Reactivity 2.1.1 – What is a Chemical Equation?

A chemical equation is a symbolic representation of a chemical reaction. It shows:

• The reactants (starting substances) on the left.
• The products (new substances formed) on the right.
• The physical states of the substances (s, l, g, aq).
• Reaction conditions above/below the arrow if needed (e.g. heat, catalyst).

Example of a Simple Equation

\( \text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(g) \)

This shows the complete combustion of propane in oxygen.

Why Do We Use Chemical Equations?

• To identify what substances react and what products are formed.
• To determine the molar ratio between reactants and products.
• To perform quantitative calculations based on the mole concept.

Stoichiometric Ratios

The numbers in front of chemical formulas are called stoichiometric coefficients.

These coefficients show the ratio in which substances react and are produced, expressed in moles.

This ratio applies to particles, moles, volumes (for gases at same T and P), and sometimes masses (if molar mass is used).

Importance of Balancing Equations

• A chemical equation must be balanced to follow the Law of Conservation of Mass.
• This law states that atoms are neither created nor destroyed in a chemical reaction.
• The total number of atoms of each element must be the same on both sides.

Using Stoichiometric Ratios in Calculations

Balanced equations allow us to calculate:

• The amount of product formed from a known amount of reactant.
• The amount of reactant required to produce a desired quantity of product.
• Which reactant is limiting (used up first) and which is in excess.

Note on Gas Volumes

At the same temperature and pressure, volumes of gases react in the same ratio as the coefficients in the balanced equation (Avogadro’s Law).

Conclusion

• Chemical equations are essential tools to represent reactions and quantify reactants/products.
• Stoichiometric ratios derived from balanced equations are used for all mole-related calculations in chemistry.

Deduce Chemical Equations When Reactants and Products Are Specified

To deduce a chemical equation means to write the correct balanced chemical equation for a reaction when the reactants and products are known or described.

This involves:

• Writing correct formulas for the given substances.
• Balancing the equation using stoichiometric coefficients.
• Including appropriate state symbols.

State Symbols in Chemical Equations

State symbols indicate the physical state of each substance:

• \((s)\) – solid
• \((l)\) – liquid
• \((g)\) – gas
• \((aq)\) – aqueous (dissolved in water)

These are written immediately after the chemical formula in parentheses.

Steps to Deduce a Chemical Equation

1. Identify the reactants and products from the question or context.
2. Write correct chemical formulas for each species (watch for polyatomic ions and valencies).
3. Balance the equation using coefficients (do not change subscripts).
4. Add state symbols based on conditions (e.g., water = (l), acids = (aq), gases = (g)).

Common Indicators for Physical States

• Metals and salts at room temperature – usually solids \((s)\)
• Acids and bases in solution – aqueous \((aq)\)
• Gases like \( \text{H}_2 \), \( \text{O}_2 \), \( \text{CO}_2 \), \( \text{NH}_3 \) – \((g)\)
• Water – \((l)\) if liquid, \((g)\) if steam

Important Notes:

• Learn common ions and charges (e.g. \( \text{NO}_3^- \), \( \text{SO}_4^{2-} \), \( \text{NH}_4^+ \))
• Be careful with diatomic elements – these occur as \( \text{H}_2 \), \( \text{O}_2 \), \( \text{N}_2 \), \( \text{Cl}_2 \), etc.
• Make sure to include state symbols if the question asks for them explicitly.