Chemical Reactions and Conservation of Mass
2. Reactants and Products
Learning outcomes
- I can define reactants and products in a chemical reaction.
- I can identify reactants and products from word equations.
- I can describe chemical reactions using appropriate scientific terminology.
- I can explain how atoms are rearranged during reactions.
- I can represent simple reactions using word equations.
Introduction
Every chemical reaction begins with one or more substances and ends with one or more new substances. Chemists use the terms reactants and products to describe these substances and to explain what happens during a reaction.
Although the substances change during a chemical reaction, the atoms themselves are not created or destroyed. Instead, the atoms are simply rearranged into new combinations. Learning to identify reactants and products and write word equations is an important first step toward understanding chemical equations and the law of conservation of mass.
What Are Reactants?
Reactants are the starting substances in a chemical reaction.
They are the substances that react together.
During the reaction:
- Chemical bonds break.
- Atoms are rearranged.
- New substances are formed.
Examples of reactants include:
- Hydrogen gas
- Oxygen gas
- Iron
- Hydrochloric acid
Reactants are always found on the left-hand side of a chemical equation.
What Are Products?
Products are the new substances formed during a chemical reaction.
Products usually have different:
- Properties
- Structures
- Uses
than the reactants.
Examples include:
- Water
- Carbon dioxide
- Rust
- Sodium chloride
Products are always written on the right-hand side of a chemical equation.
Figure 1. Reactants are converted into products during a chemical reaction.
The Reaction Arrow
Chemical reactions are written using an arrow (→).
The arrow means:
- Produces
- Forms
- Yields
Example:
Hydrogen + Oxygen → Water
This is read as:
"Hydrogen reacts with oxygen to produce water."
The arrow shows the direction of the reaction from reactants to products.
Word Equations
A word equation uses the names of substances instead of chemical symbols.
General form:
Reactant + Reactant → Product
Examples:
Hydrogen + Oxygen → Water
Iron + Oxygen → Iron oxide
Magnesium + Oxygen → Magnesium oxide
Methane + Oxygen → Carbon dioxide + Water
Word equations clearly describe what happens during a reaction without using chemical formulae.
Figure 2. Word equations use the names of substances to describe chemical reactions.
Identifying Reactants and Products
To identify the substances:
Step 1
Find the arrow.
Step 2
Everything before the arrow is a reactant.
Step 3
Everything after the arrow is a product.
Example:
Carbon + Oxygen → Carbon dioxide
Reactants:
- Carbon
- Oxygen
Product:
- Carbon dioxide
Describing Chemical Reactions
Scientists use precise scientific language to describe reactions.
Example:
Instead of saying:
"The chemicals mix together."
A chemist would say:
"Carbon reacts with oxygen to produce carbon dioxide."
Using terms such as:
- React
- Produce
- Form
- Product
- Reactant
helps communicate ideas clearly.
Rearrangement of Atoms
During a chemical reaction:
- Atoms are not created.
- Atoms are not destroyed.
- Atoms are rearranged.
Old chemical bonds break.
New chemical bonds form.
For example:
Hydrogen atoms and oxygen atoms rearrange to form water molecules.
The number of each type of atom remains the same before and after the reaction.
Figure 3. Chemical reactions rearrange atoms to form new substances without creating or destroying atoms.
Writing Simple Word Equations
Word equations should:
- List the reactants first.
- Use a plus sign (+) between reactants.
- Use an arrow (→).
- List the products after the arrow.
Examples:
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen
Copper carbonate → Copper oxide + Carbon dioxide
Hydrogen peroxide → Water + Oxygen
These equations describe the substances involved without using chemical symbols.
Everyday Chemical Reactions
Many familiar reactions can be written as word equations.
| Everyday Reaction | Word Equation |
|---|---|
| Rusting | Iron + Oxygen → Iron oxide |
| Burning magnesium. | Magnesium + Oxygen → Magnesium oxide |
| Photosynthesis | Carbon dioxide + Water → Glucose + Oxygen |
| Cellular respiration | Glucose + Oxygen → Carbon dioxide + Water |
Word equations help us describe both natural and industrial chemical processes.
Figure 4. Many everyday processes can be described using word equations.
Why Word Equations Are Important
Word equations help students:
- Identify reactants and products.
- Understand chemical reactions.
- Prepare for writing chemical formulae.
- Learn chemical equations more easily.
They provide a simple way to communicate chemical changes before introducing symbols and balancing equations.
Worked Example
Question
Consider the following word equation:
Calcium + Oxygen → Calcium oxide
Identify the reactants and the product.
Solution
Reactants:
- Calcium
- Oxygen
Product:
- Calcium oxide
The arrow shows that calcium reacts with oxygen to produce calcium oxide.
Real-World Connection
Every time a car engine burns fuel, countless chemical reactions take place. One simplified word equation is:
Petrol (hydrocarbon fuel) + Oxygen → Carbon dioxide + Water
The products of these reactions release the energy needed to move the vehicle. Similar reactions occur when natural gas burns in home heaters and when food is broken down during cellular respiration to provide energy for your body.
Did You Know?
Chemists around the world use the same reaction arrow (→) regardless of their spoken language. Whether a scientist is working in Japan, Brazil, Canada, or South Africa, chemical equations are written in essentially the same way, making chemistry a truly international language.
Key Terms
Atom – The smallest particle of an element that retains its chemical properties.
Chemical reaction – A process in which atoms are rearranged to form new substances.
Product – A new substance formed during a chemical reaction.
Reactant – A starting substance that takes part in a chemical reaction.
Reaction arrow (→) – A symbol meaning "produces" or "forms" in a chemical equation.
Word equation – A chemical equation written using the names of substances instead of chemical formulae.
Key Takeaways
- Reactants are the starting substances in a chemical reaction.
- Products are the new substances formed during the reaction.
- The reaction arrow (→) means "produces" or "forms" and separates reactants from products.
- In a chemical reaction, atoms are rearranged, not created or destroyed.
- Word equations use the names of substances to describe chemical reactions.
- Identifying reactants and products is the first step toward understanding chemical equations and the conservation of matter.