Hydrogen and water

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Section 2: Chemistry of the Elements; part e) Hydrogen and water

2.26 describe the reactions of dilute hydrochloric and dilute sulfuric acids with magnesium, aluminium, zinc and iron

Metals above hydrogen in the reactivity series will react with acids to form a salt (e.g. magnesium sulfate or zinc chloride) and hydrogen. The metals are 'displacing' hydrogen. The higher the metal in the series, the more violent the reaction. (This is why if you put copper in acids, you won't see a reaction, as it is below hydrogen in the reactivity series. However, it does react with concentrated nitric acid but we're not concerned with that now.)

Metal + dilute hydrochloric acid à metal chloride + hydrogen

Metal + dilute sulfuric acid à metal sulfate + hydrogen

Magnesium

Magnesium reacts vigorously with cold dilute acids, and the mixture becomes very warm as heat is produced. There is rapid fizzing (effervescence) and a colourless gas is evolved, which pops with a lighted splint (the test for hydrogen). The magnesium gradually disappears and a colourless solution of magnesium sulfate or chloride is formed. 

--The reactions between magnesium and hydrochloric acid or sulfuric acid are similar because it is reacting with the hydrogen ions. All acids in solutions have hydrogen ions. Although hydrochloric acid has chloride ions, and sulfuric acid has sulfate ions, these are spectator ions. They do not participate in the reaction and are unchanged by it. 

You can rewrite the equations as ionic equations. In the case of hydrochloric acid:

Mg (s) + 2H⁺ (aq) + 2Cl^- (aq) à Mg²⁺(aq) + 2Cl^- (aq) + H₂ (g)

You can see that the chloride ions weren’t changed by the reaction. It is a spectator ion, so we leave it out of the ionic equation. Leaving out the spectator ions produces the ionic equation:

Mg(s) + 2H⁺(aq) à Mg²⁺ (aq) + H₂ (g)

Repeating this with sulfuric acid:

Mg (s) + 2H⁺ (aq) + SO₄^2- (aq) à Mg²⁺ (aq) + SO₄^2- (aq) + H₂(g)

Again, leaving out the spectator ion which is the sulfate ion in this case.

Mg(s) + 2H⁺(aq) à Mg²⁺ (aq) + H₂ (g)

So the reactions look the same because they are the same. All acids in solution contain hydrogen ions. That means that magnesium will react with any simple dilute acid in the same way. 

Aluminium

Aluminium is slow to start reacting, but after warming it reacts very vigorously. There is a very thin, but very strong, layer of aluminium oxide on the surface of the aluminium, which stops the acid from getting to it. On heating, the acid removes this layer, and the aluminium can show its true reactivity. With dilute hydrochloric acid:

2Al (s) + 6HCl (aq) à 2AlCl₃ (aq) + 3H₂ (g)

Zinc and Iron

Zinc and iron react slowly in the cold, but more rapidly on heating. Their reactions are less vigorous than that of aluminium, and iron less than zinc of course, as it is below zinc in the reactivity series. Zinc forms zinc chloride or sulfate and hydrogen. The iron forms iron (II) sulfate or iron (II) chloride and hydrogen. For example:

Zn (s) + H₂SO₄(aq) à ZnSO₄ (aq) + H₂ (g)

Fe (s) + 2HCl (aq) à FeCl₂ (aq) + H₂ (g)

2.27 describe the combustion of hydrogen

Hydrogen reacts violently with oxygen in the presence of a flame to give water. It could explode if there was a lot of hydrogen. But a lighted splint placed at the mouth of a test tube of hydrogen will just give a squeaky pop as the hydrogen reacts with oxygen in the air. The lighted splint and a squeaky pop heard is the test for hydrogen. 

2.28 describe the use of anhydrous copper (II) sulfate in the chemical test for water

Anhydrous copper (II) sulfate is white, anhydrous being without water, it is dry (an--without, hydrous--related to water). Whereas hydrated copper (II) sulfate crystals are bright blue, the water is what gives it the colour, and is part of the structure. To show that the water is part of the structure, there is a '.' [dot] in the formula: 

CuSO₄·5H₂O 

^You see the dot in the middle? That shows the water is part of the copper sulfate crystal structure. 

So that is a chemical test for water, just add it to anhydrous copper (II) sulfate and watch it turn blue!

Adding water to anhydrous copper sulphate

2.29 describe a physical test to show whether water is pure

Heat the water and use a thermometer to check if it boils at exactly 100°C. Pure water boils at exactly 100°C. Or you can cool it until it freezes, it should freeze at exactly 0°C. My teacher said it's safer to state both, as pure water is the only substance that has these specific boiling and freezing points, whereas another substance might boil/freeze at either temperature. 

Hope this helped!