- Define and explain Energy in Reactions in your own words
- Use key terms such as exothermic accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Energy in Reactions: distinguish exothermic and endothermic reactions.
Distinguish exothermic and endothermic reactions.
Key ideas
Energy changes come from bond breaking and making
Breaking bonds takes in energy; making bonds releases energy. In an exothermic reaction, more energy is released making new bonds than is taken in breaking old ones, so the mixture warms up — combustion is the classic example. In an endothermic reaction, like the thermal decomposition of calcium carbonate, the surroundings must supply energy continuously, so the mixture cools.
Acids react in predictable patterns
Acids react with metals to give a salt plus hydrogen: for example, magnesium + hydrochloric acid → magnesium chloride + hydrogen. With carbonates they give a salt plus water plus carbon dioxide: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. Learn these two patterns and you can write the equation for any acid–metal or acid–carbonate reaction.
Key term — exothermic: A reaction that releases energy to the surroundings, usually as heat — the surroundings warm up.
Burning methane warms the room. Is this exothermic or endothermic? Explain in terms of bonds.
Exothermic. More energy is released when the new bonds in CO₂ and H₂O form than is taken in to break the bonds in methane and oxygen.
Answer: Exothermic. More energy is released when the new bonds in CO₂ and H₂O form than is taken in to break the bonds in methane and oxygen.
- Saying exothermic reactions make energy Energy cannot be created: in an exothermic reaction, chemical energy stored in bonds is transferred to the surroundings as heat. The total energy stays the same.
- Writing hydrogen as a product of acid + carbonate Acids reacting with carbonates produce carbon dioxide, not hydrogen — hydrogen comes from acids reacting with metals. Mixing these patterns is one of the most common exam errors.
Practice
Endothermic (thermal decomposition): calcium carbonate → calcium oxide + carbon dioxide needs continuous heat input, taking energy from the surroundings.
Zinc + sulfuric acid → zinc sulfate + hydrogen.
Iron is more reactive than copper, so it displaces copper from the solution: iron + copper sulfate → iron sulfate + copper. The blue colour fades as brown copper appears.
Yes. Magnesium is more reactive than zinc, so it can take the oxygen from zinc oxide: magnesium + zinc oxide → magnesium oxide + zinc.
Quick check
Which of these best defines "exothermic"?
How would you make pure crystals of copper sulfate from copper oxide and dilute sulfuric acid?
- Energy in Reactions: distinguish exothermic and endothermic reactions.
- Energy changes come from bond breaking and making: Breaking bonds takes in energy; making bonds releases energy.
- endothermic: A reaction that takes in energy from the surroundings — the surroundings cool down.
- Watch out for: saying exothermic reactions make energy