3.11Reactions of acids with metals
Acids react with some metals to produce salts and hydrogen.
acid + metal → salt + hydrogen
Knowledge of these reactions is limited to those of magnesium, zinc and iron with hydrochloric and sulfuric acids. Hydrochloric acid gives chlorides and sulfuric acid gives sulfates, so those six combinations are the ones to be fluent in.
For example: Mg + 2HCl → MgCl₂ + H₂, and Zn + H₂SO₄ → ZnSO₄ + H₂.
The hydrogen is seen as fizzing, and it can be tested for with a lighted splint, which gives a squeaky pop.
3.11Acid and metal reactions as redoxHigher tier
Higher tier students should be able to explain, in terms of gain or loss of electrons, that these are redox reactions, and identify which species are oxidised and which are reduced in given chemical equations.
Take Mg + 2HCl → MgCl₂ + H₂. The chloride ions are spectators, so the ionic equation is Mg + 2H⁺ → Mg²⁺ + H₂. The magnesium atom loses two electrons to become Mg²⁺, so magnesium is oxidised. Each hydrogen ion gains an electron to become part of a hydrogen molecule, so the hydrogen ions are reduced. Both happen together, which is what makes it a redox reaction.
Common mistake
The acid itself is not "reduced" — name the species. It is the hydrogen ions that gain electrons, and it is the metal atoms that lose them.
3.13–3.14Neutralisation of acids and salt production
Acids are neutralised by alkalis, such as soluble metal hydroxides, and by bases, such as insoluble metal hydroxides and metal oxides, to produce salts and water. They are neutralised by metal carbonates to produce salts, water and carbon dioxide.
The particular salt produced in any reaction between an acid and a base or alkali depends on two things:
• the acid used — hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates • the positive ions in the base, alkali or carbonate.
So the name of the salt is simply the metal from one reactant followed by the ending from the other. Copper oxide with sulfuric acid gives copper sulfate; sodium hydroxide with nitric acid gives sodium nitrate.
You should be able to predict products from given reactants, and use the formulae of common ions to deduce the formulae of salts.
The formula follows from the charges: the total positive charge must equal the total negative charge, so the numbers of each ion are whatever makes the compound neutral overall.
Worked example
Deduce the formula of calcium nitrate, given Ca²⁺ and NO₃⁻.
- The calcium ion carries 2+, and each nitrate ion carries 1−.
- For the compound to be neutral, two nitrate ions are needed for each calcium ion.
- Write the positive ion first, then the negative ion with the number needed.
- Because nitrate is a group of atoms, put brackets round it before the subscript.
Answer: Ca(NO₃)₂
Common mistake
Brackets matter. Ca(NO₃)₂ is two nitrate groups; CaNO₃₂ means nothing, and CaNO₆ is a different set of atoms entirely. Use brackets whenever more than one of a grouped ion is needed.
- Neutralisation
- the reaction of an acid with an alkali or base to produce a salt and water; hydrogen ions react with hydroxide ions to produce water
- Alkali
- a soluble base, such as a soluble metal hydroxide, whose aqueous solution contains hydroxide ions
- Base
- a substance that neutralises an acid, such as an insoluble metal hydroxide or a metal oxide
3.12Test for hydrogen
The test for hydrogen uses a burning splint held at the open end of a test tube of the gas. Hydrogen burns rapidly with a pop sound.
This is the test you meet whenever a metal reacts with water or with an acid, because hydrogen is the gas given off in both.
3.12Test for carbon dioxide
The test for carbon dioxide uses an aqueous solution of calcium hydroxide — limewater. When carbon dioxide is shaken with or bubbled through limewater, the limewater turns milky, or cloudy.
This is the test used whenever a carbonate reacts with an acid, and whenever a carbonate is heated until it decomposes.
- Limewater
- an aqueous solution of calcium hydroxide, used to test for carbon dioxide; it turns milky when carbon dioxide is bubbled through it
3.15–3.18Soluble salts
Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates. The solid is added to the acid until no more reacts, and the excess solid is filtered off to produce a solution of the salt. Salt solutions can be crystallised to produce solid salts.
Adding the solid in excess is the trick that makes this method work: it guarantees that all the acid is used up, so the filtered solution contains only the salt and water, with no acid left to contaminate the crystals.
You should be able to describe how to make pure, dry samples of named soluble salts from information provided.
Required practical 1
Preparation of a pure, dry sample of a soluble salt from an insoluble oxide or carbonate
- Measure a fixed volume of dilute acid into a beaker and warm it gently with a Bunsen burner — warming speeds the reaction up, but do not boil.
- Add the insoluble metal oxide or carbonate a spatula at a time, stirring, until no more reacts and some solid remains at the bottom. That excess is the signal that all the acid has been neutralised.
- Filter the mixture to remove the excess solid, keeping the filtrate — a solution of the salt.
- Pour the filtrate into an evaporating basin and heat it on a water bath or electric heater until crystals begin to form at the edges.
- Leave the solution to cool and crystallise slowly, then pat the crystals dry between filter papers.
Common mistake
Do not evaporate the solution to dryness over a Bunsen burner. Heating too far decomposes many salts and drives off the water of crystallisation, so the product is no longer the salt asked for.
3.19–3.21Solubility rules and insoluble salts
Whether a salt dissolves decides how it has to be made, so the solubility rules come first. They are a short list and they are worth learning exactly, because a question will give you two solutions and ask whether anything precipitates.
To predict a precipitate, swap the partners. Mix silver nitrate solution with sodium chloride solution and the possible products are silver chloride and sodium nitrate. Sodium nitrate is soluble, so it stays in solution; silver chloride is one of the two exceptions, so it comes out as a solid. That white precipitate is the reason silver nitrate is used as a test for chloride ions.
An insoluble salt is therefore made by precipitation, and the method has four steps the exam expects in order:
• mix solutions of two soluble salts that between them contain the two ions you want, in excess so that all of one is used up • filter the mixture, so the insoluble salt is left in the filter paper as the residue • wash the residue with distilled water, to rinse away the soluble salt still clinging to it • dry it, in a warm oven or between filter papers.
Washing is the step most often left out, and it is the one that makes the sample pure.
Worked example
Name two solutions that could be mixed to make barium sulfate, and say which product stays in solution.
- Barium sulfate needs a barium ion and a sulfate ion, each from a soluble compound.
- All nitrates are soluble, so barium nitrate supplies the barium.
- All sodium salts are soluble, so sodium sulfate supplies the sulfate.
- Swapping the partners also makes sodium nitrate, which is soluble.
Answer: Barium nitrate solution and sodium sulfate solution; the sodium nitrate stays dissolved and is washed away.
Common mistake
"Filter and dry" is not enough for a pure sample. Without the wash, the crystals dry with the other salt from the solution still on them, and the answer loses the purity mark.
Before you practise
- acid + metal → salt + hydrogen. acid + metal oxide or hydroxide → salt + water. acid + metal carbonate → salt + water + carbon dioxide.
- The salt is named from the metal and the acid: hydrochloric gives chlorides, sulfuric gives sulfates, nitric gives nitrates.
- Neutralisation is H⁺ + OH⁻ → H₂O — learn the ionic equation.
- Hydrogen gives a squeaky pop with a lit splint; carbon dioxide turns limewater milky.
- Soluble salt from an insoluble base: add the base in excess, filter off what is left, then crystallise.
- Soluble salt from a soluble base: titrate first to find the volumes, then repeat without the indicator.
- Insoluble salt: mix two solutions, filter, wash the residue with distilled water, then dry.
- The wash is what makes the sample pure, and it is the step most often left out.