The Reactivity Race

What reactivity actually means

Metals differ enormously in how readily they react. Potassium reacts violently with cold water; copper sits in water indefinitely and does nothing. Underneath all of it is one property: how readily the metal forms a positive ion. A metal that loses electrons easily reacts readily. That is what "reactive" means here. And every observation in this topic is a different way of measuring that same property: - How readily it reacts with oxygen to form an oxide - Whether it reacts with cold water, only with steam, or with neither - How vigorously it fizzes in dilute acid See them as three tests of one thing and you can place an unknown metal from any of them. Learn three separate lists and you cannot.

Oxidation, reduction, oxides

Four terms. Note carefully how oxidation and reduction are defined at this point in the course.

Which one is oxidation?

At this point in the course, oxidation and reduction are defined in terms of oxygen. Which statement is correct?

• Oxidation is the gain of oxygen, so a metal burning in air to form its oxide has been oxidised
• Oxidation is the loss of oxygen
• Oxidation is any reaction that gives out heat
• Oxidation is any reaction involving an acid

The order to learn

This is the one list in the topic worth committing to memory, most reactive first. potassium · sodium · lithium · calcium · magnesium · CARBON · zinc · iron · HYDROGEN · copper Carbon and hydrogen are not metals. They are in the list as reference points, and each one marks a boundary that predicts behaviour: - Above hydrogen → the metal reacts with dilute acid, giving off hydrogen gas - Below hydrogen → it does not react with dilute acid at all, which is why copper does not fizz Carbon's position matters for how a metal is extracted from its ore, which is the next module's business rather than this one's. Most people learn the order with a mnemonic. Any sentence whose words begin P S L C M C Z I H C will do, and one you invent yourself is easier to keep.

Put the metals in order

Arrange these five metals from most reactive to least reactive.

• Potassium
• Calcium
• Magnesium
• Iron
• Copper

Water and acid

The two reactions used to work out the order, and what each one can and cannot separate.

Which react with cold water?

Select the TWO metals that react with COLD water.

• Sodium
• Calcium
• Zinc
• Copper

Match each metal to what it does in cold water

Equations for the reactions

When a metal burns in oxygen it forms a metal _____, and the metal has been _____ because it has gained oxygen. When sodium reacts with cold water the products are sodium hydroxide and _____. When magnesium reacts with dilute hydrochloric acid the products are a _____ and hydrogen. A metal positioned _____ hydrogen in the reactivity series will not react with dilute acid at all.

Why copper does not fizz

A student adds dilute hydrochloric acid to magnesium and to copper. The magnesium fizzes steadily; nothing happens to the copper. Why?

• Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid, while magnesium is above hydrogen and can
• The acid was too dilute to react with copper
• The copper had an oxide coating protecting it
• Copper does react, just too slowly to see in a lesson

From observation to conclusion

Build a sentence that goes from what was seen to what it shows, which is what a practical conclusion has to do.

Deducing the order

"Three unknown metals, A, B and C, were tested. A reacted vigorously with cold water. B did not react with cold water but fizzed steadily in dilute acid. C did not react with either. Place them in order and explain. (6 marks)" "A is the most reactive. Only the most reactive metals react with cold water, so A belongs with potassium, sodium, lithium and calcium. B is next. It failed the water test but reacted with dilute acid, which places it below calcium but above hydrogen, since only metals above hydrogen displace it from an acid. Zinc and iron sit in this part of the series. C is the least reactive. Reacting with neither water nor dilute acid places it below hydrogen, like copper. The order is therefore A, B, C. The underlying reason is the same in all three cases: a more reactive metal forms a positive ion more readily, so it reacts more readily with water and with acid alike." Two things that answer does and most do not. It names the boundary each observation establishes rather than just saying which reacted more. And it gives one underlying reason covering all three, instead of three separate explanations.

Reactivity drill

Five quick questions on the series. Three lives.

Running the practical

You are placing four unknown metals in order of reactivity in the laboratory. Work through the decisions.

• All four look different: one is a bright strip, one is dull and grey, one is reddish. Where do you start?
• You add the same acid to each. Three fizz at clearly different rates; the fourth does nothing. What have you learned about the fourth?
• Two of the three that fizzed looked about equally vigorous. How do you separate them?

Your turn: order the unknowns

Four metals, W, X, Y and Z, were tested. W reacted slowly with cold water. X reacted with steam only. Y reacted vigorously with cold water. Z reacted with neither water nor dilute acid. Work with these results.

• Place the four metals in order from most reactive to least, and name a real metal each could be
• For each placement, name the boundary in the series that the observation establishes
• Explain, in terms of forming positive ions, why a more reactive metal reacts more vigorously
• Describe one thing you would keep the same if you repeated the acid test, and say why it matters