CHEMISTRY · Stage 8 · Chapter 9: Chemical Reactions 2
Chemistry 9.8 Metals and acids — Salt Formation and Hydrogen Gas
5 min read 7 parts Lesson 8 of 12
Lesson summary
9.8 Metals and acids — Salt Formation and Hydrogen Gas — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe how metals react with dilute acids, predict the salt and hydrogen formed, and evaluate an environmental issue.Equation — metal + acid → salt + hydrogen…
- Naming the salt
- Zinc and the hyena
- Zinc and iron
- Potassium, sodium and calcium
- Reactivity order
- Excess metal
Part 1 of 7The general reaction
1Equation1 point
— metal + acid → salt + hydrogen, for metals above hydrogen in the reactivity series.
- All the reactions make hydrogen gas, and the other product is a compound that dissolves in water.
2Naming the salt1 point
— it is named from the metal and the acid.
- Hydrochloric acid forms chlorides, sulfuric acid forms sulfates and nitric acid forms nitrates.
3Examples
— magnesium + hydrochloric acid → magnesium chloride + hydrogen; zinc + hydrochloric acid → zinc chloride + hydrogen; iron + hydrochloric acid → iron chloride + hydrogen.
4Zinc and the hyena1 point
— a hyena that swallowed zinc coins was poisoned because stomach hydrochloric acid reacted with the zinc.
- Zinc chloride dissolves in water and travels around the body in the blood.
Part 2 of 7Observing the reactions
- Magnesiumit bubbles vigorously, the ribbon gets smaller and disappears, and a colourless solution remains.
- Zinc and ironthey bubble slowly or steadily, and iron gives a pale green solution.
- Copperit does not react with dilute acid, and neither do silver and gold.
- Potassium, sodium and calciumthey react very vigorously with dilute acids, so these reactions are not done in school laboratories.
- Heatthe test tube becomes warm, showing the reaction is exothermic, and the more reactive the metal the greater the temperature rise.
| Metal | Observation with dilute hydrochloric acid |
|---|---|
| Magnesium | Rapid bubbling, disappears quickly, warm |
| Zinc | Steady bubbling |
| Iron | Slow bubbling, pale green solution |
| Copper | No reaction |
- Reactivity order — reactivity with dilute acids decreases from potassium to gold: potassium, sodium, calcium, magnesium, zinc, iron, copper, silver, gold.
Part 3 of 7Test for hydrogen
- Test — hold a lit splint (not a glowing splint) at the mouth of the test tube.
- Result — a squeaky pop shows hydrogen, which is burning with oxygen in the air to form water.
Part 4 of 7Making a sample of salt
- Add excess metal to the acid until no more bubbles appear, so all the acid is used.
- Filter off the leftover metal.
- Gently heat the solution to evaporate some of the water.
- Leave the solution to crystallise.
- Excess metal — it ensures the solution contains only the salt and no unreacted acid.
Part 5 of 7Ranking metals fairly
- Same conditionsuse the same acid, volume and concentration, and the same size of metal pieces.
- Measuring ratecount bubbles per minute, time how long the metal takes to disappear, or measure the volume of gas collected in a fixed time with a gas syringe.
- Grapha graph of gas volume against time shows the steepest line for the most reactive metal.
- Reliability and safetyrepeat and calculate a mean, wear eye protection and keep flames away because hydrogen is flammable.
- Potassium and sodiumthey must never be added to acid because the reaction is dangerously violent.
Part 6 of 7Explaining the pattern
- Electron transfermetal atoms lose electrons to form positive ions, while hydrogen ions from the acid gain electrons and become hydrogen gas.
- Reactive metalsthey lose electrons more easily, giving a faster reaction.
- Metals below hydrogenthey cannot push hydrogen out of the acid, so there is no reaction.
Part 7 of 7An acidic river: the Rio Tinto
- Evidence — the Rio Tinto in Spain has a pH of about 2 and is red because iron compounds are dissolved in it.
- Compounds of poisonous metals such as cadmium are also dissolved, so few plants and animals can live in the water.
- Cause — in the environment, acids can dissolve metal compounds from mine waste.
- Evaluation — scientists analysed the river water to find out exactly what is in it, hoping the knowledge will help solve the environmental problems.
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