Chemical Reactions
Types of Chemical Reaction
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to define a chemical reaction.
- ✓By the end of this lesson students will be able to identify reactants and products in a chemical reaction.
- ✓By the end of this lesson students will be able to write word equations for chemical reactions.
- ✓By the end of this lesson students will be able to classify chemical reactions as synthesis, decomposition, combustion, or neutralisation.
- ✓By the end of this lesson students will be able to give examples of each type of chemical reaction.
Key concepts
A chemical reaction is a process that involves the rearrangement of the structure of molecules or ions, resulting in the formation of new substances. During a chemical reaction, the original substances (reactants) are changed into different substances (products).
A word equation is a way of representing a chemical reaction using the names of the reactants and products. The reactants are written on the left side of an arrow, and the products are written on the right side. The arrow means 'produces' or 'forms'.
A synthesis reaction (also known as a combination reaction) is a chemical reaction where two or more simple substances combine to form a more complex substance.
A decomposition reaction is a chemical reaction where a complex substance breaks down into two or more simpler substances. This often requires energy, such as heat or light.
A combustion reaction is a rapid reaction between a substance and oxygen, usually producing heat and light. This process is commonly known as burning. If the substance contains carbon and hydrogen (like fuels), complete combustion produces carbon dioxide and water.
A neutralisation reaction is a specific type of chemical reaction that occurs when an acid reacts with a base. The products of a neutralisation reaction are always a salt and water.
Key facts to remember
- 1Reactants are the starting substances in a chemical reaction, and products are the new substances formed.
- 2The arrow (→) in a word equation means 'produces' or 'forms'.
- 3Synthesis reactions involve substances combining to form a more complex substance.
- 4Decomposition reactions involve a complex substance breaking down into simpler substances.
- 5Combustion reactions always involve a substance reacting rapidly with oxygen, often producing heat and light.
- 6Neutralisation reactions are reactions between an acid and a base, always producing a salt and water.
- 7Chemical reactions involve the rearrangement of atoms, but the total number of atoms of each element remains the same.
Worked examples
Example 1
Write a word equation for the reaction where hydrogen gas reacts with oxygen gas to form water. What type of reaction is this?
Answer
Hydrogen + Oxygen → Water. This is a synthesis reaction.
Example 2
When calcium carbonate is heated strongly, it breaks down to form calcium oxide and carbon dioxide gas. Write the word equation for this reaction and identify its type.
Answer
Calcium carbonate → Calcium oxide + Carbon dioxide. This is a decomposition reaction.
Example 3
Write the word equation for the complete combustion of methane (a common fuel) in oxygen, which produces carbon dioxide and water.
Answer
Methane + Oxygen → Carbon dioxide + Water.
This is an example of a combustion reaction.
Example 4
Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water. Write the word equation for this reaction and identify its type.
Answer
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water. This is a neutralisation reaction.
Common mistakes
- ✗Confusing the reactants (starting materials) with the products (substances formed) in a word equation.
- ✗Mixing up synthesis reactions (combining) with decomposition reactions (breaking down).
- ✗Forgetting to include oxygen as a reactant in combustion reactions, or water as a product in neutralisation reactions.
- ✗Using chemical symbols (e.g., H₂O) instead of the full names of substances (e.g., water) in word equations at this stage.
- ✗Not including all the reactants and products mentioned in the description of a reaction in the word equation.
Exam tips
- ★Always read the question carefully to identify all the reactants and products involved in a chemical reaction.
- ★Practise writing word equations for various scenarios to become familiar with the format and common reaction types.
- ★Look for keywords in the question, such as 'combines', 'breaks down', 'burns', 'acid', and 'base', as these often indicate the type of reaction.
- ★Ensure that every substance mentioned in the reaction description is included in your word equation, either as a reactant or a product.
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