focus only on the ions (and molecules) that are actively changing during the reaction. They are most commonly used for precipitation reactions (forming a solid) or neutralisation reactions.

In an ionic equation, we remove the spectator ions—ions that are present in the solution but do not take part in the reaction (they appear unchanged on both sides of the full equation).

Example: Reaction between aqueous Silver Nitrate and aqueous Sodium Chloride to form solid Silver Chloride.

1. Full Symbol Equation (with states):
\(AgNO_{3}(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_{3}(aq)\)

2. Write down all ions (only break up (aq) substances):
\((Ag^{+}(aq) + NO_{3}^{-}(aq)) + (Na^{+}(aq) + Cl^{-}(aq)) \rightarrow AgCl(s) + (Na^{+}(aq) + NO_{3}^{-}(aq))\)

3. Cancel Spectator Ions: \(Na^{+}\) and \(NO_{3}^{-}\) are the same on both sides.

4. Final Ionic Equation:
\(Ag^{+}(aq) + Cl^{-}(aq) \rightarrow AgCl(s)\)

This final equation shows that only the silver ions and chloride ions actually combine to form the precipitate.