Chemical Reactions and Equations · Lesson 2 of 9
Chemical Equations
“Atoms refuse to vanish, so every chemical equation has to balance its books.”
• Translate word equations into formula equations. • Distinguish reactants from products and interpret the reaction arrow. • Explain skeletal and balanced chemical equations using conservation of mass. • Balance chemical equations systematically without changing chemical formulae. • Add physical-state symbols and reaction conditions correctly.
Long sentences can describe chemical reactions, but chemistry becomes much clearer when the same information is written symbolically. A chemical equation tells us which substances react, which substances form and, when needed, the physical states and conditions of the reaction.
Word-Equation And Basic Representation
The burning of magnesium can first be written in words: Magnesium + Oxygen → Magnesium oxide. Magnesium and oxygen are the reactants because they undergo chemical change. Magnesium oxide is the product because it is formed by the reaction. Reactants are written on the left-hand side, products on the right-hand side, and the arrow points from reactants toward products.
Reactants are the substances that undergo chemical change during a reaction.
Products are the new substances formed during a chemical reaction.
The plus sign separates different substances on the same side of an equation. The arrow shows the direction of change from reactants to products.
Writing A Chemical Equation
Names can be replaced by chemical formulae to make an equation shorter and more precise. The word equation for magnesium burning becomes Mg + O₂ → MgO. At this stage the formulae are correct, but the numbers of atoms are not yet equal on the two sides.
A Skeletal Chemical Equation uses the correct chemical formulae of reactants and products but has not necessarily been balanced.
Balanced Chemical Equations
A chemical reaction does not create or destroy atoms. The same atoms that enter in the reactants must be present in the products, although they are rearranged into new combinations. Therefore the number of atoms of each element must be the same on both sides of a complete balanced equation.
A Balanced Chemical Equation has the same number of atoms of every element on the reactant side and the product side.
The zinc and sulphuric acid equation is already balanced. Zinc appears once on each side, hydrogen twice, sulphur once and oxygen four times.
Balancing Chemical Equations Step By Step
The safest way to balance an equation is to protect every chemical formula first. Imagine a box around each complete formula. Everything inside a box stays unchanged. Only whole-number coefficients placed before formulae may be altered. This prevents a common mistake: changing a subscript and accidentally changing the identity of a substance.
A coefficient changes how many formula units or molecules take part. A subscript is part of the chemical formula itself. To balance oxygen in H₂O, write 4H₂O when four water molecules are needed; do not change H₂O into H₂O₄.
A Reliable Balancing Routine
First, write correct formulae. Next, count the atoms of every element on both sides. It is often convenient to start with a compound containing many atoms and then select an element that is clearly unbalanced. Adjust a coefficient, recount, and continue one element at a time. When all atoms match, reduce the coefficients to the smallest whole-number ratio if necessary and perform a final check. This trial-based process is the hit-and-trial method.
The Hit-And-Trial Method balances an equation by adjusting whole-number coefficients step by step until the number of atoms of each element is equal on both sides.
Problem
Balance Mg + O₂ → MgO.
- 1.Count atoms. The left side has one Mg and two O atoms. The right side has one Mg and one O atom.
- 2.Oxygen is unbalanced. Place a coefficient of two before MgO: Mg + O₂ → 2MgO.
- 3.The product side now contains two Mg atoms, so magnesium is unbalanced.
- 4.Place a coefficient of two before Mg: 2Mg + O₂ → 2MgO.
- 5.Recheck: Mg is two on both sides and O is two on both sides. The equation is balanced.
Problem
Balance Fe + H₂O → Fe₃O₄ + H₂.
- 1.Count atoms initially: Fe is 1→3, H is 2→2 and O is 1→4.
- 2.Start with oxygen because Fe₃O₄ contains four oxygen atoms. Place 4 before H₂O: Fe + 4H₂O → Fe₃O₄ + H₂.
- 3.The left side now contains eight H atoms. Place 4 before H₂: Fe + 4H₂O → Fe₃O₄ + 4H₂.
- 4.Iron remains unbalanced. Fe₃O₄ contains three Fe atoms, so place 3 before Fe.
- 5.The result is 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
- 6.Final check: Fe 3=3, H 8=8 and O 4=4.
Problem
Balance BaCl₂ + Al₂(SO₄)₃ → BaSO₄ + AlCl₃.
- 1.Treat sulphate as an unchanged group because SO₄ appears intact on both sides.
- 2.Al₂(SO₄)₃ contains three sulphate groups, so place 3 before BaSO₄.
- 3.Three BaSO₄ units require three Ba atoms, so place 3 before BaCl₂.
- 4.The left side now has six Cl atoms. Each AlCl₃ has three Cl atoms, so place 2 before AlCl₃.
- 5.Al₂ on the left now matches the two Al atoms on the right.
- 6.The balanced equation is 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃.
- 7.Recheck every element or unchanged group before accepting the result.
Writing Symbols Of Physical States
A balanced equation can be made more informative by showing physical states. Solid is written as (s), liquid as (l), gas as (g), and a substance dissolved in water as (aq). The term aqueous means that the substance is present in a water solution.
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous solution |
Reaction Conditions
Some reactions require particular conditions such as temperature, pressure, a catalyst, sunlight or chlorophyll. These conditions may be written above or below the reaction arrow. They are not reactants or products; they tell us what conditions allow the reaction to proceed.
Never stop after matching only one element. Recount every element on both sides and confirm that the coefficients are the smallest whole numbers.
Quiz
What is a skeletal chemical equation?
Which change is allowed while balancing an equation?
What does (aq) indicate?
Which equation is balanced?
Why must chemical equations be balanced?
Practice Problems
- Translate Hydrogen + Chlorine → Hydrogen chloride into a formula equation and balance it.
- Balance Na + H₂O → NaOH + H₂ and show each step.
- Balance HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O and verify every element.
- Balance BaCl₂ + Al₂(SO₄)₃ → BaSO₄ + AlCl₃ using sulphate as an unchanged group.
- Add appropriate state symbols to the reaction between aqueous barium chloride and aqueous sodium sulphate forming solid barium sulphate and aqueous sodium chloride.
- Explain why changing H₂O to H₂O₄ would not be a valid balancing step.
Key Takeaways
• A word equation shows reactants changing into products and the arrow points toward the products. • A skeletal equation uses correct formulae but may still be unbalanced. • Balancing follows conservation of atoms and must use coefficients rather than changed subscripts. • The hit-and-trial method balances one element or unchanged group at a time and ends with a complete recheck. • State symbols and reaction conditions make a chemical equation more informative.