- Define and explain Making Salts in your own words
- Use key terms such as salt accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Making Salts: prepare pure salts by reacting acids with bases and carbonates.
Prepare pure salts by reacting acids with bases and carbonates.
Key ideas
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.
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.
Key term — salt: A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion, e.g. sodium chloride.
Write the word equation for zinc reacting with dilute sulfuric acid.
Zinc + sulfuric acid → zinc sulfate + hydrogen.
Answer: Zinc + sulfuric acid → zinc sulfate + hydrogen.
- 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.
- 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.
Practice
Warm the acid and add excess copper oxide until no more reacts; filter off the excess solid; gently evaporate the filtrate to concentrate it; leave to crystallise, then dry the crystals.
Endothermic (thermal decomposition): calcium carbonate → calcium oxide + carbon dioxide needs continuous heat input, taking energy from the surroundings.
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 "salt"?
Burning methane warms the room. Is this exothermic or endothermic? Explain in terms of bonds.
- Making Salts: prepare pure salts by reacting acids with bases and carbonates.
- Acids react in predictable patterns: Acids react with metals to give a salt plus hydrogen: for example, magnesium + hydrochloric acid → magnesium chloride + hydrogen.
- reactivity series: Metals listed in order of reactivity, from potassium (most reactive) to gold (least).
- Watch out for: writing hydrogen as a product of acid + carbonate