3.4 - Chemical Equation

Writing Balanced Chemical Equations

• A chemical equation describes a chemical reaction concisely and accurately.
• Reactants are written on the left and products on the right. The arrow $\to$ means “produces”.
• Physical states are written as:
  • solid, $(\mathrm{s})$;
  • liquid, $(\mathrm{l})$;
  • gas, $(\mathrm{g})$;
  • aqueous solution, $(\mathrm{aq})$.
• The symbol $\Delta$ over an arrow indicates that heating is required.

Method

1. Write the word equation.
2. Replace every substance with its correct chemical formula.
3. Count the atoms of every element on each side.
4. Adjust coefficients in front of formulae until the numbers of each type of atom are equal. Do not alter subscripts.
5. Add physical-state symbols.

Example:

• Equations must obey the law of conservation of mass: matter is neither created nor destroyed, so every element has the same number of atoms on both sides.

Examples of Chemical Reactions

• Heating copper(II) carbonate:

• Formation of ammonium chloride:

• Precipitation of lead(II) iodide:

Safety: Concentrated hydrochloric acid and concentrated ammonia are corrosive; handle them carefully and conduct their reaction in a fume chamber.

Interpreting Chemical Equations

• Qualitative interpretation identifies the reactants, products and physical states.
• Quantitative interpretation uses the coefficients as ratios of elementary entities and as mole ratios.
• Stoichiometry is the quantitative study of the composition of substances involved in a chemical reaction.

For:

• Qualitatively: sodium metal and chlorine gas form solid sodium chloride.
• Particle ratio: 2 sodium atoms : 1 chlorine molecule : 2 sodium chloride formula units.
• Mole ratio: $2\mathrm{mol}$ Na : $1\mathrm{mol}$ $\mathrm{C}{\mathrm{l}}_2$ : $2\mathrm{mol}$ NaCl.
• Any proportional ratio is valid; for example, $1:0.5:1$ represents the same stoichiometry.

Stoichiometric Calculations

1. Write or identify the balanced equation.
2. Convert the given quantity to moles.
3. Use the equation coefficients to find the required number of moles.
4. Convert those moles to the required mass, particle count or gas volume.

Useful relationships:

Example: aluminium burns according to

For $5.4\mathrm{g}$ Al:

The ratio $4\mathrm{mol}\mathrm{Al}:2\mathrm{mol}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ gives

Since $M(\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3)=2(27)+3(16)=102\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$:

Preparation of Oxygen in the Laboratory

• Heating potassium chlorate(V) produces potassium chloride and oxygen gas.