Chemical Reactions and Conservation of Mass
3. Writing Chemical Equations
Learning outcomes
- I can interpret simple chemical equations.
- I can use chemical symbols and formulae to represent reactions.
- I can distinguish between word equations and symbol equations.
- I can identify the substances involved in a chemical equation.
- I can explain what a chemical equation represents at the particle level.
Introduction
Chemists around the world use chemical equations as a universal language to describe chemical reactions. Instead of writing the full names of substances every time, they use chemical symbols and chemical formulae to show exactly which substances react and which new substances are produced.
Chemical equations are much more than a shorthand notation—they show how atoms are rearranged during a reaction. Every equation represents billions upon billions of particles reacting in exactly the same way. Learning to read and write chemical equations is an essential step toward understanding chemical reactions, conservation of mass, and stoichiometry.
What Is a Chemical Equation?
A chemical equation is a symbolic representation of a chemical reaction.
It uses:
- Chemical symbols.
- Chemical formulae.
- A reaction arrow.
to show:
- The reactants.
- The products.
General form:
Reactants → Products
Example:
2H₂ + O₂ → 2H₂O
This equation shows hydrogen reacting with oxygen to produce water.
Figure 1. Chemical equations use symbols and formulae to represent chemical reactions.
Word Equations vs Symbol Equations
There are two common ways to describe reactions.
Word Equation
Uses the names of substances.
Example:
Hydrogen + Oxygen → Water
Symbol Equation
Uses chemical symbols and formulae.
Example:
H₂ + O₂ → H₂O
Symbol equations are shorter, more precise, and understood internationally.
Comparing the Two
| Word Equation | Symbol Equation |
|---|---|
| Uses names | Uses symbols and formulae |
| Easier for beginners | More accurate and compact |
| Describes the reaction | Represents the reaction mathematically |
| Example: Hydrogen + Oxygen → Water. | Example: H₂ + O₂ → H₂O |
Scientists usually use symbol equations in laboratories and textbooks.
Figure 2. Word equations use substance names, while symbol equations use chemical symbols and formulae.
Using Chemical Symbols and Formulae
Every element has its own chemical symbol.
Examples:
| Element | Symbol |
|---|---|
| Hydrogen | H |
| Oxygen | O |
| Carbon | C |
| Sodium | Na |
| Chlorine | Cl |
Compounds are represented using chemical formulae.
Examples:
| Compound | Formula |
|---|---|
| Water | H₂O |
| Carbon dioxide | CO₂ |
| Sodium chloride. | NaCl |
| Ammonia | NH₃ |
Correct symbols and formulae are essential for writing chemical equations accurately.
Identifying the Substances
In every chemical equation:
- Reactants appear before the arrow.
- Products appear after the arrow.
Example:
CH₄ + 2O₂ → CO₂ + 2H₂O
Reactants:
- Methane (CH₄)
- Oxygen (O₂)
Products:
- Carbon dioxide (CO₂)
- Water (H₂O)
The arrow means "produces" or "forms."
Reading Chemical Equations
Consider:
Mg + O₂ → MgO
This can be read as:
"Magnesium reacts with oxygen to produce magnesium oxide."
Chemical equations describe:
- What reacts.
- What is produced.
They do not usually explain how the reaction occurs.
Figure 3. Reactants are written on the left of the arrow, while products are written on the right.
What Happens at the Particle Level?
A chemical equation represents the behaviour of atoms and molecules.
During a reaction:
- Chemical bonds break.
- Atoms rearrange.
- New chemical bonds form.
- New substances are produced.
The atoms themselves are not created or destroyed.
Only the way they are connected changes.
For example:
Hydrogen molecules and oxygen molecules rearrange to form water molecules.
This particle-level view explains why the products have different properties from the reactants.
Chemical Equations Represent Billions of Particles
Although a chemical equation appears small on paper, it represents an enormous number of particles.
For example:
2H₂ + O₂ → 2H₂O
Actually represents:
- Billions of hydrogen molecules.
- Billions of oxygen molecules.
- Billions of water molecules.
Chemists use equations because they apply equally well to a tiny laboratory sample or a large industrial process.
Figure 4. Chemical equations represent atoms being rearranged into new molecules during a reaction.
Examples of Chemical Equations
| Word Equation | Symbol Equation |
|---|---|
| Hydrogen + Oxygen → Water | 2H₂ + O₂ → 2H₂O |
| Carbon + Oxygen → Carbon dioxide | C + O₂ → CO₂ |
| Magnesium + Oxygen → Magnesium oxide. | 2Mg + O₂ → 2MgO |
| Iron + Sulfur → Iron sulfide | Fe + S → FeS |
Notice that symbol equations use chemical symbols and formulae instead of names.
Why Chemical Equations Are Important
Chemical equations allow scientists to:
- Describe reactions accurately.
- Predict products.
- Communicate internationally.
- Study industrial processes.
- Calculate quantities in reactions.
They are one of the most important tools in chemistry.
Figure 5. Chemical equations are the universal language used by chemists to describe reactions.
Worked Example
Question
Write the symbol equation for the following word equation.
Hydrogen + Chlorine → Hydrogen chloride
Solution
Step 1
Identify the symbols.
Hydrogen = H₂
Chlorine = Cl₂
Hydrogen chloride = HCl
Step 2
Write the equation.
H₂ + Cl₂ → 2HCl
This equation shows hydrogen molecules reacting with chlorine molecules to form hydrogen chloride molecules.
Real-World Connection
Chemical equations are used everywhere in science and industry. Engineers use them when designing processes to manufacture fertilisers, medicines, plastics, and fuels. Environmental scientists use chemical equations to study pollution and climate change, while doctors and pharmacists use them to understand how medicines react inside the human body.
Did You Know?
Every year, chemists write millions of chemical equations in scientific journals, laboratories, and factories. Because chemical symbols and equations are standardised worldwide, a chemist in Canada can read and understand a reaction written by a scientist in Brazil, India, or Germany without needing to translate it into another language.
Key Terms
Chemical equation – A symbolic representation of a chemical reaction using chemical symbols and formulae.
Chemical formula – A combination of symbols and numbers showing the types and numbers of atoms in a substance.
Chemical symbol – A one- or two-letter abbreviation representing an element.
Particle – A general term for atoms, molecules, or ions that make up matter.
Product – A substance formed during a chemical reaction.
Reactant – A starting substance in a chemical reaction.
Reaction arrow (→) – A symbol meaning "produces" or "forms" in a chemical equation.
Symbol equation – A chemical equation written using chemical symbols and formulae.
Word equation – A chemical equation written using the names of substances.
Key Takeaways
- A chemical equation uses symbols and formulae to represent a chemical reaction.
- Word equations use substance names, while symbol equations use chemical symbols and formulae.
- Reactants are written before the reaction arrow, and products are written after it.
- Chemical equations represent the rearrangement of atoms into new substances at the particle level.
- Chemical equations provide a universal way for scientists to describe chemical reactions.
- Understanding chemical equations prepares students for balancing equations and quantitative chemistry.