Chemical Reactions and Equations · Lesson 3 of 9
Types Of Chemical Reactions: Combination Reaction
“Sometimes chemistry is simple: several reactants meet and one product walks out.”
• Recognise a combination reaction from its reactant-product pattern. • Explain the reaction of calcium oxide with water. • Explain the chemistry of whitewashing. • Define exothermic reactions and identify important examples. • Explain why respiration and composting are exothermic processes.
Chemical reactions can be grouped by the way reactants rearrange to form products. In ordinary chemical reactions, atoms are not transformed into atoms of another element and they do not appear or disappear. Instead, old bonds are broken and new bonds form, producing new substances.
Types Of Chemical Reactions
A useful first question is: how many reactant substances are present and how many product substances are formed? This pattern helps distinguish combination from decomposition and prepares the way for displacement and double displacement reactions.
Combination Reaction
A Combination Reaction is a reaction in which two or more reactants combine to form a single product.
Calcium Oxide And Water
When water is added slowly to calcium oxide, also called quicklime, the reaction is vigorous and the container becomes warm. Calcium hydroxide, called slaked lime, forms as a single product. Because two reactants combine into one product, this is a combination reaction. Because heat is released, it is also exothermic.
Quicklime is calcium oxide, CaO.
Slaked Lime is calcium hydroxide, Ca(OH)₂.
Whitewashing
A solution of slaked lime is used for whitewashing walls. Calcium hydroxide reacts slowly with carbon dioxide in air and forms a thin layer of calcium carbonate. The layer develops over time and gives the wall a shiny finish. Calcium carbonate is also the chemical compound present in marble.
More Combination Reactions
Both examples fit the same structural pattern: two reactants combine to give a single product.
Exothermic Reactions
An Exothermic Reaction releases heat along with the formation of products.
The quicklime-water reaction is exothermic because heat is released and the reaction mixture becomes warm. Burning natural gas is another example. Respiration is also exothermic: glucose reacts with oxygen in cells and releases energy needed for life. Decomposition of vegetable matter into compost also releases energy.
The calcium oxide and water reaction is a combination reaction because one product forms, and it is exothermic because heat is released. Reaction type and energy change describe different features of the same reaction.
Quiz
Which pattern best represents a combination reaction?
What is formed when calcium oxide reacts with water?
Why does the beaker become warm when quicklime reacts with water?
Which substance forms the shiny layer after whitewashing?
Why is respiration described as exothermic?
Practice Problems
- Define a combination reaction and give two equations from the lesson.
- Explain the observations when water is added to quicklime and classify the reaction in two different ways.
- Write the equation responsible for the shiny finish after whitewashing.
- Explain why burning carbon is a combination reaction.
- Explain why respiration is exothermic using the glucose equation.
- Compare the meaning of combination reaction and exothermic reaction without treating them as synonyms.
Key Takeaways
• Combination reactions form a single product from two or more reactants. • Calcium oxide reacts with water to form slaked lime and releases heat. • Whitewashing depends on calcium hydroxide reacting with carbon dioxide to form calcium carbonate. • Exothermic reactions release heat or energy along with products. • Respiration and composting are important examples of exothermic processes.